H Macaigne

V G Ruggieri

L Vallet-Tadeusz

V Vernet-Garnier

L S Aho-Glélé

O Bajolet

Dr Annick Lefebvre
email: alefebvre@chu-reims.fr

Are systematic drain tip or drainage fluid cultures predictive of surgical site infections?

Keywords: Drain tip, drainage fluid, culture, prediction, surgical site infection

Background: Systematic cultures of drain tips or drainage fluids for the early detection of surgical site infections (SSI) are controversial.

Aim:

To examine the association between the results of systematic drain tip or drainage fluid cultures and the occurrence of SSI in clean or clean-contaminated surgery.

Methods:

We searched in the Pubmed, and Cat.inist databases for observational studies published before March 31 st 2017. We included studies reporting results of drain tip or drainage fluid systematic cultures and SSI after clean or clean-contaminated surgeries, and we performed meta-analyses.

Findings: Seventeen studies, including 4,390 patients for drain tip cultures and 1,288 for drainage fluid cultures, were selected. The pooled negative predictive values were high (99%, 95% CI [98-100] for drain tip cultures and 98%, 95% CI [94-100] for drainage fluid cultures).

The positive predictive values were low (11%, 95% CI [2-24] for drain tip cultures and 12%, 95% CI for drainage fluid cultures). The sensitivities were low (41%, 95% CI for drain tip cultures and 37%, 95% CI for drainage fluid cultures). The specificities were high (93%, 95% CI [88][89][90][91][92][93][94][95][96]) for drain tip cultures and moderate (77%, 95% CI [54-94]) for drainage fluid cultures.

Conclusion:

Systematic cultures of drain tips or drainage fluids appear not to be relevant, because their positive predictive values were low in the prediction of SSI.

INTRODUCTION

Surgical site infections (SSI) are still major surgical complications [START_REF] Whitehouse | The Impact of Surgical-Site Infections Following Orthopedic Surgery at a Community Hospital and a University Hospital Adverse Quality of Life, Excess Length of Stay, and Extra Cost[END_REF]. The contamination of the surgical site may occur during pre-operative, per-operative or post-operative periods.

Surgical drainage can be used to prevent hematoma formation, and thus SSI, but can also be a risk factor for SSI [START_REF] Reiffel | A Multi-Disciplinary Review of the Potential Association between Closed-Suction Drains and Surgical Site Infection[END_REF]. Indeed, many studies have found an association between the presence of surgical drainage and SSI or between the drainage duration and the proportion of SSI [START_REF] Reiffel | A Multi-Disciplinary Review of the Potential Association between Closed-Suction Drains and Surgical Site Infection[END_REF][START_REF] Barbadoro | May the drain be a way in for microbes in surgical infections?[END_REF][START_REF] Tschudin-Sutter | Drainage days-an independent risk factor for serious sternal wound infections after cardiac surgery: a case control study[END_REF].

Systematic cultures of drain tips or drainage fluids are commonly used by surgical teams for the early detection of SSI, even in the absence of clinical suspicion of infection. However, their prognostic values are controversial, and the collection and laboratory processing of these samples are costly and time-consuming [START_REF] Ahn | Suction Drain Tip Culture after Spine Surgery: Can It Predict a Surgical Site Infection?[END_REF].

We conducted meta-analyses of published comparative studies reporting on the association between results of systematic drain tip or drainage fluid cultures and the occurrence of SSI in clean or clean-contaminated surgery.

METHODS

The study was performed according to the recommendations of the Preferred Reporting Items for a Systematic Review and Meta-analysis of Diagnostic Test Accuracy Studies (PRISMA-DTA) [START_REF] Mcinnes | Preferred Reporting Items for a Systematic Review and Meta-analysis of Diagnostic Test Accuracy Studies: The PRISMA-DTA Statement[END_REF].

We searched in the Pubmed, and Cat.inist databases for articles published before March 2017. First, we identified three MeSH (Medical Subject Headings) descriptors (1. Surgical wound infection; 2. Drainage; 3. Microbiological techniques) and linked the MeSH terms that corresponded to the descriptors by "OR" and each descriptor was associated by "AND" (Supplemental material 1). A second Pubmed search was performed using the following strategy: "(surgical OR surgery) AND drain[title/abstract] AND culture AND infection[title/abstract]". The Cat.inist base was searched with the equation "suction AND drain AND culture AND infection." Finally, a manual search from the bibliography of the selected articles was carried out.

Selection

Original articles about the use of systematic surgical drain tip or drainage fluid cultures to predict SSI were selected. We excluded studies in which cultures were performed only in cases of suspicion of SSI, as well as studies that only included contaminated or dirty/infected surgeries.

We also contacted five authors of studies that were potentially eligible for inclusion in the meta-analysis in order to obtain additional information or new results [START_REF] Yamada | Drain tip culture is not prognostic for surgical site infection in spinal surgery under prophylactic use of antibiotics[END_REF][START_REF] Lindahl | Bacterial contamination and closed suction drainage in open meniscectomy of the knee[END_REF][START_REF] Anoumou | Relevance of the systematic culture of the intraoperative swab and drain tip of Redon in orthopaedictraumatology surgery[END_REF][START_REF] Cosseron | Intérêt des cultures systématiques du liquide de drainage en chirurgie de classe I et II d'Altemeier[END_REF][START_REF] Lazureanu | Drain tip cultures do not predict infections in primary total knee arthroplasty[END_REF]. Two of them replied to us [START_REF] Yamada | Drain tip culture is not prognostic for surgical site infection in spinal surgery under prophylactic use of antibiotics[END_REF][START_REF] Lindahl | Bacterial contamination and closed suction drainage in open meniscectomy of the knee[END_REF].

Quality

Two readers independently assessed the study limitations for each selected article employing the Newcastle-Ottawa Scale adapted to the specific design of the study [START_REF] Stang | Critical evaluation of the Newcastle-Ottawa scale for the assessment of the quality of nonrandomized studies in meta-analyses[END_REF]. Disagreements between the two reviewers were resolved by consensus.

Meta-analyses

Data about SSI and drain tip or drainage fluid cultures were extracted from the selected studies and analysed with Stata 11 [START_REF] Statacorp | Stata/SE 10[END_REF] (metan add-on [START_REF] Harris | metan: fixed-and random-effects meta-analysis[END_REF]). Positive predictive values (PPVs), negative predictive values (NPVs), sensitivities and specificities and their respective confidence intervals (CIs) were calculated to evaluate the performance of drain tip cultures and drainage fluids cultures to predict SSI. Odds ratios (ORs) were also calculated. Metaanalyses were also performed studying the concordance between microorganisms isolated in the drain tip or drainage fluid cultures and those isolated in SSI. We used double arcsine transformations to stabilize the variance of proportions [START_REF] Rucker | Arcsine test for publication bias in meta-analyses with binary outcomes[END_REF] and a random effect model according to DerSimonian-Laird's method [START_REF] Dersimonian | Meta-analysis in clinical trials[END_REF]. Heterogeneity was tested by the I 2 and Chi2 heterogeneity tests and was explored by subgroup analyses depending on the type of specialty. A potential publication bias was examined by means of funnel plots. We performed two sensitivity analyses. The first set of analyses included studies wherein data were available only in abstract form, but not in full-text form [START_REF] Cosseron | Intérêt des cultures systématiques du liquide de drainage en chirurgie de classe I et II d'Altemeier[END_REF][START_REF] Lazureanu | Drain tip cultures do not predict infections in primary total knee arthroplasty[END_REF]. The second set included studies wherein data were available in full-text form, with a score of five or more.

RESULTS

One hundred and ninety studies were identified. The manual search revealed six new articles.

The flow-chart is presented in Figure 1. Seventeen studies were selected (Tables I andII) [START_REF] Ahn | Suction Drain Tip Culture after Spine Surgery: Can It Predict a Surgical Site Infection?[END_REF][START_REF] Yamada | Drain tip culture is not prognostic for surgical site infection in spinal surgery under prophylactic use of antibiotics[END_REF][START_REF] Lindahl | Bacterial contamination and closed suction drainage in open meniscectomy of the knee[END_REF][START_REF] Aski | Drain tip culture following total knee arthroplasty[END_REF][START_REF] Becker | Quantitative bacteriology of closed-suction wound drainage in contaminated surgery[END_REF][START_REF] Becker | Ineffectiveness of closed suction drainage cultures in the prediction of bacteriologic findings in wound infections in patients undergoing contaminated head and neck cancer surgery[END_REF][START_REF] Bernard | The Value of Suction Drainage Fluid Culture during Aseptic and Septic Orthopedic Surgery: A Prospective Study of 901 Patients[END_REF][START_REF] Degnim | Randomized controlled trial to reduce bacterial colonization of surgical drains after breast and axillary operations[END_REF][START_REF] Girvent | The clinical significance of suction drainage cultures[END_REF][START_REF] Gunterberg | Bacterial growth on drain tips after total hip replacement. A controlled culture method[END_REF][START_REF] Krishnan | Suction Drain Tip Cultures and Predictors of Surgical Site Infections in Hip Fractures[END_REF][START_REF] Overgaard | Closed suction drainage after hip arthroplasty. Prospective study of bacterial contamination in 81 cases[END_REF][START_REF] Petsatodis | Prognostic value of suction drain tip culture in determining joint infection in primary and non-infected revision total hip arthroplasty: a prospective comparative study and review of the literature[END_REF][START_REF] Sankar | Suction drain tip culture in orthopaedic surgery: a prospective study of 214 clean operations[END_REF][START_REF] Takada | Is Drain Tip Culture Prognostic of Surgical Site Infection? Results of 1380 Drain Tip Cultures in Total Hip Arthroplasty[END_REF][START_REF] Weinrauch | Diagnostic value of routine drain tip culture in primary joint arthroplasty[END_REF][START_REF] Zamora-Navas | Closed suction drainage after knee arthroplasty. A prospective study of the effectiveness of the operation and of bacterial contamination[END_REF].

Twelve studies were prospective and fourteen studies focused on orthopaedic surgery. Drain tip cultures were analysed in eleven articles, drainage fluids were analysed in three articles, and both were analysed in three other articles.

A total of 4,390 patients were included in drain tip culture studies and 1,288 were included in drainage fluid culture studies.

The studies' quality was moderate (Table III).

Between studies, the SSI proportion varied from 0% (95% CI [0-11]) to 20% (95% CI ), and the pooled proportions were 2% (95% CI [START_REF] Whitehouse | The Impact of Surgical-Site Infections Following Orthopedic Surgery at a Community Hospital and a University Hospital Adverse Quality of Life, Excess Length of Stay, and Extra Cost[END_REF][START_REF] Reiffel | A Multi-Disciplinary Review of the Potential Association between Closed-Suction Drains and Surgical Site Infection[END_REF][START_REF] Barbadoro | May the drain be a way in for microbes in surgical infections?[END_REF][START_REF] Tschudin-Sutter | Drainage days-an independent risk factor for serious sternal wound infections after cardiac surgery: a case control study[END_REF]) for drain tip culture studies and 5% (95% CI [START_REF] Whitehouse | The Impact of Surgical-Site Infections Following Orthopedic Surgery at a Community Hospital and a University Hospital Adverse Quality of Life, Excess Length of Stay, and Extra Cost[END_REF][START_REF] Reiffel | A Multi-Disciplinary Review of the Potential Association between Closed-Suction Drains and Surgical Site Infection[END_REF][START_REF] Barbadoro | May the drain be a way in for microbes in surgical infections?[END_REF][START_REF] Tschudin-Sutter | Drainage days-an independent risk factor for serious sternal wound infections after cardiac surgery: a case control study[END_REF][START_REF] Ahn | Suction Drain Tip Culture after Spine Surgery: Can It Predict a Surgical Site Infection?[END_REF][START_REF] Mcinnes | Preferred Reporting Items for a Systematic Review and Meta-analysis of Diagnostic Test Accuracy Studies: The PRISMA-DTA Statement[END_REF][START_REF] Yamada | Drain tip culture is not prognostic for surgical site infection in spinal surgery under prophylactic use of antibiotics[END_REF][START_REF] Lindahl | Bacterial contamination and closed suction drainage in open meniscectomy of the knee[END_REF][START_REF] Anoumou | Relevance of the systematic culture of the intraoperative swab and drain tip of Redon in orthopaedictraumatology surgery[END_REF][START_REF] Cosseron | Intérêt des cultures systématiques du liquide de drainage en chirurgie de classe I et II d'Altemeier[END_REF] for drainage fluid culture studies) (Figure 2).

Figure 3 shows the results of PPV for each study. The pooled PPVs were low (11%, 95% CI for drain tip cultures and 12%, 95% CI for drainage fluid cultures).

Figure 4 shows the results of NPV for each study. The pooled NPVs were high (99%, 95% CI [98-100] for drain tip cultures and 98%, 95% CI [94-100] for drainage fluid cultures).

The sensitivities were low (41%, 95% CI (I²= 88.3%, p<0.001) for drain tip cultures and 37%, 95% CI (I²= 40.1%, p=0.154) for drainage fluid cultures).

The specificities were high (93%, 95% CI [88-96] (I²= 95.4%, p<0.001)) for drain tip cultures and moderate (77%, 95% CI [54-94] (I²= 97.9%, p<0.001)) for drainage fluid cultures.

The ORs showed an association between positive cultures and SSI, which was significant for drain tip cultures (OR=11.88, 95% CI [3.38-41.72]) but not for drainage fluid cultures (OR=3.42, 95% CI [0.70-16.63]). Subgroup analyses by specialty were performed in the presence of heterogeneity. Only the orthopedic surgery specialty saw more than two studies included. In the subgroup analysis including orthopedic studies only, heterogeneity was still high (I² =76.6%, p<0.001).

When studying the concordance between the drain tip or drainage fluid cultures and the SSI isolated bacteria, PPVs were very low (6%, 95% CI [0-23] for drain tip cultures and 7%, 95% CI [START_REF] Whitehouse | The Impact of Surgical-Site Infections Following Orthopedic Surgery at a Community Hospital and a University Hospital Adverse Quality of Life, Excess Length of Stay, and Extra Cost[END_REF][START_REF] Reiffel | A Multi-Disciplinary Review of the Potential Association between Closed-Suction Drains and Surgical Site Infection[END_REF][START_REF] Barbadoro | May the drain be a way in for microbes in surgical infections?[END_REF][START_REF] Tschudin-Sutter | Drainage days-an independent risk factor for serious sternal wound infections after cardiac surgery: a case control study[END_REF][START_REF] Ahn | Suction Drain Tip Culture after Spine Surgery: Can It Predict a Surgical Site Infection?[END_REF][START_REF] Mcinnes | Preferred Reporting Items for a Systematic Review and Meta-analysis of Diagnostic Test Accuracy Studies: The PRISMA-DTA Statement[END_REF][START_REF] Yamada | Drain tip culture is not prognostic for surgical site infection in spinal surgery under prophylactic use of antibiotics[END_REF][START_REF] Lindahl | Bacterial contamination and closed suction drainage in open meniscectomy of the knee[END_REF][START_REF] Anoumou | Relevance of the systematic culture of the intraoperative swab and drain tip of Redon in orthopaedictraumatology surgery[END_REF][START_REF] Cosseron | Intérêt des cultures systématiques du liquide de drainage en chirurgie de classe I et II d'Altemeier[END_REF][START_REF] Lazureanu | Drain tip cultures do not predict infections in primary total knee arthroplasty[END_REF][START_REF] Stang | Critical evaluation of the Newcastle-Ottawa scale for the assessment of the quality of nonrandomized studies in meta-analyses[END_REF][START_REF] Statacorp | Stata/SE 10[END_REF][START_REF] Harris | metan: fixed-and random-effects meta-analysis[END_REF][START_REF] Rucker | Arcsine test for publication bias in meta-analyses with binary outcomes[END_REF][START_REF] Dersimonian | Meta-analysis in clinical trials[END_REF][START_REF] Aski | Drain tip culture following total knee arthroplasty[END_REF][START_REF] Becker | Quantitative bacteriology of closed-suction wound drainage in contaminated surgery[END_REF] for drainage fluid cultures) (Supplemental material 2).

The funnel plot (Figure 5) did not suggest a publication bias.

Sensitivity analyses gave similar results (Supplemental material 3).

DISCUSSION

This study allowed us to determine the performances of systematic drain tip or drainage fluid cultures in the prediction of SSI and in the prediction of the microorganisms involved in SSI.

Systematic drain tip or drainage fluid cultures seem to be of little value since the PPVs that represent the probability of SSI in the presence of a positive culture were low (11%, 95% CI for drain tip cultures and 12%, 95% CI for drainage fluid cultures). Moreover, the micro-organisms identified in the cultures did not systematically correspond to the microorganisms involved in SSI (PPVs of 6%, 95% CI [0-23] for drain tip cultures and 7%, 95% CI [START_REF] Whitehouse | The Impact of Surgical-Site Infections Following Orthopedic Surgery at a Community Hospital and a University Hospital Adverse Quality of Life, Excess Length of Stay, and Extra Cost[END_REF][START_REF] Reiffel | A Multi-Disciplinary Review of the Potential Association between Closed-Suction Drains and Surgical Site Infection[END_REF][START_REF] Barbadoro | May the drain be a way in for microbes in surgical infections?[END_REF][START_REF] Tschudin-Sutter | Drainage days-an independent risk factor for serious sternal wound infections after cardiac surgery: a case control study[END_REF][START_REF] Ahn | Suction Drain Tip Culture after Spine Surgery: Can It Predict a Surgical Site Infection?[END_REF][START_REF] Mcinnes | Preferred Reporting Items for a Systematic Review and Meta-analysis of Diagnostic Test Accuracy Studies: The PRISMA-DTA Statement[END_REF][START_REF] Yamada | Drain tip culture is not prognostic for surgical site infection in spinal surgery under prophylactic use of antibiotics[END_REF][START_REF] Lindahl | Bacterial contamination and closed suction drainage in open meniscectomy of the knee[END_REF][START_REF] Anoumou | Relevance of the systematic culture of the intraoperative swab and drain tip of Redon in orthopaedictraumatology surgery[END_REF][START_REF] Cosseron | Intérêt des cultures systématiques du liquide de drainage en chirurgie de classe I et II d'Altemeier[END_REF][START_REF] Lazureanu | Drain tip cultures do not predict infections in primary total knee arthroplasty[END_REF][START_REF] Stang | Critical evaluation of the Newcastle-Ottawa scale for the assessment of the quality of nonrandomized studies in meta-analyses[END_REF][START_REF] Statacorp | Stata/SE 10[END_REF][START_REF] Harris | metan: fixed-and random-effects meta-analysis[END_REF][START_REF] Rucker | Arcsine test for publication bias in meta-analyses with binary outcomes[END_REF][START_REF] Dersimonian | Meta-analysis in clinical trials[END_REF][START_REF] Aski | Drain tip culture following total knee arthroplasty[END_REF][START_REF] Becker | Quantitative bacteriology of closed-suction wound drainage in contaminated surgery[END_REF] for drainage fluid cultures). Sensitivities were also poor; thus systematic drain tip or drainage fluid cultures are not reliable predictors of SSI in the absence of clinical signs.

Therefore, due to the lack of benefit, the associated costs (24,30 euros for a drain tip or a drainage fluid culture in France) and the risk of unnecessary or inappropriate antibiotic treatment, use of these cultures has been discouraged in our facility. The microbiology examination that should be performed is culture of biopsies, or material collected during revision surgery [START_REF]100 recommandations pour la surveillance et la prévention des infections nosocomiales[END_REF].

The low relevance of systematic drain tip or drainage fluid cultures has been shown in several studies, particularly dealing with orthopedic surgery [START_REF] Overgaard | Closed suction drainage after hip arthroplasty. Prospective study of bacterial contamination in 81 cases[END_REF][START_REF] Petsatodis | Prognostic value of suction drain tip culture in determining joint infection in primary and non-infected revision total hip arthroplasty: a prospective comparative study and review of the literature[END_REF][START_REF] Weinrauch | Diagnostic value of routine drain tip culture in primary joint arthroplasty[END_REF][START_REF] Zamora-Navas | Closed suction drainage after knee arthroplasty. A prospective study of the effectiveness of the operation and of bacterial contamination[END_REF].

However, in the study conducted by Bernard et al. [START_REF] Bernard | The Value of Suction Drainage Fluid Culture during Aseptic and Septic Orthopedic Surgery: A Prospective Study of 901 Patients[END_REF], drainage fluid cultures seemed to be promising in septic surgery with a PPV of 87%, being particularly useful in the follow-up of the efficacy of surgical and antibiotic treatments.

The NPVs were high in our meta-analysis (99%, 95% CI [98-100] for drain tip cultures and 98%, 95% CI [94-100] for drainage fluid cultures). However, the added value of this test is poor as the incidence of SSI is low in the literature (from 0.6 % in knee prosthesis surgery to 10.7% non-endoscopic colorectal surgery) [START_REF]Annual Epidemiological Report 2016 -Surgical site infections[END_REF] and in studies included in our meta-analysis.

The heterogeneity was high (>50% and p<0.001). This heterogeneity was not explained by the type of surgical specialty, but might be explained by the differences in sampling methods or microbiological techniques. The sampling methods varied significantly between studies and were not sufficiently described in several studies. It was therefore not possible to take them into account in our meta-analysis.

To our knowledge, this is the first meta-analysis to explore the performances of systematic drain tip or drainage fluid cultures after clean or clean-contaminated surgery in the prediction of SSI. We included 17 studies involving 4,390 patients for drain tip cultures and 1,288 patients for drainage fluid cultures. Although an association between positive cultures and SSI was observed in several included studies, several of them did not show the performances in terms of NPV, PPV, sensitivity and specificity of these cultures [START_REF] Aski | Drain tip culture following total knee arthroplasty[END_REF][START_REF] Degnim | Randomized controlled trial to reduce bacterial colonization of surgical drains after breast and axillary operations[END_REF][START_REF] Krishnan | Suction Drain Tip Cultures and Predictors of Surgical Site Infections in Hip Fractures[END_REF][START_REF] Weinrauch | Diagnostic value of routine drain tip culture in primary joint arthroplasty[END_REF]. Our metaanalysis allowed us to determine these performances in the prediction of SSI and to pool them.

The Newcastle-Ottawa scale was preferred to the QUADAS-2 [START_REF] Whiting | QUADAS-2: a revised tool for the quality assessment of diagnostic accuracy studies[END_REF] scale usually used for the assessment of diagnostic accuracy studies as the Newcastle-Ottawa scale allowed us to evaluate more items. Only the evaluation of the presence of SSI without knowledge of the culture was not assessed, but this information was never mentioned in selected studies [START_REF] Ahn | Suction Drain Tip Culture after Spine Surgery: Can It Predict a Surgical Site Infection?[END_REF][START_REF] Yamada | Drain tip culture is not prognostic for surgical site infection in spinal surgery under prophylactic use of antibiotics[END_REF][START_REF] Lindahl | Bacterial contamination and closed suction drainage in open meniscectomy of the knee[END_REF][START_REF] Aski | Drain tip culture following total knee arthroplasty[END_REF][START_REF] Becker | Quantitative bacteriology of closed-suction wound drainage in contaminated surgery[END_REF][START_REF] Becker | Ineffectiveness of closed suction drainage cultures in the prediction of bacteriologic findings in wound infections in patients undergoing contaminated head and neck cancer surgery[END_REF][START_REF] Bernard | The Value of Suction Drainage Fluid Culture during Aseptic and Septic Orthopedic Surgery: A Prospective Study of 901 Patients[END_REF][START_REF] Degnim | Randomized controlled trial to reduce bacterial colonization of surgical drains after breast and axillary operations[END_REF][START_REF] Girvent | The clinical significance of suction drainage cultures[END_REF][START_REF] Gunterberg | Bacterial growth on drain tips after total hip replacement. A controlled culture method[END_REF][START_REF] Krishnan | Suction Drain Tip Cultures and Predictors of Surgical Site Infections in Hip Fractures[END_REF][START_REF] Overgaard | Closed suction drainage after hip arthroplasty. Prospective study of bacterial contamination in 81 cases[END_REF][START_REF] Petsatodis | Prognostic value of suction drain tip culture in determining joint infection in primary and non-infected revision total hip arthroplasty: a prospective comparative study and review of the literature[END_REF][START_REF] Sankar | Suction drain tip culture in orthopaedic surgery: a prospective study of 214 clean operations[END_REF][START_REF] Takada | Is Drain Tip Culture Prognostic of Surgical Site Infection? Results of 1380 Drain Tip Cultures in Total Hip Arthroplasty[END_REF][START_REF] Weinrauch | Diagnostic value of routine drain tip culture in primary joint arthroplasty[END_REF][START_REF] Zamora-Navas | Closed suction drainage after knee arthroplasty. A prospective study of the effectiveness of the operation and of bacterial contamination[END_REF].

This meta-analysis also has limitations, in particular the inclusion of studies with a medium quality and/or methods poorly described and retrospective studies. Only three had a score of six or more in the Newcastle-Ottawa quality assessment scale. Moreover, some confidence intervals were wide. Finally, the limited number of studies included, as well as the heterogeneity between studies should lead one to interpret results with caution.

Although a high NPV is not interesting in the context of systematic cultures, it could be interesting in cases of clinical suspicion or clinical diagnosis of SSI, or in contaminated or infected surgery. Some studies deal with cultures in this context but were not included in our meta-analyses [START_REF] Legout | Suction drainage culture as a guide to effectively treat musculoskeletal infection[END_REF]. Such meta-analyses could be conducted to determine the performance of drain tip or drainage fluid cultures in this context. If the PPV were high, this might allow one to confirm SSI or to adapt the antibiotic treatment. However, only early SSI would be identified since drains are usually removed a few days after surgery.

CONCLUSION

Our meta-analysis showed that the systematic cultures of drain tips or drainage fluids were of low relevance since the PPV were low in the prediction of SSI. Moreover, the associated costs and the risk of useless or inappropriate antibiotic treatment should lead one to discourage the performance of systematic cultures in asymptomatic patients.

It would be interesting to provide meta-analyses that include only studies in which cultures were performed for patients who are experiencing clinical signs or in contaminated or infected surgery. Table I

Figure 1 .Figure 2 .Figure 3 .Figure 4 .

 1234 Figure 1. Flow chart for studies selection and inclusion

Figure 5 .

 5 Figure 5. Funnel plot of selected studies

Table II .

 II . Characteristics of the included studies Description of microbiologic methods in the included studies

					Prophylactic		Number
	Authors Authors Authors	Year Country Type of study Type of surgery Year Country Type of study Type of surgery Year Type of sample Incubation time Culture medium Type of sample Type of sample	Drainage duration Drainage duration Ablation if	Prophylactic antibiotic treatment antibiotic treatment duration Analysis (qualitative or quantitative)	Duration of follow-up Duration of follow-up Transport medium (yes/no)	Number of of included patients disinfection "purulent matter in the Inclusion criteria wound drained Skin Sampling mode before drain removal Inclusion criteria included spontaneously or by "The suction drain tip
	[5] Ahn et al. Sankar et al. [27] Ahn et al. 2015 Corea 2004 India 2015 Drain tip Unspecified Unspecified Retrospective Orthopaedic Prospective Orthopaedic	Drain tip 4.5 days (24-48 hours) Drainage volume < fluid and drain tip 100ml/24 hours	duration 5 days 16 Unspecified	1 year 1 year Unspecified	patients 133 214 Yes (povidone incision, serous discharge CDC criteria [35] growth of bacteria at scissors" signs of infection with single-use sterile was cut off from the wound with growth of bacteria, or approximately 5 cm from its far end using iodine)
	Aski et al. Aski et al. [17] 2015 India 2015 Drain tip Unspecified Unspecified Prospective Orthopaedic Becker et al. [18] 1990 USA Prospective Ear, nose and throat Becker et al. [19] 1985 USA Prospective Ear, nose and throat Takada et al. [28] 2015 Japan Retrospective Orthopaedic Aerobic and Anaerobic blood agar Drain tip ≤48 hours Drainage fluid Unspecified Drainage fluid Unspecified Drain tip 12 to 72hours 48 hours Unspecified 3 days 3 days During drainage Becker et al. 1990 plate, laked blood agar plate, Drainage Unspecified Fusobacterium agar plate, phenyl Quantitative fluid ethyl alcohol agar plate, chocolate agar plate, McConkey agar plate Bernard et al. [20] 2002 France Prospective Orthopaedic Drainage fluid Unspecified Unspecified Degnim et al. [21] 2013 USA Prospective Breath Drainage fluid and drain tip 4 to19 days (mean=7 days) ≤ 24 hours Weinrauch [29] 2005 Australia Retrospective Orthopaedic Drain tip 24 hours During drainage Aerobic blood agar plate, McConkey Yamada et al. [7] 2016 Japan Retrospective Orthopaedic Drain tip 48 hours 48 hours Zamora et al. [30] 1999 Spain Prospective Orthopaedic Drain tip 24 and 48 hours 2 days 10% carbon dioxide 3 groups: 12, Becker et al. 1985 Drainage fluid 48 hours agar plate, azide agar plate, chocolate agar plate, thioglycollate browth Anaerobic blood agar plate, Chocolate agar plate incubated in blood agar plate agar plate, phenyl ethyl alcohol kanamycin-vancomycin-laked blood Qualitative	reoperation " " Any possible signs of SSI Yes (povidone Unspecified "aseptic" iodine) 6 months Unspecified 338 Unspecified Presence of pus postoperatively 41 Unspecified Presence of pus postoperatively 4.7 years such as wound "on the second or discharge or dehiscence, third postoperative fever, pain, and an increase of level of C reactive protein or 1,380 day, a sample of Yes wound drainage was Unspecified aspirated from the erythrocyte sedimentation drain line into a rate " syringe" 30 1 month, 1 year if presence of implant Unspecified 843 30 days CDC criteria ("purulent drainage, positive aseptically collected culture from the wound, signs of inflammation with opening of incision and 100 8.9 months (3 months minimum) Unspecified 393 3 years (0.5-5.5 years) CDC criteria [36] -"the presence of SSI was confirmed by reoperation or by histopathologic or radiologic investigation" 1,240 the presence of purulent Unspecified healing of the wound was done taking into account 32 " The evaluation of the "on the second or third postoperative day, a sample of syringe" drain line into a aspirated from the Yes wound drainage was Unspecified

ACKNOWLEDGMENTS

We thank Marjorie Cantener and Fawn Wilderson for their editorial assistance.

Funding: none

Authors

Year Funnel plot with pseudo 95% confidence limits


Victorien De Meyer

Dr Rémy Roca
email: remy.roca@legos.obs-mip.fr

Thermodynamic scaling of extreme daily precipitation over the tropical ocean from satellite observations

Keywords: extreme precipitation, tropical ocean, satellite observations, scaling

Extreme precipitation theory has been matured over the last decade and stipulates that the intensity of the extreme precipitation scales with the surface humidity.

Surface humidity changes can further be approximated by the surface temperature changes. The analytically derived scaling coefficient based on the Clausius-Clapeyron derivative is ~6%K -1 in the tropics. While frequently confronted with observations over land, the theory has so far only been marginally evaluated against precipitation data over the ocean. Using an ensemble of satellite-based precipitation products and a suite of satellite-based SST analysis all at the 1°-1day resolution, the extreme scaling is investigated for the tropical ocean (30°S-30°N). The focus is set on the robust features common to all precipitation and SST products. It is shown that microwave constellation-based precipitation products are characterized by a very robust positive scaling over the 300 to 302.5K range of 2-day lagged SST. This SST range corresponds to roughly 60% of the tropical precipitation amount. The ensemble mean scaling varies between 5.67%K -1 +-0.89%K -1 to 6.33%K -1 +-0.81%K -1 depending on the considered period and is found very close to the theoretical expectation. The robustness of the results confirms the fitness of the current generation of constellation-based precipitation products for extreme precipitation analysis. Our result further confirms the extreme theory for the whole tropical ocean.

Yet, significant differences in the magnitude of the extreme intensity across the products prompts the urging necessity of dedicated validation efforts.

INTRODUCTION

The energy and water exchanges within the Earth system are related to the major feedback processes responsible for the fate of the Earth climate under increasing greenhouse gases concentrations (Stephens et al. 2020). The second principle of thermodynamic and the Clausius Clapeyron law indeed dictate that water vapor in the atmosphere increases at a rate of 6-7% with each degree warming which is at the heart of the strongly positive water vapor feedback. The increased loading of water vapor in the atmosphere has a strong impact on both the mean precipitation [START_REF] Stephens | Controls of global-mean precipitation increases in global warming GCM experiments[END_REF] and the distribution of its extreme [START_REF] Trenberth | Conceptual framework for changes of extremes of the hydrological cycle with climate change[END_REF].

Theory for extreme precipitation dependence on surface temperature has indeed matured significantly over the last decade and a physically based framework is now well established [START_REF] Gorman | The physical basis for increases in precipitation extremes in simulations of 21st-century climate change SCIENCES[END_REF][START_REF] O'gorman | Precipitation Extremes Under Climate Change[END_REF][START_REF] Fischer | Observed heavy precipitation increase confirms theory and early models[END_REF]. Yet cloud resolving model idealized simulations over tropical oceans exhibit diverging sensitivities and a number of open questions remains in the tropics [START_REF] Muller | Response of precipitation extremes to warming: what have we learned from theory and idealized cloud-resolving simulations, and what remains to be learned?[END_REF]. Land based studies using conventional precipitation observations over tropical land also suggests contrasting results [START_REF] Westra | Future changes to the intensity and frequency of short-duration extreme rainfall[END_REF].

Recent investigations using satellite observations nevertheless have identified robust extreme precipitation regimes in agreement with the theoretical expectations from thermodynamics [START_REF] Roca | Estimation of extreme daily precipitation thermodynamic scaling using gridded satellite precipitation products over tropical land[END_REF]. Over the ocean, on the other hand, the scarcity of conventional in-situ precipitation observations [START_REF] Serra | Precipitation measurements from the Tropical Moored Array: A review and look ahead[END_REF]) precluded much investigations of the scaling of extreme precipitation with surface temperature with notable exceptions. [START_REF] Allan | Evaluating Large-Scale Variability and Change in Tropical Rainfall and Its Extremes. Tropical Extremes: Natural Variability and Trends[END_REF] Using early instantaneous satellite observations from Special Sensor Microwave/Imager (SSM/I) and monthly SST from Hadley Centre Sea Ice and Sea Surface Temperature data set (HadISST) [START_REF] Rayner | Global analyses of sea surface temperature, sea ice, and night marine air temperature since the late nineteenth century[END_REF], [START_REF] Allan | Evaluating Large-Scale Variability and Change in Tropical Rainfall and Its Extremes. Tropical Extremes: Natural Variability and Trends[END_REF] calculated the rate of change of 2.5°x2.5° daily precipitation to tropical mean monthly SST anomalies. Extreme precipitation shows a scaling ranging from slightly above, to much larger than their value derived from the Clausius-Clapeyron response depending on the base line for the anomaly computations. The analysis further pointed out systematic underestimation of the response of the climate models prompting for a more in-depth analysis of the behavior of the extremes over the oceans. The use of tropical mean SST anomaly nevertheless prevents from further addressing the processes at play. More recently, using the OceanRAIN dataset of ship based disdrometer precipitation measurements [START_REF] Klepp | OceanRAIN, a new in-situ shipboard global ocean surface-reference dataset of all water cycle components[END_REF], the scaling of instantaneous (at 1 minute scale) extreme precipitation to the local simultaneous SST has been revisited for the global ocean [START_REF] Burdanowitz | On the sensitivity of oceanic precipitation to sea surface temperature[END_REF]. The disdrometer data exhibits a single increasing regime of the 99 th percentile with local SST. The scaling is computed by pooling available data over a large range of SST (0°C-30°C) and is shown to vary between 6%K -1 and ~9%K -1 depending on the regression technique used. Yet, the small sample of the ship-based measurements prevents a definitive conclusion and the relative role of the convective dynamics and of thermodynamics remains to be clarified at these scales.

In summary, many observation-based assessments of the scaling theory have taken place under continental conditions while model-based assessments are performed under oceanic conditions. In comparison, the tropical ocean so far benefited from very few observationally based investigations. In this paper, we propose to improve this situation by pooling recent satellite observations at the 1°x1° daily scale of both precipitation and surface temperature all over the tropical oceans.

Over the ocean, the evaluation of these satellite-based products is generally performed using the few available data from buoys networks [START_REF] Wu | Comparison of oceanic multisatellite precipitation data from tropical rainfall measurement mission and global precipitation measurement mission datasets with rain gauge data from ocean buoys[END_REF], atolls rain gauges observations [START_REF] Greene | The comprehensive pacific rainfall database[END_REF] or island based radar measurements [START_REF] Henderson | A regimebased evaluation of TRMM oceanic precipitation biases[END_REF]. More recently, the release of a new shipbased disdrometer measurements database [START_REF] Klepp | OceanRAIN, a new in-situ shipboard global ocean surface-reference dataset of all water cycle components[END_REF]) provides a complementary reference dataset over some commercial ship routes and research vessel campaigns. For large space and time scales, consistency analysis can be performed through water and energy budget conservation analysis [START_REF] L'ecuyer | The observed state of the energy budget in the early twenty-first century[END_REF]. As a consequence of these limited verification references, compared to land, the capability of these satellite-based products to describe the precipitation field is generally not very well documented [START_REF] Sun | A Review of Global Precipitation Data Sets: Data Sources, Estimation, and Intercomparisons[END_REF], not mentioning under the extreme rainfall conditions. Instead of elaborating on the difficulty to quantify the accuracy of the products, the rationale of this study is to focus on the robustness of the analysis across many satellite products to investigate the thermodynamic scaling of extreme precipitation with sea surface temperature.

Section 2 introduces a number of satellite-based products of both precipitation and SST and details the methodology followed in this study. Section 3 is dedicated to the presentation of the results and a sensitivity of the results to various assumptions.

Finally, a summary and discussion section ends the paper.

DATA AND METHOD

PRECIPITATION

The list of the various products under consideration is provided in Table 1. All the datasets are used at the same 1°x1° daily resolution over the 30°s-30°n region and originate mostly from the Frequent Rainfall Observations on GridS (FROGS) database [START_REF] Roca | Estimation of extreme daily precipitation thermodynamic scaling using gridded satellite precipitation products over tropical land[END_REF] under the DOI https://doi.org/10.14768/06337394-73A9-407C-9997-0E380DAC5598. While the products share some of the raw satellite observations, they differ in many aspects that can influence their capability of describing precipitation. They are built from different instantaneous rain rates algorithms. The daily accumulation from the various products further benefits from different sampling from single or multiple platforms, from infrared and/or microwave imagers and/or sounders. Finally, some products also incorporate in-situ corrections.

The Global Precipitation Climatology Project (GPCP) is a pioneer effort to provide satellite based precipitation estimates globally [START_REF] Huffman | The TRMM Multisatellite Precipitation Analysis (TMPA): Quasi-global, multiyear, combined-sensor precipitation estimates at fine scales[END_REF]. We here use the one degree daily climate data record v1.3 detailed in [START_REF] Huffman | The TRMM Multisatellite Precipitation Analysis (TMPA): Quasi-global, multiyear, combined-sensor precipitation estimates at fine scales[END_REF]. The GPCP 1 degree-daily (1DD) product relies on a single microwave platform, infrared measurements and the GPCC analysis over land. The daily estimates are adjusted as to mimic the GPCP monthly product when aggregated over a month. Note that the GPCP monthly product is used in various other products as an adjustment reference.

The Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks (PERSIANN) family of products has various incarnations. We here use the climate oriented product the Climate Data Record (CDR) version, also known as PERSIANN-CDR v1 [START_REF] Ashouri | PERSIANN-CDR: Daily precipitation climate data record from multisatellite observations for hydrological and climate studies[END_REF]. PERSIANN is a based on infrared imagery and it is adjusted to the GPCP monthly mean. It can be seen as an alternative downscaling of the GPCP monthly data to that of GPCP 1DD. The National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center (CPC) morphing technique (CMORPH) satellite precipitation estimates [START_REF] Xie | Reprocessed, bias-corrected CMORPH global high-resolution precipitation estimates from 1998[END_REF]) rely on the constellation of both microwave imagers and sounders and infrared derived cloud motion winds. The blending of the various data is performed thanks to a Kalman filter [START_REF] Joyce | Kalman Filter-Based CMORPH[END_REF]. Over the ocean, the product is adjusted onto the GPCP accumulations. Compared to buoys, CMORPH under (over) estimate rainfall over the Atlantic (Pacific) ocean [START_REF] Wu | Comparison of oceanic multisatellite precipitation data from tropical rainfall measurement mission and global precipitation measurement mission datasets with rain gauge data from ocean buoys[END_REF]. The Tropical Rainfall Measuring Mission Multisatellite Precipitation Analysis (TMPA) product is a widely used dataset that combines radar, infrared, microwave imagers and sounders satellite observations [START_REF] Huffman | The TRMM Multisatellite Precipitation Analysis (TMPA): Quasi-global, multiyear, combined-sensor precipitation estimates at fine scales[END_REF]). Version 7 is used here. Over tropical Atlantic (Pacific), the TMPA product shows overall under(over)-estimation [START_REF] Wu | Comparison of oceanic multisatellite precipitation data from tropical rainfall measurement mission and global precipitation measurement mission datasets with rain gauge data from ocean buoys[END_REF]. Over the northern Indian ocean, the TMPA product generally overestimated precipitation compared to the buoys but underestimates heavy precipitation events over 100mm/d [START_REF] Prakash | Validation of TRMM-3B42 precipitation product over the tropical Indian Ocean using rain gauge data from the RAMA buoy array[END_REF]. Note that earlier investigation noted that TMPA monthly means are similar to the GPCP product over tropical ocean [START_REF] Huffman | The TRMM Multisatellite Precipitation Analysis (TMPA): Quasi-global, multiyear, combined-sensor precipitation estimates at fine scales[END_REF]). The Hamburg Ocean-Atmosphere Parameters and Fluxes from Satellite Data (HOAPS) product provide a satellite based suite of fresh-water budget parameters including precipitation over sea-ice-free ocean surface [START_REF] Andersson | The Hamburg ocean atmosphere parameters and fluxes from satellite data-HOAPS-3[END_REF]. This microwave only product relies on multiplatforms inter-calibrated measurements from SSM/I and Special Sensor Microwave Imager Sounder (SSMIS) [START_REF] Fennig | A Fundamental Climate Data Record of SMMR, SSM/I, and SSMIS brightness temperatures[END_REF]). Version 3.2 of the precipitation product is used [START_REF] Andersson | Ocean Surface Freshwater Fluxes from the HOAPS Satellite Climatology[END_REF]. When compared to the oceanRAIN in situ data, HOAPS instantaneous rain rate underestimates intensity in the intertropical convergence zone especially for high rain rate but at the same time, the HOAPS retrieval overestimates the occurrence of precipitating cases [START_REF] Bumke | Validation of HOAPS rain retrievals against OceanRAIN In-situ measurements over the Atlantic Ocean[END_REF].

The Tropical Amount of Rainfall with Estimation of ERors (TAPEER) algorithm makes use of geostationary infrared imagery together with microwave imager instantaneous rain rates estimates plus the SAPHIR sounder to estimate daily-precipitation accumulation (Roca et al. 2020). The addition of the Megha-Tropiques platform in the constellation is shown to improve the product compared to imagers only implementation (Roca et al. 2018).The recently released TAPEER product has been extensively evaluated over West Africa [START_REF] Gosset | Evaluation of TAPEER daily estimates and other GPM-era products against dense gauge networks in West Africa, analysing ground reference uncertainty[END_REF] but is not well characterized over the tropical ocean yet. The Global Satellite Mapping of Precipitation (GSMaP) product provides high-resolution precipitation estimations using satellite observations from multiple platforms [START_REF] Kubota | Global Satellite Mapping of Precipitation (GSMaP) Products in the GPM Era. Satellite precipitation measurement[END_REF]) . This product is mainly based on the microwave estimation of rainfall from a suite of microwave imagers and sounders. The microwave instantaneous rain rate estimates [START_REF] Aonashi | GSMaP Passive Microwave Precipitation Retrieval Algorithm : Algorithm Description and Validation[END_REF][START_REF] Shige | The GSMaP precipitation retrieval algorithm for microwave sounderspart i: Over-ocean algorithm[END_REF]) are propagated based on cloud motion wind vectors originally derived from IR geostationary imagery [START_REF] Ushio | A Kalman Filter Approach to the Global Satellite Mapping of Precipitation (GSMaP) from Combined Passive Microwave and Infrared Radiometric Data[END_REF]). Here the near real time version 6 product is used. The Integrated Multi-satellite Retrievals for Global Precipitation Measurement mission (GPM) (IMERG) is developed at NASA based on infrared observations and on both microwave imagers and sounders data [START_REF] Huffman | Integrated Multi-satellite Retrievals for the Global Precipitation Measurement[END_REF]. It takes advantage of the TMPA product, the CMORPH-Kalman Filter approach (Joyce and Xie 2011) and the PERSIANN-Cloud Classification System algorithm [START_REF] Hong | Precipitation estimation from remotely sensed imagery using an artificial neural network cloud classification system[END_REF]). The most recent v6 release of the final run product is used. Previous evaluation efforts showed that the older version (V4) overestimates rain-rates compared to buoys observations at hourly 0.1°x0.1° scale over the north Indian Ocean (Prakash et al. 2017). Version 5 daily 1°x1° estimates are shown to underestimate the OceanRAIN measurements on average that is likely due to the inclusion of very light rain rates in the statistics [START_REF] Khan | Assessment of level-3 Gridded Global Precipitation Mission (GPM) products over oceans[END_REF]. While IMERGv5 has similar biases with respect to in-situ data than CMORPH and TMPA, it overall better performs in the estimation of the mean value with the exception of the Atlantic ocean regime over 4mm/d where the product is shown to significantly underestimate precipitation [START_REF] Wu | Comparison of oceanic multisatellite precipitation data from tropical rainfall measurement mission and global precipitation measurement mission datasets with rain gauge data from ocean buoys[END_REF] . Version 6 of the product benefits from a refined intercalibration procedure and better interpolation between the platforms that show improvements on some metrics [START_REF] Tan | Diurnal Cycle of IMERG V06 Precipitation[END_REF]) and should also reflect on these prior evaluations. The Multi-Source Weighted-Ensemble Precipitation (MWSEP) is a product that corresponds to a pragmatic approach that average existing products to provide a best estimate. Over land an optimization method based on hydrological modelling and observed stream-gauges data is used [START_REF] Beck | MSWEP: 3-hourly 0.25° global gridded precipitation (1979–2015) by merging gauge, satellite, and reanalysis data[END_REF]. Over ocean, a merging procedure is also followed. The various source of precipitation used encompass satellite-based products, rain gauge measurements and reanalysis results. We use here Version 2.2 [START_REF] Beck | MSWep v2 Global 3-hourly 0.1° precipitation: Methodology and quantitative assessment[END_REF].

From the 2000 on-wards, MSWE estimates over the tropical ocean are weighted much more towards satellite products than reanalysis. CMORPH, GSMaP-MKV v5 and TMPA v7 RT are blended in MSWE. Note that over ocean TMPA v7 RT and TMPA v7 are similar and only differ over land. The geographical distribution of the 99 th percentile of 3-hourly precipitation of MSWEP2.2 is very close to that of CMORPH over the tropical ocean [START_REF] Beck | MSWep v2 Global 3-hourly 0.1° precipitation: Methodology and quantitative assessment[END_REF].

The resulting variety of precipitation distribution from these nine products is illustrated in Figure 1. The two products that do not rely on the microwave constellation (GPCP and PERSIANN) exhibit strongly decreasing occurrence with the daily precipitation amount; GPCP not having values above 170 mm/d. The TAPEER product exhibits systematic significant underestimation of the probability of the largest rain accumulation compared to other microwave-based products. This is likely due to the relative lack of light rain situations (Roca and Fiolleau, 2020). Note that the TAPEER product is also only available for a limited time period. Owing to their outlier distribution and documented biases, those three products are not considered in the following. The figure further reveals two clusters of products showing both the occurrence declining smoothly exponentially towards the very high daily precipitation accumulation. The first cluster is composed of CMORPH, TMPA and MSWEP. The members of this cluster are very close with MSWEP being built from CMORPH and TMPA. Both CMORPH and TMPA further share the same adjustment to the common reference of GPCP pentads over the ocean. The second one includes GSMaP, HOAPS and IMERG that do not share much in terms of algorithms, methods and input data. While the two clusters show a similar distribution up to 100 mm/d, above that threshold, the first cluster systematically underestimates the occurrence compared to the second cluster. This is consistent with previous analysis that indicate that the climatological mean behavior of the products is a poor predictor of the products extreme precipitation [START_REF] Masunaga | Inter-product biases in global precipitation extremes[END_REF]). The first cluster products have further been shown to underestimate the intense rain rates compared to buoys [START_REF] Prakash | Validation of TRMM-3B42 precipitation product over the tropical Indian Ocean using rain gauge data from the RAMA buoy array[END_REF][START_REF] Wu | Comparison of oceanic multisatellite precipitation data from tropical rainfall measurement mission and global precipitation measurement mission datasets with rain gauge data from ocean buoys[END_REF] that could be related to their difference with the second cluster.

SEA SURFACE TEMPERATURE

Satellite based SST products readily provide estimations of the foundation SST, defined as the ocean temperature at a depth of ~1 meter, below the diurnal layer, at high space and time resolution over a long temporal record. Most of the products rely on optimal interpolation techniques to merge various satellites observations with insitu measurements. While such products have a well characterized accuracy under clear sky conditions of ~0.5K [START_REF] Donlon | The Operational Sea Surface Temperature and Sea Ice Analysis (OSTIA) system[END_REF], their representativity under (extreme) rainfall conditions is not well documented. Indeed, the cloudiness associated with the rainfall prevents the use of infra-red measurements and the microwave signal, while of help in cloudy but non-precipitating cases, is altered by rain drop emission during the heavy precipitation situations, also preventing SST estimation in this case [START_REF] Wentz | Satellite measurements of sea surface temperature through clouds[END_REF]. As a consequence, the SST estimates under precipitating conditions mainly rely on the result of the optimal interpolation method and are smoothed over several days and a large distance. In the following, we interpret these fields as "synoptic" SST and assess whether such synoptic SST are a good and robust proxy for the thermodynamics of the extreme precipitation events over the ocean. A suite of 3 products is used to evaluate the sensitivity of the scaling to the input data and to the optimal technique implementation and are summarized in Table 2. All SST products have been regridded at a daily 1°x1° resolution over the 30°S-30°N region to match the precipitation data using a simple conservative averaging procedure.

The Operational Sea Surface Temperature and Ice Analysis (OSTIA) [START_REF] Donlon | The Operational Sea Surface Temperature and Sea Ice Analysis (OSTIA) system[END_REF]) product provides a global foundation SST field derived from satellite observations in the microwave (2 platforms) and in the infrared (6 platforms) and insitu measurement at 1/20° and daily resolution since 2007. The analysis is a multiscale optimal interpolation using a three days assimilation window centered on the day of the analysis and the background field is the analysis of the previous day. The error correlation length scale used in OSTIA in the tropics is around 100 km. The product Optimum Interpolation Sea Surface Temperature (OISST) [START_REF] Banzon | A long-term record of blended satellite and in situ sea surface temperature for climate monitoring, modeling and environmental studies[END_REF]) uses IR measurements from the Advanced Very High-Resolution Radiometer (AVHRR) instruments on board the NOAA satellites since 1981. The interpolation method is similar to that of OSTIA. The error correlation length scales in the tropics are about 150-200km and 3 days. The nominal resolution is daily at 0.25°. Version 2 of OISST is used here. Finally, the microwave (MW) optimally-interpolated (OI) SST product from Remote Sensing Systems [START_REF] Gentemann | Passive microwave remote sensing of the ocean: An overview[END_REF]) is used. As some rain-contaminated SST estimate may persist in the microwave derived dataset, a stringent quality control is performed to prevent potentially biased SSTs retrievals.

The error correlation scales used are of 100 km and 3 days, the background field is the analysis of the previous day and the SST represents a foundation SST remapped at a daily 1°x1° resolution. This product is referred to as OIRSS in the following. Based on the dry static energy budget of the atmosphere [START_REF] Muller | Intensification of precipitation extremes with warming in a cloud-resolving model[END_REF], the rate of extreme precipitation in the tropics can be expressed as follow:

METHOD

𝑃 𝑒 = 𝜖 ∫ 𝜌𝜔 (- 𝜕𝑞 𝑠𝑎𝑡 𝜕𝑧 ) 𝑑𝑧, (1)
where 𝜖 depicts a precipitation efficiency, 𝜌 the mean density profile, 𝜔 the vertical velocity, 𝑞 𝑠𝑎𝑡 the saturation mixing ratio and with the integral taken from the surface to the tropopause. Furthermore, the fractional change of extreme precipitation .

(2)

It points out that the change in precipitation extremes is expected to be more related to the surface conditions rather than to the column integrated, and at a rate following the rate of increase of 𝑞 𝑠 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 with temperature, that is the Clausius-Clapeyron rate.

In typical tropical sea surface conditions, the expected value ranges between 5.5 and 6.5%K -1 .

2) Methodological aspects

Extreme daily precipitation is characterized using high percentiles of the wet-days (P>1mm/d) distribution. This index is chosen over others [START_REF] Zhang | Indices for monitoring changes in extremes based on daily temperature and precipitation data[END_REF]) due to its relevance for scaling investigations [START_REF] Schär | Percentile indices for assessing changes in heavy precipitation events[END_REF]. The scaling of the precipitation extremes with the sea surface temperature is calculated using the binning method which is well suited to our investigation given the large number of available observations [START_REF] Roca | Estimation of extreme daily precipitation thermodynamic scaling using gridded satellite precipitation products over tropical land[END_REF]. Indeed, for each 0.5K degree SST interval, precipitation data from the whole tropical ocean are pooled together and the 99.9 th percentile of the precipitation distribution is estimated. Then after identifying a relevant regime, linear regression (in the logarithm space) is used to compute the scaling factor defined as the rate of change of the 99.9 th percentile with sea surface temperature.

3) Sensitivity studies for robustness determination

In order to identify the robust aspects of the thermodynamic scaling estimation using satellite observations, we perform a sensitivity study in complement to the use of various precipitation and SST products. We explore how the timing between the SST and the precipitation influences the scaling and how the overall statistical analysis is sensitive to the selected period.

As already well explored for continental cases (e.g., [START_REF] Bao | Future increases in extreme precipitation exceed observed scaling rates[END_REF]) intense precipitation events can strongly alter the surface heat budget and the surface temperature. The strong gust and downdrafts associated with deep convection result in a lower surface temperature compared to the no rain case [START_REF] Lafore | Deep convection[END_REF]). As a result, the surface conditions may not represent the large-scale environment of the event but are impacted by the event itself. As discussed above, the synoptic SST products assimilation scheme prevents these impacted surface conditions to be used in the estimation, yet the risk to misattribute a rain event to a given temperature range because of this coupling is unclear. As a mean to assess the impact on our findings, the timing of the association between SST and the precipitation event is varied from simultaneous and lagged up to two days before.

The constellation-based precipitation products are characterized by a changing configuration of the constellation over the last two decades depending on the availability of microwave imagers and sounders that could influence their capability to monitor steadily the precipitation in the tropics (Roca et al. 2020).The SST products are also sensitive to the availability of the microwave imagers and the infrared radiometers. The availability of these platforms since 2001 is summarized in Figure 2. with less sampling than during the 2007-2017 that could impact our analysis. The second period is a shorter one and corresponds to a precipitation data rich era that includes the Megha-Tropiques operations (Roca et al. 2018) and partly includes the TRMM and GPM platforms along with the SSM/I and SSMIS platforms. This era is characterized with up to 14 microwave sensors operating simultaneously. The use of three periods of 18, 11 and 5 years, respectively also permits to assess the sensitivity of the estimation of the extreme percentile of the precipitation distribution.

RESULTS

SIMULTANEOUS ANALYSIS

Figure 3 shows the scaling of the 99.9 th percentile as a function of the simultaneous SST from the OSTIA product for the 6 constellation-based products. The two clusters of precipitation products previously identified are shown as well. The low cluster 99.9 th percentile ranges from 120 to 150 mm/d while the high one spans 170 to 240 mm/d. There is roughly a factor of two among the least value of CMORPH at 297K and the highest one of IMERG at 302.5K. The dependence of the high percentile to the SST is characterized by three regimes. Up to 300.25K, the products do not exhibit any robust behavior. GSMaP is mainly increasing over this SST range, while HOAPS extremes decrease; the other products rather show no sensitivity to the SST.

This regime accounts for only 19% of the total tropical precipitation accumulation.

The second regime spans SST from 300K up to 302.5K and is characterized by an increase of all the products. Almost 56% of the total rainfall belongs to this regime.

For the last regime, above 302.25K and corresponding to 25% of the total rainfall amount, the precipitation products show a robust decrease of the value of the high percentile. For the second regime, the ensemble mean of the scaling is ~5%K -1 with a small coefficient of variation of around 10% and range from 4.21 to 5.75 %K -1 (Table 3a). A robust scaling, close to the Clausius Clapeyron, is found in this case.

The replacement of OSTIA by OISST does not change the regime decomposition much (not shown) but the ensemble scaling over the regime 2 is much smaller (3.4%K -1 ) and spreads almost twice much more (Table 3b). The use of the OIRSS data, on the other hand, drastically alters the picture with no well-defined regimes at all. Over the previous SST range of regime 2, the ensemble scaling is even slightly negative (-0.45%K -1 ). The scaling obtained from simultaneous measurements of precipitation and surface temperature are hence not robust to the selection of the SST products.

LAGGED ANALYSIS

The lagged analysis using the SST of the day before the precipitation does not change much the overall picture (not shown). Quantitatively, the ensemble mean of the scaling slightly increases to 5-6%K -1 with a coefficient of variation less than 11% for all the OSTIA and OISST products (Table 3a andb). The OIRSS product now exhibits a scaling of 4.4%K -1 +-0.6%K -1 value. On the other hand, the lagged analysis at two days before confirms the previously identified low and high precipitation products clusters and reveals a very robust pattern across the SST products. Figure 4 confirms the relevance of the range of the scaling regime identified for OSTIA for all the SST products. The sensitivity of the selection of the upper bound 303K instead of 302.5K (not shown) does not modify the values significantly. With a 2 days lag, the ensemble mean scaling is very similar for each SST product: 5.9, 5.9 and 6.0 %K -1 for OSTIA, OISST and OIRSS, respectively. The ensemble variance is also similar among the products at around 10% (or 0.65%K -1 ). The multi precipitation products and multi-SST products ensemble of 18 combinations statistics reads 5.93%K -1 +-0.60%K -1 . The "cold" regime with SST < 300K remains non-robust at day-2 lag, with the precipitation products not agreeing on the sensitivity of the 99.9 th percentile to the SST. The warm regime with SST ≥ 302.5K is characterized by a robust behavior of the precipitation products does not appear robust to the SST selection. The marked decrease of the extreme in OSTIA after 302.5K is rather associated with a smooth, steady evolution for OISST and OIRSS. Note that the GSMaP product, unlike any other product, indicates a CC-like positive scaling over most of the cold regime. This unique feature deserves further attention and will be investigated in the future.

SENSITIVITY TO THE SELECTION OF THE PERIOD

The analysis for the longer period is summarized in Figure 5. The "cold" regime results still hold in this case although its upper limit can be revised to 299.5K. The "warm" regime is now more robust among the SST products but the 302.5K limit is less clearly marked than for the 2007-2017 period. The values of the 99.9th percentile is slightly higher for all precipitation products compared to the previous period. The scaling over the 300K-302.5K regime shows a smoother sensitivity than before with an ensemble mean value of 4.3 and 5.3 %K -1 for the two SST products (Table 3b andc). When the range is slightly adjusted to 299.5K and 302.0K, the regime corresponds to 45% of the total precipitation and the scaling now reads 5.65 and 5.69%K -1 for OISST and OIRSS, respectively. In this case, the spread of the ensemble scaling for OISST is diminished to ~17% instead of 24% (Table 3b) and remains the same at ~15% for OIRSS. The ensemble of 12 combinations for all SST and precipitation products in this case is 5.67%K -1 +-0.89%K -1 .

Figure 6 confirms the three regimes delineation for the shorter period. The magnitude of the extreme precipitation is close to that of the 2001-2017 period. The ensemble mean scaling is slightly larger and now ranges between 6.0 and 6.8 %K -1 while the 18 combinations for all SST and precipitation products is 6.33%K -1 +-0.81%K -1 , also in close agreement with the theoretical expectation. Owing to its availability over that period, the TAPEER products is also shown for the sake of completeness but it is not included in the ensemble statistics. It is in line with the other products, although with a much lower magnitude of the p99.9 marker as expected from Figure 1. Yet the scaling mean value is ~5.1%K -1 which is slightly less than for the other products.

SUMMARY AND DISCUSSION

The objective of this study is to explore the scaling of extreme precipitation with surface temperature over the tropical oceans. The pooling of data originating from an ensemble of 6 constellation-based precipitation products reveals two clusters of products that differ in terms of magnitude of the extreme daily accumulation. The difference between the low cluster (MSWE, TMPA and CMORPH) and the high cluster (HOAPS, GSMaP and IMERG) spans roughly a factor of two at the most. No such clustering is found over land, likely due to the inclusion of rain gauges in most of the products [START_REF] Roca | Estimation of extreme daily precipitation thermodynamic scaling using gridded satellite precipitation products over tropical land[END_REF][START_REF] Masunaga | Inter-product biases in global precipitation extremes[END_REF]. While, buoy-based comparison suggests the low cluster products indeed underestimate intense precipitation intensity, no definitive argument yet permits to prefer one cluster from the other.

Despite this magnitude difference, the present results highlight a very robust behavior of the satellite products extreme precipitation with respect to the SST. The explanation for the individual product similarities and differences would require a dedicated study and is out of the scope of the present work that focus on the precipitation products ensemble robust features.

The timing of the SST-precipitation relationship has also been explored and it is shown that the two days lagged investigation exhibits robust regimes across the SST products. This is likely due to the use of the SST analysis that blends data over 3 days and ~100km to provide an SST estimate under precipitating conditions. Our study confirms the fitness for purpose of these analyzed SST for precipitation related investigations. Specifically, the two days lagged analysis identifies three distinct regimes:

A "cold" regime with SST < 300K that corresponds to ~19% of the total tropical precipitation amount. In this case, while the results are not sensitive to the SST product selection, nor to the timing of the precipitation-temperature association nor the length of the record, the various precipitation products exhibit inconsistent behaviors. The lack of robustness of the results might be due to some structural errors in the precipitation retrievals and/or to the weak data sampling preventing a robust estimation of the high percentile of the precipitation distribution. The 99 th percentile (not shown), that ranges between 60 and 100mm/d over the cold regime, is less sensitive to the data sampling than the 99.9 th percentile and shows a lack of sensitivity that is more reproductible among the products but for HOAPS that still differs from the other five products.

A "warm" regime with SST > 302.5K corresponding to ~25% of the total precipitation amount, is characterized by a systematic decrease of the values of the 99.9 th percentile from 302.5K to the warmest SST under considerations. This warm regime is also observed over land [START_REF] Roca | Estimation of extreme daily precipitation thermodynamic scaling using gridded satellite precipitation products over tropical land[END_REF] and is usually attributed to relative humidity limited conditions at warm surface temperature that decrease the intensity of extreme precipitation over these semi-arid areas. Decrease in the wet-days duration for this regime has also been identified as a key mechanism over mid-latitudes land conditions (Utsumi et al. 2011). Further analysis is needed to assess whether these very high SST are associated with large scale subsidence and simultaneous relatively dry boundary layer that could impact the intensity and/or the duration of the precipitation events and explain the decreasing trend. [START_REF] Burdanowitz | On the sensitivity of oceanic precipitation to sea surface temperature[END_REF] do not report any such decreasing regime when analyzing instantaneous disdrometer derived precipitation rate from ship data. They attribute this departure from the behavior of the continental extreme to the absence of decreasing event duration in this regime. Yet their analysis is restricted to 1K SST bin up to 303K, and hence would miss the warm regime identified here. Furthermore, our study shows that the use of simultaneous SST is detrimental to the scaling computation. It is likely that using a time lagged analysis of the instantaneous precipitation scaling over an extended SST range would permit to better delineate the various regimes relevant for that instantaneous scale.

The third regime which we call the « Clausius-Clapeyron » regime is ranging from 300K to 302.5K and includes almost 56% of the total precipitation amount. It is characterized by a steady increase of the extremes with surface temperature. This regime is robust to the precipitation product, to the SST product and to the length of the record. When simultaneous SST data are used, the scaling is diminished but for OIRSS where it simply does not exist. The one day lagged scaling results are robust to the SST product selection and lead to a scaling around 5.17%K -1 +-0.85%K -1 . The two days lagged scaling value ranges from 5.67%K -1 +-0.89%K -1 for the 2001-2017 period to 6.33%K -1 +-0.81%K -1 for 2012-2016 for all the SST and precipitation products considered here. While the actual, direct validation or evaluation of the representation of intense rain accumulation in these products remains challenging, their common and physically sound behavior indicates that the products are fit for exploring the extreme precipitation over the ocean. The robustness of the scaling analysis across the satellite precipitation products is further very close to the theoretical expectation for the thermodynamic scaling of ~6%K -1 . This gives us even more confidence in this generation of satellite precipitation products at the 1° 1-day resolution over the ocean. The frequency of occurrence of the precipitation greater than the 99.9 th percentile and surface conditions corresponding to the Clausius Clapeyron regime have been mapped in Figure 7. The figure points out that this regime is not associated with the climatological ITCZ location as previously noted [START_REF] Masunaga | Inter-product biases in global precipitation extremes[END_REF]. On the other hand, it is associated with the known climatological rainfall maxima, off the Colombian Coast and in the northern Bay of Bengal and also seems to align with the climatological distribution of cyclones occurrence in the East Pacific, West Atlantic, South Indian Ocean and China sea.

An avenue for further research lies in identifying the contribution of the organized convection to the scaling physics (Pendergrass 2020; Muller and Takayabu 2020;

Roca and Fiolleau 2020). Our results further prompt for a dedicated investigation of the contribution of the cyclone precipitation to the scaling physics that is so far not much promoted. Our results also suggest that community efforts, possibly under the umbrella of the International Precipitation Working Group [START_REF] Levizzani | The activities of the International Precipitation Working Group[END_REF]) and the GEWEX/WCRP core-project, are needed to further characterize the absolute accuracy of precipitation products over the ocean and clarify which clusters of products is to be understood as a reference, if any. Table 3.c: Summary of the sensibility analysis with the value of the slope of the 99.9 th percentile as a function of the SST from the OIRSS product over the CC regime ([300K,302,5K[) in %.K -1 . Mean (%.K -1 ), standard deviation (%.K -1 ) and coefficient of variation (%) for the ensemble of all the products (ENS), as well as the total rainfall accumulation within the regime, are also reported.

Product

FIGURES

1 )

 1 Background on the theory The background on the scaling of extreme precipitation with surface temperature has been detailed and reviewed in various publications (O'Gorman 2015; Allan and Liu 2018; Roca 2019; Muller and Takayabu 2020) and is only briefly summarized below.

  three contributions, that is the change in microphysics related to the efficiency term, the change in dynamics through 𝜌𝜔 and the change in thermodynamics through 𝜕𝑞 𝑠𝑎𝑡 𝜕𝑧. At the daily 1°x1° scale, with the assumption that the related change of the dynamic and efficiency contributions remain low with warming and by neglecting the vertical variations in 𝜔, the scaling of extreme precipitation becomes[START_REF] Muller | Intensification of precipitation extremes with warming in a cloud-resolving model[END_REF]

  The baseline period for our investigation spans 2007-2017 which is the longest period share by all SST products. It corresponds to the homogeneous cycle of production of OSTIA[START_REF] Donlon | The Operational Sea Surface Temperature and Sea Ice Analysis (OSTIA) system[END_REF] and to the start of the systematic use of two AVHRR instruments for the OISST product[START_REF] Banzon | A long-term record of blended satellite and in situ sea surface temperature for climate monitoring, modeling and environmental studies[END_REF]. Two other different periods are considered. The first one is longer, extending back in time until 2001 for which OIRSS and OISST are available and correspond to the availability of the GSMaP precipitation product. During the 2001 to 2007 era, the microwave imagers fluctuate from 4 to 6 platforms and the sounders from 2 to 4 while the IR radiometers increases from 1 to 3. These constellation configurations are all less populated than the forthcoming period post 2007 and the products are associated

Figure 1 :

 1 Figure 1: Probability of exceedance of daily 1°x1° accumulated precipitation over the tropical ocean (30°S-30°N) for the period 2017-2017 except for the TAPEER product where it is restricted to the 2012-2016 period. The probability of exceedance is computed with respect to wet-days with precipitation above 1mm/d.

Figure 2 :

 2 Figure 2: Time series of the availability of microwave imagers and sounders used in the precipitation products of the study. Vertical dashed lines indicate the different time periods explored in the study.

Figure 3 :

 3 Figure 3: The value of the 99.9 th percentile of the 1°x1° daily accumulated precipitation as a function of the contemporaneous SST from the OSTIA product. Each color corresponds to a precipitation product. For the period 2007-2017. Regimes are separated by vertical dashed lines. The grey shaded area indicates the non-robust cold regime between precipitation products. Black dash-dotted lines correspond to the Clausius-Clapeyron 6%K -1 rate. See text for details.

Figure 4 :

 4 Figure 4: The value of the 99.9 th percentile of the 1°x1° daily accumulated precipitation as a function of the SST lagged by 2 days. Each color corresponds to a precipitation product. Solid line for OSTIA, dashed line for OISST and dash-dotted lines for OIRSS. For the period 2007-2017. Regimes are separated by vertical dashed lines. The grey shaded areas indicate the non-robust cold regime between precipitation products (left) and the non-robust warm regime between SST products (right). Black dash-dotted lines correspond to the Clausius-Clapeyron 6%K -1 rate. See text for details.

Figure 5 :

 5 Figure 5: The value of the 99.9 th percentile of the 1°x1° daily accumulated precipitation as a function of the SST lagged by 2 days. Each color corresponds to a precipitation product. Dashed line for OISST and dash-dotted lines for OIRSS. For the period 2001-2017. Regimes are separated by vertical dashed lines. The grey shaded areas indicate the non-robust cold regime between precipitation products (left) and the non-robust warm regime between SST products (right). Black dash-dotted lines correspond to the Clausius-Clapeyron 6%K -1 rate. See text for details.

shortname Product name and version

			Use of
			satellite	Period used	References
			data
	OSTIA	OSTIA Near Real Time Level 4 SST	IR + MW	2007-2017	Donlon et al. (2012)
	OISST	NOAA CDR OISST AVHRR-only v2	IR	2001-2007 Reynolds et al. (2008)
	OIRSS	RSS OI SST MW v5	MW	2001-2017	Gentemann et al. (2010)

Table 2 :

 2 List of sea surface temperature products and their acronyms. IR stands for Infra-red and MW for Microwave.

		OSTIA	OSTIA	OSTIA	OSTIA	OSTIA	OSTIA
		2007-	2007-	2007-	2012-	2012-	2012-
	Products	2017	2017	2017	2016	2016	2016
		T(t)	T(t-24h)	T(t-48h)	T(t)	T(t-24h)	T(t-48h)
		58%	56%	55%	57%	56%	54%
	GSMaP	4.21	5.37	5.38	5.43	6.09	6
	CMORPH	5.26	6.02	6.11	5.74	6.21	6.7
	MSWEP	4.69	5.61	5.52	5.89	6.69	6.77
	TMPA	5.09	5.51	5.75	4.93	5.29	5.61
	HOAPS	5.75	7.14	7.05	5.76	6.82	6.9
	IMERG	5.19	5.81	5.5	4.85	5.24	5.24
	ENS	5.04	5.95	5.9	5.41	6.05	6.17
	Mean ENS	5.03	5.91	5.89	5.43	6.06	6.2
	STD ENS	0.52	0.65	0.62	0.45	0.67	0.69
	Cvar ENS	10.43	10.93	10.62	8.26	11.1	11.09
	Table 3.a: Summary of the sensibility analysis with the value of the slope of the 99.9 th percentile as a function of the SST from the OSTIA product over the CC regime
	([300K,302,5K[) in %.K -1 . Mean (%.K -1 ), standard deviation (%.K -1 ) and coefficient of variation (%) for the ensemble of all the products (ENS), as well as the total rainfall
	accumulation within the regime, are also reported.

a.

b.

Table 3

 3

	.b: Summary of the sensibility analysis with the value of the slope of the 99.9 th percentile as a function of the SST from the OISST product over the CC regime
	([300K,302,5K[) in %.K -1 . Mean (%.K -1 ), standard deviation (%.K -1 ) and coefficient of variation (%) for the ensemble of all the products (ENS), as well as the total rainfall
	accumulation within the regime, are also reported.

c.

ACKNOWLEDGEMENTS

This study benefited from the IPSL mesocenter ESPRI facility, which is supported by CNRS, UPMC, Labex L-IPSL, CNES and Ecole Polytechnique. The authors acknowledge the CNES and CNRS support under the Megha-Tropiques program.

They also thank Dr T. Fiolleau for helpful discussions and Dr. M. Schröder for his help with the HOAPS dataset.

Figure 6 : The value of the 99.9 th percentile of the 1°x1° daily accumulated precipitation as a function of the SST lagged by 2 days. Each color corresponds to a precipitation product. Solid line for OSTIA, dashed line for OISST and dash-dotted lines for OIRSS. For the period 2012-2016. Regimes are separated by vertical dashed lines. The grey shaded areas indicate the non-robust cold regime between precipitation products (left) and the non-robust warm regime between SST products (right). Black dash-dotted lines correspond to the Clausius-Clapeyron 6%K -1 rate. See text for details.