Characterizing, modelling and understanding the climate variability of the deep water formation in the North-Western Mediterranean Sea

Type Article
Date 2018-08
Language English
Author(s) Somot Samuel1, Houpert Loic2, Sevault Florence1, Testor Pierre3, Bosse Anthony3, Taupier-Letage Isabelle4, Bouin Marie-Noelle5, 6, Waldman Robin1, Cassou Christophe7, Sanchez-Gomez Emilia7, Durrieu De Madron Xavier8, Adloff Fanny1, Nabat Pierre1, Herrmann Marine9
Affiliation(s) 1 : Meteo France, CNRM, CNRS, UMR 3589, F-31057 Toulouse, France.
2 : SAMS, Scottish Marine Inst, Oban PA37 1QA, Argyll, Scotland.
3 : Sorbonne Univ, UPMC Univ Paris 06, Lab Oceanog & Climat, CNRS,IRD,MNHN,UMR 7159,IPSL, Paris, France.
4 : Univ Toulon & Var, Aix Marseille Univ, Mediterranean Inst Oceanog MIO, IRD,UM 110,CNRS,INSU, CS 20330, F-83507 La Seyne Sur Mer, France.
5 : CNRM, 13 Rue Chatellier,CS 12804, F-29228 Brest, France.
6 : IFREMER, Lab Oceanog Phys & Spatiale, UMR 6523, CS 10070, F-29280 Plouzane, France.
7 : CNRS, CERFACS, SUC URA 1875, 42 Ave Coriolis, F-31057 Toulouse, France.
8 : Univ Perpignan, CNRS, CEFREM, UMR 5110, 52 Ave Paul Alduy, F-66860 Perpignan, France.
9 : Univ Toulouse, IRD, CNRS, CNES,UMR 5566,LEGOS, 13 Ave Edouard Belin, F-31400 Toulouse, France.
Source Climate Dynamics (0930-7575) (Springer), 2018-08 , Vol. 51 , N. 3 , P. 1179-1210
DOI 10.1007/s00382-016-3295-0
WOS© Times Cited 67
Keyword(s) Deep water formation, Open-sea deep convection, Interannual variability, Mediterranean Sea, Regional climate models, Climate trends
Abstract Observing, modelling and understanding the climate-scale variability of the deep water formation (DWF) in the North-Western Mediterranean Sea remains today very challenging. In this study, we first characterize the interannual variability of this phenomenon by a thorough reanalysis of observations in order to establish reference time series. These quantitative indicators include 31 observed years for the yearly maximum mixed layer depth over the period 1980–2013 and a detailed multi-indicator description of the period 2007–2013. Then a 1980–2013 hindcast simulation is performed with a fully-coupled regional climate system model including the high-resolution representation of the regional atmosphere, ocean, land-surface and rivers. The simulation reproduces quantitatively well the mean behaviour and the large interannual variability of the DWF phenomenon. The model shows convection deeper than 1000 m in 2/3 of the modelled winters, a mean DWF rate equal to 0.35 Sv with maximum values of 1.7 (resp. 1.6) Sv in 2013 (resp. 2005). Using the model results, the winter-integrated buoyancy loss over the Gulf of Lions is identified as the primary driving factor of the DWF interannual variability and explains, alone, around 50 % of its variance. It is itself explained by the occurrence of few stormy days during winter. At daily scale, the Atlantic ridge weather regime is identified as favourable to strong buoyancy losses and therefore DWF, whereas the positive phase of the North Atlantic oscillation is unfavourable. The driving role of the vertical stratification in autumn, a measure of the water column inhibition to mixing, has also been analyzed. Combining both driving factors allows to explain more than 70 % of the interannual variance of the phenomenon and in particular the occurrence of the five strongest convective years of the model (1981, 1999, 2005, 2009, 2013). The model simulates qualitatively well the trends in the deep waters (warming, saltening, increase in the dense water volume, increase in the bottom water density) despite an underestimation of the salinity and density trends. These deep trends come from a heat and salt accumulation during the 1980s and the 1990s in the surface and intermediate layers of the Gulf of Lions before being transferred stepwise towards the deep layers when very convective years occur in 1999 and later. The salinity increase in the near Atlantic Ocean surface layers seems to be the external forcing that finally leads to these deep trends. In the future, our results may allow to better understand the behaviour of the DWF phenomenon in Mediterranean Sea simulations in hindcast, forecast, reanalysis or future climate change scenario modes. The robustness of the obtained results must be however confirmed in multi-model studies.
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Somot Samuel, Houpert Loic, Sevault Florence, Testor Pierre, Bosse Anthony, Taupier-Letage Isabelle, Bouin Marie-Noelle, Waldman Robin, Cassou Christophe, Sanchez-Gomez Emilia, Durrieu De Madron Xavier, Adloff Fanny, Nabat Pierre, Herrmann Marine (2018). Characterizing, modelling and understanding the climate variability of the deep water formation in the North-Western Mediterranean Sea. Climate Dynamics, 51(3), 1179-1210. Publisher's official version : https://doi.org/10.1007/s00382-016-3295-0 , Open Access version : https://archimer.ifremer.fr/doc/00348/45948/