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.

Highlights

  • Open-sea deep convection and the associated deep water formation (DWF) are among the most important ocean phenomena driving the ocean thermohaline circulation (Marshall and Schott 1999)

  • Two articles have been devoted to the overall evaluation of the CNRM-RCSM4 model in the hindcast configuration, one dealing mainly with the evaluation of the atmosphere component (Nabat et al 2015) and the other with the evaluation of the air-sea fluxes, of the river discharges and of the ocean behaviour at the Mediterranean Sea scale (Sevault et al 2014)

  • We don’t repeat here the results of this overall evaluation but we describe, the mean behaviour of the model for the DWF representation in the NorthWestern Mediterranean Sea for the period 1980–2013 that is to say from winter 1980–1981 to winter 2012–2013 (33 full winters)

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Summary

Introduction

Open-sea deep convection and the associated deep water formation (DWF) are among the most important ocean phenomena driving the ocean thermohaline circulation (Marshall and Schott 1999) They occur in few places of the world including the North-Western Mediterranean Sea in which they have been observed since a long time (MEDOC Group 1970; Leaman and Schott 1991; Schott et al 1996) and still recently (Durrieu de Madron et al 2013). In this specific area, open-sea deep convection leads to the formation of the Western Mediterranean Deep Water (WMDW) with θ–S–ρ characteristics, historically close to 12.75–12.92 ◦C, 38.41–38.46 and 29.09–29.10 kg/m3 (Mertens and Schott 1998). The high salinities are due to a strong net water loss at basin scale related to the Mediterranean dry climate and to the advection of the Levantine Intermediate Water (LIW), a salty and warm intermediate water coming from the Eastern Mediterranean

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