What induced the exceptional 2005 convection event in the northwestern Mediterranean basin? Answers from a modeling study

Abstract

Open‐sea convection occurring in the northwestern Mediterranean basin (NWMED) is at the origin of the formation of Western Mediterranean Deep Water (WMDW), one of the main Mediterranean water masses. During winter 2004–2005, a spectacular convection event occurred, observed by several experimental oceanographers. It was associated with an exceptionally large convection area and unusually warm and salty WMDW. Explanations were proposed tentatively, relating the unusual characteristics of this event to the Eastern Mediterranean Transient (EMT) or to the atmospheric conditions during winter 2004–2005 in the NWMED. They could, however, not be supported until now. Here we used numerical modeling to understand what drove this convection event. The control simulation performed for the period 1961–2006 reproduces correctly the long‐term evolution of the Mediterranean Sea circulation, the EMT, and the NWMED convection event of 2004–2005. Sensitivity simulations are then performed to assess the respective contributions of atmospheric and oceanic conditions to this event. The weakness of the winter buoyancy loss since 1988 in the NWMED prevented strong convection to occur during the 1990s, enabling heat and salt contents to increase in this region. This resulted in the change of WMDW characteristics observed in 2005. The strong buoyancy loss of winter 2004–2005 was responsible for the intensity of the convection observed this winter in terms of depth and volume of newly formed WMDW. The EMT did not fundamentally modify the convection process but potentially doubled this volume by inducing a deepening of the heat and salt maximum that weakened the preconvection stratification.

Mediterranean Sea response to climate change in an ensemble of twenty first century scenarios

“…We use exactly the same settings as in Herrmann et al (2010) and Beuvier et al (2010). The largest difference with the study by Somot et al (2006) is that they used the OPAMED8 regional ocean model and ran only one 140-year long simulation.…”

“…For conciseness reasons, the model evaluation can not be exhaustive, we thus focus on the main equilibrium diagnostics and on the thermohaline circulation. The reader can find an extensive evaluation of NEMOMED8 in hindcast mode in Beuvier et al (2010), Herrmann et al (2010), Sevault et al (2009). Figure 2 represents the spatial coverage of different sub-basins of the Mediterranean and the Atlantic buffer zone in the NEMOMED8 grid.…”

“…The region ocean model used (NEMOMED8) is the same model as in Beuvier et al (2010) and Herrmann et al (2010) but it was forced in hindcast mode in these two studies. In addition, the scenario strategy and the definition of the forcings is very similar to the one applied in Somot et al (2006).…”

“…Beuvier et al 2010;Herrmann et al 2010) which is a Mediterranean configuration of the NEMO ocean model (Madec 2008). NEMOMED8 is a state-of-the-art eddy-permitting regional ocean model for climate-scale studies.…”

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“…López-Jurado et al (2005) suggested that the unusual characteristics of the 2005 convection event could be due to a change in the characteristics of the water masses advected until forming the zone of WMDWs after EMT. Herrmann et al (2010) rather suggest that the higher temperature and salinity of the new WMDW formed in 2005 are due to the absence of an intense convective winter in the 1990s and early 2000s in the NWMed, leading to an accumulation of heat and salt in the intermediate and deep layers of this area.…”