Impact of penetrative solar radiation on the diagnosis of water mass transformation in the Mediterranean Sea

International audienceWe applied a revised diagnosis of water mass formation and mixing to a 1/8° resolution ocean model of the Mediterranean Sea. The diagnosis method used and presented by Iudicone et al. (2008) is similar to that developed by Walin (1982) and applied to the Mediterranean Sea by Tziperman and Speer (1994), to which we added a penetrative solar radiation. Both the prognostic model and the diagnostic method were in agreement with respect to the solar flux parameterization. Major changes were observed in the yearly budget of water mass transformation when the penetrative solar radiation is taken into account in the diagnosis. Annual estimates of water mass formation rates were decreased by a factor of two, with values within the range [-3.7 Sv, 1.5 Sv] compared to [-6 Sv, 3 Sv]. This decrease resulted from a lower seasonal variation when penetrative solar radiation was included. This can be explained by the fact that the solar radiation flux acted over a wider range of seawater density leading to lower net values over a given density interval. The major impact of the penetrative solar radiation occurred during spring and summer. Newly formed dense water was then transformed into lighter water with a rate reaching a value about 50% of that of the water mass formation rate in winter. Another consequence was that mixing processes which counteract formation rate in yearly budget of water mass formation rates, were overestimated. We showed that, in spring and summer, about a third of the transformation took place below the surface layer

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

International audienceOpen-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