Modeling the Mediterranean Sea interannual variability during 1961–2000: Focus on the Eastern Mediterranean Transient

Abstract

This work is dedicated to the study of the climate variability of the Mediterranean Sea, in particular the study of the Eastern Mediterranean Transient (EMT) which occurred in the early 1990s. Simulations of the 1961–2000 period have been carried out with an eddy‐permitting Ocean General Circulation Model of the Mediterranean Sea, driven by realistic interannual high‐resolution air‐sea fluxes. Using different databases for the river runoff, Black Sea inflow, and Atlantic thermohaline characteristics at climatological or interannual scales, we assess the effects of the non‐atmospheric hydrological forcings on the simulation of the interannual variations of the Mediterranean circulation. The evolution of the basin‐scale heat content is in very good agreement with the observations (especially in the surface and intermediate layers), while the agreement is lower for the evolution of the salt content. Convection events in the Aegean Sea are noticed in the simulations between 1972 and 1976, in the late 1980s, and around the EMT period. The formation rates of Cretan Deep Water (CDW) are different during these periods, allowing or preventing the spreading of CDW into the eastern Mediterranean. The sequence of the EMT events is well reproduced: the high winter oceanic surface cooling and net evaporation over the Aegean Sea in the early 1990s, the high amount of dense CDW formed during these winters, and then the overflow and the spreading of this CDW in the eastern Mediterranean. Among the preconditioning processes suggested in the literature, we find that changes in the Levantine surface circulation, possibly induced by the presence in the Cretan Passage of anticyclonic eddies and a lasting period with reduced net precipitation over the eastern Mediterranean, lead to an increase of the salt content of the Aegean Sea. Changes in the Black Sea freshwater inflow or in the characteristic of the Atlantic Water entering at the Gibraltar Strait also modify the thermohaline state of the Aegean Sea before the EMT. But, as none of these preconditioning factors has a lasting impact on lowering the vertical stratification of the Aegean Sea, we conclude that concerning the EMT, the major triggering elements are the atmospheric fluxes and winds occurring in winters 1991–1992 and 1992–1993.

Dense intermediate water outflow from the Cretan Sea: A salinity driven, recurrent phenomenon, connected to thermohaline circulation changes

“…Data before 1986 -1987 in the EMed exist only from bottle casts at standard depth levels which makes the investigation on the existence of dCIW rather difficult. Nevertheless, several papers [Lascaratos et al, 1999;Theocharis et al, 2002b;Skliris and Lascaratos, 2004;Skliris et al, 2007;Beuvier et al, 2010;Vervatis et al, 2013;Theocharis et al, 2014] based on both observations and numerical modeling have reported a similar event taking place in the eastern part of the EMed during the 1970s. This event was salinity induced and seems to have originated through dense water formation taking place in the Aegean and/or Levantine Seas during a salinity preconditioning phase.…”

Improving sea level simulation in Mediterranean regional climate models

“…It is composed of LMDz4-regional as atmospheric component (Li et al 2012) and of NEMOMED8 (Beuvier et al 2010;Adloff et al 2015) as oceanic component. NEMOMED8 is a regional configuration of the NEMO ocean model (Madec 2008), setup for the Mediterranean Sea.…”

“…It has an horizontal resolution ranging from 9 to 12 km and 43 vertical levels. In this simulation, the net "evaporation-precipitation-river" (E-P-R) flux computed over the basin is added in the Atlantic buffer zone, in a way that the volume of the basin is kept constant to a fixed prescribed Atlantic level (Beuvier et al 2010). This model configuration implicitly imposes that basin averaged sea level variations will be very small.…”

“…From a modelling point of view, some ORCMs simulate the Atlantic reservoir by imposing an extra surface water flux in the nearby Atlantic of the same magnitude of the freshwater flux integrated over the Mediterranean (e.g Beuvier et al 2010). The drawback of this approach is that it does not consider the effects of Atlantic variations.…”

North-western Mediterranean sea-breeze circulation in a regional climate system model

“…The water exchanges with the Atlantic Ocean are parameterized with a 3D relaxation to this climatology between 11°W and 6°W. The runoffs and the Black Sea water input are prescribed from a climatology as precipitation (Beuvier et al 2010). The vertical diffusion is performed by the standard turbulent kinetic energy model of NEMO and in case of instabilities a higher diffusivity coefficient of 10 m 2 s −1 is used.…”