The response of Mediterranean thermohaline circulation to climate change: a minimal model

Abstract

Abstract. Physics-based understanding of the effects of paleoclimate and paleogeography on the thermohaline circulation of the Mediterranean Sea requires an ocean model capable of long integrations and involving a minimum of assumptions about the atmospheric forcing. Here we examine the sensitivity of the deep circulation in the eastern Mediterranean basin to changes in atmospheric forcing, considered a key factor in the deposition of organic-rich sediments (sapropels). To this extent we explore the setup of an ocean general circulation model (MOMA) with realistic (present-day) bathymetry and highly idealized forcing. The model proves able to qualitatively capture some important features of the large-scale overturning circulation, in particular for the eastern basin. The response to (i) a reduction in the imposed meridional temperature gradient, or (ii) a reduction in net evaporation, proves to be non-linear and, under certain conditions, of transient nature. Consistent with previous model studies, but now based on a minimum of assumptions, we find that a reduction in net evaporation (such as due to an increase in freshwater input) may halt the deep overturning circulation. The ability to perform long model integrations allows us to add the insight that, in order to have the conditions favourable for sapropel formation persist, we must also assume that the vertical mixing of water properties was reduced. The "minimal" model here presented opens the way to experiments in which one truly follows the basin circulation into, or out of, the period of sapropel formation and where forcing conditions are continously adjusted to the precession cycle.

Changes in Mediterranean circulation and water characteristics due to restriction of the Atlantic connection: a high-resolution ocean model

“…The strength of the surface overturning cell in the reference experiment is slightly larger than in most other highresolution model studies (Meijer et al, 2004;Meijer and Dijkstra, 2009;Somot et al, 2006;Stratford et al, 2000;Adloff et al, 2011). The vertical and horizontal extent, however, is similar for both the western and eastern basin.…”

“…Meijer et al, 2004;Meijer and Dijkstra, 2009). The reference experiment, set up with idealized forcing, is able to produce both.…”

“…Thus, insight gained about the change in circulation and water properties due to variation in sill depth is expected to be largely generic. A minimal model of the Mediterranean with a highly idealized surface forcing has been shown to be able to capture the important characteristics of Mediterranean circulation (Meijer and Dijkstra, 2009). A similar idealized surface forcing is applied in this study, a simplification of the model setup that is justified when the focus is on the influence of the sill depth.…”

“…The present-day Mediterranean circulation and the influence of temporal variations in the surface forcing, e.g. fresh water budget and heat flux, has also been widely studied, often in the context of sapropels (Meijer and Tuenter, 2007;Meijer and Dijkstra, 2009;Samuel et al, 1999;Myers et al, 1998a, b;Myers, 2002). Data and simple modelling studies have demonstrated the importance of the size of the Atlantic-Mediterranean connection in controlling sedimentation and circulation in the Mediterranean.…”

“…Winds over the Mediterranean are highly variable throughout the year; a yearly averaged wind pattern is therefore unusable. Since wind stresses have been shown to affect only the strength of upper ocean circulation without changing the overall circulation pattern (Myers et al, 1998b;Meijer and Dijkstra, 2009) condition is applied to the free surface elevation, a clamped inflow condition is used for the depth-averaged velocity normal to the open boundary, and a zero velocity is used for the boundary parallel velocity. For the baroclinic mode (3-D), a Sommerfeld radiation condition is applied to the velocity normal to the open boundary and a zero velocity is used, again, for the boundary parallel velocity.…”