Sensitivity of Mediterranean thermohaline circulation to gateway depth: A model investigation

Abstract

[1] The Neogene sedimentary record of the Mediterranean Sea holds evidence for changes in water properties and circulation. These paleoceanographic changes have been attributed to changes in the flow through the ocean gateway between the Mediterranean Sea and the Atlantic Ocean. We use an oceanic general circulation model to achieve quantitative, physics-based understanding of the effect of changes in the depth of the gateway such as may result from tectonic activity or variation in sea level. To isolate these effects we use idealized basin geometry and impose simplified atmospheric forcing. A reference experiment with present-day sill depth demonstrates that the model simulates reasonably well the main features of the present-day Mediterranean thermohaline circulation as inferred from observations and previous numerical studies. Subsequently, a series of sensitivity simulations is performed with different sill depths. The model results show that Mediterranean temperature, salinity, and thermohaline circulation depend strongly on sill depth. As the sill depth decreases, the upper overturning cell is quasi-linearly reduced in strength, while, contrary to what one would expect, the deep cell intensifies and does so in a nonlinear way. We find that a shoaling of the sill depth induces a "blocking effect" on the outflow waters, which creates a strong recirculation in the deep layers, strengthening the deep cell. Nevertheless, deep-water formation is reduced, and, as a consequence, the ventilation of the deep layers diminishes. We identify three different circulation modes of the Mediterranean thermohaline circulation depending on the sill depth: "shallow sill," "moderate sill," and "deep sill" modes coupled with strong, weak, and negligible blocking effects, respectively. Our results are consistent with the pre-Messinian paleoceanographic record of the Mediterranean Sea and may be useful to understanding the behavior of other land-locked basins, both extant and from the geological past.Citation: Alhammoud, B., P. T. Meijer, and H. A. Dijkstra (2010), Sensitivity of Mediterranean thermohaline circulation to gateway depth: A model investigation, Paleoceanography, 25, PA2220,

Modeling Mediterranean Ocean climate of the Last Glacial Maximum

“…Alhammoud et al (2010) found a similar dependence of the deep outflow from the sill depth in coarse resolution ocean GCM.…”

“…There is some tendency of an intensification of the deep overturning cell in the western Mediterranean. Alhammoud et al (2010) found a similar response in an idealized Mediterranean setup of on ocean GCM in response to a shallowing of the sill depth.…”

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

“…The sensitivity of circulation in the Late Miocene Mediterranean to changes in the palaeogeography has been examined by Meijer et al (2004). More recently, Alhammoud et al (2010) have examined how the sill depth of the Strait of Gibraltar affects the thermohaline circulation in a highly idealized coarse resolution ocean circulation model.…”

“…The surface forcing is idealized and constant in time, and resembles the forcing used in Alhammoud et al (2010). The surface water flux is a constant evaporation of 0.5 m yr −1 over the whole model domain, a value close to presentday evaporation -precipitation -river input (Mariotti et al, 2002), but also suitable for the Late Miocene (Gladstone et al, 2007;Schneck et al, 2010).…”

“…POM has been applied extensively to the Mediterranean and the Strait of Gibraltar (e.g. Zavatarelli and Mellor, 1995;Ahumada and Cruzado, 2007;Jungclaus and Mellor, 2000;Drakopoulos and Lascaratos, 1999;Alhammoud et al, 2010;Sannino et al, 2002;Beckers et al, 2002). In an application to the Mediterranean, POM is set up with a true evaporation instead of a virtual salt flux.…”