Stability of the Mediterranean's thermohaline circulation under modified surface evaporative fluxes

Abstract

A series of experiments with an ocean general circulation model of the Mediterranean forced by artificial (but realistic) surface fluxes of heat and fresh water is performed over 100 year periods. The model has a stable thermohaline circulation under the baseline fluxes. Small increases/decreases in excess evaporation (±8%) produce a linear strengthening/weakening of this thermohaline circulation with more/less water formation and strait transports. The water properties do not change much, and the responce is consistent with submaximal exchange at Gibraltar, where changes in volume and freshwater transport can be achieved mainly by changing the interface level between inflowing and outflowing waters. Larger changes in excess evaporation lead to nonlinear responses for increasing and decreasing values that are quite different. Reducing evaporation (−20%) causes both western and eastern deep water formation to cease, leaving a shallower intermediate circulation only. Increasing evaporation (+25%) initially strengthens the overturning but also produces a hydraulic jump‐like feature at Gibraltar, which mixes inflowing and outflowing waters, decreasing the salt flux out of the basin. As a result, the basin salinity increases greatly over 140 years, until the overturning collapses and the Gibraltar mixing ceases, leaving a shallow intermediate circulation above highly saline and stagnant deep waters. The possible relevance of these experiments to recent changes in Mediterranean fluxes and water properties is discussed.