Langfristige Variabilität der thermohalinen Zirkulation in einem gekoppelten Ozean-, Meereis-, Atmosphaerenmodell

Abstract

The heat and moisture transports by transient eddies in the EMBM are parameterized by diffusion. The coupled model reproduces many aspects of today´s oceanic circulation. The most interesting features of the coupled model are the sensitivity of the thermohaline circulation to changes in the configuration, the multidecadal variability in the ocean-sea ice system, and the behaviour of the thermohaline circulation during transitions between glacial and interglacial periods. A very strong thermohaline circulation develops in the coupled system that is not evident in the stand-alone ocean model. An interesting aspect of this behaviour is the existence of a maximum strength in the overturning. Beyond this maximum, evaporation in the subtropics cannot balance the northward salt-transport. As a result, the watermasses over the deep water production sites become fresher, leading to a collapse of the thermohaline circulation. The associated changes in the sea ice cover prevent the system to recover. Acceleration and breakdown of the thermohaline circulation is expected during glaciation periods, when freshwater is stored on continents and the oceanic stratification in the North Atlantic is weakened by a reduced continental run-off and by enhanced cooling of surface water. Under these conditions a stronger thermohaline circulation compared to the reference case was established. Salinity was redistributed such that the deep water became more saline than today. During the following equilibrium conditions without net fresh water storage on the continents, the coupled model reproduces the shallow and weak overturning cell and many other aspects of the glacial circulation. All experiments are characterised by a highly variable thermohaline circulation on multidecadal timescales. This variability is maintained by the negative feedback between the strength of the overturning and sea ice cover.