A multibasin reduced model of the global thermohaline circulation: Paleoceanographic analyses of the origins of ice‐age climate variability

Abstract

A new, multibasin reduced model of the global thermohaline circulation has been developed that builds upon the single‐basin Atlantic model recently described in Sakai and Peltier [1995]. The model comprises individual, two‐dimensional, Atlantic, Indian, and Pacific components which are linked via a circumpolar basin representative of the Southern Ocean. It also includes a complete seasonal cycle for sea surface temperature, a sea ice component, and an acceptably accurate representation of the influence of both wind stress and bottom topography. The circulation in the individual basins is described using a stretched coordinate system in order to allow the use of reduced vertical resolution where high resolution is unnecessary. Values for the control parameters of the new multibasin model are established in the same manner as was employed in analysis of the single‐basin model, and the basic conclusion of that study, that there exists a natural oscillatory mode of behavior of the deep circulation on a timescale ranging from a century to several millennia, is confirmed. The precise timescale of the internal variability is determined by the detailed nature of the hydrological cycle which is herein constrained far more realistically than was possible for the single basin model. The simulations performed with the model deliver a rather realistic facsimile of the millennium timescale Dansgaard‐Oeschger oscillations [Dansgaard et al., 1984] that were such a prominent feature of North Atlantic glacial climate according to climate proxy data from the Greenland Ice Core Project and Greenland Ice Sheet Project ice cores from Summit, Greenland, when boundary conditions are fixed to those appropriate to full glacial conditions. We also describe a sequence of paleoceanographic experiments that have been designed to explore the sensitivity of the deep circulation to the impact of the specific surface freshwater anomalies that are known to have developed during the last deglaciation event of the current ice age. The simulation of the response of the thermohaline circulation to such anomalies provides strong additional support for the notion that modulation of the strength of the overturning flow in the Atlantic basin played a very important role in the Younger‐Dryas climate reversal.