RESPONSE OF CIRCULATION AND HEAT-TRANSPORT IN THE NORTH-ATLANTIC TO CHANGES IN THERMOHALINE FORCING IN NORTHERN LATITUDES - A MODEL STUDY

To avoid an explicit simulation of the overflows across the Greenland- Scotland ridge, many models of the large-scale ocean circulation seek to include the net effect of the inflowing dense water masses by resto ring temperature and salinity near the ridge to observed conditions. I n this paper the authors examine the effect of different datasets for the northern restoring condition in two versions, eddy resolving and n on-eddy resolving, of the model of the North and equatorial Atlantic t hat has been developed in recent years as a Community Modeling Effort for WOCE. It is shown that the use of smoothed climatological fields o f temperature and salinity south of the Denmark Strait leads to strong deficiencies in the simulation of the deep flow field in the basin. A switch to actual hydrographic data from the Denmark Strait ignites a rapid dynamic response throughout the North Atlantic, affecting the tr ansport and vertical structure of the deep western boundary current an d, by virtue of the JEBAR effect, the transport of the horizontal gyre s. Meridional overturning and northward heat transport, too weak in th e cases with climatological boundary conditions, increase to more real istic levels in the subtropical North Atlantic. The initial response t o switches in the high-latitude thermohaline forcing is mediated by fa st waves along the western boundary, leading to changes in the deep we stern boundary current in low latitudes after about two years in the n on-eddy-resolving case. The initial timescale depends on the horizonta l grid spacing of the model; in the high-resolution case, the first si gnal reaches the equator in a few months. The adjustment to a new, dyn amic quasi equilibrium involves Kelvin waves along the equator and Ros sby waves in the interior and is attained in less than two decades thr oughout the North Atlantic. It is suggested that these fast dynamic ad justment processes could play an important role in possible fluctuatio ns of the thermohaline circulation, or transitions between different e quilibrium states of the coupled ocean-atmosphere system, and may have determined the timescale of the observed climatic transitions before and during the last deglaciation.