Seasonal cycle of meridional heat transport in the subtropical North Atlantic: a model intercomparison in relation to observations near 25 degrees N

Three different, eddy-permitting numerical models are used to examine the s easonal variation of meridional mass and heat flux in the North Atlantic, w ith a focus on the transport mechanisms in the subtropics relating to obser vational studies near 25 degreesN. The models, developed in the DYNAMO proj ect, cover the same horizontal domain, with a locally isotropic grid of 1/3 degrees resolution in longitude, and are subject to the same monthly-mean atmospheric forcing based on a three-year ECMWF climatology. The models dif fer in the vertical-coordinate scheme (geopotential, isopycnic, and sigma), implying differences in lateral and diapycnic mixing concepts, and impleme ntation of bottom topography. As shown in the companion paper of Willebrand et al. (2001), the model solutions exhibit significant discrepancies in th e annual-mean patterns of meridional mass and heat transport, as well as in the structure of the western boundary current system. Despite these differences in the mean properties, the seasonal anomalies of the meridional fluxes are in remarkable agreement, demonstrating a robust model behavior that is primarily dependent on the external forcing, and ind ependent of choices of numerics and parameterization. The annual range is s maller than in previous model studies in which wind stress climatologies ba sed on marine observations were used, both in the equatorial Atlantic (1.4 PW) and in the subtropics (0.4-0.5 PW). This is a consequence of a weaker s easonal variation in the zonal wind stresses based on the ECMWF analysis th an those derived from climatologies of marine observations. The similarities in the amplitude and patterns of the meridional transport anomalies between the different model realizations provide support for prev ious model conclusions concerning the mechanism of seasonal and intraseason al heat flux variations: they can be rationalized in terms of a time-varyin g Ekman transport and their predominantly barotropic compensation at depth. Analysis for 25 degreesN indicates that the net meridional flow variation at depth is concentrated near the western boundary, but cannot be inferred from transport measurements in the western boundary current system, because of significant and complex recirculations over the western half of the bas in. The model results instead suggest that the main recruitment for estimat ing the annual cycle of heat flux through a transoceanic section, and the m ajor source of error in model simulations, is an accurate knowledge of the wind stress variation. (C) 2001 Elsevier Science Ltd. All rights reserved.