Mechanisms for recent warming of the North Atlantic: Insights gained with an eddy-permitting model

The role of ocean heat transport variations in recent warming of the North Atlantic is investigated using an eddy-permitting global ocean model with prescribed atmospheric boundary conditions for 1985 - 2003. Variability of the model sea surface temperature compares favorably with satellite and in situ observations. Each data set reveals a similar pattern of significant surface warming over 1985 - 2003, across much of the North Atlantic. Significant warming trends exceed 0.1 degrees C a(-1) across a large area of the northwest Atlantic in the model simulation. Warming of the tropics and midlatitudes are considered separately. In both regions, long-term rises in temperature coincide with increasing ocean heat content in the upper few hundred meters, consistent with recent observations. The strongest link between surface temperature and heat content is found in midlatitudes. Averaged within each region, model surface heat fluxes do not show any major trends and are within the range of uncertainty in observational estimates. In midlatitudes, heat content increases are largely explained by changes in ocean heat transport across regional boundaries. While meridional heat transport is strongly linked to the meridional overturning circulation in the subtropics, such a link is weak in midlatitudes, where heat transport variability appears to be more strongly linked to fluctuations in gyre transport and eddy fluxes. It is concluded that the pattern of recent warming in the midlatitude North Atlantic is related to anomalous convergence of ocean heat transport, associated with changes in overturning and horizontal components of the circulation, in the northern subtropics and the subpolar gyre respectively.