The Active Layer of the Upper Atlantic Ocean

Abstract

The procedure to calculate the active layer depth of the upper ocean, as proposed by Van den Dool and Horel (DH), was applied to the Atlantic Ocean from 20°S to 70°N. In this method, the observed climatological annual cycle in SST is employed to invert a simple linear energy balance. The results for the Atlantic are similar to those for the Pacific Ocean in several ways. The active layer is considerably shallower than the annual mean mixed layer (which is calculated from in situ sea temperature profiles). Just as for the Pacific, however, the patterns of active and mixed layer depth show a remarkable spatial match. Using Bunker's datasets for SST and heat transfer over the Atlantic Ocean, the forcing used in the energy balance equation was made increasingly more realistic, from (i) astronomical solar radiation, through (ii) empirical estimates of absorbed solar radiation including the modifying effect of clouds to (iii) the complete empirically determined net ocean surface heat gain. No matter what forcing was used, the calculated active layer is always much shallower than the mixed layer depth. The best pattern match was found using the simplest forcing of all?the astronomical solar forcing. Increasingly, atmospheric models are being coupled to an oceanic slab in which the SST evolves in response to local heat gains and losses. The key question is how deep that slab should be. Our study implies that, in order to match the observed annual cycle in SST, the oceanic stab should be quite shallow, and certainly shallower than the mixed layer depth. The shallowness of the active layer implies that ocean heat transport contributes to the forcing of the annual cycle in SST in the midlatitudes of the Atlantic Ocean.