Estimates of the ocean heat budget in the Gulf stream
Seminar Room 1, Newton Institute
The Northwest Atlantic has the largest annually averaged oceanic surface heat loss in the world's ocean, with a wintertime spatial average of over 200 W/m2. In addition, it is at this location in the Northern Hemisphere that the zonally averaged meridional heat transport by the ocean reduces abruptly to below 0.5 PetaWatts with the atmosphere taking over the bulk of heat transport. Interannual changes of the exchange of heat between the ocean and the atmosphere are large in this region with magnitudes exceeding 50 W/m2 or more averaged over the Gulf Stream, equivalent about to 0.1 PetaWatts. To quantify the source of interannual changes in surface heat flux and heat storage, we examine the ocean heat budget in four different ocean models, a 1/10-degree prognostic ocean model (POP), a 1/3-degree assimilative model of the North Atlantic (Mercator), a diagnostic model, and a 18km degree Green;s function assimilation model (ECCO2). The last three models solutions all depend directly on the observed altimetric sea surface height. We focus the Gulf Stream once it leaves the coast. While the models disagree in detail, they do agree on some points. The ocean can locally store substantial quantities of heat on theses times scales. The changes in heat content on interannual time scales are concentrated in the upper 400 m of the water column while for monthly times scales, they are concentrated in the upper 100 m. Monthly heat content changes are controlled by the atmosphere with the ocean lagging the surface heat flux by one month. On interannual times scales, heat content changes are controlled by horizontal heat transport convergence with only a secondary role for the surface heat fluxes. At the same time, the surface fluxes are negatively correlated with the oceanic heat content, lagging it by 3 months. This indicates that the surface fluxes damp the heat content changes in the ocean instead of forcing them, suggesting an active role of ocean dynamics in the climate system. Additional comments on the use of ocean models for studies of the ocean's role in climate are also given.