The response of atmospheric heat transport to zonally-averaged SST trends

Abstract

We compute the atmospheric heat transport in a realistic atmospheric general circulation model under five different configurations of implied heat transport in the ocean. The implied oceanic heat transport is varied by changing the meridional gradient of sea surface temperature (SST). Climatological SSTs are employed for the control run. The other runs differ in that a zonally symmetric component is added to or subtracted from the climatological SST field. The meridional structure of the variation in SST gradient is based on the observed change in zonally averaged SST over the last century. The SST trend has maxima of about 1 K at high latitudes of both hemispheres. Elsewhere, the change in SST over the last century is fairly uniform at about 0.5 K. We find that in the annual mean, the atmosphere adjusts so that the total meridional heat transport (by atmosphere and ocean) is rather insensitive to the change in zonally averaged SST. Interannual variability in the annual mean heat transport is minor in each of these cases. There is a large degree of compensation even between the different components of atmospheric heat transport such that changes in latent heat transport usually go hand in hand with changes in dry static energy transport of an opposite sign. The radiative flux at the top of the atmosphere is affected the most by the change in SST in the tropics, where the shortwave component shows a strong negative feedback and the longwave component shows a weak positive feedback. Concentrating on the winter season in the Northern Hemisphere, we find that when we decrease the meridional SST gradient (i.e., warm the sea surface at high latitudes the most), the stationary waves accomplish more of the poleward heat transport than before. When we increase the meridional SST gradient, the heat flux due to both transient and stationary waves increases, although not by nearly as much as most eddy parameterization schemes would predict. The winter season in the Southern Hemisphere shows a substantial increase in heat transport by transient waves when the meridional SST gradient is increased. Their maximum heat transport is greater and extends over a wider band of latitudes than in the control case. Because the Southern Hemisphere is mostly covered by ocean, the stationary waves are weak and play a minor role in atmospheric heat transport.