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

Abstract

We compute the atmospheric heat transport in a realistic atmospheric general circulation modelunder five different configurations of implied heat transport in the ocean. The implied oceanicheat 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 zonallysymmetric component is added to or subtracted from the climatological SST field. The meridionalstructure of the variation in SST gradient is based on the observed change in zonallyaveraged SST over the last century. The SST trend has maxima of about 1 K at high latitudesof both hemispheres. Elsewhere, the change in SST over the last century is fairly uniform atabout 0.5 K. We find that in the annual mean, the atmosphere adjusts so that the total meridionalheat 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 atmosphericheat transport such that changes in latent heat transport usually go hand in hand with changesin dry static energy transport of an opposite sign. The radiative flux at the top of the atmosphereis affected the most by the change in SST in the tropics, where the shortwave component showsa 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 decreasethe meridional SST gradient (i.e., warm the sea surface at high latitudes the most), the stationarywaves accomplish more of the poleward heat transport than before. When we increase themeridional 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. Thewinter season in the Southern Hemisphere shows a substantial increase in heat transport bytransient waves when the meridional SST gradient is increased. Their maximum heat transportis greater and extends over a wider band of latitudes than in the control case. Because theSouthern Hemisphere is mostly covered by ocean, the stationary waves are weak and play aminor roôle in atmospheric heat transport.