The Climate Response To Basin-Specific Changes In Latitudinal Temperature Gradients And The Implications For Sea Ice Variability

Recent trends in sea ice have raised the question of how much is associated with greenhouse warming, and how much is the result of oscillations within the climate system. To address this issue, we investigate the effect of changes in latitudinal temperature gradients on circulation and sea ice at high latitudes in a set of experiments with the GISS GCM. Sea surface temperature gradients are increased/decreased in all ocean basins, and are also changed in the different directions in different ocean basins, without allowing sea ice to change. Additional experiments allow sea ice growth/reduction with the altered temperatures. The results show that while increased gradients deepen the subpolar lows in both hemispheres, during winter they have little effect on sea level pressure over the Arctic, unless sea ice is allowed to change. With Arctic sea ice reductions, the sea ice response acts as a positive feedback, inducing cyclonic circulation changes that would enhance its removal, as may be occurring due to the current high phase of the North Atlantic Oscillation. In the Southern Hemisphere, gradient changes in one ocean basin reduce storm intensities in that basin, by shifting storm tracks equatorward and away from the potential energy source associated with cold air advection from Antarctica. Alterations of the gradient in one ocean basin change longitudinal temperature gradients; an increased gradient in one basin from tropical heating results in subsidence in the tropics in the other basin, mimicking the effect of a decreased gradient in that basin. Hence in many respects, regional effects, such as the strength of subpolar lows in the Northern Hemisphere are amplified when the gradient changes are of opposite sign in the two ocean basins, and in the Southern Hemisphere, a gradient increase in one ocean basin strengths storms (by moving them poleward) in the other basin. This latter effect may explain observed sea ice variations (the Antarctic dipole) which are out of phase in the two ocean basins in the Southern Hemisphere, as well as upper ocean variability in the Weddell gyre.