Atmospheric response to removal of Southern Hemisphere sea ice - a model study

Abstract

The atmospheric response to the removal of Southern Hemisphere sea ice is studied using the ICTP AGCM (SPEEDY) model with an uncoupled ocean. The results show large increases in surface heat flux from the ocean to the atmosphere over areas with sea ice removed. The increased heat flux is accompanied by an intense and shallow atmospheric warming in the annual and austral winter mean. Warming in the lower troposphere is associated with a positive geopotential height response that is usually equivalent barotropic. This atmospheric response is present in the annual and the austral winter means, with the largest magnitudes during the austral winter. A baroclinic pattern is found in the late summer (March) response. A comparison between the maximum theoretical local response and the model simulated response for September and March indicates a more non-local component of the response during March. A dipole response in the zonal wind field which is consistent with the geopotential height anomalies is found in the annual and austral seasonal mean. The eddy-driven midlatitude jet is weakened and shifted equatorward, while the subtropical jet is barely affected. Using empirical orthogonal function analysis, the Southern Hemisphere geopotential height variability has been investigated. A somewhat weaker Southern Annular Mode results within the simulation of removed sea ice and this result appears to be broadly consistent with the weaker eddy-driven jet.