An increase in anthropogenic greenhouse gases influences not only surface temperature, but also sea-level pressure (SLP), specific humidity, and cloud cover. Over the Southern Hemisphere, SLP decreases significantly over the polar regions and increases over most other regions. Observations and reanalysis data reflect this SLP gradient since the 1950's, and also during the shorter period since 1979, when data became more reliable due to satellite observations. Many IPCC-AR4 general circulation models (GCM) are able to represent both positive and negative trends over the Southern Hemisphere, but the magnitude of the gradients in each direction is up to one order of magnitude lower than observed. In addition to the simulation of sea-ice cover, also the response in simulated cloud cover over the Southern Ocean and Antarctica can alter the models' response to increased greenhouse gas forcing. A cloud cover increase can diminish the radiative feedback mechanisms related to a reduced sea-ice cover.

Impacts of the SLP representation in GCMs on the simulation of atmospheric teleconnections such as the Southern Annular Mode is evaluated. The comparison of ERA40 reanalysis data and ISCCP observations to IPCC GCMs includes the high-resolution ECHAM5-MPIOM and HadGEM1. The present study quantifies the factors responsible for the reduced response over the Antarctic region in 20th century simulations. The radiative energy budget is analyzed to determine and explain the representation of this trend in surface temperature and sea-ice cover. A comparison of the radiation budget to clear-sky conditions reveals the impact of cloud radiative forcing on the simulated energy budgets as well as resulting atmospheric dynamics and teleconnections.

The analysis over the Southern Hemisphere with its more symmetric land-ocean distribution provides a clearer picture of air-sea interaction over the Southern Ocean, land-surface processes over Antarctica, and cloud cover over the Southern Hemisphere than an analysis over the Northern Hemisphere with its large land masses and related asymmetric land-surface and vegetation feedbacks. Nonetheless, the results provide vital insights into global climate change including the strong warming trend observed over the polar regions.

The present study shows that both teleconnections and trends are important factors in the description of present-day climate and the assessment of future projections with GCMs.