Compared to National Centers for Environmental Prediction (NCEP) reanalyses, the MMD ensemble annual surface temperatures are in general slightly warmer in the Southern Ocean to the north of the sea ice region. The mean bias is predominantly less than 2°C (Carril et al., 2005), which may indicate a slight improvement compared to previous models due to better simulation of the position and depth of the Antarctic trough (Carril et al., 2005; Raphael and Holland, 2006). The temperature bias over sea ice is larger. Biases over the continent are several degrees where the model topography is erroneous (Turner et al., 2006). However, as emphasized above, the biases have to be viewed in the context of the uncertainty in the observations. Changes in cloud and radiation parametrizations have been shown to change the temperature simulation significantly (Hines et al., 2004). A lateral nudging of a stretched-grid GCM (imposing the correct synoptic cyclones from 60°S and lower latitudes) brings the model in better agreement with observations but significant biases remain (Genthon et al., 2002).
The spread in the individual MMD-simulated patterns of surface temperature trends over the past 50 years is very large, but in contrast to previous models, the multi-model composite of the MMD models qualitatively captures the observed enhanced warming trend over the Antarctic Peninsula (Chapman and Walsh, 2006). The general improvements in resolution, sea ice models and cloud-radiation packages have evidently contributed to improved simulations. The ensemble-mean temperature trends show similarity to the observed spatial pattern of the warming, for both annual and seasonal trends. For the annual trend, this includes the warming of the peninsula and near-coastal Antarctica and neutral or slight cooling over the sea-ice covered regions of the Southern Ocean. While the large spread among the models is not encouraging, this level of agreement suggests that some confidence in the ensemble mean 21st-century projection is appropriate.