Accelerated Warming of the Southern Ocean and 1 Its Impacts on the Hydrological Cycle and Sea Ice 2

1 The observed sea surface temperature (SST) in the Southern Ocean shows a 2 substantial warming trend for the second half of the 20th century. Associated with the 3 warming, there has been an enhanced atmospheric hydrological cycle in the Southern 4 Ocean that results in an increase of the Antarctic sea ice for the past three decades through 5 the reduced upward ocean heat transport and increased snowfall. The simulated SST 6 variability from two global coupled climate models for the second half of the 20th century 7 is dominated by natural internal variability associated with the Antarctic Oscillation, 8 suggesting that the models' internal variability is too strong, leading to a response to 9 anthropogenic forcing that is too weak. With increased loading of greenhouse gases in the 10 atmosphere through the 21st century, the models show an accelerated warming in the 11 Southern Ocean, and indicate that anthropogenic forcing exceeds natural internal 12 variability. The increased heating from below (ocean) and above (atmosphere) and 13 increased liquid precipitation associated with the enhanced hydrological cycle results in a 14