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CGD Research: CCR Overview

Climate Change Research: Present and Future Climate Change Research

Southern Annular Mode

An observed trend in the Southern Annular Mode (SAM) during recent decades has involved an intensification of the polar vortex. The source of this trend is a matter of scientific debate with stratospheric ozone losses, greenhouses gas increases and natural variability all possible contenders. Since it is difficult to separate the contribution of various external forcings to the observed trend, a state-of-the art global coupled model is utilized here. Ensembles of 20th Century simulations forced with the observed time series of greenhouse gases, tropospheric and stratospheric ozone, sulfate aerosols, volcanic aerosols, solar variability and various combinations of these are used to examine the annular mode trends in comparison to observations, in an attempt to isolate the response of the climate system to each individual forcing. We find that ozone changes are the biggest contributor to the observed summertime intensification of the southern polar vortex in the second half of the 20th century, with increases of greenhouse gases also a necessary factor to reproduce the observed trends at the surface. Although stratospheric ozone losses are expected to stabilize and eventually recover to pre-industrial levels over the course of the 21st Century, these results show that increasing greenhouse gases will continue to intensify the polar vortex throughout the 21st century, but that radiative forcing will cause widespread temperature increases over the entire Southern Hemisphere. (Arblaster and Meehl, 2006)

 

CGD Sectional Narratives

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Ensemble mean DJFMAM seal level pressure trends.

Figure 19. (High resolution image) The 21st-century globally averaged change in (a) surface air temperature from 16 models and (b) sea level change from 6 models as compared to the period 1980–1999. The black bar and grey line in panel a) denotes the 16-model mean and 1 standard deviation, respectively.