Changes in the global divergent overturning circulation, including the Hadley and Walker Circulations

Abstract:

The interannual variations of the Walker Circulation (WC), a divergent east-west overturning circulation, affect the tropical hydroclimate directly and the extratropical hydroclimate indirectly through global teleconnections leading to changes in extratropical extremes. In addition to this crucial role in climate variations, a slowdown in the Pacific branch of the WC (PWC) has been regarded as an important indicator of climate change. Trends in the northern and southern branches of the meridional divergent circulation, e.g., the Hadley Circulation, have also been used as indicators of climate change. Global climate models suggest that water vapor will increase relatively more than precipitation as greenhouse gases (GHG) increase in the atmosphere. Consideration of top of the atmosphere and surface energetics associated with these increases suggests that the global convective mass flux must decrease to compensate. Some researchers have suggested that observations of the PWC spanning the last century provide evidence that the global convective mass flux is decreasing. Global climate models show a decrease in the PWC from the last century extending into the next. The debate surrounding this issue is complicated by different investigators using different indices to define the PWC, with some based on both the rotational and divergent components of the tropical winds to diagnose a divergent circulation. The influence and effect of tropical sea surface temperatures (SST) is also a confounding issue. We find that, in contrast to climate models, most observed aspects of the PWC show no trend or a strengthening over the last 150 years. This suggests that regional circulations cannot be used to infer global balances. We will examine the global divergent circulation, Walker Circulation, and Hadley Circulation in multiple reanalysis datasets spanning the last 150 years and in climate models to further diagnose the predicted weakening of the global overturning circulation.