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

The tropical Pacific Walker Circulation (PWC) is a planetary scale east-west overturning of the equatorial atmosphere with ascent over the western Pacific and descent over the eastern Pacific ocean. There's great interest in how the PWC has been affected by 20th century global warming. As yet, however, there's no clear observational consensus on whether it has weakened, strengthened, or stayed unchanged. On the other hand, a general slowdown of overturning atmospheric circulations has been argued to be physically necessary to explain the relatively weak response of global precipitation to global warming obtained in many climate models, but it's not clear to what extent such an argument applies to regional overturning circulations such as the PWC. Still, many climate models do also simulate a 20th century weakening of the PWC. 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. In this presentation, we will examine the global divergent circulation, the Pacific and Atlantic aspects of the Walker Circulation, and also examine the Hadley Circulation in multiple reanalysis datasets spanning the last 150 years and in climate models used in CMIP5 to further diagnose the predicted weakening of the global overturning circulation and its relationship to Pacific and Atlantic multidecadal variability.