NOAA has issued its annual forecast for the hurricane season, along with its now-standard explanation that there is a natural cycle of multidecadal (40-60 year) length in the North Atlantic circulation (often referred to as the “Atlantic Multidecadal Oscillation”–see Figure), that is varying the frequency of Atlantic tropical cyclones, and that the present high level of activity is due to a concurrent positive peak in this oscillation. More »
Second article of our 3-part series on atmospheric circulation and global warming
In Part I we outlined some general features of the tropical circulation, and discussed ways in which increases in anthropogenic greenhouse gases might affect El Niño. Now we take up the question of how global warming might affect the quasi-steady east-west overturning circulation known as the Walker Circulation. The Walker circulation affects convection and precipitation patterns, the easterly Trade Winds, oceanic upwelling and ocean biological productivity; hence, changes in this circulation can have far-reaching consequences. It also provides the background state against which El Niño events take place, and so changes in the Walker circulation should form an intrinsic part of thinking about how global warming will affect El Niño. In a paper that recently appeared in Nature, Vecchi, Soden, Wittenberg, Held, Leetmaa and Harrison present intriguing new results which suggest that there has already been a weakening of the Walker circulation in the past century, and that the observed changes are consistent with those expected as a response to increases in anthropogenic greenhouse gases. The discussion in Vecchi et al. also raises some very interesting issues regarding the way the hydrological cycle might change in a warming world.