Is the Pacific Walker Circulation Changing in Response to Global Warming?

Tuesday, May 13, 2014 - 07:00
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The interannual variations of the Pacific Walker Circulation (PWC), a divergent east-west overturning circulation in the tropical Pacific, 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 PWC has been regarded as an important indicator 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 the 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 Pacific Walker Circulation 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. With its associated wind, surface pressure, and SST variations examined separately, the PWC can be regarded as the proverbial elephant examined by a blindfolded group. 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.