Low Frequency Variations in the Hadley and Walker Circulations

In 1686 Halley advanced the idea that trade winds and monsoon circulations were tied to differential patterns in solar heating. Hadley (1735) soon added the important concept of conservation of angular momentum. Lorenz (1955) described how temperature gradients produced the potential energy, maintaining the general circulation. The differential heating patterns that drive the Hadley and Walker circulations arise through variations in insolation and water vapor-related warming. Water vapor is a very effective greenhouse gas, and the tropical-to-extra tropical and eastern-to-western Pacific water vapor gradients modulate surface longwave radiation and the associated surface temperature gradients. While increases in water vapor are accepted as one robust feature of anthropogenic climate change, there has been relatively little attention paid to the effects of water vapor-related longwave warming. While it is generally accepted that that the ‘rich will get richer' (in the sense that moist regions will see a disproportionate increase in water vapor), there has been relatively little discussion of how these differential changes in moisture will translate into differential changes in water vapor related increases in longwave radiation. Will disproportionate increases in longwave radiation result in a tendency for the ‘warm to get warmer' leading to an intensification of the Hadley and the Walker circulation? How coupled are the Hadley and Walker circulations? In this presentation we examine these issues using i) a single column plane-parallel radiative transfer model, ii) reanalysis climate fields, iii) sea surface temperature observations, and iv) historical climate change simulations drawn from the phase 5 coupled model intercomparison project (CMIP5) archive. We conclude with an examination of potential impacts on eastern Africa, central southwestern Asia and the United States.

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