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Hadley Cell (HC) Circulation response to Climate Change and Global Warming Projections

Hadley circulation was first proposed by Sir George Hadley and Sir Edmund Halley in 1735. When they proposed an explanation for the observed wind patters in the tropics. Hadley circulation is a large scale circulation over the tropics. It consists of the zonally averaged meridional circulation: north to south, in the north hemisphere; south to north in the south hemisphere; converging at the equator, and rising, and then a return flow at upper levels, back to the extra tropics, with subsidence. (See section 2.1). It has seasonal and interannual variability. Recent studies have shown the relationship between the HC circulation and other large–scale systems at higher latitudes (subtropical jets, subtropical highs, etc.) and, also interactions with other interannual oscillations as El Niño Southern Oscillation (ENSO) and the Monsoon circulation. One of the characteristics that makes the HC important to study is the Inter-Tropical Convergence Zone (ITCZ), that arises from the convergence of the wind from the north and the south at the equator; hence its intensity is directly related with the precipitation over the tropics. Changes in Hadley circulation such as strengthening, weakening and movement are important for human activities because, as it has been said before, this circulation is related with the precipitation over the tropics, and other large-scale circulations that affect human activities with changes in surface winds, precipitation, droughts, etc. The oceans play an important role as part of the Hadley circulation. It is related with the difference in sea surface temperatures (SSTs) over the ocean due to the seasonality of solar radiation. Tanaka et al. (2003) Showed that the Hadley circulation exist even in a hypothetical aqua planet (no continents and no land-sea contrast), as long as this heat contrast imposed by the sun does not change (more radiation at the tropics, less radiations at higher latitudes). They concluded land-sea temperature contrast is more related with monsoon circulations rather than Hadley circulation. Thus this circulation would be affected by the changes in SSTs over the ocean due to climate change. In the Intergovernmental Panel on Climate Change (IPCC) report of 2007, on Chapter 3: Observations: Surface and Atmospheric Climate Change, in section 3.7: Changes in the Tropics and Subtropics, and in the Monsoons; they discuss the change, that according to some studies, the Hadley circulation would have as a result of climate change: “ Tropical SSTs determine where the upward branch of the Hadley Circulation is located over the oceans, and the dominant variations in the energy transport by the Hadley cell. Examination of the Hadley Circulation in several data sets (Mitas and Clement 2005), suggest some strengthening, although discrepancies among reanalysis data sets and known deficiencies raise questions about the robustness of this strengthening, especially prior to satellite era (1979).” (Trenberth et al. 2007) Changes in the tropical circulation patterns generates a response mechanism in the atmosphere and ocean localized not just in the tropics. Mitas and Clement, 2005 explained that studies in the radiation budget in the atmosphere suggest an increase of the long-wave radiation emitted by the earth and a reduction of the short wave radiation reflected from the sun. They argued hat it had been suggested that these changes are related with the intensification of the tropical atmospheric circulation, specially the HC. An increase in the intensity of the HC is traduced in an increase of upper motion at the tropics (ITCZ), this will increase the cloudiness (cloud-fraction) and the humidity in the ITCZ, with a decrease of the cloud-fraction at the subtropics. Thus, is important to study the possible changes of this tropical feature under a climate change environment, and the consequences that they would have by the increase of anthropogenic greenhouse gases. The present paper describes the principal changes that Hadley circulation will have due to climate change, according to the result of different Atmospheric-Ocean Global Climate Models (AOGCM).
Rosenstiel School of Marine and Atmospheric Sciences, University of Miami
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Ambito Geografico