Movements of the Hadley Cell: A cause of drought?

As mentioned by Hodell et al. (2001), a possible mechanism through which fluctuations in solar activity might have caused the Classic Maya drought is through the position and strength of the Hadley cell. The significance of this potential forcing factor was highlighted by Gill et al. (2007).

According to Gill et al. (2007), the position and strength of the Hadley circulation and the annual migration of the Inter-Tropical Convergence Zone (ITCZ) are very important factors in determining the annual rainfall at locations such as the Maya Lowlands, which lie at the northern limit of tropical rainfall.

Gill et al. (2007) state that the Hadley circulation is best developed over the oceans where there are no local topographical features such as mountain chains to interfere with its flow. Every ocean in the world, therefore, has a high-pressure cell at around 308 latitude which results from the descending branch of the Hadley Cell circulation. In the North Atlantic Ocean, the high-pressure cell is known as the North Atlantic High, although some meteorologists refer to it as the Bermuda High or the Azores High. Gill et al. (2007) state that various meteorologists have related rainfall in Mexico, and the circum-Caribbean region, to the position of this North Atlantic High. It is said that research indicates that the centre of the high-pressure cell travels from year to year in a southwest–northeast direction. When the High is displaced towards the northeast, it lies closer to Europe and warm temperatures prevail over the continent. When it moves to the southwest towards the Caribbean and the Maya Lowlands, cold weather moves in over the European continent and drought comes to Mesoamerica.

This southwest–northeast travel of the North Atlantic High would, according to Gill et al. (2007) indicate that its movement is a response to the expansion and contraction of the Hadley Cell circulation. Although it is yet to be definitively established, it is said that as the Hadley Cell is thermodynamically driven, it would be logical to believe that its areal extent responds to changes in the energy driving the system. In warm periods, then, when ample energy is available, the cell expands, and the High moves toward Europe. In cold periods, when there is less energy, it contracts, bringing the High closer to Mesoamerica, diminishing rainfall in the region.

However, this theory disagrees with that of Hodell et al. (2001), who found that droughts in the Maya region were, in fact, correlated with increased solar activity.

This contradiction makes it hard to establish whether movements of Hadley Cell were responsible for the drought that played a part in the Classic Maya collapse. Without greater historical evidence, Gill et al.’s (2007) theory cannot be proved. It is therefore clear that the climatic processes behind the droughts that occur in Mesoamerica are yet to be fully understood and theories are yet to be universally accepted. This is the case because the responses of the climate system to various forcing factors are extremely complex, vary across the globe and are difficult to attribute to a particular cause. Thus, more research is necessary if the climatic reasons for Mayan drought are to be fully comprehended.