What is the Hadley Cell and Why Does it Concern Meteorologists so Much?
Hadley cells are a key player in heat transport in the atmosphere. Its expansion is increasingly evident, due to global warming.
The classical scheme of the general circulation of the atmosphere (CGA) is well known. In any book or manual on Meteorology and Climatology, both its figure and explanations of how this heat transport mechanism behaves in the Earth's atmosphere appear.
The two Hadley cells are its main engine and we are going to dedicate a few lines to them, since there is increasing evidence that they are expanding. The existence of these cells or circulation loops was first proposed by the English lawyer and amateur meteorologist George Hadley (1685-1768).
He should not be confused with the English astronomer, physicist and mathematician of the same period, Edmund Halley (1656-1742), famous for calculating the orbit of the comet that bears his name and predicting its periodic approaches to the Earth, every 76 years.
The spelling of Hadley and Halley is very similar, which is why their names are sometimes exchanged. It also happens that Halley (the astronomer) made important contributions on the trade winds (we owe to him the first map where they are represented), which are one of the key elements of the CGA and are closely linked to the Hadley cells.
Each of these cells involves a meridian circulation (according to the N-S direction), in which air is transported from the equator to approximately the 30th parallel (subtropical region) in the upper levels of the troposphere, and in the opposite direction in the lower ones, as it appears in the diagram of the figure. They are driven by the rise of air that takes place in the equatorial strip, generated by convection caused, in turn, by strong sunshine.
Global warming is expanding cells
Hadley cells play an important role in heat transfer from the equator to mid-latitudes. They are subject to a seasonal tilt, the amplitude of which varies depending on the Earth's length. If we stick to the cell located in the northern hemisphere and the geographical scope of the Canary Islands and the Iberian Peninsula, in January (boreal winter) the cell retreats towards the south, without exceeding the equator, and in July (boreal summer) it rises latitude, its descending branch being located around the Peninsula.
The natural variability of atmospheric behaviour means that these intra-annual changes in the behaviour of the Hadley cell are not the same every year. Although the winter-summer tilt of the Azores anticyclone was a fixed pattern, things are beginning to change. If until now the main modulator of this seasonal change was sunshine, for some time now, global warming is gaining more and more weight.
In its Sixth and final Report (AR6), published in 2021, the IPCC assigns a high level of confidence to the expansion of Hadley cells to high latitudes, indicating that in the particular case of the cell located in the northern hemisphere, said expansion and movement towards the north has been observed since 1980.
What are the consequences in Spain?
The above is something that is becoming increasingly clearer. Incursions of subtropical air into the Peninsula and the Balearic Islands are increasingly frequent, occurring at all times of the year. The incursions of warm ridges are nothing more than the reflection of the expansion of the Hadley cell, with the consequences that we are seeing: an increase in dry spells and droughts, as well as quasi-permanent positive temperature anomalies, with an increasing magnitude.
This observed pattern is leading us to a subtropicalisation of the climate both on the Peninsula and in the Balearic archipelago. Climate projections suggest that this expansion of Hadley cells will continue for the remainder of the century, in parallel with the continued increase in temperature.
We are witnessing the confirmation of the forcing of global warming in the general circulation of the atmosphere. Both the increase in the frequency and magnitude of droughts as well as extreme heat and rainfall irregularity are elements that, apart from making our adaptation difficult, complicate the management of a multitude of activities, closely linked to meteorological and climatic behaviour.