Hadley circulation and Climate Change
Hadley circulation is a significant atmospheric phenomenon that occurs near the equator, where hot air rises and travels toward the poles before descending around 30° latitude. This circulation pattern creates trade winds and influences global weather patterns, including precipitation and temperature distribution, particularly in tropical regions. The Inter-Tropical Convergence Zone (ITCZ) is a key area within this circulation, where the Northeast and Southeast Trades meet, leading to abundant rainfall in equatorial regions.
Climate change impacts this circulation by altering sea surface temperatures, which in turn affects precipitation patterns and atmospheric dynamics. Different climate models yield varying predictions regarding the future intensity of Hadley circulation, with some suggesting a strengthening that could lead to drier subtropical regions, while others indicate a potential weakening. Recent trends show an intensified Hadley circulation since 1950, associated with increased rainfall in equatorial areas and a drier climate in tropical and subtropical landmasses. These changes are influenced by warming ocean waters and may represent either natural fluctuations or human-induced changes, creating a complex interplay in the Earth’s climate system. Understanding these dynamics is crucial, as shifts in Hadley circulation can significantly alter weather patterns globally.
Hadley circulation and Climate Change
Definition
Named for eighteenth century meteorologist George Hadley, who explained how it worked, the is a loop of air that starts near the equator. The hot air in the equator region rises to the level and is carried toward the poles. At about 30° latitude, the air drops back to the surface, creating a high-pressure area. The air is pulled back toward the equator, completing the loop and generating the trade winds. Because the Earth is spinning, the air traveling back to the equator goes toward the southwest in the Northern Hemisphere and is called the Northeast Trades. In the Southern Hemisphere, the air movement generates the Southeast Trades. The low-pressure area where the Northeast Trades meet the Southeast Trades and where the hot air rises is called the Inter-Tropical Convergence Zone (ITCZ). The ITCZ does shift with the seasons.
The Hadley circulation is caused by solar heating. The intensity of the Hadley circulation is related to the sea surface temperature. The hot air will carry moisture aloft, but as the air rises, it cools and is able to contain less moisture, causing the large amount of rainfall in the equatorial region. Still, it is moist hot air that travels toward the poles and drier cool air that blows back toward the equator.
The Hadley circulation is one of the major regulators of the Earth’s energy budget. It spreads heat collected at the equator to the northern and southern subtropical areas. It also carries heat into the troposphere, where it can radiate into space. Hadley’s ascending limb controls the rainfall in the tropical areas, where large amounts of rain occur. The descending limb controls the dryness of the subtropical area. Although the Hadley circulation covers only part of the Earth, it covers the area where a large percentage of the people of the world live.
Significance for Climate Change
A difference in the patterns of the sea surface temperature would force a change in precipitation and cause a change in the Hadley circulation. A change in that system would cause a change in the flow of heat, momentum, and humidity along the meridians. The Earth’s overall radiative balance, along with the monsoon systems and the ocean circulation, are also affected by a change in Hadley circulation. Different climate models diverge when they project the effect of an increase in on the Hadley circulation. Some models indicate an increase of the intensity of the Hadley circulation, causing a more arid subtropical region to develop as the rising GHG concentration causes an increase in sea surface temperature. Other models indicate a weakening of the Hadley circulation.
Since 1950, there has been a more intense Hadley circulation, with a consequential increase in rainfall in the equatorial oceanic region and a drier tropical and subtropical landmass. This increase has accompanied a stronger westerly stratospheric flow and an increase in cyclones in the middle latitudes. The driving force behind these changes has been identified as the warming of the Indo-West Pacific tropical waters. The increased sea surface temperature difference between the winter and summer hemisphere tropics causes a stronger Hadley circulation.
Models indicate that the solar forcing of the increased Hadley circulation is more intense with GHGs than without. It is not clear from the models proposed whether the more intense Hadley circulation is due to a natural fluctuation, is anthropogenic, or is the combined result of natural and human factors. Solar forcing models indicate more evaporation and thus more moisture carried aloft. This causes less cloud cover and more solar heating. Thus, solar forcing seems to represent a positive feedback loop. The more solar heating, the less cloud cover and more solar heating.
Models of past data indicate that the ITCZ may have shifted over time by more than just the annual summer-to-winter position shift. Another shift would cause different areas of the Earth to be dry or to be rain-soaked. The extent of the Hadley circulation is influenced by several different factors according to models. One factor that was shown not to be related is the mean global temperature. Just increasing the Earth’s temperature will not change the area covered by the Hadley circulation. The Hadley circulation can be likened to El Niño-Southern Oscillation events. Since 1976, the increase in number and strength of El Niño events has caused an increase in the strength of the winter Hadley circulation.
Bibliography
Diaz, Henry F., and Raymond S. Bradley, eds. The Hadley Circulation: Present, Past, and Future. Dordrecht, the Netherlands: Kluwer Academic, 2004.
Hadley, G. “Concerning the Cause of the General Trade-Winds.” Philosophical Transactions of the Royal Society of London 39 (1735): 58-62.
Halley, E. “An Historical Account of the Trade Winds, and Monsoons, Observable in the Seas Between the Tropicks, with an Attempt to Assign the Physical Cause of the Said Winds.” Philosophical Transactions of the Royal Society of London 16 (1686): 153-168.
Lionello, Piero, et al. "The Hadley Circulation in a Changing Climate." Annals of New York Academcy of the Sciences, vol. 15, no. 1, Apr. 2024, pp. 69-93, doi.org/10.1111/nyas.15114. Accessed 20 Dec. 2024.
O’Hare, Greg, John Sweeney, and R. L. Wilby. Weather, Climate, and Climate Change: Human Perspectives. Harlow, England: Pearson Prentice Hall, 2005.
Watterson, Ian Godfrey, and Edwin K. Schneider. “The Effect of the Hadley Circulation on the Meridional Propagation of Stationary Waves.” Quarterly Journal of the Royal Meteorological Society 113, no. 477 (July, 1987): 779-813.