The role of the Hadley cell in the general circulation of the atmosphere and the formation of a subtropical anticyclone over Southwest Asia

Document Type : Research Article

Authors

Department of Space Physics, Institute of Geophysics, University of Tehran, Tehran, Iran.

Abstract

Subtropical high pressures play a very important role in the climate of the whole earth, especially in the Southwest Asia region. These high pressures are influential in determining the climatic conditions more than any other atmospheric component. In this research, using NCEP/NCAR reanalysis data with a horizontal resolution of 2.5 degrees during the 1991–2020 period, the impacts of various non-local factors on the formation and evolution of high pressures in the subtropical region of Southwest Asia have been investigated. Specifically, one of the main goals is to find an answer to the question: why, contrary to the initial physical expectation, despite the existence of a more intense meridional pressure gradient in the winter season, are summer high pressures in the Northern Hemisphere stronger than their winter counterparts. For this purpose, first, by using quantities such as sea level pressure, geopotential height of different pressure levels, as well as meridional and vertical components of wind speed, the state of Hadley circulation and consequently the intensity of high pressures in winter and summer during the study period have been examined and compared. In the second step of the research, according to the results of previous researches on the prominent role of the South and Southeast Asia monsoons in the formation and strengthening of anticyclones and summer subtropical high pressures, this issue has been assessed in the Southwest Asia region.
Results showed that the Hadley cell in the winter hemisphere has a greater intensity and extent than the summer hemisphere, and the subsidence caused by the Hadley mechanism in the lower levels of the winter hemisphere and Southwest Asia is considered the main factor in the formation of low-level subtropical anticyclones in these regions. Meanwhile, the powerful high-level anticyclones in the summer of Southwest Asia and the Asian region are not compatible with the weak summer Hadley cell in these regions. Among the other notable findings are the importance of Asian monsoon circulation in the formation of east-west zonal circulations in the upper levels of the Asian region and the role of local circulations induced by the high plateaus of Iran, Arabia, and Tibet to explain the summer circulation of Southwest Asia. The results also showed that the winds originating from the Asian seasonal region are responsible for the background flow of north-west upper levels in summer from East to West Asia, which can describe the formation of downward mass fluxes associated with the zonal and meridional parts of circulation in West Asia and thus explain the existence of upper levels Southwest Asia anticyclones. The high plateaus of Iran, Zagros causes upward motions and local circulations below downward flow from upper levels. The mountain effects as local enhancers of downward flux of upper background flow have also been considered.

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References

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Journal of the Earth and Space Physics
Volume 51, Issue 1
online publication date: 10 June 2025
June 2025
Pages 65-86

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