Analysis of Seasonal and Annual Variability of the Hadley Cell in the Statistical Period of 1980–2025

ABSTRACT
This study investigated the variability of the Hadley cell in both hemispheres over recent decades using historical data from the NCEP/NCAR reanalysis dataset provided by the National Centers for Environmental Prediction (NCEP) and the National Center for Atmospheric Research (NCAR), covering the period from 1980 to 2025. We employed the daily means of meridional and vertical wind components, on 12 pressure levels from 1000 to 100hPa. The data are on a horizontal resolution of 2.5 ×2.5 degrees. Therefore, 144×37 points were considered in the regular grid in each hemisphere. The mass stream function was employed to analyze the Hadley cell. The results indicate no consistent trend in the latitudinal positions of the equatorial boundary, subtropical boundary, or central core of the Hadley cell in either hemisphere. In contrast, the intensity of the northern Hadley cell's central core displays a positive trend, whereas that of the southern core exhibits a negative trend. The rate of change per decade, however, remains contingent upon both the chosen data source and the statistical period under consideration. Furthermore, the average area under the 5-unit curve in the southern Hadley cell is about 1.6 times greater than that of the northern Hadley cell. This study revealed that in the summer of each hemisphere, when the Inter-tropical convergence zone is at its highest position in the summer hemisphere, the ascending branch of the Hadley cell is located in the summer hemisphere and its descending branch is located in the winter hemisphere, and the Hadley cell is weaker in the summer hemisphere.
Extended Abstract
Introduction
The Hadley circulation is a key element of the global weather and climate systems, responsible for the energy, angular momentum and moisture transport from the equatorial region to the poleward of both hemispheres. Although a circulation consisting of Hadley circulation extending from the equator to the pole in each hemisphere is mathematically possible - in the sense that such a circulation would not violate the laws of physics - the observed Hadley circulation is confined to the tropics. So in three-cell model of the atmospheric circulation in the second half of the 19th century, in each hemisphere were defined Hadley, Ferrel, and Polar cells in which air circulates through the entire depth of the troposphere.
Hadley cell is a large-scale thermally driven enclosed atmospheric circulation over tropics, with warmer air rising around the equator and cooler air sinking over subtropics of both hemispheres. This deep convection circulation is characterized by ascending motions of warm and moist air converging near the equator, followed by poleward flows in the upper troposphere in both hemispheres, which descend over the dry subtropics and return equatorward near the surface in the form of trade winds.
Numerous studies have investigated the intensity and width Hadley cell. Using climate models or statistical methods, these studies have detected varying trends in the intensity and expansion of the Hadley cell during recent decades. The reason for the difference can be attributed to different definitions, different statistical periods, and different data sources. A comprehensive review of previous studies exhibited that the long-term process of Hadley cell changes remains controversial, and the poleward expansion of this cell also remains unclear.
Therefore, this study has attempted to identify some of the Hadley cell changes by using previous methods and supplementing them. What distinguishes this study from previous related studies is the use of new and appropriate definitions for Hadley cell edges.
 
Methodology
This study investigated the variability of the Hadley cell in both hemispheres over recent decades using the historical data of NCEP/NCAR from the National Centers for Environmental Protection and the National Center for Atmospheric Research, covering 1980 to 2025. We used the daily means of meridional and vertical wind components, on 12 pressure levels from 1000 to 100 hPa. The data are on a horizontal resolution of 2.5 ×2.5 degrees. To analyze Hadley cell and evaluate meridional circulation over 1980-2025 period, we used - similar to previous studies - the mass stream function, average of zonal mean of vertical velocity (Omega) and mass stream function gradient. The mass stream function was being easily obtained via a mass-weighted vertical integral of the zonal mean meridional velocity. These parameters are usually calculated as an annual mean or an average over specific months or seasons. In the analysis of the northern and southern edges of the Hadley cell, the sign change of the meridional mass stream function was utilized in the vertical direction.
The edge of the Hadley cell in the Northern (Southern) Hemisphere was defined in such a way that at each latitude, from 12 points corresponding to different pressure levels, the sign of the meridional mass flow function was positive (negative) at least in 10 points and this sign changes at the next latitude.
The use of the average of zonal mean of vertical velocity (omega) analysis and the definition of the Hadley cell edge are considered as innovations of this research. To analyze the development or weakening of the Hadley cell during the warm (December, January, and February) and cold (June, July, and August) seasons, the area inside (and outside) the line equal to 5 units (-5 units) of the meridional stream function for the northern (southern) Hadley cell was estimated and its changes with time were examined.
 
Results and Discussion
From the investigation of average of zonal mean of meridional mass stream function, it was observed that the Hadley cell was developed in the winter of each hemisphere. In the northern hemisphere winter, the southern Hadley cell is weakened beyond detection, and conversely, in the southern hemisphere winter, the northern Hadley cell is at its weakest compared to other months.
The result revealed, in statistical period, in 84.8% of cases the equatorial edge of the northern Hadley cell is at latitude 7.5 degrees north, and in 45.7% of cases its subtropical edge is at latitude 27.5 degrees north.
Moreover, in 80.4% of cases the center of the Hadley cell is located at 15 degrees’ north latitude. In addition, in 39.1% of cases the equatorial edge of the southern Hadley cell is at latitude 5 degrees south, and in 80.4% of cases its subtropical edge is at latitude 27.5 degrees south. The location of the center of the southern Hadley cell in 89.1% of cases was estimated at 12.5 degrees’ south latitude.
 
Conclusion
This study indicated that in the summer of each hemisphere, when the Inter-tropical convergence zone is at its highest position in the summer hemisphere, the ascending branch of the Hadley cell is located in the summer hemisphere and its descending branch is located in the winter hemisphere, and the Hadley cell is weaker in the summer hemisphere.
The results also exhibited that the changes in latitudes related to the equatorial, subtropical edges, and central core of the Hadley cell in both hemispheres do not follow a specific trend. While the intensity of the northern Hadley central core displays a positive trend and the southern core a negative trend, the decadal rate of change remains contingent upon both the data source and the statistical period employed in the study. Furthermore, the intensity in southern Hadley cell is 1.3 times larger than the northern’s. In the DJF season, the intensity in the northern Hadley cell is about 4 times greater than the southern Hadley cell, but in the JJA season, the intensity of the southern Hadley cell is estimated to be about 8 times greater than the northern’s.
 
Funding
There is no funding support.
 
Authors’ Contribution
Authors contributed equally to the conceptualization and writing of the article. All of the authors approved the content of the manuscript and agreed on all aspects of the work declaration of competing interest none.
 
Conflict of Interest
Authors declared no conflict of interest.
 
Acknowledgments
We are grateful to all the scientific consultants of this paper.