واکاوی تغییرات فصلی و سالانه سلول هادلی در دوره آماری 2025-1980

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 equator to pole in each hemisphere is mathematically possible in the sense that such a circulation would not violate the laws of physics, but 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 the hemispheres, which descend over the dry subtropics and return equatorward near the surface in the form of trade winds.

Many studies have investigated the intensity and width Hadley cell and using climate models or statistical methods was being detected different trend of the intensity and expand Hadley cell during recent decades. The reason for the difference can be traced to different definitions, different statistical periods, and different data sources. A comprehensive review of previous studies showed 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.



Materials and methods

In this study we investigated the variability of the Hadley cell in the past decades in both hemispheres, and were used 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. For analyzed of Hadley cell and evaluation of meridional circulation in 1980-2025 period, similar to previous studies we used 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 as 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 in the vertical direction was used.

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 in the warm (December, January, and February) and cold (June, July, and August) seasons, the area inside (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 seen 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 shown, 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.

Also, 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 shown 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 of this study also showed 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 (southern) Hadley central core has an upward (downward) trend, but the amount of the trend in each decade depends on the data source and statistical period was used in the study. Furthermore, the intensity in southern Hadley cell is 1.3 times larger than the northern Hadley cell. 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 Hadley cell.