Document Type : Research
Faculty of Geography, University of Tehran
Subtropical Anticyclones are among the large-scale atmospheric centers of action in the northern hemisphere in the east of the oceans. Clockwise flow and high surface pressure are two prominent features of these systems. These systems have an annual trend and usually achieve maximum flow and surface pressure in the summer, especially in July. Understanding the factors influencing the development and intensification of these anticyclones has been the favorite of many researchers. One of these factors has been the general circulation of the atmosphere. In this study, a climatological study of the general atmospheric circulation, including the Hadley and Walker circulation, has been performed. Their role in the development and strengthening of subtropical anticyclones has been investigated. The research has been done in three parts; 1- Mean Meridian Circulation, 2- meridional circulation in the North Atlantic and Pacific, and 3- Walker circulation in the North Atlantic and Pacific. In this study, the meridional component of wind, vertical velocity (omega), and horizontal wind divergence have been used. Data at 27 pressure levels with a horizontal resolution of 0.25 × 0.25 ° were extracted from the European Center for Medium Weather Forecasting (ECMWF) and the ERA5 version. The monthly mean of the data used was conducted over 40 years, from 1979 to 2018. The Mass Stream Function (MSF) method has been used to quantify the meridional and walker circulation.
The Mean Meridian Circulation showed that the meridional circulation in the equinox months consists of a pair of Hadley cells in which air rises in the tropics and subsides in the subtropics. Also, a solstitial cell with the ascent in the outer tropics of the summer hemisphere and subsidence in the outer tropics of the winter hemisphere. Although the Mean Meridional Circulation showed that mass transfer takes place in the summer of the Northern Hemisphere to the Southern Hemisphere and the Hadley circulation could not explain and justify the maximum activity of the subtropical anticyclones, but the meridional circulation at smaller cross-sections in the East Atlantic and Pacific showed that the Hadley cells play a vital role in mass transfer to the subtropics and mid-latitudes. The mean walker circulation (20- 40 ° N) showed that the source of this circulation is only the latent heat released on the waters and the lands of the western oceans have no role in mass transfer to the east. westerly and southwesterly winds form mass transfer in the Walker circulation to the northeast of the oceans. Heating in northwestern Africa and North America is another phenomenon that plays a role in subsidence in the North Atlantic and Pacific. The subsidence induced from heating on African lands is much more severe than in North America. This may depend on the climate and extent of these areas. Therefore, as a result of this research, it can be said that three processes: Hadley circulation, Walker circulation, and heating on the lands adjacent to the eastern oceans, are effective in mass transfer and subsidence in the east Atlantic and Pacific. These conditions form strong northerly winds in the eastern oceans and trade winds in the tropics and effectively develop and strengthen subtropical anticyclones.