aliakbar garmsiri mahvar

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 circulations, 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 is found 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 over the waters and the lands of the western oceans that have no role in mass transfer to the east. Westerly and southwesterly winds also 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 that 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.

This study presents a climatological study of a subtropical anticyclone from North Africa to Iran, focusing on maximum counter-clockwise current. Atmospheric data with a horizontal resolution of 0.25 × 0.25 ° from the European Center for Medium Weather Forecasting (ECMWF) and the ERA5 version have been used. The monthly mean of the data used was conducted over 40 years, from 1979 to 2018. The results show three separate anticyclones in the middle atmospheric levels in Africa, Saudi Arabia, and Iran. These anticyclones have a seasonal evolution and reach their maximum intensity in the boreal summer. The maximum counter-clockwise flow in these systems occurred at approximately 650 to 500 hPa levels. High cells in the middle levels also correspond to the maximum counter-clockwise flow, and outside of these levels, the gradient of high cells in the pressure levels is reduced. Air subsidence occurs on the east side of the anticyclones and concentrates northeast of the center of the systems, and does not correspond to the high cells and ridges in the middle levels. Therefore, adiabatic heating due to air subsidence does not play a role in forming high cells and ridges, and their structure is due to the intense sensible heating of the surface and the lower levels in these areas.

Subtropical high pressure systems are among the atmospheric large scale centers of action in the northern hemisphere in the east of the oceans. The high pressure in the Atlantic called "Azores" or "Bermuda" and in the Pacific called "Hawaii". The clockwise flow around these systems in the south comes from easterly trade winds and in the north from the westerly belt of the mid-latitudes. Both of these currents are influential in the formation and development of this systems. Surface pressure and clockwise flow in these systems reach the maximum in the warm season, especially in July. While at this time there are thermal low pressures on most continental. For the first time Bergeron (1930) in context of air mass and frontal development argue that a surface high pressure belt exist in the subtropics around the earth. This belt has generally been attributed to the descent in the pole-ward branch of the meridionally overturning Hadley Cell. Bjerknes (1935) argued for such a belt structure from “stability considerations,” and discussed the organization of the subtropical anticyclones, including continentally anchored cols. Rodwell and Hoskins (2001) emphasize that the zonal-mean of the Hadley Cell in the summer of the northern hemisphere is much weaker than in the summer of the Southern Hemisphere, and that the circulation is not strong enough to produce the observed summer peaks of the intensity of the subtropical high pressure. Therefore, the classical Theory of the Hadley Cell Theory not be able to describe the existence of the maximum sea level pressure at the sub-tropic of the northern hemisphere. Study of this systems that affecting the oceans and continents in the hot season requires analyzing many phenomena in the atmosphere. These include the Hadley meridional circulation, warming in the atmosphere, monsoon events on the continents, and latent heat released in the upper levels of the atmosphere and etc. The structure and mechanism of each of these phenomena are also complex. Therefore, it is not possible to address many of the above in the context of research at this level. Internal investigations do not seem to have much tangible and close relevance to subtropical high-pressure systems. These studies are based on the geopotential pattern in different levels, and most of the studies focus on seasonal variations, displacement, and the relationship of this pattern to other atmospheric phenomena. While a fundamental question arises in the mind, "how much this quantity depends on the mechanism of subtropical high pressure systems and whether the formation of geopotential patterns can be related to the subtropical high pressure systems or their origin?" This study attempts to provide a convincing answer and analysis based on the findings and foundations.

In this research, sea level pressure, wind vector, omega, geopotential height and horizontal divergence are used. Initially, the monthly mean of sea level pressure and wind vector averages at 6 levels (700, 750, 775, 800, 825, 850 hPa) in the North Atlantic and North Pacific were analyzed. Sea level pressure in has been used as a measure of the intensity, development, and displacement of subtropical high pressure systems. The mean wind in the above 6 levels is also used as a current in these systems. Due to the high resolution of the data, the wind cannot be represented in vector form and this quantity is presented as stream lines. Monthly intensities and displacements of high-pressure centers on the North Atlantic and Pacific are investigated based on the maximum monthly mean sea level pressure. In the second part, in order to identify the extent and subsidence of these systems, the pattern of the vertical cross section of the meridional wind and omega is investigated at the position of these systems. Finally, the vertical cross-section of geopotential height and horizontal velocity divergence in the position of these systems in July is analyzed. To better understand and analyze these systems, the study was conducted in hot (April to September) and cold (October to March) periods. The data were extracted from the European Center for Medium-Range Forecasts (ECMWF) and the ERA5 version with a horizontal resolution of 0.25 * 0.25 degrees. This data is a reanalysis of stationary data and outputs of numerical models. Monthly averages of quantities used over a 40 year period from 1979 to 2018. result and Discussion In July, at the sea level the center of the Anticyclone corresponds to the maximum of pressure and at higher levels corresponds to the maximum of Geopotential height. Therefore, the counter clockwise flow and sea level pressure are the two main factors in the formation of height cells in the lower levels. Another point is that the maximum sea level pressure and the height of the geopotential at the lower levels do not correspond to the maximum divergence and subsidence of the air, respectively. Therefore, on the one hand, the subsidence of air flow cannot be considered as the main factor for the formation of maximum sea surface pressure, and on the other hand, the idea of the effect of adiabatic heating due to subsidence in the formation of height cells is negated. At this time, the maximum height of the Geopotential at the upper levels occurs on the western flank of Anticyclone. The ascent of hot air and the latent heat released on the western flank are the main factors in formation the maximum height or ridge in this area. The extent status of Anticyclone in July exhibits prominent patterns of counter clockwise flow, sea level pressure, geopolitical height, ascent and descent of air, divergence, warm and cold advection, and other atmospheric quantities. These patterns show the effects of these systems on the wide thickness of the atmosphere. 

conclusion: 

In this study, an attempt is made to display a more comprehensive analysis of the structure of Subtropical High Pressure Systems (Azores and Hawaii) using atmospheric data, which will certainly be effective in our knowledge and analysis of Anticyclone systems on continents. This study includes variability and vertical cross-section of flow, vertical velocity, divergence and geopotential height in these systems. It seems necessary to distinguish between Azores high-pressure and upper level atmospheric high systems with anticyclonic rotation.