An Analysis of Subtropical High Pressure Systems (Azores and Hawaiian)

Document Type : Full length article


Department of Physical Geography, Faculty of Geography, University of Tehran


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.
Material and method:
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.
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.


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