Across the subtropical regions of both hemispheres are zones of high pressure, or high geopotential height, commonly called the subtropical high-pressure belts. They are not perfectly continuous belts. Mountains, land–sea heating contrasts, and resulting changes in the atmosphere's energy budget break them into separate high-pressure cells.
These cells are among the most prominent and persistent features of tropospheric circulation in the subtropics. They influence atmospheric circulation and water-vapour transport, affecting weather and climate not only locally but across much larger regions. [source] [source]
During the warm half of the year, particularly summer, subtropical highs occupy extensive portions of the lower, middle, and upper troposphere. As an example, the source notes that Northern Hemisphere summer subtropical high-pressure cells over the North Pacific can cover roughly 20–25 percent of the hemisphere. Anomalies in the extent or intensity of such systems can therefore accompany regional or global circulation anomalies, extreme weather, and climate irregularities with economic and social consequences. [source]
Why are summer subtropical highs so strong?
Classical descriptions commonly relate subtropical highs to descending air on the poleward side of the Hadley cell. Yet the source points to a physical puzzle: stronger meridional pressure gradients in winter might seem to imply stronger winter highs, while important subtropical highs are particularly strong in summer. This question motivated research into their formation, including extensive studies of the South Asian and western North Pacific subtropical highs.
Climatological literature has long emphasized the role of subtropical high-pressure centres in the climate of the Middle East, with some studies treating them as among the region's most important climatic controls. Iranian studies had examined relationships between these highs and regional climate characteristics, but fewer had focused directly on their physical formation mechanisms.
A study of the Iranian subtropical high
The article summarizes research by Azar Zarrin, formerly associated with the University of Wisconsin's climate research community, and Abbas Mofidi, an assistant professor of climatology in the Geography Department at Ferdowsi University of Mashhad. Their work was presented at the Eleventh Congress of the Iranian Geographers Association in 2011 at Shahid Beheshti University.
Using NCEP/NCAR reanalysis data, the researchers examined the geographical characteristics and frequency of summer subtropical high-pressure centres over the Middle East across a 30-year period (1971–2001) at lower, middle, and upper tropospheric levels. They then investigated thermal forcing and used the RegCM4 regional climate model to evaluate how the high mountains of the Iranian Plateau influence the formation and strengthening of the high. [source] [source]
The frequency analysis indicated an independently identifiable Iranian subtropical high in the middle and upper troposphere. Examination of sensible, local, horizontal- and vertical-advection, and diabatic heating showed that maxima in heating corresponded closely with the elevated mountain ranges of the Iranian Plateau. [source]
The Zagros experiment
In the model experiment, removing the Zagros Mountains demonstrated their importance as an elevated heat source supporting the formation and persistence of Iran's summer high. With the Zagros removed and regional heating reduced, upward vertical motion in the lower levels changed toward downward motion over the region.
Removing the mountains also weakened anticyclonic circulation in the middle and upper troposphere and altered lower-level cyclonic circulation, in some cases turning it anticyclonic. The modelling therefore emphasized the Zagros Mountains' prominent role in regional-scale atmospheric circulations over the Middle East.
The study's interpretation was that the anticyclonic centre over western Iran was influenced more strongly by regional surface thermal forcing than by external forcing.
Source cited in the original article: Azar Zarrin and Abbas Mofidi (2011), a study asking whether Iran's summer subtropical high is an extension of the Azores subtropical high, presented at the Eleventh Congress of the Iranian Geographers Association, Shahid Beheshti University.