- Subtropical ridge
The subtropical ridge is a large belt of high pressure situated around 30°N in the
Northern Hemisphereand 30°S in the Southern Hemisphere. It is characterized by mostly calm winds, which acts to reduce air qualityunder its axis by causing fogovernight, and hazeduring daylight hours caused by the stable atmosphere found near its location. Air flows out from its center toward the upper and lower latitudesof each hemisphere, creating both the trade winds and the westerlies. It moves poleward during the summer, reaching its most northern latitude in early fall, before moving equatorward during the cold season. The ENSO climatecycle can displace the subtropical ridge, with La Niñas allowing for a more northerly axis for the ridge, while El Niños show flatter, more southerly ridges. The change of the ridge position during ENSO cycles changes tracks of tropical cyclones which form around their southern and western peripheries. As the subtropical ridge varies in position and strength, it can enhance or depress monsoonregimes around their southern periphery.
Heating of the earth near the equator leads to large amounts of upward motion and convection along the
monsoon troughor Intertropical convergence zone. The divergence over the near-equatorial trough leads to air rising and moving away from the equator aloft. As it moves towards the Mid-Latitudes, the air cools and sinks, which leads to subsidence near the 30th parallel of both hemispheres. This circulation is known as the Hadley cell and leads to the formation of the subtropical ridge. [Dr. Owen E. Thompson. [http://www.atmos.umd.edu/~owen/CHPI/IMAGES/circs02.html Hadley Circulation Cell.] Retrieved on 2007-02-11.]
The subtropical ridge starts migrating poleward in late spring reaching its zenith in early
autumnbefore retreating equatorward during the late fall, winter, and early spring. Note that this is strongly correlated with the progression of the monsoon troughor Intertropical Convergence Zone.
tropical cyclones form on the side of the subtropical ridge closer to the equator, then move poleward past the ridge axis before recurving into the main belt of the Westerlies. [Joint Typhoon Warning Center. [http://www.nrlmry.navy.mil/forecaster_handbooks/Philippines2/Forecasters%20Handbook%20for%20the%20Philippine%20Islands%20and%20Surrounding%20Waters%20Typhoon%20Forecasting.3.pdf 3.3 JTWC Forecasting Philosophies.] Retrieved on 2007-02-11.] When the subtropical ridge shifts due to ENSO, so will the preferred tropical cyclone tracks. Areas west of Japanand Koreatend to experience much fewer September-November tropical cyclone impacts during El Niñoand neutral years, while mainland Chinaexperiences much greater landfall frequency during La Niñayears. During El Niño years, the break in the subtropical ridge tends to lie near 130°E which would favor the Japanese archipelago, while in La Niña years the formation of tropical cyclones, along with the subtropical ridge position, shift west, which increases the threat to China. [M. C. Wu, W. L. Chang, and W. M. Leung. [http://ams.allenpress.com/perlserv/?request=get-document&doi=10.1175%2F1520-0442(2004)017%3C1419:IOENOE%3E2.0.CO%3B2 Impacts of El Nino-Southern Oscillation Events on Tropical Cyclone Landfalling Activity in the Western North Pacific.] Retrieved on 2007-02-11.] In the Atlantic basin, the subtropical ridge position tends to lie about 5 degrees farther south during El Niño years, which leads to a more southerly recurvature for tropical cyclones during those years.
Atlantic Multidecadal Oscillation's mode is favorable to tropical cyclone development (1995-present), it amplifies the subtropical ridge across the central and eastern Atlantic. [Climate Prediction Center. [http://www.cpc.noaa.gov/products/outlooks/hurricane2004/May/hurricane.html NOAA: 2004 Atlantic Hurricane Outlook.] Retrieved on 2007-02-11.]
Importance to global monsoon regimes
When the subtropical ridge in the northwest Pacific is stronger than normal, it leads to a wet
monsoonseason for Asia. [C.-P. Chang, Yongsheng Zhang, and Tim Li. [http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2F1520-0442(2000)013%3C4310:IAIVOT%3E2.0.CO%3B2 Interannual and Interdecadal Variations of the East Asian Summer Monsoon and Tropical Pacific SSTs. Part I: Roles of the Subtropical Ridge.] Retrieved on 2007-02-11.] The subtropical ridge position is linked to how far northward monsoon moisture and thunderstorms extend into the United States. Typically, the subtropical ridge across North Americamigrates far enough northward to begin monsoon conditions across the Desert Southwest from July to September.Arizona State University. [http://geography.asu.edu/aztc/monsoon.html Basics of the Arizona Monsoon & Desert Meteorology.] Retrieved on 2007-02-11.] When the subtropical ridge is farther north than normal towards the Four Corners, monsoon thunderstorms can spread northward into Arizona. When suppressed to the south, the atmosphere dries out across the Desert Southwest, causing a break in the monsoon regime. [David K. Adams. [http://geochange.er.usgs.gov/sw/changes/natural/monsoon/ Review of Variability in the North American Monsoon.] Retrieved on 2007-02-11.]
Role in haze and fog formation
When surface winds become light, the subsidence produced directly under the subtropical ridge can lead to a build up of particulates in urban areas under the ridge, leading to widespread
haze. [Myanmar government. [http://www.kjc.gov.my/english/education/weather/haze01.html Haze.] Retrieved on 2007-02-11.] If the low level relative humidityrises towards 100 percent overnight, fogcan form. [Robert Tardif. [http://www.rap.ucar.edu/staff/tardif/Documents/CUprojects/ATOC5600/fog_characteristics.htm Fog characteristics.] Retrieved on 2007-02-11.]
Atlantic Multidecadal Oscillation
*Intertropical Convergence Zone (The Doldrums)
* [http://www.websters-online-dictionary.org/su/subtropical+ridge.html Webster's Online Dictionary]
* [http://www.us.oup.com/us/companion.websites/0195160223/studentresources/ch10/?view=usa Physical Geography - The Global Environment]
* [http://www.wiley.com/college/strahler/sc/strach5.html Winds and the Global Circulation System]
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