Subtropical ridge

The subtropical ridge is a large belt of high pressure situated around 30°N in the Northern Hemisphere and 30°S in the Southern Hemisphere. It is characterized by mostly calm winds, which acts to reduce air quality under its axis by causing fog overnight, and haze during daylight hours caused by the stable atmosphere found near its location. Air flows out from its center toward the upper and lower latitudes of 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 climate cycle 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 monsoon regimes around their southern periphery.

Formation

Heating of the earth near the equator leads to large amounts of upward motion and convection along the monsoon trough or 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.]

Migration

The subtropical ridge starts migrating poleward in late spring reaching its zenith in early autumn before retreating equatorward during the late fall, winter, and early spring. Note that this is strongly correlated with the progression of the monsoon trough or Intertropical Convergence Zone.

Most 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 Japan and Korea tend to experience much fewer September-November tropical cyclone impacts during El Niño and neutral years, while mainland China experiences much greater landfall frequency during La Niña years. 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.

When the 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 monsoon season 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 America migrates 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 humidity rises towards 100 percent overnight, fog can form. [Robert Tardif. [http://www.rap.ucar.edu/staff/tardif/Documents/CUprojects/ATOC5600/fog_characteristics.htm Fog characteristics.] Retrieved on 2007-02-11.]

See also

*Atlantic Multidecadal Oscillation
*Hadley Cell
*Intertropical Convergence Zone (The Doldrums)

References

External links

* [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]