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Subtropical ridge
From Wikipedia, the free encyclopedia
The subtropical ridge is a large belt of high pressure situated around 30ºN in the Northern Hemisphere and 30ºS in the Southern Hemisphere. It is characterized by mostly calm winds. Air flows out from its center toward the upper and lower latitudes of each hemisphere, creating both the trade winds and the westerlies.
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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.[1]
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.
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.[2] If the low level relative humidity rises towards 100 percent overnight, fog can form.[3]
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.[4]
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 130E which would favor the Japanese archepelago, while in La Niña years the formation of tropical cyclones, along with the subtropical ridge position, shift west, which increases the threat to China.[5]
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.[6]
When the subtropical ridge in the northwest Pacific is stronger than normal, it leads to a wet monsoon season for Asia.[7]
Its 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.[8] 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.[9]
- Atlantic Multidecadal Oscillation
- ENSO
- Hadley Cell
- Monsoon
- Tropical cyclone
- Intertropical Convergence Zone (The Doldrums)
- ^ Dr. Owen E. Thompson. Hadley Circulation Cell. Retrieved on 2007-02-11.
- ^ Mynamar government. Haze. Retrieved on 2007-02-11.
- ^ Robert Tardif. Fog characteristics. Retrieved on 2007-02-11.
- ^ Joint Typhoon Warning Center. 3.3 JTWC Forecasting Philosophies. Retrieved on 2007-02-11.
- ^ M. C. Wu, W. L. Chang, and W. M. Leung. Impacts of El Nino-Southern Oscillation Events on Tropical Cyclone Landfalling Activity in the Western North Pacific. Retrieved on 2007-02-11.
- ^ Climate Prediction Center. NOAA: 2004 Atlantic Hurricane Outlook. Retrieved on 2007-02-11.
- ^ C.-P. Chang, Yongsheng Zhang, and Tim Li. 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.
- ^ Arizona State University. Basics of the Arizona Monsoon & Desert Meteorology. Retrieved on 2007-02-11.
- ^ David K. Adams. Review of Variability in the North American Monsoon. Retrieved on 2007-02-11.