persistent areas of high and low air pressure that are planetary-scale systems. They undergo some important seasonal changes in location and surface air pressures and include subtropical anticyclones, the intertropical convergence zone, subpolar lows, and polar highs.
semipermanent high-pressure systems centered over subtropical latitudes (on average, near 30 degrees N and S) of the Atlantic, Pacific, and Indian Oceans. These warm-core systems extend from the ocean surface up to the tropopause.
areas of persistent light winds or calm air between about 30 and 35 degrees N and S under subtropical anticyclones
prevailing planetary-scale winds in the middle and upper troposphere between about 30 and 60 degrees of latitude; blow on average from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere out of the poleward flanks of the subtropical anticyclones
prevailing planetary-scale surface winds in tropical latitudes; blow from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere out of the equatorward flanks of the subtropical anticylcones
an east-west equatorial belt of light and variable surface winds where the trade winds of the two hemsipheres converge
intertropical convergence zone (ITCZ)
discontinuous low-pressure belt of thunderstorms paralleling the equator and marking the convergence of the Northern and Souther Hemisphere surface trade winds
the latitude (about 10 degrees N) of highest mean annual surface air temperature. The mean position of the intertropical convergence zone approximately corresponds to the heat equator.
high-latitude, semipermanent cyclones marking the convergence of planetary-scale surface southwesterlies of midlatitudes with surface northeasterlies of polar latitudes in the Northern Hemisphere or midlatitude northwesterlies and polar southeasterlies in the Southern Hemisphere. the Icelandic low and Aleutian low are Northern Hemisphere examples.
narrow transition zone where the relatively mild midlatitude westerlies meet and override the relatively cold polar easterlies. When the temperature gradient across the front is teep, the front is well defined and is a potential site for development of extratropical cyclones
a cold-core anticyclone that originates in a source region for continental polar air; this shallow system is the product of intense radiational cooling over the snow-covered continental interior of North America well north of the polar front.
thermally-driven air circulation in tropical and subtropical latitudes of both hemispheres resembling a huge convective cell with rising air near the equator in the intertropical convergence zone and sinking air in the subtropical anticyclones. Equatorward blowing surface winds and poleward directed upper-level winds complete the circulation.
trade wind inversion
an elevated stable air layer that occurs on the eastern flank of subtropical anticyclones in the vicinity of the trade winds; a persistent and climatically significant feature. Formed when the subsiding, compressionally warmed air in a subtropical anticyclone encounters the marine air layer, a layer of cool, humid,and stable air formed where sea-surface temperatures are relatively low. Atemperature inversion develops at the altitude where air subsiding from above meets top of the marine air la
large-scale roughly circular surface ocean current systems, centered near 30 degrees latitude in the North and South Atlantic, the North and South Pacific, and the Indian Ocean.
roughly circular surface ocean current systems that occur at high latitudes of the Northern Hemisphere.
seasonal reversals in prevailing winds that cause wet summers and relatively dry winters. The most vigorous monsoon circulation occurs over Africa and southern Asia.
also called the North American Monsoon System (NAMS); circulation brings summer rainfall to the American Southwest from moisture sources in the Gulf of California and the Gulf of Mexico.
series of long-wavelength troughs and ridges that characterize the planertary-scale westerlies (above the 500-mb level) as they encircle the globe; also called long waves. Typically, between 2 and 5 waves encircle the hemisphere at one time.
zonal flow pattern
flow of the planetary-scale westerlies almost directly from west to east; westerlies exhibit little amplitude. In this pattern, the north-south exchange of air masses is minimal.
term used to describe cool, humid maritime polar air swept inland from the Pacific Ocean that undergoes air mass modification over the Rocky Mountains, emerging milder and drier to the east of the mountains. During a zonal flow pattern, Pacific air floods the eastern two-thirds of the U.S. and southern Canada, causing mild and generally dry weather.
meridional flow pattern
flow of the planetary-scale westerlies in a series of deep troughs and sharp ridges; westerlies exhibit considerable amplitude. In this pattern, cold air masses surge southward and warm air masses stream northward, leading to strong temperature gradients.
split flow pattern
wave pattern in the planetary-scale circulation regime where westerlies to the north have a wave configuration that differs from that of westerlies to the south.
a cyclone or anticyclone cutoff from the main westerly airflow that blocks the usual west-to-east progression of weather systems. A blocking system may be responsible for weather extremes, such as drought or flooding rains or excessive heat and cold.
relatively small short-wavelength ripples (troughs and ridges) superimposed on Rossby saves in the planetary-scale westerlies; they propogate with the airflow in the middle and upper troposphere. Typically, a dozen or more short waves encircle the hemisphere at one time.
polar fron jet stream
a corridor of strong westerlies in the upper troposphere between the midlatitude tropopause and the polar tropopause and directly over the polar front.
an area of accelerated air flow within a jet stream; the wind may strengthen by as much as an additional 100 km per hr (62 mph). Jet streaks occur where surface horizontal temperature gradients are particularly steep and play an important role in the generation & maintenance of synoptic-scale cyclones. The strongest jet streaks develop during winter in the polar front jet stream along the East Coasts of North America & Asia.
subtropical jet stream
a zone of relatively strong winds aloft situated between the tropical tropopause and the midlatitude tropopause, on the poleward side of the Hadley cell.
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