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| Semi-Permanent Pressure Systems |
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Definition
| Persistent areas of high & low air pressure that are planetary-scale systems. They undergo some important seasonal changes in location & surface air pressures & include subtropical anticyclones, the intertropical convergence zone, subpolar lows, & polar highs. |
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Definition
| Semipermanent high-pressure systems centered over subtropical latitudes (on average, near 30 degrees North & South) of the Atlantic, Pacific, & Indian Oceans. These warm-core systems extend from the ocean surface up to the tropopause. |
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Definition
| Areas of persistent light winds or calm air between about 30 & 35 degrees North & South under subtropical anticyclones. |
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Definition
| Prevailing planetary-scale winds in the middle & upper troposphere between about 30 & 60 degrees of latitude, blowing, on average from the southwest in the North Hemisphere & from the northwest in the Southern Hemisphere out of the poleward flanks of the subtropical anticyclones. |
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Definition
| Prevailing planetary-scale surface winds in tropical latitudes blowing from the northeast in the Northern Hemisphere & from the southeast in the Southern Hemisphere out of the equatorward flanks of the subtropical anticyclones. |
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Definition
| An east-west equatorial belt of light and variable surface winds where the trade winds of the two hemispheres converge. |
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Term
| Intertropical Convergence Zone (ITCZ) |
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Definition
| Discontinuous low-pressure belt of thunderstorms paralleling the equator & marking the convergence of the Northern & Southern Hemisphere surface trade winds. |
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Definition
| The latitude (about 10 degrees North) of highest mean annual surface air temperature. The mean position of the intertropical convergence zone approximately corresponds to the heat equator. |
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Definition
| 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 & polar southeasterlies in the Southern Hemisphere. The Icelandic low & Aleutian low are Northern Hemisphere examples. |
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Definition
| Narrow transition zone where the relatively mild midlatitude westerlies meet & override the relatively cold polar easterlies. When the temperature gradient across the front is steep, the front is well defined & is a potential site for development of extratropical cyclones. |
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Definition
| Cold-core anticyclones originating in a source region for continental polar air; this shallow system is the product of intense radiational cooling over the snow-covered interior of North America, well north of the polar front. |
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Definition
| Thermally-driven air circulation in tropical & subtropical latitudes of both hemispheres resembling a huge convective cell with rising air near the equator in the intertropical convergence zone & sinking air in the subtropical anticyclones. Equatorward blowing surface winds & poleward directed upper-level winds complete the circulation. |
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Definition
| An elevated stable air layer that occurs on the eastern flank of subtropical anticyclones in the vicinity of the trade winds; a persistent & climatically significant feature. Formed when the subsiding, compressionally warmed air in a subtropical anticyclone encounters the marine air layer, a layer of cool, humid, & stable air formed where sea-surface temperatures are relatively low. A temperature inversion develops at the altitude where air subsiding from above meets the top of the marine air layer. |
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Definition
| Large-scale roughly circular surface ocean current systems, centered near 30 degrees latitude in the North & South Atlantic, the North & South Pacific, & the Indian ocean. |
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Definition
| Roughly circular surface ocean current systems that occur at high latitudes of the Northern Hemisphere. |
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Definition
| Seasonal reversals in prevailing winds that cause wet summers & relatively dry winters. The most vigorous monsoon circulation occurs over Africa & southern Asia. |
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Definition
| Also called the North American Monsoon System (NAMS); circulation brings summer rainfall to the American Southwest from moisture sources in the Gulf of California & the Gulf of Mexico. |
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Definition
| Series of long-wavelength troughs & ridges that characterize the planetary-scale westerlies (above the 500 milibar level) as they encircle the globe; also called long waves. Typically, between 2 & 5 waves encircle the hemisphere at one time. |
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Definition
| Relatively small short-wavelength ripples (troughs & ridges) superimposed on Rossby waves in the planetary-scale westerlies; they propagate with the airflow in the middle & upper troposphere. Typically, a dozen or more short waves encircle the hemisphere at one time. |
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Definition
| Flow of the planetary-scale westerlies almost directly from west to east, exhibiting little amplitude. In this pattern, the north-south exchange of air masses is minimal. |
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| 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 & drier to the east of the mountains. During a zonal flow pattern, Pacific air floods the eastern two-thirds of the United States & southern Canada, causing mild & generally dry weather. |
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Definition
| Flow of the planetary-scale westerlies in a series of deep troughs & sharp ridges, exhibiting considerable amplitude. In this pattern, cold air masses surge southward & warm air masses stream northward, leading to strong temperature gradients. |
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Definition
| 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. |
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| 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. |
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Definition
| A corridor of strong westerlies in the upper troposphere between the midlatitude tropopause & the polar tropopause & directly over the polar front. |
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Definition
| An area of accelerated air flow within a jet stream; the wind may strengthen by as much as an additional 100 kilometers per hour (62 miles per hour). Jet streaks occur where surface horizontal temperature gradients are particularly steep & play an important role in the generation & maintenance of synoptic-scale cyclones. The strongest jet streaks develop during winter in the polar front jet streak along the East Coasts of North America & Asia. |
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Definition
| A zone of relatively strong winds aloft situated between the tropical tropopause & the midlatitude tropopause, on the poleward side of the Hadley cell. |
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Definition
| An anomalous warming of surface ocean waters in the eastern tropical Pacific accompanied by suppression of upwelling off the coasts of Peru & Ecuador. |
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| A period of particularly strong trade winds & unusually low sea-surface temperatures (SST) in the central & eastern tropical Pacific; opposite of El Nino. The strong trade winds induce exceptionally vigorous upwelling in the eastern tropical Pacific, which causes unusually low SST. |
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Definition
| Opposing swings of surface air pressure between the western & central tropical Pacific Ocean. When air pressure is low at Darwin (Australia) it is high at Tahiti (a south Pacific island) & when air pressure is high at Darwin it is low at Tahiti. |
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Term
| Southern Oscillation Index |
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Definition
| An index calculated from the monthly or seasonal fluctuations in the air pressure between Darwin, Australia & Tahiti. Positive values indicate La Nina conditions & negative values indicate El Nino conditions. |
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Definition
| The relationship between El Nino & the Southern Oscillation. An El Nino episode begins when the weakening of the surface air pressure gradient between the western & central tropical Pacific heralds the slackening of the trade winds. |
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Definition
| A spiral in water motion in the top 100 meters (330 feet) or so of the ocean produced by the directional change and decreasing horizontal motion of successively lower layers of water. Due to a balance between the Coriolis Effect & frictional drag, the surface ocean layer moves at an angle of up to 45 degrees to the right or left (depending on the hemisphere) of the surface wind direction. Due to the same balance of forces, the layer immediately below the surface ocean layer moves slower & at an angle to the motion of the layer above. This spiraling effect is responsible for the Ekman transport. |
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Definition
| The net horizontal movement of water in the top 100 meters (330 feet) or so of the ocean induced by the coupling of surface winds with ocean surface waters. The Ekman spiral causes this net transport to be about 90 degrees to the right of the surface wind direction in the Northern Hemisphere & to the left of the surface wind direction in the Southern Hemisphere. |
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Definition
| The upward movement of cold, nutrient-rich water from depths of 200 to 1000 meters (about 650 to 3300 feet) toward the ocean surface & associated with the offshore flow of near-surface waters. |
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Definition
| Where Ekman transport moves surface waters toward the coast, piling them & thus forcing them to sink. |
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Definition
| Direct zonal tropical circulation, thermally driven, in which air rises over the warm western Pacific Ocean & sinks over the cool eastern Pacific. |
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Definition
| The loss of color by coral polyps & their subsequent death caused by unusually high sea-surface temperature. |
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Definition
| A linkage between changes in atmospheric circulation occurring in widely separated regions of the globe, often many thousands of kilometers apart. |
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Definition
| Consists of an array of moored & drifting instrumented buoys, island & coastal tide gauges, ship-based measurements, & satellites. |
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Definition
| Launched by NOAA in 2009, it issues watches for El Nino & La Nina weather patterns. |
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Term
| North Atlantic Oscillation (NAO) |
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Definition
| The seasaw variation in sea level air pressure between the Azores & Iceland. The air pressure gradient between the Azores high & Icelandic low governs the strength & direction of westerly winds, the middle latitude jet stream, & storm tracks across the North Atlantic. |
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Definition
| A seasaw variation in sea level air pressure between the Arctic and the portion of the middle latitudes centered from 37 to 45 degrees North. Changes in the surface air pressure gradient alter the strength of the bands of winds aloft (the polar vortex) blowing in a mostly zonal pattern counterclockwise around the Arctic. |
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Term
| Pacific Decadal Oscillation (PDO) |
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Definition
| Long lived variation in climate over the North Pacific & North America sea surface temperatures between the north central Pacific (north of 20 degrees N) & the west coast of North America. |
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