| Section E | E index | 321-329 of 498 terms |
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equatorial trough—1. The quasi-continuous belt of low pressure lying between the subtropical high pressure belts of the Northern and Southern Hemispheres. This entire region is one of very homogeneous air, probably the most ideally barotropic region of the atmosphere. Yet humidity is so high that slight variations in stability cause major variations in weather. The position of the equatorial trough is fairly constant in the eastern portions of the Atlantic and Pacific, but it varies greatly with season in the western portions of those oceans and in southern Asia and the Indian Ocean. It moves into or toward the summer hemisphere. It has been suggested that this name be adopted as the one general term for this region of the atmosphere. Thus, the equatorial trough would be said to contain regions of doldrums; portions of it could be described as intertropical convergence zones; and within it there might be detected intertropical fronts. However, one weakness of this nomenclature is that it alludes specifically and only to the existence of a trough of low pressure. Perhaps an even more general term might be preferable, for example, atmospheric equator. 2. Same as meteorological equator.
Riehl, H., 1954: Tropical Meteorology, p. 238.
Berry, F. A., E. Bollay and N. R. Beers, Eds., 1945: Handbook of Meteorology, 776–777.
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Equatorial Undercurrent—(Abbreviated EUC.) A subsurface current flowing eastward along the equator. A narrow, swift-flowing ribbon with a thickness of 200 m and a width of at most 400 km, it displays the largest current speeds of the equatorial current system. In the Pacific, where it is also known as the Cromwell Current, it flows with a speed of 1.5 m s−1 at a depth of 200 m in the west, rising to a depth of 40 m in the east. In the Atlantic its core is at a depth of 100 m and its speed exceeds 1.2 m s−1. In the Indian Ocean it exists as a flow ribbon centered on a depth of 200 m during the northeast monsoon season (December–April); during the remainder of the year this flow gets incorporated into the eastward flowing southwest monsoon current. In all oceans the EUC swings back and forth between two extreme positions 90–150 km either side of the equator with a two- to three-week period.
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equatorial upwelling—The rising of water along the equator from about 200 m to the surface. It occurs in the Atlantic and Pacific Oceans where the Southern Hemisphere trade winds reach into the Northern Hemisphere, giving uniform wind direction on either side of the equator. Because the surface currents of the Ekman spiral are deflected to different sides of the wind in both hemispheres, the surface water is drawn away from the equator, causing the colder water from deeper layers to upwell.
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equatorial water—The water mass of the permanent or oceanic thermocline (100–600 m) in the tropical Pacific from 20°S to 15°N. South Pacific Equatorial Water (SPEW) occupies the region south of the equator; it is derived from central water and an injection of high-salinity water formed by evaporation near Polynesia that sinks to a depth of 200 m. North Pacific Equatorial Water (NPEW) is found north of the equator; it is formed by subsurface mixing between SPEW and central water. Indian Central Water is a historical term used for Australasian Mediterranean Water in the western Indian Ocean before the Indonesian Seas were recognized as its source region.
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