Coastal Lows along the Subtropical West Coast of South America: Mean Structure and EvolutionReport as inadecuate




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The typical conditions of the eastern boundary of the subtropical anticyclone e.g., well-defined marineboundary layer MBL, equatorward low-level flow that prevail along the mountainous west coast of subtropicalSouth America are frequently disrupted by shallow, warm-core low pressure cells with alongshore and crossshorescales of 1000 and 500 km, respectively. These so-called coastal lows CLs occur up to five times permonth in all seasons, although they are better defined from fall to spring. Marked weather changes along thecoast and farther inland are associated with the transition from pressure drop to pressure rise.The mean structure and evolution of CLs is documented in this work, using a compositing analysis of 57episodes selected from hourly pressure observations at a coastal station at 308S during the austral winters of1991, 1993, and 1994, and concurrent measurements from a regional research network of nine automatic weatherstations, NCEP–NCAR reanalysis fields and high-resolution visible satellite imagery. Coastal lows tend to developas a migratory surface anticyclone approaches southern Chile at about 408S producing a poleward-orientedpressure gradient and geostrophically balanced offshore component in the low-level wind. At subtropical latitudesthe transition from negative to positive geopotential anomalies occurs around 850 hPa. Enhanced mid- and lowlevelsubsidence near the coast and downslope flow over the coastal range and Andes Mountains leads to thereplacement of the cool, marine air by adiabatically warmed air, lowering the surface pressure at the coast andoffshore. As the midlatitude ridge moves to the east of the Andes, the alongshore pressure gradient reverts backand the easterly wind ceases to act. The recovery of the surface pressure toward mean values occurs as thecool, cloud-topped MBL returns to the subtropical coast, although the pressure rise can be attenuated by midlatitudetroughing. The return of the MBL resembles a Kelvin wave propagating along the coast from northernChile where the MBL eventually thickened into subtropical latitudes in about a day.Nota general

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Author: Garreaud Salazar, René Darío; - Rutllant Costa, José Ángel; -

Source: http://repositorio.uchile.cl/



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