Coastal Lows along the Subtropical West Coast of South America: Mean Structure and Evolution

Abstract

The typical conditions of the eastern boundary of the subtropical anticyclone [e.g., well-defined marine boundary layer (MBL), equatorward low-level flow] that prevail along the mountainous west coast of subtropical South America are frequently disrupted by shallow, warm-core low pressure cells with alongshore and cross-shore scales of 1000 and 500 km, respectively. These so-called coastal lows (CLs) occur up to five times per month in all seasons, although they are better defined from fall to spring. Marked weather changes along the coast 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 57 episodes selected from hourly pressure observations at a coastal station at 30°S during the austral winters of 1991, 1993, and 1994, and concurrent measurements from a regional research network of nine automatic weather stations, NCEP?NCAR reanalysis fields and high-resolution visible satellite imagery. Coastal lows tend to develop as a migratory surface anticyclone approaches southern Chile at about 40°S producing a poleward-oriented pressure gradient and geostrophically balanced offshore component in the low-level wind. At subtropical latitudes the transition from negative to positive geopotential anomalies occurs around 850 hPa. Enhanced mid- and low-level subsidence near the coast and downslope flow over the coastal range and Andes Mountains leads to the replacement of the cool, marine air by adiabatically warmed air, lowering the surface pressure at the coast and offshore. As the midlatitude ridge moves to the east of the Andes, the alongshore pressure gradient reverts back and the easterly wind ceases to act. The recovery of the surface pressure toward mean values occurs as the cool, cloud-topped MBL returns to the subtropical coast, although the pressure rise can be attenuated by midlatitude troughing. The return of the MBL resembles a Kelvin wave propagating along the coast from northern Chile (where the MBL eventually thickened) into subtropical latitudes in about a day.