Topographically trapped lows along the subtropical west coast of South America. Part I: Climatology

The prominent topography along the west coast of South America (coastal range and Andes cordillera) represents a significant barrier to the tropospheric circulation and generates a variety of mesoscale phenomena. At subtropical latitudes (25°-35°S) the most relevant disturbances are topographically trapped, warm-core low pressure cells (or troughs) propagating from north to south along the coastline. These so-called coastal lows (CLs) occur up to 5 times per month in all seasons, although they are better defined from fall to spring. The passage of the pressure minimum is associated with marked weather changes along the coast and further inland, namely a sharp transition in the wind field (from southerly-offshore flow to northerly-onshore flow) and pronounced fluctuations in the depth of the marine boundary layer and the strength of the subsidence inversion. Coastal lows in this region, however, have received little attention compared with topographically trapped disturbances (or CTDs) along the mountainous coastlines of western North America, South Africa and eastern Australia.

In this work we present a climatology of CLs along the west coast of subtropical South America and compare them with CTDs elsewhere. The climatology is based on a compositing analysis of 50 episodes selected from hourly barometric measurements in a coastal station at 30°S between 1991 and 1994. Concurrent measurements from a regional research network and synoptic stations allowed a detailed description of regional-scale signatures of the coastal lows, including the composite amplitude and propagation speed of the surface pressure perturbations and their relationship with other meteorological variables. Reanalysis data and satellite imagery were used to document the large-scale circulation attending CLs, characterized by an upper level ridge approaching the Andes cordillera and a surface anticyclone moving from the south eastern Pacific to the Atlantic. Interesting, this large-scale pattern is also conducive of cold surges along the east side of subtropical Andes, explaining a tendency for their simultaneous occurrence with CLs to the west of the Andes. We also found that geostrophically balanced easterly winds associated with the surface anticyclone is instrumental in forcing downward flow over the western slope of the subtropical Andes, that in turn leads to lee troughing, although other mechanisms (foehn wind, barrier wind) might act inconcert.