Abstract

Subsynoptic, warm core low pressure areas are frequently observed along the west coast of subtropical South America during austral winter. These so-called coastal lows (CLs) tend to develop as an upper-air, midlatitude ridge is approaching the subtropical Andes and, therefore, while pressure is increasing aloft and farther to the south. These CLs have a profound impact in the coastal weather associated with a rapid transition from clear skies and stronger than average equatorward low-level flow to overcast conditions and relaxed equatorward (or even poleward) flow. Weather conditions inland mostly reflect the associated changes in the strength and height of the base of the subsidence inversion. In this work, a mesoscale simulation of a typical CL episode is performed using a numerical weather prediction model [the fifth-generation Pennsylvania State University-NCAR Mesoscale Model (MM5)] Comparison with observations reveals that the model simulation properly captures the large-scale pattern as well as many of the mesoscale features that characterize the CL. The model results were then used to diagnose the CL. It is found that the coastal troughing is largely due to the marked adiabatic warming of the lower troposphere (including a significant strengthening of the temperature inversion). The large-scale subsidence ahead of the incoming upper-air ridge axis is enhanced as the low-level easterly flow is constrained by the western slope of the subtropical Andes. The low-level wind off the subtropical coast is close to geostrophic balance and it is fed by air parcels that 1-2 days before had been located in the middle troposphere over the Pacific Ocean. The easterly flow is set up as the alongshore pressure gradient becomes poleward oriented because of the extratropical ridging. This gradient is further enhanced as the CL develops at subtropical latitudes. As soon as the ridge axis crosses, the low-level easterly flow vanishes and a shallow, narrow tongue of northwesterlies and stratocumulus clouds propagates poleward from northern Chile. Shortly thereafter, the trapped wind reversal merges with the incoming synoptic-scale, tropospheric deep-cyclonic circulation.