The Influence of the Andes on Cutoff Lows: A Modeling Study

Abstract

A cutoff low (COL) pressure system that occurred in March 2005 (late austral summer) over the subtropical southeast Pacific is examined by means of numerical simulations using the Weather and Research Forecasting (WRF) model. The episode exhibited typical features of COLs in this region, including its formation from an elongated northwest?southeast extratropical trough and subsequent intensification off the west coast of South America. During the developing stage, the cyclonic circulation did not extend into the lower troposphere and only upper-level, nonprecipitating clouds were observed at and around the system. When the COL reached the continent it produced moderate but unseasonal rainfall along the semiarid western slope of the Andes cordillera [summit level at ?5000 m above sea level (ASL)] at the same time that the system experienced a rapid decay. The control simulation used full physics, full topography, and a single domain (54-km grid spacing) laterally forced by atmospheric reanalysis. Model results are in general agreement with upper-air, surface, and satellite observations, and allow a detailed description of the three-dimensional structure of the COL, as well as an evaluation of the vorticity and temperature budgets. A quasi-stationary, amplifying warm ridge over the South Pacific appears as the key precursor feature, in agreement with studies elsewhere. Once the COL formed, it drifted eastward mostly driven by vorticity advection induced by its own circulation, and there was close balance between vertical and horizontal temperature advection near its center. The jet streak along the COL?s periphery migrated from upstream of the COL axis, during the developing stage, to downstream later on. Four sensitivity experiments?reducing/removing topography, suppressing hydrometeors, and using an enlarged domain?were performed to assess the influence of the Andes, the importance of latent heat release, and the effect of the boundary conditions. Comparison among the control and sensitivity runs indicates that the COL formation occurs regardless of the presence of the Andes, and COL dissipation is mainly due to latent heat released in the deep clouds that form over the mountainous terrain. Nevertheless, the Andes cordillera delayed the COL demise by blocking the inflow of warm, moist air from the interior of the continent that otherwise would initiate deep convection in the region of ascending motion downstream of the COL.