Circulation variations of the atmosphere over Chile associated to rainfall anomalies

In recent years it has become apparent that during the warm phase of El Niño-Southern Oscilation (ENSO) central Chile rainfall tends to be larger than normal while the opposite seems to be true for the cold phase. These changes have profound effects on the country's productive activities because they are associated to significant damage and economic loss. Variations in the pressure and wind fields at the lower atmospheric levels lead to changes in the humidity transport, in the divergency fields and in the vorticity advection, which in turn regulate rainfall distribution.

The climatic vertical structure of the atmosphere over Chile is first derived using a 30-year NCEP reanalysis database. This includes monthly mean values of geopotencial height, wind, divergence and vorticity advection, for all standard pressure levels along the Chilean coast. Anomalies are then computed corresponding to years for which central Chile rainfall is larger and smaller than climatically expected, in relation to the different ENSO phases. Analyses of these anomalies indicate significant changes of circulation at several pressure levels and of vorticity advection corresponding to the different ENSO phases. Variations of divergence related to variations of the humidity supplied to the rainfall process are also identified.