Wave activity flux and kinetic energy analysis applied to a cut-off low event in the Southern South America

ALEJANDRO ANIBAL GODOY; CAROLINA S. VERA; CLAUDIA M. CAMPETELLA

Denver

Conferencia; WCRP open science conference; 2011

Cut-off lows (COLs) are upper level synoptic systems affecting the South American region with a mean frequency of 17 episodes per year. COLs were observed to develop more frequently off the subtropical coast of Chile, in a region between 30º-45ºS and 68º-80ºW. Several COL episodes are associated to significant weather events as hail, heavy rains and/or snowfalls, strong winds as has been reported in southern Europe, eastern Australia and South America.To further understand the physical processes associated with maintenance and stationary characteristic of the COLs that occur in the vicinity of the west coast of South America, we performed the analysis of the Ke equation during the life cycle of a COL occurred in that region between March 25 and April 2, 2007. The dataset used is the operational analysis NCEP-GDAS with 1ºx1º horizontal resolution. In order to understand the origin of the wave packet and the stationary circulation pattern, we analyze the wave activity flux, zonal wind, meridional wind, outgoing longwave radiation and the stream function during March 2007. In this case we have used the NCEP-NCAR reanalysis with 2.5ºx2.5º horizontal resolution in 300 hPa level.The COL’s beginning started when an upper level through increase its amplitude and deepens while a cold low develops over south-western South America. This COL event establishes a quasistationary circulation pattern favouring the low level advection of an unstable warm and moist air mass over central Argentina. When the upper level low pressure system is cut-off on the equatorward side of the maximum westerly flow the system has reached its mature stage.The results show that the advections of Ke by the mean flow (AKM), and by the eddies (AKP) as well as the divergence of the geopotential flux term (DFG) dominate throughout the life cycle of the cut-off low. During the initial stage, the maximum displacement of Ke to the east is dominated by AKM, while AKP led to a extension of the energy center to the north east and an energy increase at the northern portion of the through. In this stage DFG contribute considerably to a Ke transfer from extratropical regions to place where the cut-off low develops. During the segregation stage, there is a balance between essentially the dominant terms keeping stationary the Ke center at the region where the low is segregated. DFG is important in the formation of a new Ke center on the cut-off low eastern side and tends to cancel AKP. The barotropic term contribution is always secondary while the conversion baroclinic term is not important at any stage of the system. At this stage, AKM decreases extensively in the cut-off low region but dominates at higher latitudes, promoting the eastward displacement of a wave train, and thus the isolation the cut-off low. The pentad wave activity flux and eddy stream function shown that the wave packet have their origin in the south of Australia eight days before the COL’s segregation. Energy spreads through a Rossby wave that remains stationary during the following days in the South Pacific around 50 °S. While at the same time some of that energy spreads to the central tropical Pacific, without affecting the system pressure. The OLR indicates that there is no influence of the tropics in the formation of the COL system. A 3-wave pattern in higher latitudes, during March 2007, seems to have an important role in maintaining the stationary large scale atmospheric pattern.