Lowlevel circulation characteristics during two extreme precipitation regimes over South America

Previous studies have emphasized the evidences of the existence of a strong warm and moist northerly current east of the Andes and the related convergence in the southward water vapor flux contributing to downwind heavy summer precipitation over Southeastern South America. The objective of this paper is to describe and compare the low level wind and precipitation anomalies during two extreme January precipitation regimes over South America. Monthly mean 850 hPa wind field is obtained from the ERA data (1979-1993 ECMWF reanalyses) to determine the average location and intensity of the east of the Andes poleward wind maximum. Analysis of the 850 hPa horizontal wind speed at the four synoptic times for the summer reveals a strong diurnal cycle in the wind speed and in the meridional moisture transport with speeds maximizing at 06:00 and 12:00 UTC. This ERA characterization of the 850 hPa wind speed diurnal variability explains the use of the synoptic time 12:00 UTC for the current study. The mean precipitation field for the same period is depicted from the Xie and Arkin data archive (freely available at Climate Diagnostic Center Web site). Using the ERA data, interannual variation in the meridional v wind component is explored examining the different Januarys 850 hPa wind fields and the time cross-sections at different latitudes over the region where the two continental v maxima south of 5ºS are found in the mean pattern. V wind and precipitation anomalies are obtained and examined for the two summer months: January 1985 qualified as part of an La Niña event and January 1987 and 1988 assigned as part of the long El Niño event that extended from August 1986 to February 1988. These two years exemplify two opposite January events in terms of suppression (January 1985) and enhancement (January 1987 and 1988) of precipitation over continental latitudes extending from 20º to 35ºS. January 1985 v wind fields exhibit a positive anomalies center (northerlies deflected to the east without penetration to subtropical latitudes) extended over the whole continent in the W-E direction upwind of a center with negative precipitation anomalies accompanied by an eastward shift of the western flank of the South Atlantic anticyclone. On the other hand, January 1987 and January 1988 are characterized by a more inland penetration of this system and by a similar correlation (but opposite sign) between precipitation and low-level circulation anomalies. A criterion similar to Bonner's (1968) has been used to identify low-level jet (SALLJ) cases within the poleward current to the east of the Andes. January mean frequency of its occurrence has been determined as well as the corresponding frequencies for the extreme Januarys. Again, higher (lower) frequencies in the SALLJ occurrence than the average are associated with enhancement (suppression) in precipitation downwind of the maximum.