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South American Monsoon
1. Influence of Amazon convection on the Atlantic ITCZ
The tropical Atlantic variability involves land-atmosphere-ocean interaction. The oceanic influence on land precipitation is well recognized. For example, the Atlantic ITCZ has a strong control of precipitation in the northeast Brazil. How continental rainfall influences the Atlantic is still under debate, however. Use TRMM daily rainfall data, we have detected systematic changes in intensity and location of the Atlantic ITCZ along the Equator during boreal spring. We demonstrate that convectively coupled Kelvin waves, originating from deep convection in the equatorial Amazon, propagate eastward, modulate convection within the Atlantic ITCZ and result in a zonal variation of the ITCZ. This study has presented evidence that synoptic-scale variation of the Atlantic ITCZ is directly linked to precipitation anomalies in South America through the convectively coupled Kelvin wave, suggesting a mechanism of land influence of the tropical Atlantic variability.
Figure 1. Observed mean daily precipitation for two consecutive four-day periods during April 13-16 and 17-20, 2000 (left) and reconstructed precipitation patterns associated with two different phases of an eastward propagating Kelvin wave (right), using the TRMM data. The zonal variation of the ITCZ is obviously caused by the passage of the convectively coupled Kelvin wave.
Publication:
Wang, H., and R. Fu, 2006: Variability of the Atlantic ITCZ associated with Amazon rainfall and convectively coupled Kelvin wave, J. Climate, (in press). [Download PDF]
2. Mechanism and predictability of South American low-level jet
South American low-level jets (SALLJs) frequently occur to the east of the Andes throughout the year, and they are strongest in austral winter. These features are different from LLJs occurring in many other places that exist only during summer. By analyzing 15-year ECMWF reanalysis data, we have revealed that the seasonal and synoptic time-scale variations of the SALLJ are largely controlled by an upper-level trough and associated low-level zonal flow, rather than by horizontal temperature gradients along the eastern slope of the Andes. The SALLJs are maintained by strong zonal pressure gradients caused by the trough and westerly flow crossing the Andes through lee cyclogenesis. The process involves both baroclinic development of the upper-level trough and mechanical deflection of the westerly flow by the Andes. The dependence of the LLJ upon the upstream wind pattern helps explain the seasonal variation of the SALLJ that is related to the seasonal changes of the large-scale circulation pattern over the eastern South Pacific. We have developed a statistical model for up to five-day forecast of the LLJ using real-time observations of the QuikSCAT ocean surface winds over the eastern South Pacific. A cross validation indicates a significant degree of predictability for SALLJs. Our analysis suggests that the upstream flows over the South Pacific should be closely monitored to determine the variability of the SALLJ.
Figure 2. Composites of (a) 925-hPa temperature, (b) 850-hPa height (contours) with 850-hPa wind (vectors), and (c) vertical profile of horizontal wind at (20S, 60W), based on top 20% of northerly LLJs. Contour intervals are (a) 2 K and (b) 20 m. Shadings indicate topography. The LLJ is clearly maintained by strong zonal pressure gradients (geostrophic wind, b), rather than by horizontal temperature gradients through the thermal wind (a).
Publication:
Wang, H., and R. Fu, 2004: Influence of cross-Anders flow on the South American low-level jet, J. Climate, 17, 1247-1262.
Wang, H., R. Fu, W. Tan, and W. T. Liu, 2004: Influence of cross-Andes flow on the SALLJs and application of real-time scatterometer observations to forecasting the SALLJs, CLIVAR Exchanges, No. 29.
3. Seasonal and intraseasonal variability of South American monsoon
A reliable seasonal prediction of precipitation over South America relies upon an improved understanding of spatial and temporal variations of precipitation in this region. From our study of 15-yr ECMWF reanalysis data, we have found that changes in the low-level cross-equatorial flow over the western Amazon correlate well with the latitudinal shift of precipitation over the tropical and subtropical South America on submonthly, seasonal and interannual timescales. This cross-equatorial flow is thus identified as a monsoon index for the South American region (hereinafter the V index). Our analysis shows that when the V index is southerly, precipitation is mainly located to the north of the equator. When the V index is northerly, precipitation shifts towards the Amazon basin and the subtropics. The V index is predominately southerly in austral winter and northerly in summer. The onset (demise) of the Amazon rainy season is led by an increase in the frequency of the northerly (southerly) V index. Hence, the V index is a good indicator for precipitation change over tropical and subtropical South America. Our study suggests that the precise date of the wet season onset may largely depend on the submonthly activities. The wet and dry seasons are brought on by more frequent and persistent occurrences of circulation patterns similar to those during the wet and dry intraseasonal periods, as a result of modulations in the atmospheric circulation caused by external forcing, such as solar radiation and SST in adjacent oceans.
Figure 3. Composite patterns of January precipitation (shading) and 925-hPa wind (vectors) associated with (a) southerly and (b) northerly cross-equatorial winds in the equatorial western Amazon, which are constructed based on linear regressions of 15-year daily mean ECMWF reanalysis data vs. the V index.
Publication:
Wang, H., and R. Fu, 2002: Cross-equatorial flow and seasonal cycle of precipitation over South America, J. Climate, 15, 1591-1608.
@ School of Earth and Atmospheric Sciences, Georgia Institute of Technology