Extended Abstract (128K)2.5
Intraseasonal modes of Variability affecting the SACZ
Christopher A. Cunningham Castro Sr., Center for Weather Forcast and Weather Forecast and Weather Prediction/INPE, Cachoeira Paulista, Brazil; and I. F. A. Cavalcanti
In this study the interaction among the tropical convection in Indonesian region, the extratropical flow and the tropical convection in the South Atlantic Convergence Zone (SACZ) was analysed regarding the intraseasonal variability. The analyses were performed applying statistical techniques to the daily NCEP/NCAR reanalysis data set and to daily OLR obtained from NOAA satellites covering the period from 1979 to 1999. The data were filtered to retain low frequency fluctuations between 30 and 90 days. An association between tropical convection and atmospheric circulation anomalies was identified through correlation, EOF and composites analyses. The results show teleconnections linking the Indonesia/Pacific region to South America, through the Madden-Julian Oscillation (MJO), and a tropical/extratropical wave train like the Pacific South America (PSA) pattern. Thus, these two modes, equatorial MJO and PSA-like, are related to convective activity on the intraseasonal timescale, over South America. The analyses show the MJO propagating over the equatorial belt, with a period close to 50 days and a remarkable zonal dipole over the Indonesia region and West Pacific. Furthermore an opposite convective behavior was observed between Indonesia and tropical South America. The convective activity on SACZ was analysed considering three areas of occurrence, a climatological location, and positions displaced northwards and southwards. It was noticed that a SACZ occurring in the northermost position is influenced by the MJO, while the other two positions are influenced by a tropical-extratropical PSA-like interaction. The convective events occurring in the SACZ region showed a characteristic north-south dipole between the convective region and the region to the south. The daily data were also filtered to enhance the high frequency between 2 to 10 days. The dominant patterns showed wave trains close to and over South America and Australia, indicating the influence of synoptic systems, like cold fronts. It is well known that such systems have sometimes an interaction with tropical convection and initiate a SACZ episode. It is suggested that if these two frequencies, high with the synoptic systems and low with the MJO or PSA, come in phase, they are able to establish appropriated conditions to develop a SACZ episode. Lag analyses indicate a signal transference from the Pacific to South America in 20 to 25 days, through the PSA mode. It has been noticed too that the activity on the SACZ depends on the positioning of the individual circulation anomalies composing the PSA pattern. This pattern has its own periodicity, changing the signal of the anomalous circulation. It is suggested that the wave trains result from the interaction between the tropical convection and the anomalous circulation of the extratropical latitudes.
Session 2, Tropical-Extratropical Interactions and Teleconnections over the Southern Hemisphere I
Monday, 24 March 2003, 10:30 AM-3:00 PM
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