Extended Abstract (280K)2.8
Inter-El Niņo variability of the Southern Hemisphere circulation Part I: Observational data
Gabriel E. Silvestri, Consejo Nacional de Investigaciones Cientificas y Tecnicas, Buenos Aires, Argentina; and V. R. Barros and C. S. Vera
The typical response of the Southern Hemisphere (SH) circulation to El Niņo (EN) events consists in wave trains extending southeastward from the tropical Pacific before they turn equatorward into South America. They have elements of the second (PSA1) and third (PSA2) leading modes of SH circulation variability on interannual time scales which are characterized by well-defined wave trains emanating from central and western tropical Pacific respectively.
It is well known that wavelike response during EN events is particularly noticeable over the SH circulation during the austral winter and more amplified during austral spring while it becomes more zonally symmetric during the austral summer.
Based on previous knowledge that SST variations over the subtropical south central Pacific (SSCP) region produce strong modulation among EN events, a stratification of the springs associated with those events was performed according to SST conditions over El Niņo 3.4 sector and over the SSCP region. Using the NCEP/NCAR reanalysis dataset for the period 1958-2000, the analysis of the inter-El Niņo differences show that EN events associated with cold SST anomalies in the SSCP (WC) exhibit enhanced convection not only in equatorial but also in the subtropical regions of the southeastern Pacific Ocean which maintains a strong PSA1-like circulation pattern, characterized by a Rossby wave train stretching between the equatorial central Pacific and the SH midlatitudes. Neither the diabatic heating source nor the related PSA1-like circulation pattern are present over the south central Pacific extratropics during EN events associated with warm conditions in the SSCP (WW). Also, in both cases there is present a PSA2-like pattern initiated off the west of Australia, that is therefore the common circulation feature of all EN events.
The seasonal evolution of those circulation patterns was explored. Composites fields averaged over all the EN events show that the PSA2-like pattern is clearly noticeable from MJJ while the PSA1-like structure begins to be distinguished from August-September-October and acquiring its conspicuous structure in the following trimesters. Moreover during EN events associated with cold (warm) conditions over the SSCP, a clear manifestation of PSA1 (PSA2)-like circulation anomaly is evident from the austral winter while the PSA2 (PSA1)-like signal is not that evident. It can be then concluded that the differences in the austral spring SH circulation between EN events occurring under different conditions in SSCP already are detectable from the previous winter. On the other hand, in all the EN composites, regardless the SST conditions in the SSCP, the wavelike structure of the SH circulation break down by the austral summer of the next year. The exception are the anticyclone anomaly located at the tropical central Pacific and the cyclone anomaly over the SSCP region, still present in the summer circulation resembling a PSA1-like structure but with no evidences of PSA2 characteristics.
Session 2, Tropical-Extratropical Interactions and Teleconnections over the Southern Hemisphere I
Monday, 24 March 2003, 10:30 AM-3:00 PM
Previous paper Next paper