NOTES AND CORRESPONDENCE Variability of Southern Hemisphere Cyclone and Anticyclone Behavior: Further Analysis

This paper presents some additional results on the use of an automatic scheme for tracking surface cyclones and anticyclones. The Southern Hemisphere (SH) total amount of synoptic tracks (every 12 h) was analyzed for the 1973-96 period using sea level pressure from the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis. Composites for seven El Nino (EN) and La Nina (LN) years were constructed in order to analyze the association between the hemispheric cyclone and anticyclone propagation and the phase of the El Nino-Southern Oscillation (ENSO) phenomenon. A climatological view of cyclone and anticyclone tracks and orphan centers superposed on the same map is presented and analyzed. A large area with overlapped cyclone and anticyclone tracks is seen between 308 and 608S, which is approximately the climatological position of the SH transient activity. To the north of 30 8S, the subtropical South Atlantic high is embedded in a region with just a few cyclone tracks. This feature is not evident for the Pacific and the Indian Oceans' high. The subtropical cyclones dominate most of the west Pacific and north of Australia. Orographic and heat lows are well spread over the tropical regions of South America and Africa. Finally, the storm track region appeared as a very marked feature around the Antarctic continent. In accordance with some previous studies, the total number of the SH cyclones and anticyclones during the austral winter season has shown an overall decline, particularly at the end of the 1970s. Nevertheless, a more complex behavior shows up when the weak systems are eliminated and the intense end of the spectrum is analyzed. For the anticyclone tracks above 1020 hPa, there is still some tendency toward an overall decline, but it is small and not statistically significant. For the stronger anticyclones this tendency rapidly disappears. On the other hand, the cyclone tracks presented a different behavior, since the decreasing trend turned into a significant increase for those stronger than 980 hPa. These results also emphasize how sensitive the tracking scheme is to capturing low and high pressure centers, and it presents another perspective for the interpretation of cyclone and anticyclone trends. The ENSO composites indicated a higher anticyclone concentration near the subtropical South Atlantic high during EN years, while in the subtropical South Pacific high it occurs during LN years. On the other hand, the cyclone tracks showed a higher variability, with an excess of lows over the subtropical Pacific, west of South America and southern Argentina during EN years and a more pronounced activity over the subtropical Atlantic and southeastern Australia during LN years. Nevertheless, the trends and the average of the total hemispheric number of cyclones and anticyclones are not significantly affected by the ENSO phase.