A study of teleconnectivity patterns for boreal summer and winter

It is well established that the sea level pressure in the tropics and subtropics undergoes slow, irregular fluctuations, with a characteristic time scale of a week or longer with a well defined spatial pattern indicative of standing oscillations. The Asian summer monsoon season exhibits intra-seasonal variabilities in two dominant modes; one on a 10-20 day time period and another on 30-60 day period. In this study the teleconnection patterns arising from the geopotential anomalies during summer and winter monsoon periods is presented. Daily data of geopotential heights, at 12 GMT 850 mb pressurelevel in the period 1980 to 1998, from NCEP/NCAR reanalysis is used in our study. The horizontal resolution of the data is 2.5x2.5 degree latitude-longitude grids. All the results in this work pertain to the global tropical region lying between 45S and 45N. The analysis was performed with both the weekly and monthly means derived from the daily data.

The analysis of 19 years data of geopotential height anomolies reveals that there exist well defined teleconnectivity patterns for the tropical region just like the various seesaw or standing oscillation patterns of the northern hemisphere. This study suggests that the orientation and the connectivity in the tropics changes for winter and summer monsoon seasons. For example, the south Asian region shows strong teleconnection with Africa during summer monsoon; and for winter the connection of south Asia is found to be with the south west coast of south America. First two empirical orthogonal functions (EOFs) explain about 30% of the observed variabilities. Various experiments done by averaging over time scales for different decades has shown that the statistics is capable of detecting the planetary waves of different time scales.

In order to check the statistical stability of the results, the entire data from 1980 to 1998 was broken in to two sets for the period (a) 1980 to 1989 and (b) 1990 to 1998. The teleconnectivity map and one point correlation map was constructed for these two periods. The results show that the teleconnectivity pattern are similar to the those seen in the data for the entire 19 year period. Thus we believe our results are statistically significant and are reproducible.