ON THE VARIABILITY OF THE INDIAN OCEAN DURING THE 20TH CENTURY

Unlike the Atlantic and the Pacific, our know ledge on the Indian Ocean is relatively deficient. Some evidences indicate that the Indian Ocean might have connections with the Asian-Australian monsoon system, ENSO phenomena and the African monsoon system. Thus, detecting the role of the Indian Ocean in global climate variability is of great value. Based on the Global sea-ice and Sea Surface Temperature dataset (GISST), which has a relatively ideal time consistency, the variability of the Indian Ocean during the 20th century, together with its link to east equatorial Pacific and Western Equatorial Warm Pool (WEWP), has been surveyed in this paper. The results show that the seasonal variation of northern Indian Ocean surface temperature (SST) is dominated by the monsoon. During the mature period of the southwest monsoon, the northern Indian Ocean surface temperature experiences its coldest stage of annual cycle. The southern Indian Ocean undergoes a routine seasonal progression, and the SST maximum lags behind the annual cycles of solar heating by about 2 months. While the seasonal cycle of equatorial western Indian ocean is strongly affected by the monsoon, the seasonal variation of middle and eastern equatorial Indian Ocean is not apparent. The Indian Ocean, especially the tropical Indian Ocean north of 20°S, experienced a typically unanimous variation and had a linear trend of sustained warming in the recent 50 years, the mean surface temperature of equatorial Indian Ocean has increased by about 0.6℃ since 1950. The warm anomaly over the Indian Ocean associated with that taking place over East equatorial Pacific concent rates mainly in the tropical Indian Ocean north of 20°S. The strongest response of the equatorial Indian Ocean SST lags the Nino 3 index about 4-5 months. It is also found that the variation of WEWP SST is positively correlated with the equatorial northern Indian Ocean and the west wind drift of the southern Indian Ocean. The decadal scale oscillation of the WEWP SST is similar to that of the equatorial Indian Ocean. While there are evidences indicating the linkage of the sout hern Indian Ocean to its counterpart of the northern Indian Ocean, unique features of the southern Indian Ocean have also been found. The year-to-year deviation of SST over the southern Indian Ocean is far stronger than the tropical part. In the view of statistics, the variation of southern Indian Ocean links coherently with the WEWM, especially on decadal scale.