the sea surface temperature ( sst ) over the Indian Ocean was affected due to the El Nino phenomenon in 1997. SST anomalies have been observed in various parts of the Indian Ocean. The year 1997 witnessed one of the most severe El Nio events of the past century. El Nino, which is also known as the enfant terrible or the boy terror, is an unusual event that leads to severe climatic and geomorphic repercussions around the globe.
It refers to the increase in SST near the coast of Peru in the Pacific Ocean. In a normal year, upwelling of cold dense seawater takes place in this area. But, in an El Nino year, this area of the ocean is unusually warm, leading to a wide range of climatic changes all around the globe. The SST over the Indian Ocean is also affected due to this. A change in the pattern of monsoon cycle leading to decreased rainfall is also said to take place because of these changes.
The mean SST over the Indian Ocean region, bounded from 20 N latitude to 10 S latitude and from 50 E longitude to 100 E longitude, is computed in a 5 square grid. This is done by using observations from ships that are archived by the National Data Centre of the Indian Meteorological Department ( IMD ), Pune. The anomaly values are computed with respect to the sst mean monthly value for the period 1961-1998. After this the SST maps are plotted and analysed to identify the regions of positive anomaly (warm region) and those of negative anomaly (cool region).
Various anomalies were observed in SST variation over Indian Ocean concurrent to the 1997 El Nio. In August 1996, that is around six months prior to the beginning of warming over the Pacific, predominant cooling was observed over parts of northern Indian Ocean. Throughout the El Nino's stages of developing, maturing and decaying, the northern Indian Ocean was observed to be continuously warm. Predominant warming of the ocean was observed over a wide area in the northern Indian Ocean.
During the peak stage of the El Nino, warming was observed over the northern Indian Ocean, whereas cooling was observed over its southeastern parts. It was also observed that the sst s decreased over northern Indian Ocean from July 1996 to January 1997. However, such a fall in sst s, especially over the northern Indian Ocean east of 80 E longitude, was not observed during July and December of 1997. This region was unusually warm since January to December of 1997. April 1997 onwards, a westwards spreading of sst anomalies higher than 0.5 c over northern Indian Ocean was observed, according Medha Khole of the imd , Pune ( Current Science , Vol 77, No 10).
As far as monsoons are concerned, the annual cycle was suppressed during 1997 and 1998. Monsoons also failed to produce the monsoonal cooling over the Arabian Sea and the Bay of Bengal during these years. However this did not influence the total seasonal precipitation. The warming over the Pacific showed a similar trend in the same period.
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