1997: The El Nino of the century and the response of the Indian summer monsoon

The year 1997 was characterized by the rapid development of an El Nino whose strength exceeded any previously observed this century. The basic understanding of the influence of El Nino on the Asian summer monsoon suggested that the monsoon should be substantially deficient, yet the all-India rainfall (AIR) was 2% above normal. The reasons for this have been investigated in terms of both the seasonal-mean, large-scale circulation anomalies and the subseasonal, regional weather events. By comparing the results with a similar analysis of two previous major El Nino events in 1982 and 1987, the common and disparate features of the response have been identified. On the large scale, the basic hypothesis that, in El Nino years, the strength of the monsoon is influenced by a modulation of the Walker circulation, in which there is implied additional subsidence over the west Pacific and southeast Asia, is generally supported by the results. However, the results have shown that the modulation of the local Hadley circulation over the Maritime Continent may also play an important role. In both 1982 and 1997, the suppression of convection over the Maritime Continent and the equatorial Indian Ocean was marked. As a consequence, the tropical convergence zone (TCZ) to the north over the Indian subcontinent and extending out into the west Pacific was preferentially more active, particularly in July and August. The development of this local Hadley circulation over the Maritime Continent resulted in large-scale convergence over the monsoon trough region, aiding in the generation of vorticity in the lower troposphere and in the formation of intense tropical storms over the northwest Pacific and deep monsoon depressions over northern India. These tropical storms were a notable feature of the 1997 monsoon season. The change in the local Hadley circulation is potentially driven by the suppression of convection over the Maritime Continent and equatorial Indian Ocean, which itself is caused by the modulation of the Walker circulation as a direct response to Ei Nino. The results of the present study suggest that it may be possible, depending on the strength of the ENSO forcing, for the circulation to enter different regimes. In 1987, and to a large extent in 1982 also, the main impact of El Nino was a modulation of the Walker circulation and consequently deficient monsoon rains over India. In 1997, however, when the El Nino was so intense, the circulation may have entered a regime in which the local Hadley circulation over the Indian Ocean and Maritime Continent was sufficiently modified by the substantial change in the Walker circulation that the TCZ was preferentially located to the north over the Asian summer monsoon domain and therefore resulted in above-average precipitation and subseasonal synoptic activity.