Influence of El Niño on the 1997 Indian Summer Monsoon

Abstract

The 1997 Indian Summer Monsoon is investigated in the context of interannual variations. It is evidenced that the 1997/98 El Niño, one of the strongest in history, suppressed the large-scale monsoon circulations. On the contrary, however, the Indian summer monsoon rainfall was not much reduced. In fact, it slightly exceeded the climatological seasonal average.
An inquiry into circulation and precipitation data indicates that it was the intraseasonal variability of the monsoon system that brought the above-normal rainfall over India. Furthermore, it is shown that the 1997 El Niño not only suppressed the large-scale Asian monsoon circulations, but also produced a convectively unstable area off the east coast of Somalia through the modifications in sea surface and lower tropospheric conditions. Anomalous convection was triggered, amplified over this area, then intruded northward to the Indian subcontinent as it propagated eastward.
A moisture budget analysis using 41 years (1958-98) of the NCEP/NCAR reanalysis data set confirmed that the transient part of moisture transport anomaly in the 1997 summer was larger than the stationary one, contributing positively to the small positive rainfall anomalies over India. The 1997 summer appears to be one of the singular cases in the context of the general positive correlation between the monsoon circulation and precipitation indices over the recent 41 years.