Evaluation of dynamical and thermal anomalies associated with the summer monsoon of 1997 and 1999

Abstract

The year 1997 was characterized by rapid development of a strong El Nino event and during 1999, a La Nina episode prevailed. The response of all India monsoon rainfall  to these events  in 1997 (above normal rainfall; >100%) and 1999 (below normal rainfall; <100%)  was of unusual nature. In this paper, the reasons for this have been investigated in terms of planetary scale dynamical and thermal  anomaly  indices. The major findings of this study are (a) The early onset of south west monsoon  in 1999  was associated with the stronger positive vertical shear of zonal anomaly over the north Indian Ocean, (b) Development of an active inverse Hadley circulation over Indian longitude appears to play  an important role to reactivate monsoon during  August and September  in 1997,  (c) Positive anomalies of tropospheric temperature occupied larger area over the Tibetan  Plateau  in May 1999,  (d)  The geopotential height   anomaly at 200 hPa over the Tibetan Plateau during the month of May in 1999, in the year of early  onset, was considerably higher compared to 1997 and (e) The dominance of negative anomalies of mean sea level pressure during May in 1999 over Indian region  reflected the  La Nina influence.