Seasonal variations of synoptic features over the north Indian Ocean during dipole years

Abstract

Recent studies found that the atmospheric circulation over the Indian Ocean region is highly influenced by the Indian Ocean Dipole (IOD) forcing. In the present study, we analyzed National Center for Environmental Prediction and National center for Atmospheric Research (NCEP-NCAR) reanalysis data to observe the IOD related variability at different pressure levels from surface to upper troposphere during the spring season. The study period (1982-2001) includes three positive IOD (1982, 1994 and 1997) and two negative IOD years (1992 and 1996). It is observed that during the spring season the variations in the lower tropospheric winds were stronger in the positive IOD years than the negative dipole mode years. Signals associated with IOD were weak over upper tropospheric fields. In order to strengthen the IOD events in the upcoming boreal autumn, the warming in the central and western Indian Ocean is necessary right from spring season onwards. The extreme western Indian Ocean (west of 60E) warming and eastern cooling are not sufficient to strengthen the dipole mode structure. Although the variations in surface wind stress is small over the central Indian Ocean during spring, it has significant effect on equatorial Wyrtki Jets and the resultant variation may occur in thermal structure of equatorial Indian Ocean in the summer monsoon period. During strong dipole mode years the seasonal variability was found to be more in various fields (surface wind, sea surface temperature, outgoing long wave radiation and surface heat flux) than weaker dipole mode years.