Increasing influence of ENSO and decreasing influence of AO/NAO in the recent decades over northwest India winter precipitation

The interannual variability of northwest India winter precipitation (NWIWP) is examined in association with the variability of surface temperature, mean sea level pressure, tropospheric geopotential height, and wind patterns over the globe to study the changes in the large-scale circulation features associated with NWIWP. The data for the study have been considered from 1950 to 2008 (for a period of 59 years). The analysis is based on correlations and composites performed using India Meteorological Department records based on station data. We find that the interannual variability of NWIWP is influenced by Arctic Oscillation/North Atlantic Oscillation (AO/NAO) and El Nino-Southern Oscillation (ENSO) phenomena. Above-normal NWIWP is associated with the positive phase of AO/NAO and the warm phase of ENSO, but showing strong secular variations. During the recent decades, the influence of ENSO over NWIWP has increased while the influence of AO/NAO has decreased. This conclusion is supported by a consistency across the different observation data sets employed in the study. A physical mechanism for such effect is proposed, by which western disturbances are intensified over NW India by the intensification of Asian westerly jet stream over Middle East during positive phase of AO/NAO and shift and intensification of Asian jet to the lower latitudes during the warm phase of ENSO