Impact of Nino Phases on the Summer Monsoon of Northwestern and Eastern Himalaya

Abstract

Summer monsoon rainfall (SMR) and it’s ENSO linkages over Indian Himalayan Region (IHR) is investigated assuming that local effects of complex Himalayan terrain can substantially attenuate the coupling effect of SMR and ENSO. Hence, relationships between the SMR of northwestern (NWH) and eastern Himalayan (EH) region and nino indices (Nino 3.0 and Nino 3.4 indicating tropical Pacific Ocean sea surface temperature) are investigated using six cases of El-Nino (EN), La-Nina (LN) and normal (NN) events during 1981–2005. Particular objectives of this study are: (i) to compare impact of three nino phases (i.e. EN, LN and NN) on the monthly average rainfall of NWH and EH region and (ii) to assess relationships between dominant modes of two nino phases (i.e. EN and LN) and rainfall of NWH and EH region. The relationships are further investigated with respect to latitudinal transacts representing changes in the terrain characteristics. Results of this study indicate existence of an inverse relationship between monthly rainfall and nino indices for NWH and EH region. Over the NWH region, the area averaged monthly rainfall index is found to have statistically significant (p-value <0.05) correlation coefficients of −0.48 and −0.49 for EN cases of Nino 3.0 and Nino 3.4; similarly, over the EH region, statistically significant (p-value <0.05) correlation coefficients of 0.46 and 0.57 for LN cases of Nino 3.0 and Nino 3.4 are observed. It is noted that the rainfall modes of NWH, obtained from the Empirical Orthogonal Function analysis, associated to LN events have higher spread of heavy rainfall towards higher latitudes in Uttarakhand than the rainfall modes associated to EN events. Generically, the dominant modes of EN events are found to negatively impact rainfall distribution of mountainous regions of Himachal Pradesh and Uttarakhand states of India, whereas, marginally positive impact on the rainfall distribution of Khasi-Garo hills and Brahmaputra river basin area of EH region is observed. The transect-wise analysis of correlation coefficients indicated that the EN events having negative impacts on rainfall distribution of NWH, whereas, the LN events had positive impacts on rainfall distribution of EH region.