Impact of arctic oscillation on Indian winter monsoon
The Himalayas in the north part of Indian sub-continent is characterized by variable topography, heterogeneous landuse and climate variations. It acts as a barrier to atmospheric circulation for Indian summer and winter monsoon, thus modulating the associated precipitation over these regions. Western disturbances embedded in Indian winter monsoon (IWM) arriving from Mediterranean region result in one-third of annual precipitation over Himalayas. In the recent decade, studies have shown increased forcings associated with Arctic Oscillations (AOs). In the present study, impact of AO on IWM (December, January, February—DJF) is analyzed. During positive AO years, Polar region is having low surface pressure and colder conditions in comparison with higher pressure with cold surges to midlatitudes during negative AO years. For detailed analysis, National Center of Environmental Prediction/National Center for Atmospheric Research, US reanalysis data is used. Analysis of positive and negative AO year composites shows stronger westerlies over Arabian Sea and neighboring Indian region. Stronger north–south temperature gradient due to warmer Indian Ocean and colder Tibetan region is seen during positive AO years. In addition, weakening of Hadley circulation is seen during positive AO years. This weakening helps in Polar and/or extratropical mass to move southwards having mid-latitude interactions leading to enhanced precipitation over Indian regions. Extensive and intense wave train seen during positive AO years indicates Rossby wave response. Such forcings associated with AOs led to increased precipitation over the northern Indian region.
The authors acknowledge the National center for Environmental Prediction/National Centre for Atmospheric Research for providing dataset. Interpolated OLR and reconstructed SST data used in the study are provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA. One of the authors (Midhuna) also thanks Department of Science and Technology for providing Junior Research Fellowship(JRF).
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