Tropical-Extratropical Interactions and Intrasasonal Oscillations in the Indian Monsoon System in a Warmer Planet

Abstract:

The India summer monsoon (ISM) experiences long periods of wet and dry conditions frequently associated with floods and long dry spells. These events are largely governed by northward propagating boreal summer monsoon intraseasonal oscillations (MISO). Here we investigate intraseasonal variability of the ISM in the climate of the 20^th century using the Climate Forecast System Reanalysis (1979-2013) and examine future scenarios of climate change using models of the Coupled Model Intercomparison Project Phase-5 project. ISM is characterized with a large-scale index obtained by performing combined EOF analysis of precipitation, low level circulation, specific humidity and temperature. This index realistically defines the monsoon’s onset and withdrawal, is well correlated with seasonal precipitation in India and exhibits variance on intraseasonal timescales that are related to MISO and extreme wet and dry conditions in India. With similar approach we investigate the skill of the CMIP5 models in realistically simulating MISO in the ‘historic’ run (1951-2005) and examine projected changes in the amplitude and persistence these events in the high-emission representative concentration pathway 8.5 (RCP8.5) (2006-2100). MISO is well characterized in CMIP5 models that indicate significant increase in the intensity and frequency of extremely dry and wet conditions affecting India by 2050. We show that the main mechanism driving MISO in CMIP5 models are linked to the propagation of extratropical wave trains and interactions with the tropics. In a warmer planet, the increase in polar temperatures weakens the tropical-extratropical temperature gradient and decreases the intensity of the upper tropospheric jet. These changes in the jet and in the baroclinic structure of the atmosphere result in enhanced extratropical wave activity and more extreme events. We use a wave tracking algorithm to demonstrate these differences and explore physical and dynamical mechanisms underlying these changes. This research was supported by the National Science Foundation (1116105).