Indian Monsoon Precipitation Variation: Role of Northern Indian Ocean Warming and inter ocean thermal gradient

Mohanty, Sarthak (2019) Indian Monsoon Precipitation Variation: Role of Northern Indian Ocean Warming and inter ocean thermal gradient. Masters thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

North-east monsoon (NEM) popularly known as Indian winter monsoon plays a vital role in the social and financial development of Southern India and SriLanka by providing food and nourishment throughout the Boreal fall. This plays a crucial role in agriculture as well as on the recharge of the groundwater which is helpful in overcoming the dry winter and hot summer. During NEM, high pressure over Tibetan plateau and low pressure in the south-east coast of India drives the wind over the Bay of Bengal thus picking the moisture. NEM proves 40-50% of total precipitation within southern India. Further, However, NEM precipitation exhibits much higher inter-annual variability (28%) compared to ISM precipitation (11%), and thus southern peninsular India often witnesses extreme precipitation events. The Indian northeast monsoon rainfall (NEMR) displays considerable inter-annual variability (IAV), significantly affecting the agricultural activity in southern India region. Compared to the Indian summer monsoon (ISM), relatively very little work has been done to explore the NEMR variability and its teleconnections. So the present study focused to predict the variation in winter monsoon on the basis of numerous oceanographic and atmospheric parameters. The precipitation regression analysis over southern Indian subcontinent has been performed using the gridded datasets obtained from “Global Precipitation Climatology Center (GPCC)” and “Central Research Unit (CRU)” during 1958-2016. There is a significant positive increase in the precipitation amount inside the southern part of India has been observed in which the maximum increase (Approx. 20-30%) in precipitation has been observed in the southernmost part of India which includes some part of Kerala and Tamilnadu along with the coast of Andhra Pradesh. In contrast, Sri Lanka exhibits a reduction in precipitation. This plays a significant role in the social and financial development in that area. Alongside this, no significant difference in precipitation amount has been found in the central and western portion of southern India. In order To evaluate the controlling factors for this change in precipitation, analysis of sea surface temperature (SST), Tropical cyclone heat potential (TCHP) and Mixed Layer Heat Content (MLHC) inside Bay of Bengal (BoB) have been performed. These three factors exhibit an increasing trend inside BoB and have a positive correlation with NEMR. Among these variables, in time series plot MLHC behaves similarly as NEMR. On the contrary, SST and TCHP show a monotonic increasing trend in past half-century. This trend of SST and TCHP are observed due to enhanced downward solar radiation flux and weakening of wind-stress curl. In addition, the changes in mixed layer heat content (MLHC) in south-eastern Bay of Bengal (9-16 N, 88-97 E) is attributed due to higher freshwater influx from the continent regulate the inter-annual variability of NEM precipitation over southern peninsular India. In addition, convection over the BoB has a higher correlation with the monsoon circulation. Enhancement in convection over BoB can be modulated by the walker zonal- vertical circulation. A long-term regression analysis of walker circulation exhibits an increase in the upward movement of precipitable water inside BoB which is compensated by an increase in downward transport inside southern India. It has been further observed that the strengthening of NEM precipitation is related to the atmospheric temperature gradient between Arabian Sea Warm Pool (ASWP, 5°-15° N, 55°-72° E, lower pressure) and north-western Pacific Ocean (5°-15° N, 110°-150° W, higher pressure). The pressure difference between these two regions has been increased significantly in the past half-halfcentury resulting in an increase in the zonal wind strength that is subsequently responsible for the increase in the zonal transport of precipitable water and is responsible for the increase in precipitation. The fresh water flux which is responsible for MLHC variation is mainly controlled by the summer monsoon precipitation. So in this study the precipitation variation within Indian subcontinent has been analysed. During summer (Jun-Sep), low pressure over the Tibetan plateau drives moisture from the Indian Ocean resulting in heavy rainfall over the most part of the Indian subcontinent. However, monsoonal rainfall has a large spatio-temporal variation that is difficult to comprehend and predict as the factors governing the variability is poorly understood. Therefore, the present study aims to underscore the causative factors controlling the long-term variations in the ISM rainfall. In this context, precipitation trend of the ISM from 1958 to 2015 from the Indian subcontinent, has been calculated using gridded datasets from Climate Research Unit (CRU). A prominent decreasing trend (10-20%) has been observed over central India (75°-85°E, 20°-30°N). On the contrary, the rainfall amount in the East-coast, the northernmost part of the Indian subcontinent along with a few parts of western and southern India exhibit an increasing trend (10-20%). In order to understand the factors controlling the spatial and temporal heterogeneity in the ISM rainfall over the Indian subcontinent, regression analyses between precipitation and SST, TCHP and MLHC within Western Indian Ocean (WIO) region (50°-65°E, 5°S-10°N) have been conducted. The SST and TCHP exhibit a significant negative correlation with central India rainfall, whereas MLHC exhibits a positive correlation. Previous studies have shown that warm SST and TCHP have accompanied by large variation in lower and upper troposphere due to enhanced latent heat aloft from convection over ocean which has a higher correlation with the monsoon circulation. A recent study points out an enhancement of convection inside northern Indian ocean as a result of the spatial extension of Indian ocean warm pool by introducing a dry bias over the land by modulating the Hadley zonal-vertical circulation. Therefore, the present study indicates the role of Hadley and Walker circulation in weakening the ISM over central India. A long-term (1958–2015) vertical wind velocity values over the ASWP (55°–72°E,5°-15°N) show large-scale upward movement of moisture laden wind (expanding up to the upper troposphere) resulting in intense local convective activity. This enhanced upward movement is compensated by the subsidence of air parcel over the continental region. The maximum subsidence is observed between 0°-10° N enhancing the local precipitation and diminishing precipitable water. As the present study area lies at 20°-30°N, it receives less rainfall due to the absence of convergence. Similar observations is also made in case of Walker circulation where a positive upward movement towards the troposphere has been detected in the ASWP. Interestingly, in the studied region i.e. 75°-85°E, an upward motion has been observed. Therefore, it can be assumed that the cumulative effect of SST, TCHP, MLHC and the weakening of Hadley and Walker circulation has resulted in the decrease in rainfall amount in the central Indian subcontinent. Apart from this seasonal variation of MLHC and TCHP have also been observed in which winter and summer monsoon correspond to mean maximum MLHC and minimum TCHP whereas pre monsoon and post monsoon correspond to mean minimum MLHC and maximum TCHP respectively. Further, an Auto-Regressive Integrated Moving Average (ARIMA) has been performed to predict the monthly MLHC and TCHP values up to 2 years ahead with minimum error. The seasonal prediction of MLHC within BoB has a significant importance in the prediction of winter monsoon precipitation.

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