Date of Graduation
Eberly College of Arts and Sciences
Geology and Geography
Amy E Hessl
North-East Indian Monsoon rainfall (NEIMR) during October-December is of immense socio-economic importance to the agriculture dependent population in the southeastern peninsular India. NEIMR is subject to extreme year-to-year and intra-seasonal variability that needs to be understood to enhance climate resilience. An understanding of NEIMR variability and the physical processes behind this variability would help to improve the predictability of NEIMR variation over the southeastern peninsular of India. An insight into past societal responses to monsoon variability would be a great lesson for adapting to future climate challenges. This dissertation seeks to examine the intra-seasonal variability of NEIMR and its driving ocean-atmospheric conditions, and simultaneously understand the past societal responses to these conditions. In so doing the research addresses some of the key scientific knowledge gaps in NEIMR and provides recommendations on how to respond to monsoon variability.;The investigation of intra-seasonal variability of pentad NEIMR using the Hidden Markov Model indicated the presence of three dominant rainfall 'states' in the southeastern peninsular of India: wet, coastal wet, and dry. These three rainfall states are associated with distinct atmospheric circulation and surface temperature. The wet conditions are characterized by enhanced cyclonic activities and increased moisture convergence at 850 hPa over the Indian southeastern peninsular and its neighboring oceanic regions -- the Bay of Bengal and Indian Ocean. In contrast, the dry conditions are associated with anticyclonic circulation and the reduced moisture convergence at 850 hPa.;The examination of ocean and atmospheric conditions associated with wet NEIMR conditions over the southeastern peninsular of India revealed that sea surface temperatures (SSTs) significantly increased over the Bay of Bengal during all the simultaneous and lagged time steps. The SSTs were significantly increased over the equatorial western Indian Ocean during the lag-2, -3, and -4 time steps before the occurrence of wet NEIMR conditions. At the same time, the latent heat fluxes (LHF) decreased over the Bay of Bengal (all time steps) and increased over the Indian Ocean (same and lags-1 and -2). The differences in the relationships with wet NEIMR between SST and LHF over the two ocean basins are possibly due to the differing atmospheric moisture demands and lower level winds. The investigation of moisture transport processes indicates that moisture transport was dominant over the lower atmospheric levels (1000 to 850 hPa). The moisture from the equatorial Indian Ocean region was transported to the moisture-abundant Bay of Bengal region through westerly anomalies and subsequently the moisture was carried to the southeastern peninsular of India through strong cyclonic circulation.;An investigation of instrumental and proxy climatic data sets between 500 and 2010 CE indicates that the period between ~850 and ~1300 CE, a time of frequent El Nino-like conditions, was associated with a substantial increase in NEIMR, whereas South-West Indian monsoon rainfall (SWIMR) suffered substantial deficits. The spatial pattern and chronology of water harvesting infrastructure developed under Chola rule indicate that these features were concentrated in the NEIMR-receiving regions of southeastern India and that their construction peaked during El Nino-dominated intervals. Overall, enhanced NEIMR conditions and adaptation strategies practiced in the Chola's territory, combined with less favorable climatic conditions over the neighboring kingdoms, appears to have underpinned the well-documented political and economic strengths of the Chola Kingdom. I infer that the water management infrastructure promoted by the Chola rulers helped to buffer the consequences of climatic extremes in later history, whether from droughts (e.g. the El Nino-related mega-drought of 1876--1878) or floods (because the reservoirs contained surplus runoff).;Overall, the spatio-temporal characterization of the three rainfall states, and the ocean and atmospheric processes associated with the wet NEIMR conditions over the southeastern peninsular of India could provide a valuable scientific input for enhancing rainfall predictability, which is of huge socioeconomic value to agriculture and water resource management sectors in the Indian southeastern peninsular. Further, the lessons from the past would be a valuable input for adapting to future climate challenges in the southeastern peninsular of India. Especially for the planners and policymakers who would gain from restoring value to the existing tank system as a means of managing climatic risk in Southern India, and for ensuring the related goals of food security, food sovereignty, and urban flood hazards are suitably managed within a "capable" landscape.
Shanmugasundaram, Jothiganesh, "North-East Indian Monsoon Rainfall Variability over Southeastern Peninsular India" (2017). Graduate Theses, Dissertations, and Problem Reports. 6616.