Impact of irrigation on South Asian monsoon climate

The land surface and atmosphere have multiple pathways to interact with each other, which include the coupled water and energy cycle. This coupling plays an imperative role in the monsoon climate of South Asia which is manifested as a strongly coupled land-atmosphere-ocean circulation system. Di erent feedbacks occur when land and atmosphere interact and the understanding of these feedbacks is very crucial to explain past climatic changes and to improve the knowledge of seasonal weather forecasts and future climate. Considering the strongly coupled nature of the South Asian summer monsoon (SASM), the soil moisture conditions become very crucial both for interannual as well as intraseasonal precipitation. For South Asia, the water used for irrigation plays a major role in modifying soil moisture conditions, which in turn e ects the monsoon circulation and precipitation. The Indian subcontinent is one of the most intensely irrigated regions of the world and state of the art climate models do not account for the representation of irrigation. Sensitivity studies with the regional climate model REMO show distinct feedbacks between the simulation of the monsoon circulation with and without irrigation processes. It is found that the temperature and mean sea level pressure, where the standard REMO version without irrigation shows a signi cant bias over the Indus and Ganges basins, are highly sensitive to the water used for irrigation. In the sensitivity test with the representation of irrigation in REMO, the removal of this bias is achieved, which resulted in less di erential heating between land and sea masses. This in turn reduces the westerlies entering into the land from the Arabian Sea, hence creating conditions favorable for currents from the Bay of Bengal to intrude deep into western India and Pakistan that have been unrealistically suppressed before. Therefore, it is concluded that the representation of irrigated water is unavoidable for the realistic simulation of South Asian summer monsoon and its response under global warming. Further, considering the complex spatial and deep vertical structure of SASM, the existing literature on the validation of a climate model dealing with only a few surface variables over the region is considered insu cient. Therefore, an evaluation framework has been proposed for the better assessment of the capability of a climate model in capturing the fundamental structure of SASM. This framework has been applied to the regional climate model REMO using ERA40 lateral boundary conditions for the period 1961 2000. The application of the framework yielded satisfactory performance of REMO in capturing the lower, middle, and upper components of the SASM circulation. REMO has higher correlation between di erent SASM indices as compared to ERA40, showing its ability in capturing the dynamical link between these indices better than ERA40. We have employed di erent criteria for the assessment of the monsoon onset, and the movement of the Intertropical Convergence Zone (ITCZ) during the boreal summer and REMO has captured these phenomena reasonably well. The model has also shown the association of the meridional temperature gradient with the easterly shear of zonal winds. These results lead to the conclusion that REMO is well suited for long-term climate change simulations to examine projected future changes in the complex SASM system. Afterwards, the attention is given to the Indus river which has the largest contiguous irrigation system in the world. Since much of the ow of the Indus river originates in the Himalayan, Karakoram and Hindu Kush mountains, an understanding of weather characteristics leading to precipitation over the region is essential for water resources management. The study examines the in uence of upper level mid-latitude circulation on the summer precipitation over the upper Indus basin (UIB). Using reanalysis data, a geopotential height index (GH) is de ned at 200 hPa over central Asia, which has a signi cant correlation with the precipitation over UIB. GH has also shown signi cant correlation with the heat low (over Iran and Afghanistan and adjoining Pakistan), easterly shear of zonal winds (associated with central Asian high) and evapotranspiration (over UIB). It is argued that the geopotential height index has the potential to serve as a precursor for the precipitation over UIB. In order to assess the in uence of irrigation on precipitation over UIB, a simpli ed irrigation scheme has been developed and applied to the regional climate model REMO. It has been shown that both versions of REMO (with and without irrigation) show signi cant correlations of GH with easterly wind shear and heat low. However, contrary to reanalysis and the REMO version with irrigation, the REMO version without irrigation does not show any correlation between GH index and evapotranspiration as well as between geopotential height and precipitation over UIB. It is concluded that although atmospheric moisture over the coastal Arabian sea region, triggered by wind shear and advected northward due to the heat low, also contribute to the UIB precipitation. However for the availability of necessary moisture for precipitation over UIB, the major role is played by the evapotranspiration of water from irrigation. From the results, it may also be inferred that the representation of irrigated water in climate models is unavoidable for the representation of SASM climate.