| dc.description.abstract | The Indian summer monsoon is a coupled land-ocean-atmosphere phenomenon that supplies about 80% of the annual rainfall that the region receives. The monsoon season typically begins on June 1 st with a steep rise in rainfall in the southern state of Kerala. The system gradually progresses poleward as the conditions become favorable for convection to happen. The characteristics of the rainfall onset display intricate differences between land and ocean.The heterogeneous nature of the land surface could have a role to play in this. The spatial distribution of rainfall during the season is non-uniform, with rainfall peaks in the Indian land observed in the Western Ghats and the core monsoon zone in north India. Orography plays a significant role in the rainfall aggregation over the Western Ghats, while a major chunk of the rainfall in the core monsoon zone is associated with rain-bearing systems travelling northwestward from the Bay of Bengal. Climate models typically underestimate the seasonal mean rainfall over the core monsoon zone, which can be hypothesized to be partly due to inaccuracies in land surface parametrizations. In this thesis, we will elucidate the role of land-atmosphere interactions in modulating the seasonal mean and early-season rainfall using coupled climate model experiments.
Land use and land cover patterns in India have changed dramatically in the last century. This can have implications for spatiotemporal patterns of monsoon rainfall. We propose the use of an energetics framework to delineate the physical mechanisms linking land cover changes to seasonal mean rainfall. Using experiments with idealized vegetation
cover and with the help of the energetics framework, we identify three primary pathways through which land cover changes modulate seasonal mean precipitation. The precipitation response happening via the first pathway, namely evaporation, is found to be spatially uniform throughout the Indian region. The other two pathways, namely, net radiation at the top of
the atmosphere and moist stability, are found to exhibit regional preferences. The radiative pathway is found to be most active in the vicinity of the north Bay of Bengal. This is related to the climatological presence of deep convective clouds in the region. The stability pathway is found to be most active in the land regions on the eastern side of the Bay of Bengal and in some regions of peninsular India. Regional characteristics influencing rainfall, such as low-level circulation and orography, appear to be influencing the spatial preference exhibited by this pathway. In addition, we find that enhanced surface evaporation need not result in an increase in rainfall of similar magnitude everywhere. The spatial heterogeneity exhibited by the different pathways indicates the need to account for proper planning while undertaking land cover modifications in different regions.
Land cover changes are also shown to influence the onset phase of the monsoon over the land regions in central and northwest India. The onset of the monsoon over these regions is associated with the northwestward march of the onset isochrones from the Bay of Bengal. Accurate land cover representation enhances the westward moisture flux from the Bay of Bengal and improves the simulation of onset isochrones over land in the coupled climate model. Precipitation efficiency is found to increase over northwest India in the simulations with improved vegetation cover. This might be associated with the favorable phase of high-frequency intraseasonal fluctuations, which aid in the onset of monsoon over these regions. Our study shows that land cover can influence intraseasonal oscillations at these timescales, and this suggests the need to refine land models to ameliorate monsoon onset simulations.
Soil moisture is an important intermediary in these land-atmosphere interactions and may have a role in the dry bias exhibited over land by climate models, which makes it essential to understand the processes controlling soil moisture evolution in these models. A groundwater model is integrated with the land surface model to study its effects on soil
moisture evolution. In this context, uncoupled land model simulations subjected to the same precipitation forcing but with and without groundwater effects are performed to characterize soil moisture evolution. Soil moisture decay rates decrease in the regions where groundwater effects are important when these effects are accounted for. Analytical approximations
to quantify the soil moisture evolution under certain idealized scenarios are also derived as part of this study, which highlights the nonlinear nature of the impact of adding the groundwater model. Our experiments also unveil the spatiotemporal variability in the impact of groundwater effects on soil moisture evolution.
In order to reveal the role of soil moisture in the monsoon, the groundwater model is incorporated into a coupled general circulation model. An enhancement in seasonal mean precipitation is observed over the central Indian region. The energetics framework is utilized to expose the role of the three pathways. The pathways are found to exhibit spatial
preferences similar to that in the vegetation experiments, confirming the preferred locations of each pathway. The seasonal cycle of precipitation is also found to have changed. This is associated with changes in evaporation, surface fluxes, and top-of-the-atmosphere radiative fluxes. The changes in evaporation are found to be dominant during the first three months of the monsoon season, while radiative changes at the top of the atmosphere are found to be significant during the peak monsoon months of July and August. This suggests that the pathways may exhibit temporal preferences as well. It is also evident from the seasonal cycle that the most significant changes are observed during the onset phase of the monsoon, where conditions are similar to a soil moisture-limited regime. The relatively unstable nature of the system during the transition from dry to wet conditions then allows soil moisture perturbations to influence precipitation.
The present study has shown the significance of land processes during the monsoon season. The findings indicate the need to refine land cover representations and the simulation of soil moisture in climate models to improve monsoon simulations since the moisture flux from land is found to supplement the oceanic moisture sources and enhance precipitation, especially during periods when surface evaporation can modulate rainfall. In addition, our study proposes a framework that can be employed to understand the impact of other forcings like irrigation on the monsoon. | en_US |
