Amey Pathak

Weakening of Indian Summer Monsoon Rainfall due to Changes in Land Use Land Cover

Weakening of Indian summer monsoon rainfall (ISMR) is traditionally linked with large-scale perturbations and circulations. However, the impacts of local changes in land use and land cover (LULC) on ISMR have yet to be explored. Here, we analyzed this topic using the regional Weather Research and Forecasting model with European Center for Medium range Weather Forecast (ECMWF) reanalysis data for the years 2000–2010 as a boundary condition and with LULC data from 1987 and 2005. The differences in LULC between 1987 and 2005 showed deforestation with conversion of forest land to crop land, though the magnitude of such conversion is uncertain because of the coarse resolution of satellite images and use of differential sources and methods for data extraction. We performed a sensitivity analysis to understand the impacts of large-scale deforestation in India on monsoon precipitation and found such impacts are similar to the observed changes in terms of spatial patterns and magnitude. We found that deforestation results in weakening of the ISMR because of the decrease in evapotranspiration and subsequent decrease in the recycled component of precipitation.

Precipitation Recycling in the Indian Subcontinent during Summer Monsoon

AbstractThe Indian summer monsoon rainfall is dominated by oceanic sources of moisture. However, land surface processes also have a significant role in the generation of precipitation within the Indian subcontinent. Evapotranspiration over a region supplies moisture to the atmosphere, which may lead to precipitation in the same region. This is known as recycled precipitation. The role of evapotranspiration as an additional source of moisture to precipitation has been investigated in earlier studies at continental scales; however, the amount of monsoon precipitation generated from evapotranspiration has not been quantified at the daily scale for the Indian subcontinent. To examine the role of land surface hydrology in regional precipitation and to quantify recycled precipitation, the dynamic recycling model at a daily scale with NCEP Climate Forecast System Reanalysis (CFSR) data for the period of 1980–2010 is used. A high precipitation recycling ratio, that is, the ratio of recycled precipitation to total...

A threefold rise in widespread extreme rain events over central India

Socioeconomic challenges continue to mount for half a billion residents of central India because of a decline in the total rainfall and a concurrent rise in the magnitude and frequency of extreme rainfall events. Alongside a weakening monsoon circulation, the locally available moisture and the frequency of moisture-laden depressions from the Bay of Bengal have also declined. Here we show that despite these negative trends, there is a threefold increase in widespread extreme rain events over central India during 1950–2015. The rise in these events is due to an increasing variability of the low-level monsoon westerlies over the Arabian Sea, driving surges of moisture supply, leading to extreme rainfall episodes across the entire central subcontinent. The homogeneity of these severe weather events and their association with the ocean temperatures underscores the potential predictability of these events by two-to-three weeks, which offers hope in mitigating their catastrophic impact on life, agriculture and property.Against the backdrop of a declining monsoon, the number of extreme rain events is on the rise over central India. Here the authors identify a threefold increase in widespread extreme rains over the region during 1950–2015, driven by an increasing variability of the low-level westerlies over the Arabian Sea.

Role of Oceanic and Land Moisture Sources and Transport in the Seasonal and Interannual Variability of Summer Monsoon in India

AbstractThree key issues of moisture supply and Indian summer monsoon rainfall (ISMR) variability are discussed in the present work: identification of the oceanic and terrestrial sources of moisture; the extent to which each source affects the ISMR; and their individual contributions to the interannual variability of ISMR. The modified Dynamic Recycling Model, based on a Lagrangian trajectory approach, is used to estimate the relative contributions from 27 terrestrial and oceanic moisture source regions to the monsoon during 1979–2013. ERA-Interim data are used for the study. The results show that the ISMR is strongly influenced by the land–ocean–atmosphere interactions, and a significant fraction of atmospheric moisture to the ISMR comes from five main moisture sources: the western Indian Ocean (WIO), central Indian Ocean (CIO), upper Indian Ocean (UIO), Ganges basin (GB), and Red Sea and the neighboring gulf (RDG). The moisture flux from WIO is very high during the initial period of monsoon seasons. Fro...

Lack of Dependence of Indian Summer Monsoon Rainfall Extremes on Temperature: An Observational Evidence

The intensification of precipitation extremes in a warming world has been reported on a global scale and is traditionally explained with the Clausius-Clapeyron (C-C) relation. The relationship is observed to be valid in mid-latitudes; however, the debate persists in tropical monsoon regions, with the extremes of the Indian Summer Monsoon Rainfall (ISMR) being a prime example. Here, we present a comprehensive study on the dependence of ISMR extremes on both the 2 m surface air temperature over India and on the sea surface temperature over the tropical Indian Ocean. Remarkably, the ISMR extremes exhibit no significant association with temperature at either spatial scale: neither aggregated over the entire India/Tropical Indian Ocean area nor at the grid levels. We find that the theoretical C-C relation overestimates the positive changes in precipitation extremes, which is also reflected in the Coupled Model Intercomparison Project 5 (CMIP5) simulations. We emphasize that the changing patterns of extremes over the Indian subcontinent need a scientific re-evaluation, which is possible due to availability of the unique long-term in-situ data. This can aid bias correction of model projections of extremes whose value for climate adaptation can hardly be overemphasized, especially for the developing tropical countries.

Use of Atmospheric Budget to Reduce Uncertainty in Estimated Water Availability over South Asia from Different Reanalyses

Disagreements across different reanalyses over South Asia result into uncertainty in assessment of water availability, which is computed as the difference between Precipitation and Evapotranspiration (P–E). Here, we compute P–E directly from atmospheric budget with divergence of moisture flux for different reanalyses and find improved correlation with observed values of P–E, acquired from station and satellite data. We also find reduced closure terms for water cycle computed with atmospheric budget, analysed over South Asian landmass, when compared to that obtained with individual values of P and E. The P–E value derived with atmospheric budget is more consistent with energy budget, when we use top-of-atmosphere radiation for the same. For analysing water cycle, we use runoff from Global Land Data Assimilation System, and water storage from Gravity Recovery and Climate Experiment. We find improvements in agreements across different reanalyses, in terms of inter-annual cross correlation when atmospheric budget is used to estimate P–E and hence, emphasize to use the same for estimations of water availability in South Asia to reduce uncertainty. Our results on water availability with reduced uncertainty over highly populated monsoon driven South Asia will be useful for water management and agricultural decision making.

Role of Oceanic and Terrestrial Atmospheric Moisture Sources in Intraseasonal Variability of Indian Summer Monsoon Rainfall

Summer Monsoon Rainfall over the Indian subcontinent displays a prominent variability at intraseasonal timescales with 10–60 day periods of high and low rainfall, known as active and break periods, respectively. Here, we study moisture transport from the oceanic and terrestrial sources to the Indian landmass at intraseasonal timescales using a dynamic recycling model, based on a Lagrangian trajectory approach applied to the ECMWF–ERA–interim reanalysis data. Intraseasonal variation of monsoon rainfall is associated with both a north-south pattern from the Indian landmass to the Indian Ocean and an east-west pattern from the Core Monsoon Zone (CMZ) to eastern India. We find that the oceanic sources of moisture, namely western and central Indian Oceans (WIO and CIO) contribute to the former, while the major terrestrial source, Ganga basin (GB) contributes to the latter. The formation of the monsoon trough over Indo-Gangetic plain during the active periods results in a high moisture transport from the Bay of Bengal and GB into the CMZ in addition to the existing southwesterly jet from WIO and CIO. Our results indicate the need for the correct representation of both oceanic and terrestrial sources of moisture in models for simulating the intraseasonal variability of the monsoon.

Land-Surface Feedback and Impacts of Land-Use Change to Indian Monsoon Rainfall

Indian summer monsoon rainfall (ISMR) being mainly controlled by the large-scale oceanic processes is also influenced by the land–atmosphere interactions. At the regional scale, land–atmosphere interactions govern the fate of precipitation by modulating regional hydro-climate variables such as evapotranspiration, snow cover, soil moisture, and surface temperature. The contribution of local evapotranspiration to the precipitation in the same region is known as precipitation recycling. The quantitative estimates of recycling in terms of regional ratio highlight the importance of land-surface feedback in the regional precipitation. In the present chapter, we discuss the role of land-surface feedback (through evapotranspiration) over the India subcontinent during summer monsoon rainfall. We also discuss how a significant change in land-surface condition due to changes in land use–land cover (LULC) could affect the summer monsoon rainfall in India with the help of a sensitivity experiment.

Understanding the role of moisture transport on the dry bias in indian monsoon simulations by CFSv2

We analyse the bias present in the Indian Summer Monsoon Rainfall (ISMR), as simulated by Climate Forecast System Model 2 (CFSv2), the operational model used for monsoon forecasts in India. In the simulations, the precipitation intensity is redistributed within the ITCZ band with southward shifts of precipitation maxima. We observe weakening of maximum intensity of precipitation over the region between 20°N and 14°N. In the simulations by CFSv2, there exists two rain bands: the northern one located slightly southward compared to reanalysis dataset and the southern one over the equator with intensified precipitation. This results in dry bias over land and wet bias over the ocean. We use a Dynamic Recycling Model, based on Lagrangian approach, to investigate the role of various moisture sources in generating these biases. We find that, the dry bias during June exists due to the delayed monsoon onset and reduced moisture flow from the Arabian Sea. As the monsoon progresses, deficiency in the simulated contributions from South Indian Ocean becomes the key source of bias. The reduced supply of moisture from oceanic sources is primarily attributed to the weaker northward transport of moisture flux from the Southern Ocean, associated with a weaker southward energy flux. Inefficiency of the model in simulating the heating in Tibetan plateau during the pre-monsoon period leads to this reduced cross equatorial energy flow. We also find that, towards the end of monsoon season, moisture contributions from land sources namely, Ganga Basin and North-Eastern forests become significant and underestimations of the same in the simulations by CFSv2 result into biases over Central and Eastern India.