Deeksha Rastogi

Sources of errors in the simulation of south Asian summer monsoon in the CMIP5 GCMs

Accurate simulation of the South Asian summer monsoon (SAM) is still an unresolved challenge. There has not been a benchmark effort to decipher the origin of undesired yet virtually invariable unsuccessfulness of general circulation models (GCMs) over this region. This study analyzes a large ensemble of CMIP5 GCMs to show that most of the simulation errors in the precipitation distribution and their driving mechanisms are systematic and of similar nature across the GCMs, with biases in meridional differential heating playing a critical role in determining the timing of monsoon onset over land, the magnitude of seasonal precipitation distribution and the trajectories of monsoon depressions. Errors in the pre-monsoon heat low over the lower latitudes and atmospheric latent heating over the slopes of Himalayas and Karakoram Range induce significant errors in the atmospheric circulations and meridional differential heating. Lack of timely precipitation further exacerbates such errors by limiting local moisture recycling and latent heating aloft from convection. Most of the summer monsoon errors and their sources are reproducible in the land–atmosphere configuration of a GCM when it is configured at horizontal grid spacing comparable to the CMIP5 GCMs. While an increase in resolution overcomes many modeling challenges, coarse resolution is not necessarily the primary driver in the exhibition of errors over South Asia. These results highlight the importance of previously less well known pre-monsoon mechanisms that critically influence the strength of SAM in the GCMs and highlight the importance of land–atmosphere interactions in the development and maintenance of SAM.

Extreme hydrological changes in the southwestern US drive reductions in water supply to Southern California by mid century

The SouthwesternUnited States has a greater vulnerability to climate change impacts onwater security due to a reliance on snowmelt driven importedwater. The State of California, which is themost populous and agriculturally productive in theUnited States, depends on an extensive artificial water storage and conveyance systemprimarily for irrigated agriculture,municipal and industrial supply and hydropower generation.Herewe take an integrative high-resolution ensemblemodeling approach to examine near term climate change impacts on all imported and local sources of water supply to SouthernCalifornia.While annual precipitation is projected to remain the same or slightly increase, rising temperatures result in a shift towardsmore rainfall, reduced cold season snowpack and earlier snowmelt. Associatedwith these hydrological changes are substantial increases in the frequency and the intensity of both drier conditions andflooding events. The 50 year extreme dailymaximum precipitation and runoff events are 1.5–6 timesmore likely to occur depending on thewater supply basin. Simultaneously, a clear deficit in total annual runoff overmountainous snow generating regions like the SierraNevada is projected. On one hand, the greater probability of drought decreases importedwater supply availability. On the other hand, earlier snowmelt and significantly stronger winter precipitation events pose increased flood risk requiringwater releases from control reservoirs, whichmay potentially decrease water availability outside of thewet season. Lack of timely local water resource expansion coupledwith projected climate changes and population increasesmay leave the area in extended periods of shortages.

Drought and immunity determine the intensity of west nile virus epidemics and climate change impacts

The effect of global climate change on infectious disease remains hotly debated because multiple extrinsic and intrinsic drivers interact to influence transmission dynamics in nonlinear ways. The dominant drivers of widespread pathogens, like West Nile virus, can be challenging to identify due to regional variability in vector and host ecology, with past studies producing disparate findings. Here, we used analyses at national and state scales to examine a suite of climatic and intrinsic drivers of continental-scale West Nile virus epidemics, including an empirically derived mechanistic relationship between temperature and transmission potential that accounts for spatial variability in vectors. We found that drought was the primary climatic driver of increased West Nile virus epidemics, rather than within-season or winter temperatures, or precipitation independently. Local-scale data from one region suggested drought increased epidemics via changes in mosquito infection prevalence rather than mosquito abundance. In addition, human acquired immunity following regional epidemics limited subsequent transmission in many states. We show that over the next 30 years, increased drought severity from climate change could triple West Nile virus cases, but only in regions with low human immunity. These results illustrate how changes in drought severity can alter the transmission dynamics of vector-borne diseases.

Dominating Controls for Wetter South Asian Summer Monsoon in the Twenty-First Century

AbstractThis paper analyzes a suite of global climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) archives to understand the mechanisms behind a net increase in the South Asian summer monsoon precipitation in response to enhanced radiative forcing during the twenty-first century. An increase in radiative forcing fuels an increase in the atmospheric moisture content through warmer temperatures, which overwhelms the weakening of monsoon circulation and results in an increase of moisture convergence and therefore summer monsoon precipitation over South Asia. Moisture source analysis suggests that both regional (local recycling, the Arabian Sea, the Bay of Bengal) and remote (including the south Indian Ocean) sources contribute to the moisture supply for precipitation over South Asia during the summer season that is facilitated by the monsoon dynamics. For regional moisture sources, the effect of excessive atmospheric moisture is offset by weaker monsoon circulation and uncertaint...

Effects of climate change on probable maximum precipitation: A sensitivity study over the Alabama-Coosa-Tallapoosa River Basin

Probable maximum precipitation (PMP), defined as the largest rainfall depth that could physically occur under a series of adverse atmospheric conditions, has been an important design criterion for critical infrastructures such as dams and nuclear power plants. To understand how PMP may respond to projected future climate forcings, we used a physics-based numerical weather simulation model to estimate PMP across various durations and areas over the Alabama-Coosa-Tallapoosa (ACT) river basin in the southeastern United States. Six sets of Weather Research and Forecasting (WRF) model experiments driven by both reanalysis and global climate model projections, with a total of 120 storms, were conducted. The depth-area-duration relationship was derived for each set of WRF simulations and compared with the conventional PMP estimates. Our results showed that PMP driven by projected future climate forcings is higher than 1981–2010 baseline values by around 20% in the 2021–2050 near-future and 44% in the 2071–2100 far-future periods. The additional sensitivity simulations of background air temperature warming also showed an enhancement of PMP, suggesting that atmospheric warming could be one important factor controlling the increase in PMP. In light of the projected increase in precipitation extremes under a warming environment, the reasonableness and role of PMP deserves more in-depth examination.

Sensitivity of Probable Maximum Flood in a Changing Environment

This study uses integrated hydrometeorological simulations over the Alabama-Coosa-Tallapoosa (ACT) River Basin in the southeastern United States to understand the impact of climate change on probable maximum precipitation.

Characteristics of Bay of Bengal Monsoon Depressions in the 21st Century

We show that 21st century increase in radiative forcing does not significantly impact the frequency of South Asian summer monsoon depressions (MDs) or their trajectories in the Coupled Model Intercomparison Project Phase 5 general circulation models (GCMs). A significant relationship exists between the climatological occurrences of MDs and the strength of the background upper (lower) tropospheric meridional (zonal) winds and tropospheric moisture in the core genesis region of MDs. Likewise, there is a strong relationship between the strength of the meridional tropospheric temperature gradient in the GCMs and the trajectories of MDs over land. While monsoon dynamics progressively weakens in the future, atmospheric moisture exhibits a strong increase, limiting the impact of changes in dynamics on the frequency of MDs. Moreover, the weakening of meridional tropospheric temperature gradient in the future is substantially weaker than its inherent underestimation in the GCMs. Our results also indicate that future increases in the extreme wet events are dominated by nondepression day occurrences, which may render the monsoon extremes less predictable in the future.

The Second Assessment of the Effects of Climate Change on Federal Hydropower

.................................................................................................................................. iii ACKNOWLEDGEMENTS ............................................................................................................ v LIST OF FIGURES ....................................................................................................................... xi LIST OF TABLES ........................................................................................................................ xv EXECUTIVE SUMMARY ........................................................................................................ xvii ABBREVIATIONS, ACRONYMS, AND INITIALISMS ........................................................ xxv LIST OF VARIABLES.............................................................................................................. xxxi