Title: The Impact of a Warmer Climate on Atmospheric Circulation with Implications for the Asian Summer Monsoon
Description: Warming of both the high latitudes and tropical sea surface temperatures are present in modern observations and projected under future climate change scenarios. These conditions were also present in the Warm Pliocene (3.3 - 3.0 million years ago), a paleoclimatic interval that bares resemblance to future global warming. This dissertation investigates the impact of both tropical and high latitude warming on regional atmospheric circulation using GISS global climate model simulations of the Pliocene and sensitivity tests. Chapter 1 discusses the initial approach used to investigate how a warmer climate impacts regional atmospheric circulation. A general circulation model (GCM) was utilized to assess the contribution from both high latitude and tropical warming to regional Pliocene climatic patterns. It was found that both a warming of the high latitudes and Indo-Pacific tropical region are needed to reproduce the regional Pliocene climates indicated by terrestrial paleo-proxy data. These results suggest that the tropical atmospheric circulation of the Indo-Pacific region during the warm Pliocene may have been different from modern mean conditions.

These findings are corroborated by Pliocene paleo-data, a luxury not afforded by future climate projections, and provide insight into possible regional atmospheric circulation processes in a future warmer climate. Chapter 2 (Shukla et al., 2011) investigates how exactly the Indo-Pacific circulation and global teleconnections differed from modern day conditions. GCM generated teleconnections from the Indo-Pacific region were examined from origin to their impact on the extra-tropics under warm Pliocene conditions. The exact forcing source was not assumed a-priori, and it was found that while warmer SSTs in the eastern tropical Pacific generated weak El Niño-like teleconnections to North America, their effects over the Indian Ocean region were attenuated, primarily by the warmer SSTs there. Teleconnections to the extra-tropics were largely blocked from the Indian Ocean region, and most of the energy generated by the SST patterns went into maintaining an anomalous atmospheric overturning circulation.

This altered background circulation of the Indian Ocean region can impact the South Asian Summer Monsoon (SASM) system. In these simulations, the dynamic monsoon intensity experienced the greatest decrease with tropical warming alone. Lesser SASM weakening occurred when both tropical and high latitude warming were imposed. Given the potential Indo-Pacific SSTs changes under Pliocene and warm climate conditions, Chapters 3 and 4 focus on the implications these changes have for the South Asian Summer Monsoon circulation. Chapter 3 examines the GISS suite of GCMs' ability to reproduce the major features of the South Asian Summer Monsoon (SASM) system. The GISS Model E (atmosphere only), Middle Atmospheres Model 3 (atmosphere only) and the ocean-atmosphere coupled Model E were run using forcings from 1960-2008. Major indices and features of the SASM were evaluated and compared to NCEP/NCAR and ECMWF reanalysis data. It was found that the atmosphere-only Model E better simulated, both in magnitude and variability, the circulatory (wind, vorticity, etc.) components of the SASM, whereas the coupled ModelE better simulated the magnitude of rainfall over the Indian sub-continent.

Chapter 3 highlighted the SASM features in the models that need improvement, specifically in the overproduction of rainfall and the underestimation of windspeeds. Given the relatively accurately modelE simulated SASM intensity variability, and acknowledging its underestimation of wind strength, continuing modelE studies of the SASM will focus on large-scale circulation processes, rather than the rainfall distribution and variability. Chapter 4 compares SASM changes under both Pliocene conditions and future climate projections, the latter dictated by the Representative Concentration Pathways (RCPs). A tropical SST forcing, in the form of warmer western tropical Indian Ocean and eastern tropical Pacific Ocean SSTs, was additionally tested in isolation from globally warmer conditions. It was found that the SASM weakens under globally warmer conditions, but the greatest weakening occurred under tropical forcing alone. This suggests the importance of the relative regional temperature gradients of the Indian Ocean region.

Although both simulations served to weaken the SASM system, the regional climatic patterns differed between Pliocene and future simulations and warrant further investigation. Future studies must focus on obtaining more data from the Indian Ocean region for the Pliocene period in order to corroborate modeled climate processes in that region. In addition, more assessments must be done to understand difference between climate processes in future projections and past warm climate intervals as this will aid in model development and our understanding of the climate's response and sensitivity.