Date of Award

Spring 1-1-2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Civil, Environmental & Architectural Engineering

First Advisor

Balaji Rajagopalan

Second Advisor

Peter H. Molnar

Third Advisor

Thomas M. Marchitto

Fourth Advisor

Harihar Rajaram

Fifth Advisor

Joseph R. Kasprzyk

Abstract

Although model projections suggest an intensified Indian summer monsoon under a warmer climate, the monsoon has remained below average in recent decades despite strong warming trends. An understanding of Indian summer monsoon variability in past climate might provide clues for the future. Paleoclimate proxies suggest that parts of India were wetter during the early- to mid-Holocene period (~10 to 6 thousand years ago, ka), a period that was warmer than present day due to increased summer insolation. Enhanced wetness has been attributed to land heating, a key ingredient of a strong summer monsoon. Recent paleoclimate proxy evidence, however, suggests that the tropical Pacific was in a cooler (La Niña) state, which guarantees a strong monsoon in present day. This thesis advances the hypothesis that the El Niño Southern Oscillation (ENSO) could explain the aridification of India since 10 ka. To test this hypothesis, modern-day spatial and temporal monsoon-ENSO teleconnections are used to reconstruct Holocene Indian monsoon hydro-climate variability. Specifically, this dissertation asks: (1) What is the present day signature of ENSO on the Indian summer monsoon?; (2) Could the wet mid-Holocene India be explained by teleconnections from a cooler Pacific?; and (3) How much wetter was it during the early- to mid-Holocene? Some key findings include: (1) present-day ENSO writes a distinct spatial signature over India whereby a 1°C cooling of the equatorial Pacific can contribute 30-100% greater monsoon precipitation; (2) maximum cold sea surface temperatures (SSTs) and zonal wind anomalies occurred at 10 ka across the equatorial Pacific, and maximum zonal SST differences at 10 and 6 ka, as revealed by a multi-proxy reduced dimension field reconstruction; (3) maximum winds and 40-60% greater precipitation over the core monsoon region occurred at 10 ka, which have both decreased to present day; and (4) lakes in the northwestern region of India, which is currently desert, would require 40-65% greater precipitation than present day, based on the results of a hydrological water balance lake model. These findings provide the basis for research extensions that have the potential to improve seasonal monsoon forecasting efforts and to provide mechanistic insights into future monsoon variability.

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