The behavior of the South Asian summer monsoon in climate model simulations of pre-industrial, modern, and projected future climate
The Asian summer monsoon (“ASM”), as one of the most important components of the climate system, has crucial economic and societal relevance to the most densely populated regions over South Asia and East Asia. The primary goal of this dissertation is to analyze the changes in the ASM in a variety of model simulations over various time intervals for insights into the factors that govern interannual and longer-term variations in the ASM. Specific issues addressed are (i) the degree to which current generation climate models can reproduce the real world ASM variations recorded in observations and proxy reconstructions, (ii) the physical mechanisms responsible in the models for ASM variability and ASM responses to external forcing, and (iii) the reliability of projections of the future changes in the ASM in light of these investigations.
The ASM is first analyzed in a simulation of the NCAR Climate System Model, version 1.4 (CSM 1.4) driven with natural and anthropogenic radiative forcing over the past millennium. The focus of this analysis is the isolation of potential factors, including both natural forcing and internal variability, governing the long-term, prehistoric changes in the ASM. A newly developed integrative monsoon index (IMI) is applied to represent the strength of the monsoon circulation, with a focus on the most robust, South Asian Summer Monsoons (“SASM”) component of the ASM. The direct radiative effect of solar forcing on the SASM is found to be too weak to be detected in the context of the model simulation, whereas volcanic forcing gives rise to a significant short-term decrease of the SASM strength in the year of the largest radiative forcing anomaly associated with the volcanic eruption. It is evident that forced changes in the eastern tropical Pacific sea surface temperature play an important role in the long-term changes in the SASM, but certain potential real world dynamical responses of the tropical Pacific ocean-atmosphere system to radiative forcing may not be well reproduced in this model simulation.
Over the latter half of the 20th century during which (reanalysis) observations of the SASM are available, a parallel analysis is performed of the historical, 20th century experiment (‘20C3M’) simulations of the suite of coupled Atmosphere-Ocean General Circulation Models (AOGCMs) used in the Coupled Model Intercomparison Project, phase 3 (CMIP3). The models are found to capture some of the features of the observed SASM including aspects of the mean circulation, associated precipitation influences, interannual variability, trend, and relationship with ENSO. However, there is also a fair amount of divergence in the trends found within the multi-model ensemble experiments. There is some evidence for greater consistency among the “best performing” subgroup of models (based on comparison between observed and modeled SASM-related precipitation climatological statistics).
A similar analysis is applied to the projections of future changes in the SASM during the 21st century in the CMIP3 720ppm stabilization experiment (‘SRES A1B’). The seven “best performing” models (as determined in the ‘20C3M’ analysis described above) indicate a clear tendency for both weakening of SASM circulation, and increase in SASM-related precipitation.