Impacts of SST Warming in tropical Indian Ocean on CMIP5 model-projected summer rainfall changes over Central Asia

Huqiang Zhang,Yong Zhao

doi:10.1007/s00382-015-2765-0

Abstract

Based on the historical and RCP8.5 experiments from 25 Coupled Model Intercomparison Project phase 5 (CMIP5) models, the impacts of sea surface temperature (SST) warming in the tropical Indian Ocean (IO) on the projected change in summer rainfall over Central Asia (CA) are investigated. The analysis is designed to answer three questions: (1) Can CMIP5 models reproduce the observed influence of the IO sea surface temperatures (SSTs) on the CA rainfall variations and the associated dynamical processes? (2) How well do the models agree on their projected rainfall changes over CA under warmed climate? (3) How much of the uncertainty in such rainfall projections is due to different impacts of IO SSTs in these models? The historical experiments show that in most models summer rainfall over CA are positively correlated to the SSTs in the IO. Furthermore, for models with higher rainfall-SSTs correlations, the dynamical processes accountable for such impacts are much closer to what have been revealed in observational data: warmer SSTs tend to favor the development of anti-cyclonic circulation patterns at low troposphere over north and northwest of the Arabian Sea and the Bay of Bengal. These anomalous circulation patterns correspond to significantly enhanced southerly flow which carries warm and moisture air mass from the IO region up to the northeast. At the same time, there is a cyclonic flow over the central and eastern part of the CA which further brings the tropical moisture into the CA and provides essential moist conditions for its rainfall generation. In the second half of twenty-first century, although all the 25 models simulate warmed SSTs, significant uncertainty exists in their projected rainfall changes over CA: half of them suggest summer rainfall increases, but the other half project rainfall decreases. However, when we select seven models out of the 25 based on their skills in capturing the dynamical processes as observed, then the model projected changes are much closer. Five out of the seven models predicted more rainfall over CA. Such a result is helpful for allowing us to attribute part of the observed upward rainfall trend in the CA region in the last several decades to the IO SST warming.

Highlights

The central Asia (CA) is one of the largest arid and semiarid areas in the world, with an area over 4 × 106 km2, across Kazakhstan, Kyrgyzstan, Tajikistan, Turkmenistan, and Uzbekistan and part of northwest China (Huang et al 2014)
Warmed Indian Ocean (IO) sea surface temperature (SST) enhances the Indian monsoon and its cyclonic circulation pattern in the tropics; (2) Without the IO warming, much warmed SSTs over tropical central Pacific Ocean could lead to significant rainfall decreases in the Rainfall is very low over CA
Existing studies showed that one of the most important social and political risks associated with climate change pertains to water availability, especially over CA (Dinar and Dinar 2003; Wolf et al 2003)

Summary

Introduction

The central Asia (CA) is one of the largest arid and semiarid areas in the world, with an area over 4 × 106 km, across Kazakhstan, Kyrgyzstan, Tajikistan, Turkmenistan, and Uzbekistan and part of northwest China (Huang et al 2014). Observational analysis showed that a weakened south Asian summer monsoon could result in the cooling in middle and upper troposphere over CA and generate favorable anomalous circulations for generating more summer rainfall in east part of CA (Yang et al 2009; Zhao et al 2014), we do not know if the same processes operate in the models. Following these evaluations, we select models to assess if the models with reasonable skill in capturing these fundamental observed features can offer reasonable agreement in their projected changes of rainfall over CA. Impacts of SST Warming in tropical Indian Ocean on CMIP5 model-projected summer rainfall

Datasets used in the analysis

Observed and model‐simulated linkages between IO SSTs and CA rainfall

Projected change in summer rainfall over CA

Findings

Conclusion and discussions