Role of atmospheric teleconnection processes in the observed relationship between the Atlantic Multidecadal Oscillation and Indian summer monsoon

Abstract

The Indian summer monsoon (ISM) exhibits considerable variance in the multi-decadal timescales in observational and paleoclimate proxy records. The summer-mean all-India rainfall exhibits multidecadal fluctuations remaining periodically above the long-term mean for several decades followed by periods when it is below the long-term mean. The multidecadal variance of monsoon is generally attributed to the multidecadal variability over the Atlantic - the Atlantic multi-decadal Oscillation (AMO). Though a robust relationship between the two is observed in the instrumental records, with the limited span of observational records we can capture only two cycles of AMO and the AMO-ISM relationship is inconsistent among the paleoclimate proxy records. While several hypotheses have been put forth to explain the observed relationship, several questions remain with respect to the robustness of the AMO- ISM relationship and the teleconnection pathways. In this thesis, I revisit the AMO-ISM relationship and try to understand the underlying large-scale atmospheric teleconnection processes which connect these climate modes. A combination of observational and hierarchical modelling approaches using intermediate complexity models and long simulations from state- of-the-art global climate models (GCM) were used to infer the observed AMO-ISM relationship. The first question addressed is understanding how well the proposed modes of AMO- ISM teleconnection are represented in current generation GCM simulations. Last millennium simulations from the Climate Model Intercomparison Project/Paleoclimate Model Intercomparison Project (CMIP5/PMIP3) were analyzed to examine two broad atmospheric teleconnection mechanisms which connect the Atlantic and the ISM. AMO-induced Pacific Ocean variability and associated changes in Walker circulation are found to be robust in different model simulations. However, the models are found to exhibit considerable disagreement in the upper-tropospheric planetary wave response to AMO and the associated impact it can have on the ISM. The study highlights the strength and deficiencies of current climate models in simulating the teleconnection processes which control the decadal and longer-term variability of monsoon and is of great value as these models are also used for future climate projections. The second question addressed is how the planetary wave responses to AMO modulates the ISM. SST anomalies over the extratropical North Atlantic can drive vortices with equivalent barotropic vertical structures. Idealized model experiments indeed show that equivalent barotropic Rossby wave response across the Euro-Atlantic sector can reach west central Asia (WCA). Upper-level circulation anomalies associated with the Rossby waves over WCA impact ISM through anomalous tropospheric temperature and easterly shear. The study demonstrates significant modulation of WCA anomalies by the AMO in the multi-decadal time scale. It is also found that the observed out-of-phase relationship between AMO and ISM in the recent decades, may be attributed to the anomalous equivalent barotropic responses to the relatively stronger high latitude warming over the North Atlantic during the recent AMO warm phase which altered the entire downstream teleconnection pattern producing cyclonic anomalies over WCA and in turn weaken the ISM. The third central question addressed is the relative roles of the equatorial and extratropical North Pacific in the observed AMO-ISM relationship. Simple model experiments with an intermediate complexity GCM highlight the vital role played by the Pacific Ocean in the AMO-ISM relationship, in agreement with the more comprehensive GCM simulations. In response to the warm North Atlantic, the equatorial Pacific is found to constrain the Walker circulation resulting in enhanced low-level convergence over India. The North Pacific contribution is found to be weak during summer. Analysis of observational records shows a strong lagged AMO-ISM relationship consistent with the development of a strong meridional gradient in SST over the Pacific Ocean. Further sensitivity experiments are carried out with prescribed forcing confined over tropical and extratropical North Atlantic. Results suggest tropical North Atlantic is the critical driver of the lagged response.