Exploring the circum-global teleconnection—Indian summer monsoon interactions in the interannual and multidecadal timescales

The Northern hemispheric circum global teleconnection (CGT) pattern is thought to be maintained by two main forcings—viz-diabatic heating associated with the Indian summer monsoon (ISM) and barotropic instability generation over the jet exit region over the North Atlantic. The CGT and ISM impacts one another through the circulation responses over West central Asia (WCA). In this study we revisit the CGT-ISM interactions focusing on the WCA region and try to understand whether the downstream impact of CGT on ISM dominates over the ISM feedback on CGT. Analysis indicates that the Atlantic forced CGT responses play a lead role in modulating the ISM in the interannual timescale, by modulating the upper-level anticyclones over WCA and in turn affecting the ISM easterly vertical wind shear. Atlantic multi-decadal oscillation (AMO) is a major driver of ISM variability in the multi-decadal time scale and the AMO is associated with an arching wave-train of teleconnection across Eurasia. Our analysis indicates significant modulation of WCA anomalies by the AMO in the multi-decadal time scale, implying that the Atlantic-CGT-WCA-ISM pathway of teleconnection has a low frequency counterpart. We further demonstrate that the observed out of phase relationship between AMO and ISM in the recent decades, may be attributed to the relatively stronger high latitude warming over the north Atlantic during the recent AMO warm phase. The equivalent barotropic responses to the extra-tropical north Atlantic sea surface temperature (SST) anomalies alter the entire downstream teleconnection pattern producing cyclonic anomalies over WCA and in turn weakening the ISM.