Abstract
The dominant interannual SST variability in the eastern equatorial Atlantic is referred to as the Atlantic Zonal Mode (AZM), which peaks in boreal summer impacts global weather patterns. The cold (warm) phase of this ocean-atmospheric coupled phenomenon enhances (weakens) the intensity of the Indian Summer Monsoon Rainfall (ISMR). Observational studies show a strengthening relationship between AZM and ISMR in recent decades, providing a predictive signal for the ISMR. However, a suite of Coupled Model Intercomparison Project Phase 6 (CMIP6) model simulations in the highest emission scenario (SSP58.5) show a weakening relationship between ISMR and AZM in the future (2050–2099). The strengthening of atmospheric thermal stability over the tropical Atlantic in the warming scenario weakens the associated convection over the eastern equatorial Atlantic in response to the warm phase of AZM. This leads to weakening velocity potential response over the Indian subcontinent, resulting in a weak AZM–ISMR relationship. There is no convincing evidence to indicate that either the tropical Atlantic SST bias or the AZM–ISMR teleconnection bias plays a crucial role in the potential weakening of this relationship. These results imply that ISMR prediction will become more challenging in a warming scenario as one of the major external boundary forces that influence monsoon weakens.
Highlights
India receives ∼80% of its annual rainfall during boreal summer months
These results give an inkling that the recent strengthening of Atlantic Zonal Mode (AZM)-Indian summer monsoon rainfall (ISMR) teleconnection in the observational data is not an after-effect of greenhouse warming, but instead, it is induced by the increase in Atlantic SST variability, as shown in Sabeerali et al (2019)
The Atlantic Zonal Mode (AZM) is a dominant mode of interannual climate variability in the tropical Atlantic Ocean that emerges from the air-sea coupled interaction similar to El Niño-Southern Oscillation (ENSO) in the Pacific
Summary
India receives ∼80% of its annual rainfall during boreal summer months. A large majority of people in an agrarian-based society like India depend on the seasonal rainfall. Observations indicate that the teleconnection between AZM and ISMR strengthens in recent decades (Sabeerali et al, 2019) Both these studies highlight the significance of a realistic simulation of tropical Atlantic variability for the skillful prediction of ISMR. The large eastern equatorial Atlantic warm SST bias in coupled models limit the proper simulation of interannual variability of SST (Richter and Xie, 2008; Wahl et al, 2011; Wang et al, 2014; Ding et al, 2015a,b). This SST bias is closely related to the boreal spring tropical Atlantic westerly wind stress and associated deepening of thermocline in eastern equatorial Atlantic (Chang et al, 2007; Richter and Xie, 2008; Tozuka et al, 2011).
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