Fingerprint of volcanic forcing on the ENSO-Indian monsoon coupling

Coupling of the El Niño-Southern Oscillation (ENSO) and Indian monsoon (IM) is central to seasonal summer monsoon rainfall predictions over the Indian subcontinent, although a nonstationary relationship between the two nonlinear phenomena can limit seasonal predictability. Radiative effects of volcanic aerosols injected into the stratosphere during large volcanic eruptions (LVEs) tend to alter ENSO evolution; however, their impact on ENSO-IM coupling remains unclear. Here, we investigate how LVEs influence the nonlinear behavior of the ENSO and IM dynamical systems using historical data, 25 paleoclimate reconstructions, last-millennium climate simulations, large-ensemble targeted climate sensitivity experiments, and advanced analysis techniques. Our findings show that LVEs promote a significantly enhanced phase-synchronization of the ENSO and IM oscillations, due to an increase in the angular frequency of ENSO. The results also shed innovative insights into the physical mechanism underlying the LVE-induced enhancement of ENSO-IM coupling and strengthen the prospects for improved seasonal monsoon predictions.

Falling monsoon depression frequency: a Gray-Sikka conditions perspective

In this study, we show that the annual monsoon depression (MD) frequency making landfall on the east coast of India shows a statistically significant decreasing trend for the period 1979-2010. Importantly, about 80% of this fall is confined to the south of 20°N. To understand the plausible reason(s) for the weakening frequency of MDs in the southern Bay of Bengal in recent decades, we examine some of the seasonal average in-situ atmospheric parameters important for tropical cyclogenesis; we use various observational data from the IMD, and three atmospheric climate reanalysis datasets to account for possible quality constraints in them. Our findings suggest that the observed weakening of MD frequency south of 20°N in the Bay of Bengal since 1950s is likely due to a declining trend in the mid-tropospheric relative humidity over the Indian region. Our numerical sensitivity experiments support this finding.