Asymmetric response of the Indian summer monsoon to positive and negative phases of major tropical climate patterns

Spatial diversity of atmospheric moisture transport and climate teleconnections over Indian subcontinent at different timescales

Regional weather and climate are generally impacted by global climatic phenomenon's. Understanding the impact of global climate phenomenon's on an atmospheric branch of the hydrological cycle is crucial to make advances in skillful precipitation forecast. The present study adopts a multiscale approach based on wavelets for unravelling the linkages between teleconnections and atmospheric moisture transport over homogeneous regions of Indian sub-continent. We investigated linkages between atmospheric moisture transport quantified as monthly integrated water vapor transport (IVT) during 1951-2022 over selected homogeneous regions and eight large scale climate oscillations using wavelet and global wavelet coherence. Our results indicate significant heterogeneity in linkages across different regions and across multiple timescales. In particular, the Indian Ocean Dipole (IOD) influence monthly IVT at intra-annual to inter-annual scale over all regions. The El Niño-Southern Oscillation (ENSO) have strong connection to monthly IVT at inter-annual scale whereas over west central region both IOD and ENSO strongly influence IVT at inter-decadal scale. While the Atlantic Multi-Decadal Oscillation and Pacific Decadal Oscillation have an impact on IVT in the north-east and southern regions, the Arctic Oscillation and North Atlantic oscillation have a strong inter-annual connection to IVT, majorly in the northwest and hilly regions. Overall, the methodology offers an effective approach for capturing the dynamics of atmospheric moisture transport in time-frequency space and provide a practical reference for prediction of atmospheric moisture transport linked precipitation over different regions of Indian subcontinent.

Varying contributions of fast and slow responses cause asymmetric tropical rainfall change between CO2 ramp-up and ramp-down

Tropical rainfall is important for regional climate around the globe. In a warming climate forced by rising CO₂, the tropical rainfall will increase over the equatorial Pacific where sea surface warming is locally enhanced. Here, we analyze an idealized CO₂ removal experiment from the Carbon Dioxide Removal Model Intercomparison Project (CDRMIP) and show that the tropical rainfall change features a stronger pattern during CO₂ ramp-down than ramp-up, even under the same global mean temperature increase, such as the 2 °C goal of the Paris Agreement. The tropical rainfall during CO₂ ramp-down increases over the equatorial Pacific with a southward extension, and decreases over the Pacific intertropical convergence zone and South Pacific convergence zone. The asymmetric rainfall changes between CO₂ ramp-down and ramp-up result from time-varying contributions of the fast and slow oceanic responses to CO₂ forcing, defined as the responses to abrupt CO₂ forcing in the first 10 years and thereafter, respectively, in the abrupt-4xCO2 experiment. The fast response follows the CO₂ evolution, but the slow response does not peak until 60 years after the CO₂ peak. The slow response features a stronger El Niño-like pattern, as the ocean dynamical thermostat effect is suppressed under stronger subsurface warming. The delayed and stronger slow response leads to stronger tropical rainfall changes during CO₂ ramp-down. Our results indicate that returning the global mean temperature increase to below a certain goal, such as 2 °C, by removing CO₂, may fail to restore tropical convection distribution, with potentially devastating effects on climate worldwide.