Jasti Sriranga Chowdary

Land surface hydrological processes influence climate predictability through soil moisture persistence and coupling efficiency, the capacity of surface flux anomalies to generate spatially coherent atmospheric forcing at large scales. We examine coupling efficiency using century‐long free‐running simulations of a fully coupled Earth system model, comparing two configurations that differ only in land surface parameterizations. The Noah‐MP configuration differs from the Noah 2.7.1 control in subsurface hydrology, vegetation physiology, and surface‐layer exchange, and produces larger‐scale and more coherent atmospheric responses despite nearly identical soil moisture memory. Because every other model component is shared, the contrast isolates the effect of the land surface formulation. The Amazon–Pacific teleconnection is absent in the control but is reproduced closely with Noah‐MP. Drought composites with nearly equal rainfall show higher latent heat flux over tropical evergreen forests with accessible subsurface water with Noah‐MP than control. Over the Amazon, sustained evapotranspiration anomalies are accompanied by a Matsuno–Gill type response and a reduction in the Pacific Walker Circulation bias. The El Niño–Southern Oscillation (ENSO) spatial structure is also more realistic, with a stronger east–west sea surface temperature gradient. Atlantic Niño variability becomes less tied to ENSO, closer to the observed relationship. These results suggest that coupling efficiency, rather than soil moisture persistence alone, governs how strongly land surface processes influence large‐scale variability, and that a more realistic representation of land surface processes can strengthen that influence with potential implications for long‐term predictability.

The South Asian summer monsoon has exhibited a pronounced Northwest India‐Indo‐Gangetic Plains rainfall dipole since 1999, with northwest India experiencing a 24.6% increase, while rainfall in the Indo‐Gangetic Plain has decreased by 4.4%. This dipole pattern is absent in historical climate model simulations, and the underlying physical mechanisms responsible for it are not yet fully understood. Using synthesis of observational and assimilated data and climate model simulations, we demonstrate that this dipole is driven by North Atlantic Sea Surface Temperature (SST) changes, which are transmitted through a “barotropic governor mechanism” and modulate Asian jet stream dynamics. Enhanced barotropic energy conversion after 1999 generated momentum focusing in the jet core while weakening east‐west flanking regions. This fundamentally altered the monsoon circulation by modulating the local Hadley cell. We find that this barotropic governor activation coincides with the North Atlantic “cold blob” attributed to a slowdown in the Atlantic Meridional Overturning Circulation (AMOC). Coupled climate simulations over the historical era systematically fail to reproduce North Atlantic SST changes; thus, this atmospheric dynamical mechanism instead showed reversed barotropic energy conversion patterns and failed to reproduce this key teleconnection mechanism. Prescribed SST experiments forced by observed North Atlantic SST changes support the proposed mechanism, successfully reproducing both observed jet dynamics and rainfall trends. The identification of a North Atlantic‐Asian teleconnection pathway modulated by the barotropic governor effect directly links the behavior of the AMOC and monsoon tipping elements to each other.

Compound climate extremes, where temperature and precipitation extremes occur together are increasing globally and pose growing risks to human and natural systems. India is already experiencing more frequent extreme weather events, underscoring the urgency of assessing how such events may evolve with continued warming. This study uses high‐resolution, statistically downscaled, and bias‐corrected CMIP6 climate projections from 30 different models to assess the future compound extremes such as Cold‐Dry (CD), Cold‐Wet (CW), Warm‐Dry (WD), and Warm‐Wet (WW) under 1.5°C and 2.0°C global warming levels for two future climate scenarios (SSP2‐4.5 and SSP5‐8.5) during the pre‐monsoon season (March‐June). The projections indicate that cold‐related extremes (CD, CW) will become rare while warm‐related extremes (WD, WW) will increase significantly across India, with the largest changes emerging by the mid‐21st century across the West Central India (WCI), North East India (NEI), and Central North East India (CNEI). The occurrence of WD events has approximately doubled while WW events have increased nearly threefold across India during this season. Temperature increases, reinforced by land‐atmosphere coupling and thermodynamic constraints such as higher vapor pressure deficit and greater moisture holding capacity, intensify wet and dry extremes, while circulation‐driven precipitation changes play a lesser role. Population exposure to WD and WW events is projected to rise sharply, particularly in densely populated and climate‐sensitive regions. These findings highlight the need for targeted, region‐specific adaptation planning and risk‐informed decision‐making to manage the escalating risks from compound extremes in a warming climate.

Northwestern India (NWI) received anomalously heavy rainfall from 21 to 30 August, 2024, despite the monsoon being in a break phase. This study reveals how midlatitude dynamics triggered this unusual event. Using reanalysis, we perform a moisture budget analysis and local wave activity (LWA) diagnostics and find that the enhanced mid‐tropospheric baroclinic instability peaked during 13–17 August in the North Atlantic, 7–10 days before the NWI rainfall event. This instability fueled the North Atlantic westerly jet and reinforced a “” shaped blocking high over the Ural Mountains (“Ural block”) that developed over 21–22 August. This block initiated a wave train that propagated eastward with increasing amplitude over subsequent days. The downstream distorted Rossby waves broke, triggering a pronounced Caspian Sea trough that deepened by 25–26 August, strengthening a subtropical jet streak north of the Tibetan Plateau, with maximum geostrophic wind speeds exceeding 120 . The NWI was in the jet streak's right entrance region, where the upper‐level divergence and compensatory upward motion intensified a low‐pressure system that had been steered northwestward into the region. Despite monsoon break conditions, the elevated mid‐tropospheric humidity allowed sustained deep convection, which combined with the quasigeostrophic ascent arising from the jet streak entrance region and led to extreme rainfall over NWI. Our analysis provides a case study showing the importance of the growing influence of mid‐latitude circulation changes traceable to alterations in the North Atlantic for Indian monsoon variability. The physical linkages demonstrated here may be crucial for improving monsoon behavior in changing climate.

The spring heatwave over the Indian region in 2022 was unprecedented in intensity and scale, severely affecting millions of people and various sectors, including health and agriculture. In contrast, spring 2023 experienced fewer heatwaves, with higher than normal precipitation and widespread hailstorms across central and northwest India. This study explores the physical mechanisms that are responsible for these contrasting patterns. In the spring of 2022, La Niña conditions prevailed, leading to a poleward shift of the subtropical westerly jet (STWJ). This shift, combined with an upper‐air anticyclonic circulation and associated divergence over central and northwest India, resulted in subsidence and adiabatic heating, leading to extreme heatwave conditions and below‐normal precipitation. In contrast, during the spring 2023, the emergence of El Niño conditions caused the STWJ to shift equatorward. This shift invigorated a cyclonic circulation, with a trough in the upper‐level westerlies and a lowering of the zero‐degree isotherm over central and northwest India. In addition to upper‐level circulation changes, the convergence of moisture at lower levels due to moisture inflow from the Bay of Bengal and the Arabian Sea caused convective instability, leading to hailstorms. The fidelity of coupled dynamical climate models in forecasting the contrasting temperature and precipitation patterns during spring 2022 and 2023 has also been evaluated. This study highlights the significant role of the El Nino–Southern Oscillation (ENSO) in influencing spring weather and climate variability across the Indian region.

The relationship between meridional displacement of the Asian jet and Indian summer monsoon (ISM) is examined in the observations and 40 state‐of‐the‐art Coupled Model Intercomparison Project, phase 6 (CMIP6) historical simulations. The southward displacement of the Asian jet (SWDAJ) is found to be linked with reduced rainfall over central and north India including monsoon trough region in both the observations and most of the CMIP6 models. In general, models are able to simulate the characteristics of the SWDAJ during summer with some differences in zonal asymmetry. Detailed analysis elucidated two observed pathways that connect the SWDAJ with ISM rainfall variability. The first is the west Asian jet–easterly jet pathway, where the SWDAJ is linked with the southeastward shift of the South Asian high, contributing to the weakening of tropical easterly jet. This pattern aligns with changes in low‐level ISM circulation and contributed to reduced rainfall over India. The west Asian jet–easterly jet pathway is well represented by most of the CMIP6 models. The second pathway involves the East Asian jet–Pacific Japan pattern, in which the SWDAJ over East Asia strengthens the western North Pacific anticyclone through the barotropic–baroclinic coupling. The anomalous westward extended easterlies originating from the western North Pacific anticyclone hinder the southwesterly monsoon flow and suppress the rainfall. It is found that CMIP6 models largely failed to capture the East Asian jet–Pacific Japan pathway, likely due to the models' tendency to overestimate the simulation of the Indian Ocean dipole, and westward extension of El Niño‐related sea‐surface temperature anomalies in most of the models. Further, the combined influence of El Niño and the Pacific decadal oscillation and El Niño alone on the SWDAJ and ISM rainfall is also discussed in detail. Results obtained in this study are useful to understand the ability of CMIP6 models in representing the midlatitude circulation associated with ISM rainfall variability.

The present study explores the multi‐decadal variability of Severe Cyclonic Storm (SCS) frequency over the Bay of Bengal (BoB) during post‐monsoon season for the period 1950–2022. The SCS frequency displays noticeable decadal/multi‐decadal variability during the study period, and shows a strong relationship with mid‐tropospheric relative humidity (RH) and sea surface temperature (SST) over BoB. Influence of the large‐scale climate modes, such as Atlantic Multi‐decadal Oscillation (AMO) in modulating the BoB SCS frequency on multi‐decadal time scale is highlighted. It is found that AMO exhibits a significant out‐of‐phase relationship with SCS decadal frequency over the BoB. Detailed analysis suggests that large‐scale circulation changes associated with the negative phase of AMO (strong SCS decade) induced low‐level convergence extending from western Indian Ocean to southern BoB favour increased SCS frequency. A negative AMO (strong SCS period) induced zonally oriented upper‐level convergence and divergence centres, corroborated by the Walker circulation, helped to modulate the low‐level circulation over BoB. High RH and warm SSTs associated with the negative AMO further supported the increased SCS frequency over BoB. In the case of weak SCS decade, positive AMO induced low‐level divergence over southern BoB through atmospheric teleconnections provides unfavourable conditions for SCS formation. In addition to this, changes in the tropospheric temperature over the mid‐latitudes caused by mid‐latitude Rossby waves also contributed to the differences in SCS frequency on the multi‐decadal time scale. This study highlights the role of AMO in modulating the multi‐decadal SCS frequency over the BoB. The results presented here are useful for improving the decadal prediction of cyclone frequency over the BoB.