Monsoon Storms Can Thrive Even When Rains Fade, Model Study Shows

Monsoon low-pressure systems, the sprawling rain-bearing disturbances that drift across South Asia each summer, may not depend on abundant seasonal rainfall as much as scientists long assumed. A new modeling study published in Geoscience Letters finds that these systems can form, intensify, and propagate even when mean monsoon precipitation is substantially reduced, drawing their energy instead from wind shear and horizontal advection of moist static energy. The finding, based on stylized climate simulations in which the height of the Tibetan and Himalayan orography was artificially lowered, suggests that extreme precipitation events linked to these storms could still occur even in drought years.

Monsoon low-pressure systems, or LPS, are synoptic-scale tropical disturbances with horizontal extents of roughly 1,000 to 2,000 kilometers, vertical reaches of about 10 kilometers, and lifetimes of three to five days. Around 14 of them form each year over South Asia, and about half grow into the stronger systems known as depressions and deep depressions. Most originate over the warm waters of the Bay of Bengal and track northwestward, while roughly 20 percent form over the Arabian Sea. They deliver much of the region’s seasonal rainfall, contributing 60 to 70 percent of total monsoon precipitation and about 80 percent of extreme precipitation over India’s core monsoon zone. Yet they are also implicated in more than half of the hydrometeorological disasters of the summer season, including the Pakistan floods of 2010 and 2022, the Kerala floods of 2018, the Mumbai floods of 2017, and the Uttarakhand floods of 2013.

For decades, convection has been regarded as the principal energy source that keeps these systems moving. Early theoretical work described northwestward propagation as the coupling of cumulus convection with low-level cyclonic vorticity, in which latent heat release generates a Rossby-gravity wave response that shifts moisture convergence to the northwest quadrant of the storm. Later studies reframed LPS as mid-tropospheric potential vorticity maxima advected westward by nonlinear horizontal adiabatic processes, and idealized experiments identified barotropic instability as the key energy provider during the initial stages of formation. What remained unresolved was how these systems sustain themselves when the background convection weakens, a question the new study set out to answer directly.

The research team, Tresa Mary Thomas and Mankulam Sivaprasad, used NCAR’s Community Earth System Model version 1.2.2, a fully coupled climate model whose atmospheric component, CAM4, runs at a horizontal resolution of 0.9 by 1.25 degrees with 26 vertical levels. Following an earlier study, they performed four simulations in which the height of the Tibetan and Himalayan orography, or THO, was scaled to 1.5, 1.0, 0.5, and 0.0 times its present height within a rectangular domain bounded by 60 to 120 degrees east and 25.9 to 43.8 degrees north. The topographic modification altered three variables in the model’s boundary condition file, surface geopotential, the standard deviation of surface elevation, and surface roughness, with a Gaussian smoothing kernel applied to soften sharp transitions at the domain edges. After a 197-year spin-up to near-equilibrium and branch runs of 8 to 12 years to re-equilibrate, each case was run for an additional 37 years at a six-hourly time step, producing simulations labeled M1.5, M1.0, M0.5, and M0.0.

As expected from previous orography experiments, mean monsoon precipitation and convective precipitation declined as the THO was lowered, with a significant decrease over the Indian inland emerging only when the mountains were removed entirely in the M0.0 simulation. Convective available potential energy, convective inhibition, and upper-level cloud fraction all decreased as well, confirming a reduction in deep convective activity. Counterintuitively, however, the number, intensity, and lifetime of LPS increased as the mountain height dropped, echoing the earlier finding that LPS frequency rises even as seasonal rainfall falls. The paradox demanded an explanation: if convection was weakening, what was feeding the storms?

The answer begins with barotropic instability. The researchers computed the kinetic energy budget of LPS-associated wind anomalies at 850 hPa, the level near which composite depressions show their maximum relative vorticity and tangential velocity. In the initial days of LPS evolution, the growth rate of perturbed kinetic energy increased drastically in the M0.0 simulation compared with the others, driven by positive pressure work and barotropic conversion terms. Among the barotropic conversion components, the interaction of velocity anomalies with the meridional shear of the mean zonal wind, a term proportional to u’v’ times the meridional gradient of zonal wind, dominated. This shear strengthens as the THO is lowered because dry extratropical air intrudes into the Indian mainland, raising surface temperatures, intensifying the sensible heat-driven low-level trough, and sharpening the geostrophically associated westerlies. The LPS anomalies, in effect, extract kinetic energy directly from the mean flow, and the extraction grows more efficient as the mountains shrink.

Barotropic energy conversion explains the birth of the storms, but not their later intensification. To probe that phase, the team constructed a moist static energy budget, defining perturbed MSE as the sum of the dry static energy term and the latent energy term, with the geopotential component neglected because it is orders of magnitude smaller for slowly propagating tropical disturbances. Vertically integrated over 1,000 to 100 hPa and averaged over the western quadrant of each storm, the total MSE associated with LPS was largest in the M0.0 simulation despite the reduced mean precipitation. The moist component dominated over the dry component in every simulation, underscoring that moisture remains central to storm growth even when the seasonal average rainfall declines.

The budget decomposition revealed a striking phase dependence. In the simulations that retained some orography, vertical advection accounted for the major share of the MSE trend. In the mountain-free M0.0 run, however, horizontal advection emerged as the dominant term in the later stages of propagation. During the initial days, enhanced meridional winds, themselves a consequence of the intensified low-level trough, strengthened the horizontal advection of MSE over the storm’s western quadrant, inducing isentropic ascent and vertical advection comparable to that in the other simulations. This moistens the atmosphere to the west of the storm and builds up the MSE anomaly. Once sufficient moisture has accumulated, enhanced mean zonal winds, which strengthen alongside rising surface temperatures as cloud cover diminishes, take over, driving horizontal advection of both the moist and dry MSE components upward by about 0.01 watts per kilogram per second, expressed as 0.01 joules per kilogram per second, between day one and day ten of propagation. The terms describing advection of the mean-state MSE by wind anomalies and advection of MSE anomalies by the mean wind proved especially significant.

The authors are careful to frame these results within their limitations. The experiments are highly idealized: a single climate model, a single automated tracking algorithm based on geopotential criteria, and a coarse resolution of roughly one degree that may not fully resolve mesoscale convection. The design also cannot capture projected climate change scenarios, in which multimodel studies using CMIP5 and CMIP6 ensembles project a weakening of mean monsoon circulation alongside an increase in mean precipitation over India, whereas the orography experiments weaken circulation and precipitation together. Moisture source trajectories were not analyzed, and the teleconnections of the simulated changes to planetary-scale circulation and land-atmosphere-ocean coupling remain to be evaluated. Multimodel comparisons with multiple tracking algorithms at finer resolutions, along with observational analyses of drought years, are proposed as tests of robustness.

Even so, the implications are provocative. A recent observational study covering 1901 to 2020 found that although El Nino events suppress total summer monsoon rainfall over India, the intensity of daily extreme precipitation increased, and those extremes were primarily associated with LPS. The new simulations offer a physical mechanism consistent with that picture: strong LPS and their flood-producing downpours do not require a wet seasonal background. Barotropic shear can seed the storms, and horizontal MSE advection, sustained by shifting winds and warmer surfaces, can drive them to maturity. If the mechanism holds beyond these stylized experiments, drought years may offer no protection against the most dangerous rains the monsoon can deliver.

Subject of Research: Energy sources sustaining monsoon low-pressure systems under reduced mean monsoon precipitation in orography-forced climate simulations

Article Title: Energetics of monsoon low-pressure systems in stylized simulations of reduced mean monsoon precipitation through orography forcing

Article References: Thomas, T. M., & Sivaprasad, M. (2026). Energetics of monsoon low-pressure systems in stylized simulations of reduced mean monsoon precipitation through orography forcing. Geoscience Letters, 13(1), Article 58. https://doi.org/10.1186/s40562-026-00509-7

Image Credits: AI Generated

DOI: 10.1186/s40562-026-00509-7

Keywords: monsoon low-pressure systems, Tibetan orography, barotropic instability, moist static energy, CESM1.2.2, South Asia monsoon, extreme precipitation, climate modeling, wind shear, drought years, Bay of Bengal, convection