arXiv subjects

D. L. Suhas

Publications and source records attributed to D. L. Suhas.

5 recordsLinked to original sources

arXiv2026

Marine Heatwaves in the Arabian Sea: Drivers and Impacts on Atmospheric Circulation and Extreme Precipitation

Marine heatwaves (MHWs) threaten marine ecosystems and significantly impact weather patterns. In the Arabian Sea, summer MHWs are of particular concern due to their potential impacts on the Indian summer monsoon, a lifeline for nearly a billion people. However, the drivers of these MHWs and their influence on atmospheric circulation and monsoon rainfall remain poorly understood. Using satellite observations, reanalysis datasets, and numerical model experiments, we investigate the key drivers of MHW events and assess their impacts. When SST warming trends are retained, the eastern and northern Arabian Sea emerge as MHW hotspots, showing rapid increases during 1982-2023, largely due to anthropogenic warming. On detrending the SSTs to remove the influence of anthropogenic warming on individual MHWs, we find that most MHWs are short-lived (lasting <= 20 days) and are initiated by enhanced surface shortwave radiation and reduced latent heat loss associated with the suppressed convection phase of the Boreal Summer Intraseasonal Oscillations (BSISOs). Interannual SST anomalies, including ENSO and Indian Ocean Dipole (IOD), further modulate the year-to-year MHW variability. Conversely, the warm SSTs during MHWs exert strong atmospheric feedbacks. MHWs in the eastern Arabian Sea drive cyclonic winds, intensify moisture convergence and increase the risk of extreme precipitation along the southwest coast of India. In the northern Arabian Sea, MHW-induced cyclones trigger intense rainfall over northwestern India and Pakistan, contributing to extreme events like the 2022 Pakistan floods. These findings improve our capacity to predict Arabian Sea MHWs and assess their risks, offering significant socio-economic and ecological benefits.

physics.ao-ph↗

arXiv2021

Turbulence and equatorial waves in moist and dry shallow water systems

Turbulence and large-scale waves in the tropical region are studied using the spherical shallow water equations. With mesoscale vorticity forcing, both moist and dry systems show kinetic energy scaling that is dominated by rotational modes, has a -5/3 exponent. At larger planetary scales, the divergent component of the energy increases and we see a footprint of tropical waves. The dry system shows a signature of the entire family of equatorial waves, while the moist simulations only show low frequency Rossby, Kelvin and mixed Rossby gravity waves with an equivalent depth that matches rapid condensation estimates. Further, runs with interactive moisture exhibit spontaneous aggregation with the equilibrium moist energy spectrum obeying a -2 power-law. Synoptic scale moisture anomalies form in heterogeneous background saturation, and are sustained by advection and convergence, within rotational gyres that dominate the tropical region. In that case, along with equatorial waves and turbulence, a large-scale eastward moving moisture wave appears in the midlatitudes. In all simulations with upscale energy transfer, a systematic equatorial zonal mean zonal flow develops which is easterly and westerly for the dry and moist ensembles, respectively.The interaction of this zonal mean flow with a spatially heterogeneous saturation field results in the formation of a moist stationary tropical wave. The super-rotating flow is driven by rotational eddy momentum fluxes due to enhanced equatorial Rossby wave activity in the moist runs. The nature of eddies is such that the tropical circulation in the dry and moist cases tends to homogenize and exaggerate potential vorticity gradients, respectively. These experiments demonstrate the co-existence of tropical waves and turbulence, and highlight the fact that the vortical and divergent wind are inextricably linked with the evolving moisture field.

physics.ao-ph↗

arXiv2021

Moist Shallow Water Response to Tropical Forcing: Initial Value Problems

The response of a spherical moist shallow water system to tropical imbalances in the presence of inhomogeneous saturation fields is examined. While the initial moist response is similar to the dry reference run, albeit with a reduced equivalent depth, the long time solution depends quite strikingly on the nature of the saturation field. For a saturation field that only depends on latitude, specifically, one with a peak at the equator and falls off meridionally in both hemispheres, height imbalances adjust to large-scale, low-frequency westward propagating modes. When the background saturation environment is also allowed to vary with longitude, in addition to a westward quadrupole, there is a distinct eastward propagating response at long times. The nature of this eastward propagating mode is well described by moist potential vorticity conservation and it consists of wave packets that arc out to midlatitudes and return to the tropics and are predominantly rotational in character. In all moist cases, initially formed Kelvin waves decay, and this appears to be tied to the off-equatorial organization of moisture anomalies by rotational modes. Many of these basic features carry over to the response in the presence of realistic saturation fields derived from reanalysis based precipitable water. In boreal summer, long time eastward response is restricted to northern hemisphere and takes the form of a wavetrain that passes over Indian landmass into the subtropics, reaching across the Pacific to North America. In boreal winter, the eastward mode consists of a subtropically confined rotational quadrupole along with midlatitudinal disturbances. Thus, in addition to circumnavigating westward Rossby waves, slow eastward propagating modes appear to be a robust feature of the shallow water system with interactive moisture in the presence of saturation fields that vary with latitude and longitude.

physics.ao-ph↗

arXiv2021

Dry and Moist Atmospheric Circulation with Uniform Sea-Surface Temperature

The steady and transient response of "dynamically" dry and moist atmospheres to uniform sea-surface temperature (SST) is studied. Specifically, the latent heat (Lv) of water vapor is varied, so that for small Lv, water substance is essentially a passive tracer from a dynamical point of view. Despite the lack of SST gradients, a general circulation with Hadley and Ferrel cells is observed for relatively stronger moist coupling. Organized precipitation patterns via equatorial waves appear to play a significant role in tropical ascent, and along with the equatorial deformation radius, the Hadley cell width increases with coupling strength. An abrupt switch to a much shallower tropical cell is noted when the system becomes completely passive. In all cases, the Hadley cell is thermally indirect and is influenced by eddy fluxes which are strong in the upper and lower troposphere. Moist static energy is transported equatorward in the tropics and a larger amount is directed poleward in the midlatitudes. Transient extratropical activity is seen in the form of intense warm-core vortices for strong coupling, and these systems become weaker and smaller as Lv decreases. The drift of these moist vortices results in the observed poleward energy transport in the midlatitudes. In the tropics, intraseasonal variability is dominant and systematically shifts to longer time periods with stronger coupling. In fact, large-scale, low-frequency Kelvin waves and MJO-like modes disappear as water vapor becomes passive in nature. Finally, extreme rainfall events associated with cyclonic storms vanish as water vapor becomes dynamically inactive. Tropospheric heating due to a saturation of the outgoing longwave radiation results in an increase in the stability of the atmosphere for strong coupling, and provides a plausible physical mechanism for interpreting the behavior of precipitation.

physics.ao-ph↗

arXiv2016

Low frequency modulation of jets in quasigeostrophic turbulence

Quasigeostrophic turbulence on a beta-plane with a finite deformation radius is studied nu- merically, with particular emphasis on frequency and combined wavenumber-frequency do- main analyses. Under suitable conditions, simulations with small-scale random forcing and large-scale drag exhibit a spontaneous formation of multiple zonal jets. The first hint of wave-like features is seen in the distribution of kinetic energy as a function of frequency; specifically, for progressively larger deformation scales there are systematic departures in the form of isolated peaks (at progressively higher frequencies) from a power-law scaling. Con- comitantly, there is an inverse flux of kinetic energy in frequency space which extends to lower frequencies for smaller deformation scales. The identification of these peaks as Rossby waves is made possible by examining the energy spectrum in frequency-zonal wavenumber and frequency-meridional wavenumber diagrams. In fact, the modified Rhines scale turns out to be a useful measure of the dominant meridional wavenumber of the modulating Rossby waves; once this is fixed, apart from a spectral peak at the origin (the steady jet), almost all the energy is contained in westward propagating disturbances that follow the theoretical Rossby dispersion relation. Quite consistently, noting that the zonal scale of the modulating waves is restricted to the first few wavenumbers, the energy spectrum is almost entirely contained within the corresponding Rossby dispersion curves on a frequency-meridional wavenumber diagram. Cases when jets do not form are also considered; once again, there is a hint of Rossby wave activity, though the spectral peaks are quite muted. Further, the kinetic energy scaling in frequency domain follows a -5/3 power-law and is distributed much more broadly in frequency-wavenumber diagrams

physics.ao-ph↗