• R N Keshavamurty

      Articles written in Journal of Earth System Science

    • Role of barotropic, baroclinic and combined barotropic-baroclinic instability for the growth of monsoon depressions and mid-tropospheric cyclones

      B N Goswami R N Keshavamurty V Satyan

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      A detailed barotropic, baroclinic and combined barotropic-baroclinic stability analysis has been carried out with mean monsoon zonal currents over western India, eastern India and S.E. Asia. The lower and middle tropospheric zonal wind profiles over western India are barotropically unstable. The structure and growth rate of these modes agree well with the observed features of the midtropospheric cyclones. Similar profiles over eastern India and S.E. Asia, however, are barotropically stable. This is attributed to weak horizontal shear, inherent to these profiles. The upper tropospheric profiles, on the other hand, are barotropically unstable throughout the whole region. The features of these unstable modes agree with those of observed easterly waves. The baroclinic and combined barotropic-baroclinic stability analyses show that the baroclinic effects are not important in tropics.

      Though the barotropic instability of the mean zonal current seems to be res ponsible for the initial growth of the mid-tropospheric cyclones, neither barotropic nor baroclinic instability of the mean zonal current seem to explain the observed features of the monsoon depressions.

    • Some aspects of the 30–50 day oscillation

      S V Kasture R N Keshavamurty

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      The structure and interannual variability of the 30–50 day oscillation over the Indian region have been studied during the monsoon season. The power spectra of the zonal component of wind show large power in the 30–50 day time scale. The oscillation has a meridional wavelength of about 25° latitude and a slow northward phase speed of about 0.7° latitude per day. The oscillation also has some interannual variability. The periods are somewhat longer during the drought years.

    • Moist baroclinic instability and the growth of monsoon depressions— linear and nonlinear studies

      V Krishnakumar R N Keshavamurty S V Kasture

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      The objective of this study was two-fold: the first to investigate the role of moist convection and nongeostrophic effects on the growth of the monsoon depressions using a linearized multi-level moist primitive equation (PE) model and quasi-geostrophic (QG) model with only vertical shear. The second was to study the nonlinear evolution, growth, movement and detailed energetics of the monsoon depressions using a nonlinear moist global spectral model.

      Our linear studies using both models revealed lower as well as upper tropospheric growing modes. For the lower tropospheric modes the shorter scales were found to grow faster. While the PE model showed faster growth rate for shorter scales, as compared to longer scales, the QG model showed less tendency for scale selection. The shorter scales in PE model had phase speeds ranging from 4 to — 1 ms−1 and in QG model from 8 to — 4 ms−1. The nongeostrophic effects were found to be, in general, important. One of the lower tropospheric modes with wavelength 2500 km was found to have many features similar to the observed monsoon depression of the Bay of Bengal.

      In the upper troposphere the PE model showed much faster growth rates compared to the QG model. Also the fastest growing mode with a doubling time of 2.5 days had a scale of 6000 km. This was shorter than the scale predicted in the QG model. This mode had many characteristics similar to the observed features of the monsoon upper tropospheric easterly waves.

      Using a nonlinear global spectral model, we simulated the monsoon depression around 21°N starting from an antisymmetric heating distribution (with respect to the equator) and with a specific vertical structure with and without basic flows. The model was integrated for a period of five days incorporating a simple form of cumulus heating. The simulated model disturbance showed a pronounced growth and a westward movement in the presence of cumulus heating. The detailed energetics calculations revealed that the baroclinic energy exchange is the primary energy exchange process and cumulus heating is the driving force for the generation of available potential energy.

    • Dynamics of upper tropospheric stationary wave anomalies induced by ENSO during the northern summer: A GCM study

      R Krishnan C Venkatesan R N Keshavamurty

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      Ensemble seasonal integrations are carried out with the COLA GCM, with a view to understand the dynamical connection between warm SST anomalies in the equatorial central-eastern Pacific Ocean and the upper level stationary wave anomalies seen during drought years over the Indian summer monsoon region. In addition, experiments with and without orography are performed in order to examine the role of the Himalayas in modulating the El Niño induced stationary wave anomalies over the summer monsoon region.

      The GCM simulations show a statistically significant weakening of the summer monsoon activity over India in response to the SST forcing in the equatorial Pacific Ocean. This weakening of the summer monsoon appears to be largely related to modifications of the local Hadley and Walker cells over the summer monsoon region. In addition, it is seen that the anomalous ENSO divergent forcing over the tropical Pacific Ocean can act as a potential source for Rossby wave dispersion. Here one finds the possibility of meridionally propagating Rossby waves, which emanate from the ENSO forcing region, to interact with the subtropical westerlies and generate anomalous highs and lows in the subtropics and extratropics. The quasi-stationary perturbations seen over west Asia, Pakistan and northwest India during drought years, seem to be generated by the above mechanism. An alternate mechanism that could be important for the persistence of the quasi-stationary perturbations seems to be based on the dynamic excitation of middle latitude normal modes which can extract energy from the zonally varying unstable basic flow.

      It is seen from the GCM simulations, that the Himalayan orography plays a crucial role in anchoring the El Niño induced extratropical westerly troughs far to the west in the high latitude belt. In the absence of orography it is seen that the ENSO induced extra-tropical cyclonic anomalies tend to intrude southward into the monsoon region thereby destroying the regional scale circulations completely. Another effect due to the Himalayas is to generate lee waves on the eastern side of the topographic barrier which encircle the globe in the subtropics and midlatitudes.

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