On the maintenance of the mean monsoon trough over north India

The different terms in the equation for the balance of standing eddy kinetic energy are estimated in the region of the monsoon trough over north India. The main contribution to the maintenance of the trough against frictional dissipation is from the work done by the horizontal pressure forces. The contribution by the mean and transient advection terms is of a smaller order of magnitude. There is loss of standing eddy kinetic energy by the rising of cold air and sinking of warm air in the x-p plane in the lower troposphere. The circulation is forced by the flux of potentia1 energy from outside, mainly from the bottom and southern boundaries

On the vertical tilt of monsoon disturbancy

It is known that disturbances in the extratropics grow by the process of baroclinic instabilit

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

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 Nino 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 Nino 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

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

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

The four varieties of South Asian monsoon low-pressure systems and their modulation by tropical intraseasonal variability

South Asian monsoon low-pressure systems (LPSs) are synoptic-scale cyclonic vortices that produce substantial precipitation over the Indian subcontinent. In this study, a catalogue of four regional varieties of LPSs, which occur during June–September 1979–2018, is used for 4 examining the track statistics, thermal and moisture structure, and precipitation contribution of each variety. The modulation of LPS activity by tropical intraseasonal variability, which is commonly monitored by the Boreal Summer Intraseasonal Oscillation, Monsoon Intraseasonal Oscillation and Madden-Julian Oscillation indices, is investigated

On Identifying the role of Sun and the El Nino Southern Oscillation on Indian Summer Monsoon Rainfall

Open accessA solar influence on Indian Summer Monsoon (ISM) rainfall, identified in previous studies using the method of solar peak year compositing, may not be robust and can be influenced by other factors such as the El Nino Southern Oscillation (ENSO) and trends. Regression analysis, which takes into account variations across the whole solar cycle rather than just the minimum /maximum solar years, fails to detect any direct solar influence on the ISM during June–August. Regression suggests that the spatial pattern of ENSO, as imprinted in the sea level pressure in the Indian Ocean region, covering parts of Australia, has changed during the second half of last century. Thus ENSO impacts via variations in the local Hadley circulation may have played a role in modulating the ISM during that period. Finally, we discuss a possible indirect connection between the solar cycle and monsoon rainfall, which are different since the 1950s.Natural Environment Research Council (NERC

The effect of horizontal resolution on Indian monsoon depressions in the Met Office NWP model

Monsoon depressions are synoptic scale features that are responsible for a significant fraction of the rain over northern India during the summer monsoon season, and, as such, it is important to quantify their structure and behaviour in numerical weather prediction models. It is known that increasing model resolution is strongly correlated with improved forecasts in the short term and global circulation in the longer term, as well as better representation of tropical cyclones; here, we explore the sensitivity of depressions to changes in resolution using the Met Office Unified Model. Seven NWP case studies of depressions from 2013-15 were run at eight resolutions corresponding to equatorial grid spacing of between 16 and 208 km, and compared with data for the same events from TRMM and ERA-Interim reanalysis. We found that at the low resolution end of the spectrum, increases in resolution led to improvements in the composite structure, but with diminishing returns. The model also persistently overestimated the depression intensity, in particular the wind speed and the warm core aloft -- with the source appearing to originate in the mid-troposphere. The sensitivity of the diurnal cycle to resolution was also explored: the stratiform component was found to be very well represented by the model, whereas the convective component was described quite poorly. Improvement in most components of structure with increasing model resolution were marginal beyond N320 (63 km) and N512 (39 km) for dynamic and thermodynamic fields respectively