Indian Summer Monsoon Activity Research Articles

Characteristics of low clouds over the Arabian Sea

AbstractThis paper studies climatically important low clouds (with cloud top below 680 hPa) over the Arabian Sea using the Kalpana‐1 satellite data and International Satellite Cloud Climatology Project cloud data. Characteristics and possible mechanisms behind low‐cloud formation in the summer monsoon season are presented. The dominant lower tropospheric circulation over the Arabian Sea during the summer season is the monsoon low‐level jet (LLJ). Low clouds are predominantly found on the exit portion of the LLJ with decelerating winds. We postulate that the moisture carried by the LLJ from the entrance region with accelerating winds converge at the exit region with decelerating winds. This low‐level convergence (observed ~925 hPa level) may be conducive for moisture convergence, parcel uplifting, and low‐cloud formation. These clouds are unable to grow vertically due to the presence of lower tropospheric thermal inversion. Change in the spatial extent of low‐level convergence is found to influence the spatial coverage of the low clouds. It is found that low cloud cover is inversely related to the Indian summer monsoon activity. It is speculated that the changes in the strength of the lower tropospheric stability and spatial extent of convergence may be possible causes behind the observed inverse relation between low cloud cover and monsoon activity. Based on Clouds and the Earth's Radiant Energy System data, we find that these low clouds exert a peak cooling of about −55 Wm−2.

Summer temperature and summer monsoon history on the Tibetan plateau during the last 400 years recorded by tree rings

Global circulation models predict an increase of summer monsoon precipitation in High Asia as a consequence of global warming. The shortness of available meteorological records requires the reconstruction of past climate variability. However, high‐resolution climate proxy records from the Tibetan plateau are scarce and of limited spatial representativeness. Here we present first evidence of increased summer monsoon intensity from the Tibetan plateau based on reconstructions of late summer (August and September) temperature and rainfall from a network of 22 maximum latewood density (MLD) chronologies of high‐elevation conifer sites. After 1980, a decrease in MLD points to an increase of Indian summer monsoon activity in southern Tibet unprecedented during the past 350 years.

Remotely and Regionally Forced Pre‐Monsoon Signals Over Northern India and Neighbourhood

AbstractA detailed diagnostic examination of the National Centre for Environmental PredictiodNational Centre for Atmospheric Research Reanalysis data supplemented by a theoretical study is undertaken with the goal of investigating the dynamics of pre‐monsoon stationary wave anomalies occurring over northern India and adjoining region prior to weak Indian summer monsoons. It is seen from the analysis that the rotational wind anomalies preceding weak monsoon years are primarily forced by tropical divergent wind anomalies located over the tropical central‐eastern Pacific ocean, south Indian ocean and locally over north‐west India. the divergent wind anomalies over the central‐eastern Pacific ocean can be linked to anomalous changes in the tropical Pacific sea surface temperature arising due to El Nioñ/Southern Oscillation (ENSO). the upper level divergent wind forcing overlying the south Indian ocean seems to be locally connected to the warm sea surface temperature anomalies over that region.Results of simple experiments using a forced divergent barotropic vorticity model establish that the maintenance of rotational wind anomalies over north India prior to weak monsoons is attributable to the existence of anomalous divergent wind forcing over the aforementioned regions in the tropics. A normal mode experiment shows that the climatological basic flow alone is not sufficient to support these pre‐monsoon vorticity perturbations. Furthermore, calculation of vorticity generation terms suggests that the divergent wind advection and Rossby source terms make significant contributions to the production of rotational anomalies over north India during the pre‐monsoon months. Based on the above results, it is conjectured that energy injection into the subtropics via Rossby wave dispersion could be a foremost dynamical mechanism which efficiently sustains rotational wind perturbations over north India and neighbourhood. an assessment of the relative importance of the ENSO and non‐ENS0 divergent forcings, shows that the latter tend to exert a larger influence on the pre‐monsoon signals over north‐west India.The implications of using the pre‐monsoon signals for long range forecasting of Indian summer monsoon activity are determined by two factors viz., (i) the robustness and intensity of the pre‐monsoon signals and (ii) the persistence of the signals during the following months. the first factor in turn depends on the manner in which the different forcing anomalies co‐ordinate. the amplitude of the pre‐monsoon signals can be large (small) depending on whether the ENSO and non‐ENSO boundary forcings act in a mutually co‐operative (competitive) manner. the second factor, which is related to the maintenance of anomalous quasi‐stationary patterns during the following months, depends crucially on how the different boundary forcings evolve during the summer monsoon season.

North-Eastward propagating 25–50-day mode and its link with Indian summer monsoon activity

Spatio-temporal characteristics of the 25–50-day oscillations are investigated using the empirical orthogonal function (EOF) decomposition and spectral analysis with the Maximum Entropy Method (MEM). Daily pressure values over India during 1978 are used in this study. Power spectra of the temporal coefficients of the two leading EOFs show the existence of a low frequency oscillation with a period range 25–50-day over all India. An analysis using extended empirical orthogonal function (EEOF) indicates North-Eastward propagating 25–50-day mode. The analysis EOF has allowed to establish a relationship between the 25–50-day oscillation and the activity of the summer monsoon. The North-Eastward propagation of this mode is also confirmed by the simple EOF analysis.

Characteristics of certain surface meteorological parameters in relation to the interannual variability of Indian summer monsoon

With an objective to understand the influence of surface marine meteorological parameters in relation to the extreme monsoon activity over the Indian sub-continent leading to flood/drought, a detailed analysis of the sea level pressure over the Southern Hemisphere and various surface meteorological parameters over the Indian seas is carried out. The present study using the long term data sets (Southern Hemispheric Sea Level Pressure Analysis; Comprehensive Ocean Atmospheric Data Set over the Indian Seas; Surface Station Climatology Data) clearly indicates that the sea surface temperature changes over the south eastern Pacific (El Nino/La Nina) have only a moderate impact (not exceeding 50% reliability) on the Indian summer monsoon activity. On the other hand, the sea level pressure anomaly (SOI) over Australia and the south Pacific has a reasonably high degree of significance (more than 70%) with the monsoon activity over India. However, these two parameters (SLP and SST) do not show any significant variability over the Indian seas in relation to the summer monsoon activity. Over the Indian seas, the parameters which are mainly associated with the convective activity such as cloud cover, relative humidity and the surface wind were found to have a strong association with the extreme monsoon activity (flood/drought) and thus the net oceanic heat loss over the Indian seas provides a strong positive feed-back for the monsoon activity over India.

Upper air circulation and thermal anomalies over India and neighbourhood vis-à-vis Indian summer monsoon activity

The three dimensional circulation and thermal anomaly features associated with droughts and floods India are examined using 20 year of upper wind data over India and neighbourhood. The analysis reveals that years of droughts (floods) in India are associated with cyclonic (anticyclonic) circulation anomalies and cold (warm) thermal anomalies in the troposphere between 500 hPa and 200 hPa over northwestern India. In drought years (flood years) upper tropospheric westerly (easterly) anomalies are observed in the lower latitudes in the months of May and June. Tibetan anticyclone shifted to east of its normal position in drought years , during the month of June. The correlation coefficient between the meridional component of the wind at 200 hPa over northwestern India and the summer monsoon rainfall found to be -0.72 which is significant.

Importance of Upper Tropospheric Thermal Anomalies for Long-Range Forecasting of Indian Summer Monsoon Activity

Upper tropospheric monthly mean values of geopotential height at selected Indian stations during the pre-months of April and May, and the monsoon months of June, July and August, have been used to study the interannual variation in the upper tropospheric thermal field for the years 1968–77. The 300–100 mb thickness anomalies averaged over these stations reveal that 1) the anomalies observed during the pre-monsoon period of April-May generally persist through the subsequent monsoon months of June, July and August; and 2) a warm (cold) anomaly over northern India during the pre-monsoon months is followed by above-normal (below-normal) monsoon rainfall activity. The study may be useful in long-range forecasting of Indian summer monsoon rainfall activity.