Satellite derived sea surface temperature variability in the Bay of Bengal

Bay of Bengal is associated with disturbances like tropical cyclones and monsoon depressions during pre and post monsoon and southwest monsoon seasons respectively. The Sea Surface Temperature (SST) variability over the Bay of Bengal plays an important role in the genesis of these disturbances. Satellite based SST climatologies, though based on shorter duration of data, have enabled study of interannual variabilities of SST over the smaller regions of Bay of Bengal which are associated with different types of weather disturbances in different seasons. Interannual variabilities and recent trends in SSTs over different regions of Bay of Bengal have been presented using a reliable satellite based climatological data for the 14 year period from 1985-1998.The annual SST over the Bay of Bengal has risen at the rate of about 0.2°C /decade during the period from 1985-1998. Maximum rising trend of 0.71°C/decade has been found over south Bay of Bengal during June. Frequency of monsoon depressions has decreased considerably in recent years in spite of increasing SST trends over Bay of Bengal in southwest monsoon season.

On the onset of summer monsoon over India in relation to interactions of the monsoon stationary wave with transient baroclinic waves leading to monsoon cyclogenesis

The important problem of the early or late onset of summer monsoon over India is addressed in the present study and found to be related to the structure and behaviour of a monsoon stationary wave that forms over the region due to land-sea thermal contrast and interacts with travelling wave disturbances in the westerlies and the easterlies associated with the subtropical belt over Asia. Depending upon the type of coupling and decoupling that occurs between the interacting waves, monsoon advances towards India either slowly or speedily. Since northward-propagating monsoon depressions are found to accelerate the onset processes. the study carries out a detailed analysis of the interaction processes which give rise to such disturbances and determine their development and movement.

Rainfall distribution over Bangladesh stations during the monsoon months in the absence of depressions and cyclonic storms

Monsoonal rainfall plays an important role in the annual rainfall distribution over Bangladesh. It is generally believed that monsoon depressions and cyclonic storms significantly affect the rainfall distribution over Bangladesh during the monsoon months and their absence causes deficient rainfall during the individual monsoon months. This aspect has been examined by computing the average rainfall for 32 meteorological observatories of Bangladesh Meteorological Department during the period 1948.91 for those monsoon months which were free from depressions and cyclonic storms. It has been found that the absence of monsoon depressions and cyclonic storms is not the main factor which causes deficient rainfall and consequent drought conditions in the individual monsoon months over different stations of the country. All the stations in the country experienced normal rainfall conditions inspite of the absence of depressions and cyclonic storms in the monsoon season (June-September).

Secular variations and trends in the occurrence of monsoon synoptic systems and its impact on central India rainfall

Importance of monsoon depressions, Low Pressure Systems (LPS) and the number of LPS days on rainfall and hence indirectly on agriculture and hydrology, is well recognized. In this paper the pattern of annual variability in these systems have been examined using data from 1901-2000. The above mentioned parameters have been subjected to decadal analysis to detect presence of any regular pattern. An attempt has been made to find its tendency with time. Impact of these systems on central India rainfall has been determined and discussed. The study endorses the earlier findings that there is a decreasing trend in the frequency of depressions which has been compensated with increase in LPS days over Indian region in recent years. The rainfall over central India is more significantly related with a number of LPS days over Indian region.

Simulations of frequency, intensity and tracks of cyclonic disturbances in the Bay of Bengal and the Arabian Sea

The paper presents the results of simulation experiments conducted for the assessment of likely changes in the cyclogenesis pattern in the Bay of Bengal (BOB) and the Arabian Sea (AS) resulting from global climate change. Two experiments were performed, namely the ‘control’ (CTL) experiment in which the greenhouse gas concentration in the atmosphere was fixed as per 1990 levels and the ‘greenhouse gas’ (GHG) experiment in which an annual compound increase of 1% from 1990 onwards was introduced. CTL and GHG experiments of 20 years length were performed for the period 2041-2060. The model used is the regional climate model Had RM2 of the Hadley Centre of Climate Prediction and Research, U.K. The results have brought out some significant changes in the cyclogenesis pattern in the North Indian Ocean (BOB and AS). The most significant likely change is the increase in the frequency of post-monsoon storms in the Bay of Bengal. The experiments show an increase of about 50% in the post-monsoonal cyclogenesis by 2041-2060 as a result of increased greenhouse gas concentrations in the atmosphere. The frequency of monsoon depressions / storms in the BOB is likely to decrease considerably during June-August. Due to varying impacts in different seasons, the annual frequency of cyclonic disturbances may change marginally in the BOB. In the Arabian Sea, however the model has simulated a significant reduction in the frequency which may be halved by the period 2041-2060. The results show intensification of storms during May-June and September-November. The monsoon depressions during July-August are likely to become less intense. In GHG experiment most of the post-monsoon storms have a tendency to strike north Andhra-Orissa coasts whereas in CTL experiment the storms strike coast from Tamilnadu to south Orissa. Thus, the focus of post-monsoon storms in the BOB is likely to shift northwards from Tamilnadu-Andhra Pradesh coast to north Andhra Pradesh-south Orissa coast. Another important simulated change in storm tracks is that more number of pre-monsoon storms in the BOB may have a tendency to recurve north or northeastwards by 2041-2060.

Value addition in district level dynamical forecast during monsoon depressions and storms

ABSTRACT. The trials of district level forecasts yielded encouraging results during 2005 monsoon. The purpose of this paper is to document the methodology followed in the value addition during the periods of monsoon depressions and storms. The focus is on the use of Mean Sea Level (MSL) positions and the 850 hPa circulation features predicted by different model centres, especially the European Centre for Medium-Range Weather Forecasts (ECMWF). The ECMWF-predicted 72 hr MSL position of the monsoon depression centre was found to be significantly correlated to the actual position of the system and the central location of the realized rainfall zone associated with the system. Even the predicted location of the system at 850 hPa by the ECMWF has been found useful in identifying the districts that received heaviest rainfall associated with the monsoon systems.MM5 and T-80 – predicted locations of the system at 850 hPa yielded lower correlations with the location of the actual rainfall zone associated with the system. As ECMWF – predicted rainfall was not available the rainfall predicted by MM5 and T-80 were used in the computations of the correlations with actual rainfall amounts associated with monsoon depressions and storms. The correlations between MM5 and T-80 – predicted average and maximum rainfall associated with systems and corresponding actual were poor. Though it is not difficult to identify the districts that are likely to be affected by the heavy rainfall associated with monsoon depressions/storms, the prediction of exact rainfall amount for each district (beyond heavy, very heavy or exceptionally heavy categories) is difficult from the model outputs which makes such forecasts a very challenging task. Therefore, the value addition using other inputs such as satellite information, synoptic charts, climatology etc. are very useful in the prediction of rainfall amounts associated with monsoon systems.

Evolution of idealized vortices in monsoon-like shears: application to monsoon depressions

This study examines processes fundamental to the development of South Asian monsoon depressions using an array of integrations of an idealized convection-permitting numerical model. In each integration, a wave of initially small amplitude is subjected to a different amount of vertical and meridional wind shear, with temperature and moisture fields constructed according to realistic constraints. Based on the evolution of this disturbance into monsoon depression-like vortices, two features of the background environment emerge as important: the low-level gradient of moist static energy (MSE) and the low-level meridional shear. As the low-level MSE gradient steepens, the disturbance becomes stronger and produces more rain. This strengthening results from the interaction of the vortex with latent heat release by convection that is in turn organized by positive MSE advection in the northerly flow west of the vortex. In this region of advection, moister air from the north ascends along upward sloping isentropes, driving moist convection. The disturbance also becomes stronger with increasing meridional shear, which makes the environment more barotropically unstable. The absence of either of these two features of the background environment prevents substantial growth of the disturbance. Our results suggest that monsoon depression growth in South Asia is fostered by the coexistence of a strong low-level MSE gradient with strong meridional wind shear associated with the monsoon trough.