Intra-seasonal oscfffations of radio refractive index during southwest monsoon over India

The study deals with the spatial and temporal variations of intra-seasonal oscillations in radio refractive index during southwest monsoon season over India and islands over Indian seas. Average daily radio refractive index data from 1 June to 30 September and that of the individual years for the period 1969-1986 were subjected to harmonic analysis to investigate the contributions of various periodicities in monsoon radio refractive index. The inter-annual variability of various intra-seasonal oscillations have been studied for each 5° latitudinal strip from 50 oN to 30° N with the help of variance explained by various frequency modes for different years. Variance explained by 30-60 day and 10-20 day modes were studied in relation to monsoon performance. The northward and eastward propagation of30.60 day mod~ was noticed. The 10.20 day mode and seasonal mode dominate at latitudinal belts 5°N.10oN and 25°N-30°N respectively. Between 10°N and 25°N, both 30-60 day and 10-20 day modes occur.

Energy exchanges between ocean and atmosphere in relation to southwest monsoon performance over India

Energy fluxes over Indian seas have been computed for pre-monsoon and monsoon months of the years 1987 and 1988 using bulk aerodynamic equations with exchange coefficients which vary with wind speed and stability, The years 1987 and 1988 have bee~ chosen due to the constrasting nature of the performances of Indian summer monsoon during these years. Besides energy fluxes several other oceanographic parameters, viz., SST, sea-air temperature difference, 'Bowen ratio'and 'rate of wind work' have been computed and examined. The distributions of these parameters over Indian seas during 1987 and 1988 reveal several constrasting features. Some of these features provide predictive indications of monsoon performance.

PRESSURE FIELDS OVER INDIAN SEAS AND THEIR RELATIONSHIPS WITH INDIAN SUMMER MONSOON RAINFALL

Monthly ano mal ies of surface pressu re for quadrangula r grids over Indian Seas ha ve been computedulilisin& 15 )'~ars' I NCU data (1976-90). The relationships between the grid pressure anomalies and summermonsoon rainfall over India ha ve beendiscussed. Good correlations have bee n found between the anomalies ofMayand the monsoon rainfall.

Latitudinal distribution of water vapour over the Arabian Sea and Bay of Bengal using Bhaskara-II SAMIR data

The main purpose of the present work is to establish the reliability of the SAMIR-derived water vapour (WV) data over the Arabian Sea and the Bay of Bengal for the entire 18-month from January 1982 to June 1983 period of the in-orbit operation of the SAMIR system. The average latitudinal distributions of WV over the Arabian Sea and Bay of Bengal for different months, derived from the SAMJR data were found to be broadly consistent with the climatological data on WV from the coastal and island radiosonde stations. A significant latitudinal gradient in WV has been found during the northern winter months (Dec-Feb) with the highest value of 4-5 gm/cm2 near the equator and thc lowest value of about 2 gm/cm2 at about 20oN over the Indian seas. This gradient gradually decreases during the subsequent months and almost vanishes during the southwest monsoon months (Jun-Sep) when the WV has nearly uniform value of 4-5 gm/cm2 in the entire latitude range from the equator to 20oN over the Indian seas. Finally, it has been found that WV values over the Bay of Bengal are generally higher than those over the Arabian Sea at co-latitudinal positions. The implications of this result are discussed in the light of other considerations.

Oceanic response to cyclone moving in different directions over Indian Seas using IRG model

The IITM Reduced Gravity (IRG) ocean model is employed to investigate the influence of tropical cyclone moving in different directions in Indian Seas. Some of the observed storm tracks in the Arabian Sea and Bay of Bengal are considered which have northward and westward movement. Sensitivity study is carried out for initial position of the storm at (90° E, 10° N) and moving in different directions. For westward moving cyclones the right bias in the model upper-layer thickness deviation (ULTD) field disappears. In an another experiment of westward moving cyclone originating at different latitudes, the ocean response is found to be sensitive to the Coriolis parameter (f). The surface currents as well as ULTD reduce, as f increases. The amplitude and the wavelength of inertia gravity wave increase with decrease in f, in the wake of the cyclone. This study helps to determine the upwelling region arising due to movement of the cyclone.