Surface meteorological parameters acquired during the field phase experiment, BOBMEX-99, for the stationary periods (SP I and II) of the ship ORV Sagar Kanya over Bay of Bengal have been analysed. Active and weak monsoon conditions were observed during the first and the second phase of the experiment respectively over India as well as over the stationary ship location. The phase mean sea surface temperature (SST) is found to be the same in both the phases, however large differences have been observed in the phase mean values of wind speed, mean sea level pressure, latent heat and momentum fluxes at air-sea interface. Synoptic scale monsoon disturbances formed only during the period of strong north-south pressure gradient over the Bay region. Events of prominent fall in SST and in the upper 15 m ocean layer mean temperature and salinity values during typical rainfall events are cited. The impact of monsoon disturbances on ocean-atmosphere interface transfer processes has been investigated.
Based on NCEP/NCAR reanalysis data, kinetic energy and momentum transport of waves 0 to 10 at 850 hPa level are computed from monthly mean zonal (u) and meridional (v) components of wind from equator to 90• N. Fourier technique is used to resolve the wind field into a spectrum of waves. Correlation analysis between All India Seasonal Monsoon Rainfall (AISMR) and energetics of the waves indicates that effective kinetic energy of waves 1, 3 and 4 around 37.5• N in February has significant correlation (99.9%) with the subsequent AISMR. A simple linear regression equation between the effective kinetic energy of these three waves and AISMR is developed. Out of 47 years ' (1958-2004) data, 32 years (1958-1989) are utilized for developing the regression model and the remaining 15 years (1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004) are considered for its verification. Predicted AISMR is in close agreement with observed AISMR. The regression equation based on the dynamics of the planetary waves is thus useful for Long Range Forecasting (LRF) of AISMR. Apart from the regression equation, the study provides qualitative predictors. The scatter diagram between AISMR and effective kinetic energy of waves 1, 3 and 4 around 37.5• N indicates that if the kinetic energy is more (less) than 5 m 2 s −2 , the subsequent monsoon will be good (weak). Stream function fields indicate that high latitude trough axis along 40• E (70 • E) leads to a good (weak) monsoon over India.
The southwest monsoon of 2002 has three unusual features viz., (i) delay in advance over westernmost parts of India by one month, (ii) absence of depressions/storms over Bay of Bengal and Arabian Sea and (iii) -51% departure of all India rainfall in July. These features of intra-seasonal variability have been studied in this paper. Apparent heat source (Q1) and apparent moisture sink (Q2) over Indian region have been estimated using daily NCEP-reanalysis data, for June - September 2002 to study the intra-seasonal variations of the secondary heat sources in relation to observed intra-seasonal variations of circulation and rainfall over India.The intensities of vertically integrated heat source < Q1 > and moisture sink < Q2 > are found to coincide with the excess rainfall zone over India in June 2002, whereas the intensities of < Q1 > and < Q2 > over central parts of the country in July 2002 are not comparable. This indicates absence of convective rainfall and increased sensible heating over India in July. The x-t diagram of heating and drying rates at 500 hPa along 22.5° N shows large positive heating and drying rates of the order of 16° C per day over the western trough region around 26 June. Following this event, low positive or negative values are observed over central India in July.The y-t diagram of vertical velocity at 500 hPa shows strong subsidence near the foot hills of Himalayas during the season. This has affected the formation of depressions or storms over Bay of Bengal and Arabian Sea. Adiabatic and diabatic heating as well as drying in the troposphere has led to the break-like situation over India in July 2002 and weaker monsoon circulation during August and September. The analysis has brought out the impact of northern hemispheric mid-latitude circulation on intra-seasonal variability of southwest monsoon 2002 more clearly.
The role of low-frequency oscillations in the northward movement of the Equatorial Trough during pre-monsoon period over extreme south peninsular India is studied in relation to the associated enhancement of meso-scale convective activities over the region. Main objective of the present study is to bring out usefulness of this information in predicting the onset of summer monsoon over Kerala, well in advance. Thunderstorm data of selected stations over south peninsular India for the months March-June and for the years 1961-92 have been used in this analysis. A characteristic peak in weekly cumulative thunderstorm frequency is seen over these stations about 5-8 weeks prior to the onset of monsoon almost every year, suggesting the dominance of 30-60 day oscillation in the convection associated with these two events. A regression equation has been developed to predict the onset date. The relationship is found to be significant at 0.1% level. The results have been validated using an independent data set of five recent years. A characteristic fall in OLR values over the region, well in correspondence with the characteristic peak in thunderstorm activity suggests that OLR also can be used as a supporting tool in identifying such signal which is useful for the onset prediction.
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