Ð In this paper, we examine the large-scale balances of kinetic energy, vorticity, angular momentum, heat and moisture over the Asian summer monsoon region. The ®ve year (1986±1990) uninitialized daily analyses for the summer season comprising June, July and August (JJA), produced at the European Centre for Medium Range Weather Forecasts (ECMWF) under the aegis of Tropical Ocean and Global Atmosphere (TOGA) have been considered to carry out the study.The following features characterize the Asian summer monsoon domain. It acts as the source of kinetic energy as well as vorticity, and sink of heat and moisture. Kinetic energy and vorticity are produced in the monsoon region and transported horizontally. On the contrary, heat and moisture are transported into the monsoon region. The zonal and meridional components of adiabatic generation of kinetic energy contribute to the production of kinetic energy over the Arabian Sea and Bay of Bengal, respectively. The horizontal advection of relative vorticity is balanced by sub-grid scale generation. The angular momentum generated due to pressure torque (east-west pressure gradient) is balanced by the¯ux convergence of omega momentum. Further, the angular momentum budget delineates that¯ux convergence of relative momentum is necessary to maintain the surface westerlies against the friction. The horizontal convergence of heat and moisture facilitates enhancement of diabatic heating, and also leads to the formation of diabatic heat sources, which are crucial to sustain the summer monsoon circulation.

The motivation for this study stems from the fact that there are no uniform criteria to identify the onset of the summer monsoon at a particular location. Furthermore, proper understanding

An endeavour is made to distinguish the mean summer monsoon features during surplus and deficient monsoon seasons. Based on all-India summer monsoon rainfall, over 42 years , seven surplus and ten deficient monsoon seasons are identified. Making use of daily averaged (00 Z and 12 Z) reanalysis data sets from the National Center for Environmental Prediction-National Center for Atmospheric Research for the corresponding surplus and deficient monsoon seasons, the mean circulation characteristics and large-scale energetics are examined.The circulation features denote that the cross equatorial flow, low-level jet and tropical easterly jet are stronger during a surplus monsoon. Further, strong Tibetan anticyclonic flow characterizes a surplus monsoon. The large-scale balances of kinetic energy, heat and moisture show a significantly large quantity of diabatic heating, adiabatic generation of kinetic energy, and horizontal convergence of heat and moisture during the surplus monsoon season compared with the deficient state. The regions with statistically significant difference between surplus and deficient monsoon seasons are delineated by a Student's t-test at the 95% confidence level. The remarkable aspect noticed in this study is that the Arabian Sea branch of the monsoon circulation is more vigorous during a surplus monsoon season, whereas the eastern Bay of Bengal branch is stronger during a deficient monsoon. The various large-scale budget terms of kinetic energy, heat and moisture are found to be consistent and in agreement with the seasonal monsoon activity over India.

In this study, we elucidate the temporal characteristics of the onset and withdrawal of the Indian southwest monsoon, making use of the model integration and daily analyses of the National Centre for Medium Range Weather Forecasting, India. The onset of the Indian southwest monsoon over the Bay of Bengal is discernable by a gradual increase in the adiabatic generation of kinetic energy, while over the Arabian Sea it is first noticeable by a steep and abrupt increase of generation. The horizontal transport of heat indicates a convergence regime over the Bay of Bengal prior to onset, while over the Arabian Sea a convergence regime is indicated by a change from the divergence to the convergence regime. The withdrawal of the southwest monsoon is characterized by the horizontal transport of heat and moisture that evince a transition from the convergence to divergence regime; similarly, diabatic heating noticed during the active period changes to cooling. The withdrawal over the Arabian Sea is characterized by the divergence regime of the horizontal transport of moisture. This change precedes even the circulation changes over northwest India, which may be regarded as a precursor. The withdrawal is further supported by a monotonic decrease in the net tropospheric moisture over the Arabian Sea, followed by a similar change at land locations.

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The thermodynamic characteristics of the Asian summer monsoon are examined with a global analysis-forecast system. In this study, we investigated the large-scale balances of heat and moisture by making use of operational analyses as well as forecast fields for June, July and August (JJA), 1994. Apart from elucidating systematic errors in the temperature and moisture fields, the study expounds the influence of these errors on the large-scale budgets of heat and moisture over the monsoon region. The temperature forecasts of the model delineate predominant cooling in the middle and lower tropospheres over the monsoon region. Similarly, the moisture forecasts evince a drying tendency in the lower troposphere. However, certain sectors of moderate moistening exist over the peninsular India and adjoining oceanic sectors of the Arabian Sea and Bay of Bengal.The broad features of the large-scale heat and moisture budgets represented by the analysis/forecast fields indicate good agreement with the observed aspects of the summer monsoon circulation. The model forecasts fail to retain the analyzed atmospheric variability in terms of the mean circulation, which is indicated by underestimation of various terms of heat and moisture budgets with an increase in the forecast period. Further, the forecasts depict an anomalous diabatic cooling layer in the lower middle troposphere of the monsoon region which inhibits vertical transfer of heat and moisture from the mixed layer of the atmospheric boundary layer to the middle troposphere. In effect, the monsoon circulation is considerably weakened with an increase in the forecast period. The treatment of shallow convection and the use of interactive clouds in the model can reduce the cooling bias considerably.

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