A study on dynamic and thermodynamic aspects of breaks in the summer monsoon over India

Abstract

The rainfall associated with the Indian summer monsoon shows large intraseasonal and interannual variability. Break-monsoon conditions are one of the important epochs of the monsoon, and they contribute significantly to the intraseasonal variability of the monsoon. The National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis data sets are used to investigate the significant energy budget terms during the pre-break (5 days prior to the commencement of the break), break and post-break (5 days after the cessation of the break) periods. In the present study, certain dynamic and thermodynamic characteristics of the monsoon circulation during break-monsoon conditions are investigated. The important terms in the various energy budget equations are analysed between the surface and 100 hPa for the break and its departures from pre- and post-break for the period 1968-96. The statistical significance of these departures is also examined by Student's t-test at the 95% confidence level. The volume integral of the budget terms is also examined in four sectors, i.e. the Arabian Sea, Bay of Bengal, northern India and central India. Significant changes in the wind field and vorticity at 850 hPa take place in the monsoon trough zone, coastal regions of the western coast of India and the southwestern Bay of Bengal off the southern Indian coast. The vertically upward rising arm of the Hadley cell weakens during the break phase. The strong flux convergence of kinetic energy in the central Arabian Sea and flux divergence in the northeastern Bay of Bengal weakens during pre- and post-break periods. Significant changes in the diabatic heating horizontal flux of heat and moisture are observed in the monsoon trough zone, central and northwestern Bay of Bengal. The Bay of Bengal and central India sectors show higher magnitudes and changes in respect of dynamic and thermodynamic parameters compared with the Arabian Sea and northern India.