Dynamics of the monsoon system covering onset and withdrawal phases including break monsoon

Abstract

In this work monsoon onset, monsoon period, monsoon withdrawal and break monsoon were studied in and around Bangladesh. The OOOOUTC rawinsonde data at Dhaka station was utilized to compute atmospheric energy components and their fluxes per unit mass from May to September in 1994, 1995 and 1996. Dynamicity of monsoon system was observed using these energy components and their fluxes. As was found before monsoon onset sensible heat as well as total energy of atmosphere per unit mass reached its highest value from lower to middle troposphere for all the years. lbis condition was prevailed till fIrst week of June. It established a heat low over the country and neighborhood that enhanced the monsoon onset. After onsetting monsoon period(June-August) started over the country and neighborhood. During this period sensible heat as well as total energy were fluctuated. Occurrences of rainfall increase and total energy decrease and vice versa. Latent heat content was highest from 2nd week of June to last week of August. Latent heat content was almost same for May and September and was lower than monsoon period. In withdrawal phase(September) sensible heat and total energy were lower than any other months. Zonal fluxes of energy components were fluctuated from positive to negative (east to westward) and vice versa from lower to middle troposphere. At the upper troposphere it was completely positive( eastward) for May. It was completely negative (westward) at all levels after fIrst week of June. And this condition prevailed up to last week of August. At the beginning of September again it was positive( eastward) for lower levels and at last week of September it was completely positive( eastward) for all levels. 1bis is the indication of monsoon withdrawal. Meridional fluxes of energy components were fluctuated at all levels for the study period. Seven cases of break monsoon were studied in 1994 and 19915. Out of them three cases occurred in 1994 and four cases occurred in 1995. Among seJen break cases break period is observed from 5 to 16 days. It is observed that zonalfluJes have no effect on break monsoon. During break monsoon meridional fluxes were ndgative(southward) at upper troposphere and positive(northward) at lower troposphere wicl less intensity.