Mousam, (19J3). 41. J.23 1·238

551.515 .1:551.511

Variations in intensit y and st ructu reof a westward-propagatingmonsoon depression

KSHUDIRAM SAHA

27. BsRoad, Maharani Bagh, Nell' Delhi (Illdla)and

SURANJA A SAHA

Developmen t Division, Nat ional M el . Center, Washingloll. D.C. (U.s.A.)

(Received 14 No vember / 990, M odified / 7 March /993)

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A BSTRACT. The study deals with a mo nsoo n dep ression which developed over the Bay of Bengal, movedwestward across central India and turned into a mid-tropospheric disturbance (1'.ITD) over the northea sternco rner of the Arabian Sea. Its interactions with the thermal fields associated with the basic monsoon, subtro­pical westerly troughs and a new depression oyer the Bay of Bengal arc examined. Evidence suggests the invol..vement of all the three factors in causing the observed varia tions in its intensity and structure. The low pressuresystem turned into a mid-tropospheric disturbance when it re-entered the wa rm sector of the basic monsoon fieldandreceived increased warm advection from the north to the west of its centreand cold advection from the southto the east in mid-troposphere. The importance of thermal advection is confirmed by co mputation of a heatbudget. The role of co ndensation heating is also briefly discussed.

Kc)' l'ords-Monson depression, w estward-propagating depression, Intensity of depression, Structure o fdepression.

2. Data and anab~h

Sea three days later (F ig. 1). The specific period studiedis from 25 through 27 June during which it interactednot only with the bas ic monsoon field over India but alsowith sub-tropical westerly troughs which moved cast.ward acro ss northern India. Possible effect of the entryof a fresh depression o ver the Bay of Bengal from thecast is also considered. An explosive upsurge of intensitya n~ a change of structure of the low pressure systemwhich occurred at 12 UTe On 27 June receive specialattention.

J. Introduction

Mon soon dep ressions which are low pressure systemsof moderate intensity (for details of oflicial classificationof low pressure systems over Ind ia and adjo ining seaareas. sec, e.g., Rao 1976. Saha et 01. 1981) usually formover the Head Bay of Bengal during the month' of J uneto August and move westnorthwestward into India.However, observations show that a major ity of thesedisturbances weaken or die soon after crossing the coastand in relatively few cases o nly the ir track extend s beyond80"E longitude (Mooley and Shukla 1989). The questionas to why a majority should die prematurely with in a The depression for med during the Arabian Sea phaseshort distan ce of th e east coas t of Ind ia while others are of the Monsoon Experiment (MONEX)-1979 andable to move on and reach the west coast of India or a hence specia l observations, both surface and upp.;r airlongitude even beyon d has not been addressed so far, were available from several observing platforms such a;to the best knowledge or the autho rs. The present st udy ships. instrumented aircrafts and satellites over this sea.deal s with a Bay of Bengal monsoon depre ssion which Unfortunately. the Bay of Bengal pha se of MONEX-1979after crossing the cast coast of India o n 24 June 1979 had not started till I July. so no special observationsmoved across centra l India and emerged overthe Arabian except satellite cloud imagery, were available while th~

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