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Int. J. Struct. & Civil Engg. Res. 2014 Balamurali Arumugam, 2014

STUDY ON POST-MONSOON IN TAMIL NADU

Balamurali Arumugam1*

1 M-545,EllisNagar west, Madurai-625016, Tamilnadu.

*Corresponding author:Balamurali Arumugam � balamurali.arumugam@gmail.com

ISSN 2319 – 6009 www.ijscer.com

Vol. 3, No. 2, May 2014

© 2014 IJSCER. All Rights Reserved

Int. J. Struct. & Civil Engg. Res. 2014

Research Paper

INTRODUCTION

The south-west monsoon has to withdraw fullyto create enabling conditions for the onset ofthe monsoon in reverse (or north-eastmonsoon). The ongoing disturbed weatheractivity over the Mumbai latitude would onlyprolong the sway of westerlies over large partsof the peninsula. The westerlies would have to

ultimately yield place for monsoon easterlies.This can take time, and is not expected tohappen until the disturbed weather activitymigrates to the Chennai latitude. The spinningup of a fresh ‘low’ over East-central Bay ofBengal on Tuesday may have onlycompounded matters. Normal date of onsetis October 20 (Wednesday) but ongoing

During the North-East-monsoon months of October to December, a different monsoon cycle,the northeast (or "retreating") monsoon, brings dry, cool, and dense Central Asian air masses tolarge parts of India. Winds spill across the Himalayas and flow to the south-west across thecountry, resulting in clear, sunny skies. Though the India Meteorological Department (IMD) andother sources refers to this period as a fourth ("post-monsoon") season, other sources designateonly three seasons. Depending on location, this period lasts from October to November, afterthe south-west monsoon has peaked. Less and less precipitation falls, and vegetation begins todry out. In most parts of India, this period marks the transition from wet to dry seasonal conditions.Average daily maximum temperatures range between 28 and 34°C (82 and 93°F). The north-east monsoon, which begins in September, lasts through the post-monsoon seasons, and onlyends in March, carries winds that have already lost their moisture while crossing central Asiaand the vast rain shadow region lying north of the Himalayas. They cross India diagonally fromnorth-east to southwest. However, the large indentation made by the Bay of Bengal into India'seastern coast means that the flows are humidified before reaching CapeComorin and rest ofTamil Nadu, meaning that the state, and also some parts of Kerala, experience significantprecipitation in the post-monsoon and winter periods. However, parts of West Bengal, Orissa,Andhra Pradesh, Karnataka and North-East India also receive minor precipitation from the north-east monsoon.

Keywords:Peninsula, Seasonal Transition, Onset, Monsoon

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cyclonic disturbances in the Arabian Sea andthe Bay of Bengal are seen fending offmonsoon easterlies from establishing over thesoutheast Coast. Onset of the north-eastmonsoon over Tamil Nadu and the rest of thepeninsula could be delayed by as much as aweek, according to experts.

ONSET CONDITIONS

According to IMD specifications,commencement of north-east monsoonassumes withdrawal of southwest monsoon upto 15º latitude, onset of persistent surfaceeasterlies over Tamil Nadu coast to requireddepth, fairly widespread rainfall over coastalTamil Nadu, South Coastal Andhra Pradeshand adjoining areas. These conditions arelikely to be met as the week progresses, withsurface easterlies notching up cruising speedby Friday. Rainfall would also have becomemore widespread over the peninsula by then.

WEATHER WARNING

A weather warning valid for Monday said thatisolated heavy rainfall would occur over CoastalAndhra Pradesh and South Coastal Orissa.The causative upper air cyclonic circulationover West-central Bay of Bengal and adjoiningCoastal Andhra Pradesh has been persistingover the past couple of days. This system couldbe the fulcrum around which the seasonaltransition of weather would come about overPeninsular India with westerlies gettingincreasingly replaced by north easterlies tomonsoon easterlies. Additionally, the IMD haspicked the crucial “shear zone” formation alongthe 15o latitude, which defines the playgroundfor northeast monsoon weather systems. Theshear line is a line or narrow zone across which

there is an abrupt change in the horizontal windcomponent; a line of maximum horizontal windshear.

CONVECTIVE CLOUDS

Scattered rain or thundershowers have beenforecast for Rayalaseema, remaining parts ofOrissa, Konkan and Goa. Satellite cloudimagery on Monday revealed the presence ofconvective clouds over parts of CoastalAndhra Pradesh and West-central Bay ofBengal. International Research Institute (IRI) forClimate and Society at Columbia Universityhas indicated wetter than average weather forRayalaseema, parts of Telangana, North andSouth Interior Karnataka and parts of North andInterior Tamil Nadu during the six days endingFriday. Other international models signalled tothe setting up of a strong “pulse” entering theBay of Bengal from upstream South China Sealater in the week. They also indicated that, onsetof northeast monsoon during the week may nothave the back-up of a wet phase of the periodicMadden-Julian Oscillation (MJO) wave asforecast earlier. The onset, if at all, wouldhappen on the monsoon’s own inherentstrength and dynamics. The MJO wave is nowforecast to settle over Equatorial Indian Oceanand adjoining peninsular seas aroundNovember 7. Transiting the upper levels of theatmosphere, the wave has been known to setup monsoon onsets, low-pressure areas,depressions and even cyclones. Given this, thenortheast monsoon could likely see a pick upafter November 7, according to as per variousMJO models surveyed. The wave, which hasimplications for ground weather, is seenparticularly strong during November 12 and 16.Two days into its onset, the north-east monsoon

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has been steadily consolidating its presencealong the country’s south-east coast. Theseasonal weather system has been ridingpiggyback on an upper level cycloniccirculation, which has since moved southwardalong the Tamil Nadu coast to a perch overSouth-west Bay of Bengal.

RAIN ALERT

According to the UK Met Office weather model,the system was expected to cross land andthe South Peninsula towards CoastalKarnataka over the next few days. A weatherwarning issued by India MeteorologicalDepartment (IMD) on Sunday said that isolatedheavy to very heavy rainfall would occur overCoastal Andhra Pradesh during the next twodays. It would be isolated heavy over TamilNadu, Interior Andhra Pradesh, South CoastalOrissa and Kerala during this period. The 24hours ending Sunday morning said thatwidespread rainfall was reported from CoastalOrissa and Andhra Pradesh while it was fairlywidespread over the rest of South PeninsularIndia.

‘LOW’ BREWING

The IMD said that the first full-fledged low-pressure area of the season may emerge overSouth Andaman Sea by Wednesday. Thiswould be remnant of a tropical depressionlocated across India’s territorial waters in theGulf of Thailand on Sunday. It was headed in awest-northwest direction to move for itsonward hop into the South Andaman Sea. Anumber of weather models surveyed—including Canadian Meteorological Centre, USNational Centres for Environment Prediction/Global Forecast System and European Centrefor medium-Range Weather Forecasts—

indicated its generally west-northwest track andintensification into a likely depression.

DIFFERING VIEWS

The models differed in their outlook for itslandfall, positing it along the Southeast Coastat any point from Central coastal Tamil Naduto the Coastal Andhra Pradesh. The US Navy’sNGP proved the odd model out by suggestingthat the system might track straight to the west,cross northern Sri Lanka and weaken butproceed to curl into extreme South IndianPeninsula. The International Research Institute(IRI) for Climate and Society at ColumbiaUniversity said in its six-day outlook endingThursday that very heavy recorded rainfall islikely over South Coastal Andhra Pradesh,Central Tamil Nadu coast and adjoining interior.

ABOVE NORMAL

The region around Chennai and South CoastalAndhra Pradesh may receive up to 60% abovethe normal rainfall during this six-day period, itsaid. ‘Wetter than normal conditions’ havebeen forecast for North Coastal and InteriorTamil Nadu. But the Climate Prediction Centreof the US National Weather Services indicatedthat monsoon north easterlies and easterliesover the Bay of Bengal would give way to aburst of westerlies turning south easterlies fromFriday. This might rob the Tamil Nadu coastsome of the rains, though they are expectedto fall over Kerala as the south easterlies mopup moisture from the Bay and blow in aswesterlies to north westerlies to complete thecirculation in the larger trough.

CLOUD IMAGERY

On Sunday, satellite imagery showedconvective clouds over parts of West-central,

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Northeast and Southwest Bay, South AndamanSea, Southeast Arabian Sea, Orissa, SouthChhattisgarh, Andhra Pradesh, Kerala andTamil Nadu. This is expected to triggerwidespread rain or thundershowers overCoastal Andhra Pradesh. It would be fairlywidespread over Tamil Nadu, Andhra Pradesh,Kerala, Coastal Orissa, South Chhattisgarhand Andaman and Nicobar Islands. Scatteredrain or thundershowers would occur over SouthKarnataka and Lakshadweep on Monday andincrease thereafter. Isolated rain orthundershowers would also occur over theNortheastern States on Monday and increasethereafter. An extended forecast until Fridaysaid that fairly widespread rainfall activitywould occur over Andhra Pradesh, Karnataka,Tamil Nadu, Kerala and Andaman and NicobarIslands.

CONCLUSION

Depending on location, this period lasts fromOctober to November, after the southwestmonsoon has peaked. Less and lessprecipitation falls, and vegetation begins to dry

out. In most parts of India, this period marksthe transition from wet to dry seasonalconditions. Average daily maximumtemperatures range between 28 and 34°C (82and 93°F). The northeast monsoon, whichbegins in September, lasts through the post-monsoon seasons, and only ends in March,carries winds that have already lost theirmoisture while crossing central Asia and thevast rain shadow region lying north of theHimalayas.

REFERENCES

1. Business Daily from THE HINDU groupof publications, Tuesday, Oct 26, 2010

2. Business Daily from THE HINDU groupof publications, Monday, Nov 01, 2010

3. Das M R, Mukhopadhyay R K, DandekarM M and Kshirsagar S R (2002), “Pre-Monsoon Western Disturbances inRelation to Monsoon Rainfall, ItsAdvancement over Northwestern Indiaand Their Trends” (PDF), CurrentScience, Vol. 82 , No. 11, pp. 1320–1321,retrieved on October 1, 2011