MAUS AM . 52 . I {January 200 1).37-.46

551. 5 15.12 (267)

Trends and fluctuations of the cyclonic systems over orth Indian Ocean

D.V. BHASK AR RAO. C. V. NAID U

and

B. R. SRINIVASA RAO

Department of Meteorology & Oceanography

Andhra University. Yisakhapa tnam. India

m< - 1877 ~ 1998 <1<li 'll\ 3l'I1il ili~ 'll'RIT'R ili iiffi it mIiirnii 'll\ 'llftrq;~ ili~ q;j fllffi'f'l iWllI .-Itlilil~ili ~ <l>lT ;,jf.'l~f.'ld(\IJit q;j 3lWlA 'IR"l ;); ft:I'1 fll;m 'l'I1 t I

~<Ji 3lT'!fu>ii il> WHIM~ 'llII '>«.! ~ 'fl .~ 'llII '>«.! ;); <:'RA ~ iiffi~ 'llII it"" ~Tiil ~ mIiirnii q;j 3IOflT~~ fll;m 'l'I1 t 1~~ ~ 11 qQ ili~ aItmiiili WI ,,"1I~ ~ ~ ~ ~ 'W"l ~ lTc-l ~ ~ <t\~ q;j tItIT <'I'1Tm 'l'I1 t I 1950 ;);111<: l¥"iR 'lC<ft ~ ~ \fi.P \l<><'l<!l '~~ fll'ir«<n t, I

2 1 'llII ~ 3!filqi ili '1iTC'I--.:;hii ili~ ili ~ -.rn;<IT<j <t\ 'lTftrili • ;); iliTOHiiii oi'I r.rt'1m~ ;); Jf1Ul'l ~ \l'l'Jffi <RIm 'l'I1 t, ~ lim ll"'J<;I 3IT'Iffi<I1Jit q;j tItIT <'flTT'l ;); fot ~~11

<ti;J1f\ (ENTROPY) ~ il> \J1nlTq ~l ~ 3!jW'l fllffiY(11f fll;m 'l'I11 2.2 ~ 2.8; 3.5 ~ 6.5~10 ~ 15 il'il q\j <!M~ 3IT'Iffi<I1Jit q;j tItIT <'I'1Tm 'l'I1 t ~ ;,j~ qI~ ili <:T<A ("'l..<ft .Jil l IRl1~ Gl1 m 3lT'!fu>ii ;); -"q it 'IRT VlT~ t, I

ABST RACT . The data of the monmty cyc lone frequencies over North Indian Ocean for the period 1H77·1998 hasbeen analys ed 10 slud)' the long-term trends ;lnd fluctuatices Analr.-is has been made separately feW" depres..ions andcyclones for the pre-monsoon. monsoon and post-monsoon seasons along with the ann ual frequencies. The wL1 W3S

subjected 10 11·)e3r mo ..ing averages end the epochv of increasing and dec reasing trend.. have been idcnuficd , Acco si srcm decreasing tr m.t after 1950s is a notable featu re.

The 11Ilk' series or the monthly cyclone frequency 'toloCle pOb~ throuPt 3 high-~ filler 10 ehrnmate periodspealer than :! I yeah and then subjected to spectrum an.:alysis using ~' 3.\ imum Enuopy Method 10 obum dominantpcrUldlClIlCS Three significant penoncuies .:al .! 2·:! ; J .5-6 ~ and 10-15 )'C3n. have teee Kk'llllfiC'tJ .... luch could heannbcted to QRO, E:-OSO and decadal frequencies

I. Introd uction

nderstanding the patterns of the frequ ency ofcyclonic systems over the tropical oce ans assumedimportance in the scenario o f global climate change . Inassociation with an increasi ng trend in the globa ltemperatures. it is of interes t to exa mine the frequency o fcyclonic systems. For the Indian region surro unded bythe North Indian Ocean. cyclonic systems provide mostof the annual ra infall esse ntial for agriculture and waterresources management . The frequencies o f dep ressions(maximum wind speed between 16-33 knots) is moreduring tbe summer monsoon season and prov ide well

distributed rainfall for a major part of the country which isuseful for agricultural operations . Cyclonic storms(maximum wind speed grea ter than 33 knots) . generallyform dur ing the pre and post-mo nsoo n seasons. causedevas tation but also prov·ide subs tan tial rai nfall 10 thecoastal region s useful for water resources mana gement .

In this background. a study o f the trend s andvariability in the frequency o f the cyclon ic sys tems forindivi dual Ocean basins is useful for understand ing thevario us effects of climate change. Recent studies reportcon flicting results such as that the global warmi ng maynot co ntribute for increase e ither in freq uency or intensit y

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