The first two chapters cover the design and operation of the ore microscope and the preparation of po

atmospheric science lecture

1. Monsoon
By
Prof. Nagaraju Chilukoti
Department of Earth & Atmospheric Sciences
NIT Rourkela
Introduction
to
Atmosphere & Ocean Sciences

2. Introduction
❖Theterm‘Monsoon’isderivedfromtheArabic/Hindiword
‘Mausam’forseason.ThetermwasfirstusedbyArabsailorsfor
thelargevariationinthedirectionofthewindswithseason,
observedovertheArabianSea.
❖Systematicobservationsoftheatmospherictemperature,pressure,
rainfalletc.overtheIndianregionareavailableforoveracentury.

3. ❖Withtheanalysisofthedataofthefirstfewyears,alucidaccount
ofthenatureofrainfallanditsvariabilitybyBlanfordwas
publishedin1886.
❖SincethenourknowledgeabouttheweatherandclimateofIndia
hasincreasedcontinuouslywiththeeffortsofmanymonsoon
meteorologists.
Introduction

4. ❖Withtheadventofmeteorologicalsatellitesinthemid-sixties,there
wasaquantumjumpinourunderstandingofthenatureofthe
systemresponsibleforthemonsoonandofthelinksoftheIndian
monsoontoeventsoverthesurroundingseasandtheotherpartsof
thetropicalbelt.
❖Observationsofthetropicaloceansandtheatmosphereabovewith
oceanbasedplatforms(buoys)establishedinthe90shavefurther
addedtoourknowledgeofthemonsoonandits‘teleconnections’.
Introduction

5. ❖Thusmuchofourcontinuouslygrowingknowledgeofthemonsoonhas
beengainedempirically.
❖Suchanindepthknowledgeofthenatureofthesystemisessentialfor
modelingandhencepredictingthemonsoon.
❖However,forthebillionsinhabitingthemonsoonalregionsofthe
world,themarkedvariationoftherainfallwiththeseasonisoffar
greaterinterestthantheseasonalvariationofthewind.
Introduction

6. ❖Understandingandpredictingthemonsoonanditsvariabilityis
oneofthemostchallengingproblemsinatmosphericscience
today.
❖Themonsoongovernsthepulseoflifeforthemillionsofhumans
aswellasthefloraandfaunalivingontheIndiansubcontinent.
❖UnderstandingandpredictingthevariabilityoftheIndian
monsoonisveryimportantbecauseithasalargeimpactonthe
agricultureandeconomyofthecountry.
❖Inthecolonialera,theIndianeconomywasdescribedasa
gambleonthemonsoonrains.
Introduction

7. ❖Evenaftermorethansevendecadesofdevelopment,themonsoon
continuestohavealargeimpactonoureconomy.
❖Thustheimportanceofidentifyingandadoptingstrategieswhichare
appropriateforthemonsoonvariabilityweexperiencecannotbe
overemphasized.
Introduction

8. ❖Considerfirst,thevariationoftheobservedmean(averaged
overaboutahundredyears)monthlyrainfallaveragedoverthe
Indianregion.
❖OvertheIndianregion,thereishardlyanyrainfallduring
DecembertoAprilandmostoftherainfalloccursduringJune-
Septemberwhichisthesummermonsoonseason.
Introduction

9. The variation of the observed mean monthly rainfall (averaged
over 1901-2012) on the country as a whole
Source: https://www.ijert.org/research/impact-of-climate-variability-over-mahanadi-river-
basin-IJERTV3IS071028.pdf
Introduction

10. ❖InIndia,thewordmonsoonincommonparlancereferstothe
systemthatvisitsthecontinenteveryyearandgivesusrain.
❖Thecommencementofthe‘rainyseason’isassociatedwiththe
onsetofthemonsoonandtheendoftherainyseasonwiththe
retreatofthemonsoon.
❖Thelifeofplants,animalsaswellashumanbeingsontheIndian
subcontinent,istunedtotheseasonalcycle(Rutuchakra)ofthe
rainfallassociatedwiththeannualvisitofthemonsoon.
Introduction

11. Introduction

12. TheChataka,thepiedcrestedcuckoo,drinksonlyrain-water.
Chataka’sintenseyearningfortherainsisoftencomparedwith
piningbyhumansfortheirlovedones
Introduction

13. Thefinalretreatofthemonsoonfromthesoutheasternpartsof
theIndianPeninsulaiscelebratedwiththePongalfestival
Introduction

14. ❖Sofarwehaveseenhowlifeonthesubcontinentrevolvesaroundthe
meanseasonalcycle.
❖Theseasonalcycleitselfisnotthesameoverdifferentpartsofthe
country.
❖Thusthereisaspatialvariationinthedatesoftheonsetofthe
monsoonaswellasitsretreat.
❖Forstudyingthenatureoftheobservedrainfallanditsvariability
overtheIndianregionrainfalldataforalargenumberofstations
withmodernraingaugesfortheIndianregionareavailablefrom
1876onwards.
Introduction

15. ❖Thesehavebeenextensivelyanalyzed.Monthlydataforaverages
overtheIndianregionaswellasoverthemeteorological
subdivisionsareavailablefromthewebsiteofIndianInstituteof
TropicalMeteorology.
❖Inaddition,agriddeddatasetatonedegreeandhigher
resolutions,preparedbyRajeevanet.al.(Rajeevan,M.,J.Bhate,
J.D.Kale,andB.Lal,Highresolutiondailygriddedrainfall
datafortheIndianregion:Analysisofbreakandactivemonsoon
spells,CurrentScience,2006,91,3,296–306)isnowavailable
fromtheIndiaMeteorologicalDepartment(IMD).
Introduction

16. Rainy Season
TherainyseasoncommenceswiththemonsoononsetoverKerala(MOK)whichis
recognizedasarapid,substantialandsustainedincreaseinrainfallasseenfrom
theplotofthemeandailyrainfall(1901to1980)ofsouthKeralawithrespectto
MOKas0day

17. Intheonsetphaseofthemonsoon,whichlastsforabout45days,the
monsoonadvancesnorthwardoverthesubcontinentfromits
commencementoverKeralatoitsculminationoverthenorth-western
partofIndia.

18. Rainy Season

19. TheretreatofthemonsooncommencesinSeptemberfromthenorth-western
partswiththefinalphaseoverthesouthpeninsulacompletedbymid-
December.
Rainy Season

20. IMD onset and withdrawal
(old dates)

21. ❖Itisseenthattheentirecountrycomesundertheswayofthe
monsoonattheendoftheonsetphase,aroundtheendofJune.
❖Themonsoonbeginsitsretreatfromthenorthwesternparts
aroundthebeginningofSeptember.
❖JulyandAugustare,therefore,consideredpeakmonsoonmonths.
BythefirsthalfofOctober,themonsoonisrestrictedtothe
peninsulasouthof15N.
Rainy Season

22. ❖Asnotedearlier,forthecountryasawhole,mostoftherainfall
occursduringthesummermonsoonseasonJune-Septemberand
thefocusofmostofthestudieshasbeentheIndiansummer
monsoon.
Rainy Season

23. Onset & Advance of Monsoon
Theguidelinestobefollowedfordeclaringtheonsetofmonsoonover
Keralaanditsfurtheradvanceoverthecountryareenlistedbelow:
a)Rainfall
Ifafter10thMay,60%oftheavailable14stationsenlisted*,viz.
Minicoy,Amini,Thiruvananthapuram,Punalur,Kollam,Allapuzha,
Kottayam,Kochi,Thrissur,Kozhikode,Thalassery,Kannur,Kudulu
andMangalorereportrainfallof2.5mmormorefortwoconsecutive
days,theonsetoverKeralabedeclaredonthe2ndday,providedthe
followingcriteriaarealsoinconcurrence.

24. b) Wind field
Depthofwesterliesshouldbemaintainedupto600hPa,inthebox
equatortoLat.10ºNandLong.55ºEto80ºE.Thezonalwindspeed
overtheareaboundedbyLat.5-10ºN,Long.70-80ºEshouldbeof
theorderof15–20Kts.at925hPa.Thesourceofdatacanbe
RSMCwindanalysis/satellitederivedwinds.
c)OutgoingLongwaveRadiation(OLR)
INSATderivedOLRvalueshouldbebelow200wm
-2
inthebox
confinedbyLat.5-10ºNandLong.70-75ºE.
Onset & Advance of Monsoon

25. NorthernLimitofMonsoon(NLM)
SouthwestmonsoonnormallysetsinoverKeralaaround1st
June.Itadvancesnorthwards,usuallyinsurges,andcoversthe
entirecountryaround8
th
July.TheNLMisthenorthernmost
limitofmonsoonuptowhichithasadvancedonanygivenday.
Onset & Advance of Monsoon

26. Withdrawal of SW Monsoon
a)Withdrawalfromextremenorth-westernpartsofthecountryisnot
attemptedbefore1stSeptember.
b)After1
st
September:
Thefollowingmajorsynopticfeaturesareconsideredforthefirst
withdrawalfromthewesternpartsofNWIndia.
i)Cessationofrainfallactivityovertheareaforcontinuous5days.
ii)Establishmentofanticycloneinthelowertroposphere(850hPaand
below)
iii)Considerablereductioninmoisturecontentasinferredfrom
satellitewatervapourimageriesandtephigrams.

27. Further Withdrawal from the Country
❖Furtherwithdrawalfromthecountryisdeclared,keepingthe
spatialcontinuity,reductioninmoistureasseeninthewater
vapourimageriesandprevalenceofdryweatherfor5days.
❖SWmonsoonisfromthesouthernpeninsulaandhencefromthe
entirecountryonlyafter1stOctober,whenthecirculationpattern
indicatesachangeoverfromthesouthwesterlywindregime.
Withdrawal of SW Monsoon

28. Monsoon Activity

29. Monsoon Activity

30. JJAS rainfall climatology (mm/day)

31. ❖Itisseenthatthemeanrainfallpatternduringthesummer
monsoonischaracterizedbyamajorrain-beltoverthe‘monsoon
zone’(markedbytheredlineinthelastslide),withtherainfall
decreasingnorthwestwardfromthemaximumovertheeastern
part.
❖Inaddition,heavyrainfalloccursalongthewestcoastofthe
peninsulaandoverthenortheasternregion.
Monsoon Activity

32. Thevariationofthemeanall-Indiarainfallduringtheseasonisstrikingly
similartothatofthemeanmonsoonzonerainfall.

33. The topography of the region plays an important role in the very high rainfall over the
northeastern region and over the Western Ghats & the west coast of the peninsula.
AlthoughthefocusofmostofthestudiesoftheIndianmonsoonhasbeentheIndiansummermonsoon,
thesummermonsoonisnotthemajorrainyseasonfortheentirecountry.Considerthemeanmonthly
(i.e.averageover30ormoreyears)rainfallatstationsacrosstheIndianregionnextslide

34. Monthly Mean Rainfall (cm) at some stations

35. Thank you