Down to Earth - - SCIENCE -

The dis­cov­ery of itcz cracked open a win­dow into global tele­con­nec­tions, which dic­tate the flow of mon­soonal winds. One such con­nec­tion is with El Niño Southern Os­cil­la­tion (enso). It denotes an in­ter­decadal, al­ter­nat­ing pat­tern of heat­ing and cool­ing in the Pa­cific Ocean. Dur­ing the heat­ing phase,El Niño is char­ac­terised by the heat­ing of the East­ern Pa­cific that ex­tends as a tongue to­wards the Cen­tral Pa­cific re­gion. “The heat­ing up of the East­ern Pa­cific causes a de­flec­tion in the path of the South-East­ern trade winds and sup­presses the In­dian mon­soon,” ex­plains Ra­jee­van. An ex­am­ple of this was the drought year of 2009. “The in­tense oceanic heat­ing to the east of the In­dian sub­con­ti­nent caused winds to gush east­ward to­wards the Pa­cific Ocean in­stead of South Asia.The mon­soon winds com­pletely missed the In­dian sub­con­ti­nent and a con­vec­tion zone was cre­ated over the ocean in­stead.This is what caused the se­vere deficit in 2009,” says R Kr­ish­nan,pro­gramme man­ager at the Cen­tre for Cli­mate Change Re­search Cen­tre at iitm.

Cli­ma­tol­o­gists be­lieve there is a strong cor­re­la­tion be­tween enso and the oc­cur­rence of drought years in In­dia (see 'El Niño ef­fect' on p28). A brief anal­y­sis of rain­fall records re­veals that eight of the 15 drought years ex­pe­ri­enced be­tween 1951 and 2014 co­in­cided with enso (see ‘Off to an­other bad start’, DownTo Earth, 16-30 June,2015).“This year, enso sig­nals have been the dom­i­nant sig­nals and that’s why they have fea­tured in all the fore­cast­ing mod­els across the world,”says Sury­achan­dra A Rao, as­so­ciate di­rec­tor of the Na­tional Mon­soon Mis­sion.

Cer­tain ob­ser­va­tions also show mon­soons that are nor­mal or even marginally above nor­mal dur­ing El Niño years.This has fu­elled spec­u­la­tion in re­cent years that the link be­tween enso and the In­dian mon­soon is weak­en­ing.But such spec­u­la­tion may be pre­ma­ture as enso is hardly the only fac­tor in de­ter­min­ing the strength of the mon­soon.An­other fac­tor is dif­fer­en­tial heat­ing be­tween the Ara­bian Sea and the Bay of Ben­gal in the In­dian Ocean, known as the In­dian Ocean Dipole (iod).It has been ob­served to oc­cur in alternate phases in the Ara­bian Sea and the Bay of Ben­gal.

iod and its as­so­ci­ated at­mo­spheric com­po­nent known as the Equa­to­rial In­dian Ocean Os­cil­la­tion (equinoo) must be given part of the credit for off­set­ting the im­pact of El Niño in cer­tain years,ac­cord­ing to Ra­jee­van. “We have seen that dur­ing El Niño years, if the Ara­bian Sea ex­hibits higher tem­per­a­tures than the Bay of Ben­gal, it can off­set the mon­soon-sup­press­ing ef­fect of enso to a cer­tain de­gree. But if the Bay of Ben­gal heats up more than the Ara­bian Sea dur­ing an El Niño event,it can fur­ther re­duce mon­soon rains in In­dia,” says Ra­jee­van. Gadgil be­lieves it is equinoo that neu­tralises the ef­fect of El Niño in cer­tain years.The mon­soon of 1997-98 is a clas­sic ex­am­ple of this,she says.(See ‘Un­mask­ing the mon­soon’on p18.) While the con­ver­gence zone is crit­i­cal to rain­fall in In­dia,it is also re­spon­si­ble for the in­trasea­sonal vari­abil­ity. Since the mon­soon was first stud­ied, cycli­cal pat­terns of al­ter­nat­ing mon­soonal strengths within a sea­son have been ob­served.The most crit­i­cal of th­ese are the three-nine days’cy­cles of “ac­tive” and “break”phases ob­served as a kind of reg­u­lat­ing mech­a­nism. Th­ese phases are caused due to the move­ment of the con­vec­tive sys­tem aris­ing out of the con­ver­gence zone over the In­dian Ocean, par­tic­u­larly along the Bay of Ben­gal.

While ac­tive phases are char­ac­terised by deep de­pres­sions and cy­clonic storms, es­pe­cially over the Bay of Ben­gal, no such sys­tems de­velop dur­ing break phases.Break phases oc­cur due to a north­ward prop­a­ga­tion of the con­vec­tive cloud zones due to move­ments of the itcz. It is char­ac­terised by rain­fall over Hi­malayan foothills and in the south­east­ern part of the penin­sula, while the rest of cen­tral In­dia and the core mon­soon zone in the sub­con­ti­nent ex­pe­ri­ence drier con­di­tions. “Breaks oc­cur when air in the up­per tro­po­sphere de­scends to dis­turb mon­soon cir­cu­la­tion.The up­per tro­po­sphere is driven by global con­nec­tions, al­though the mech­a­nisms are un­clear which makes con­clu­sive pre­dictabil­ity im­pos­si­ble,” ex­plains J Srini­vasan, chair­per­son of the Divecha Cen­tre for Cli­mate Change at the In­dian In­sti­tute of Sci­ences in Ben­galuru.

Un­for­tu­nately, the pre­dictabil­ity of th­ese short du­ra­tion synop­tic cy­cles is most cru­cial for farm­ers.

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