Met of­fi­cials mon­i­tor­ing IOD fac­tor

The Times of India (New Delhi edition) - - | POLITICS POLICY -

the likely ab­sence of El Nino and La Nina this year, met of­fi­cials are closely watch­ing an­other fac­tor — the In­dian Ocean Dipole (IOD) — to see how it im­pacts the cru­cial June-Septem­ber rains in the coun­try. IOD is the mea­sure of the tem­per­a­ture dif­fer­ence in sur­face wa­ters in the east and west equa­to­rial In­dian Ocean. IOD is in the pos­i­tive phase when wa­ters in the west are warmer, and vice-versa.

A pos­i­tive In­dian Ocean Dipole is seen to aid the mon­soon while a neg­a­tive one could ad­versely im­pact mon­soon. This year a weak neg­a­tive IOD is ex­pected to de­velop dur­ing the sec­ond half of the sea­son.

Al­though In­dian Ocean Dipole’s con­nec­tion with the In­dian mon­soon isn’t seen to be as strong as that of El Nino/La Nina, it is known to have af­fected rains in the past. In 1997, a strong pos­i­tive IOD over­rode the neg­a­tive in­flu­ence of an El Nino, re­sult­ing in nor­mal rains. year data shows, there are years when this re­la­tion­ship doesn’t hold.

How­ever, the mon­soon’s per­for­mance varies sig­nif­i­cantly dur­ing the neu­tral years as well. These vari­a­tions are a re­sult of many other lo­cal and large-scale fac­tors. Among the large scale fac­tors is the In­dian Ocean Dipole (IOD), which is ex­pected to be weakly neg­a­tive dur­ing the sec­ond half of this year’s mon­soon. A pos­i­tive In­dian Ocean Dipole phase is seen to gen­er­ally aid the mon­soon while a neg­a­tive phase could de­press rains.

An­other highly un­pre­dictable con­di­tion with sharp, al­though short, im­pacts on rain­fall is the Mad­den Ju­lian Os­cil­la­tion (MJO), a pe­ri­odic east­ward mov­ing weather dis­tur­bance close to the equa­tor. MJOs can de- press or en­hance rain­fall for a week or two, de­pend­ing on their po­si­tion and strength. Slow-mov­ing or sta­tion­ary MJOs can have longer im­pacts. A well-po­si­tioned MJO can in­vig­o­rate the mon­soon while its ab­sence tends to pro­long breaks in mon­soon rains. MJOs, how­ever, are very hard to pre­dict.

Fi­nally, the dis­tri­bu­tion and in­ten­sity of mon­soon rains comes down to the num­ber of low-pres­sure sys­tems and de­pres­sions com­ing in­land from the Bay of Ben­gal. Dur­ing ac­tive mon­soon pe­ri­ods, the fre­quency of these sys­tems are usu­ally high. On some oc­ca­sions, even winds from the north­west (west­ern dis­tur­bances) af­fect rain­fall.

The in­ter­play of all these fac­tors make mon­soon fore­cast­ing a highly haz­ardous pro­fes­sion.

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