The weekly indices over the equatorial Pacific Ocean have remained unsteady, ranging between +0.2°C to -0.3°C. The Nino 3.4 index value has remained more than -0.5°C, except once on 28th October 2024. Under the guidance of CPC ENSO Outlook, the conditions favour the onset of La Nina but remain weak and brief, than other historical episodes. A weak La Nina would be less likely to result in conventional winter impacts.
It is highly unusual for the La Nina event to commence during the fall of the year. Since 1950, it happened only once in 2005, when the La Nina episode started with a modest ONI of -0.6°C in Oct-Nov-Dec and concluded by Feb-Mar-Apr 2006. The likely La Nina event of 2024 may get further curtailed. The event giving a complete amiss, remains a possibility. The objective IRI model-based ENSO Outlook forecasts indicate the continuation of ENSO -Neutral conditions for Nov-Jan 2025. Weak and brief La Nina episodes are intrinsically marred with the low accuracy of forecasts.
ENSO: During the last four weeks, equatorial SSTs were above average across the Western Pacific Ocean, around the Maritime Continent, across the Eastern Indian Ocean, and in most of the Atlantic Ocean. Near-to-below-average SST’s were evident in the east-central and eastern Pacific Ocean. Overall, a mix of positive and negative SST anomaly changes were observed, rather insufficient to draw conclusive inferences on the emergence of La Nina.
All the four Nino indices this week, have become either zero-zero or positive. Nino 3.4, the marker for ONI has once again become 0°C, after staying negative for the last 12 weeks. This is a big change and does not augur well for the onset of La Nina, anytime soon. The average value of ONI for the last 8 weeks is -0.3°C. To have an average of -0.5°C for a period of 12 weeks is an essential condition to begin the process of La Nina. In another 4 weeks time, the sun will start an equatorward journey from the Tropic of Capricorn. The equatorial sea surface may start warming up due to increased insolation and therefore, negate the build-up of La Nina. The longer it gets delayed, the lesser become the chances of a La Nina event.
IOD: The Indian Ocean Dipole is neutral. The IOD index for the week ending 17th Nov was -0.73°C, making it the 6th consecutive week below the negative threshold (-0.4°C). To be classified as a negative IOD event, the index needs to be at values below the negative IOD threshold for a sustained period of at least 6-8 weeks. It is a matter of another one week or so, when the BoM may announce the negative IOD event in vogue. A negative IOD at this point in time does not sound well for winter monsoon rains over the South Peninsula.
MJO: The Madden-Julian Oscillation pulse was fairly active over the Indian Ocean during this week. It has gained amplitude and portends well for an increase in convection over the Indian Seas. The tropical cyclone ‘Fengal’ is evolving over the South/ Southwest Bay of Bengal. MJO is likely to propagate eastward rapidly, over the Maritime Continent, during the last phase of November. An uptick in the northeast monsoon activity is expected over the South Peninsula for the next 4-5 days.
This has been an action-packed week for the oceanic parameters. Nino indices turned warmer all across the equatorial central Pacific. A negative IOD event appears inevitable and may find an announcement soon. The MJO has brought back the stormy conditions in the Indian Seas. The next two weeks are going to be interesting, impending conclusive communique on the status of La Nina and IOD.