El Nino is an atmospheric-oceanic interaction phenomenon. Its prediction is done using hydrodynamical coupled ocean-atmosphere models and statistical models. The atmospheric component brings out speed and directional changes of the trade winds. The oceanic constituent triggers ocean upwelling resulting sub surface warming. Also, warming of the Pacific occurs as a result of weakening of trade winds that normally blow westward from South America towards Asia.
Nino indices are traced and tracked in the equatorial Pacific Ocean to establish sea surface temperature (SST's) anomalies. All four Nino indices cover an approximate sea surface of 17 million sq. km.
Under the Tropical Ocean Global Atmosphere Programme (TOGA), a number of Buoyes or equipment are deployed between 7degN and 7degS latitude in the Pacific Ocean. These Buoyes placed in array fashion, constantly record temperatures which are further verified with satellite derived data.
El Nino forecasts are made utilising blend of hydro and statistical techniques. Warm water on the western boundary of Pacific begins to drift eastward. This small perturbation decreases the temperature gradient which in turn subsides the Walker's Circulation. As trade winds decrease and Walker's Circulation diminishes, the temperature gradient starts shrinking. A +ve feedback loop is created.
Classification - El Nino
Standard El Nino: The eastern and central Pacific become warm and rainy; western Pacific becomes cooler and drier; warm equatorial water flows down the West Coast of South America. Commonly, Nino 3.4 warms up and travel to the coast. As and when, one month +ve SST anomaly of 0.5deg C or greater is observed in Nino 3.4 region and an expectation that the 3 month Oceanic Nino Index (ONI) threshold will be met, an El Nino 'watch' is issued. These oceanic conditions need to be qualified with an appropriate atmospheric response, typically associated with El Nino in the Pacific.
Modoki El Nino: During 'Modoki' El Nino , warming is observed more over central Pacific and cooling on either side over east and west Pacific over the equatorial region. Position of warming in the Pacific affect the 'Pacific Walker Cell'. Walker cell is an atmospheric bridge over which the parcel of air rises from its centre, travels on it and then climb down, on the other side. During 'Modoki', the ascending part of this cell is observed over central Pacific which gets shifted eastward during standard El Nino. This affects the position of the sinking part of the cell over Indian region. This descending part over the Indian region seem to be more dominant during 'Modoki' event and hence, rainfall in India gets reduced more than it does during Standard El Nino event.
Canonic El Nino: During 'Canonic' El Nino, eastern Pacific Ocean gets warmer, more than the central and western Pacific Ocean. When the El Nino is full blown, the west and central Pacific may even observe -ve SST anomalies. Data is rather sparse of such events and therefore the findings remain inconclusive. Year 2014 was 'Canonic' El Nino when Nino 1+2 region warmed up much earlier than the rest.
Current Conditions: Since December 2022, -VE SST anomalies have weakened across most of the equatorial Pacific Ocean. However, the SST is still below the average across Nino region. The marathon session of Triple Dip La Nina is likely to conclude soon. It has already weakened from its peak. Transition from La Nina to ENSO - Neutral is expected during Feb-Apr 2023 season. Eastern parts of the Nino region has warmed up faster than the rest, which still remain below the threshold mark of -0.5deg. No firm pattern is emerging as yet and best read is likely after the spring barrier gets over.
Climate Change
ENSO is earth's main source of year to year climate variability but its response to global warming remains highly uncertain. Possibly, all the naturally occurring climate events like El Nino/ La Nina/IOD/SAM now take place under the shadow of human induced climate change. Cooling influence of La Nina and warming phase of El Nino could be temporarily retarding or accelerating the rise in the global temperature but can not reverse the trend. Cooling of La Nina during 2020-2022 did not suffice and these years continue to be listed among the 10 hottest years on record.
Impact
El Nino is invariably linked with poor Indian southwest monsoon, the largest season from June to September. On an average, 80% of the El Nino years end up with 'below normal' rainfall and 60% of these lead to 'drought'. El Nino episode usually result in below average rainfall in palm oil producers: Indonesia and Malaysia. It could hurt crops in Thailand as well, the biggest rice exporter. El Nino can bring dry weather in Australia. The worst El Nino of 1997-98 was blamed for massive flooding along China's Yangtze river that killed over 1500 people.