The consistent rise in the sea temperature on account of El Nino warming seem to be increasing the sea level in the eastern equatorial Pacific Ocean. Thermal expansion of the warm waters in this part of the Pacific basin, resulting change in the sea level is being monitored constantly and measured by the satellite sensors. Variations in the sea level are reliable indicators of emergence and cessation of both, El Nino and La Nina.
ENSO: In October, November and early December 2023, Sea Surface Temperature (SST) anomalies continued to increase in Nino 3.4 region. However, in the last week, SST anomalies have weakened across most of the Pacific. While the drop in temperature over Nino 4 and Nino 3.4 may be marginal, but Nino 1+2 has plunged below 1.5°C, first time since March 2023. Climate Prediction Center ENSO probabilities of December 2023 continue to wield spike of El Nino till the quarter Feb-Mar- Apr and starts tapering thereafter. Notwithstanding, model read around the ‘spring barrier’ is always feared for low accuracy and therefore treated with caution.
The intensity of 2023 El Nino had a steady rise. Apparently, it has reached its peak. It may retain these peak values for another 4-6 weeks before mellowing down, around the commencement of spring 2024. El Nino 2023 at one time had semblance of Super El Nino, but the intensity does not quite match that of the largest events in recent decade. In the previous events of 1997 and 2015, the sea level were much higher and these high levels ( depicted by red & white) were spread over a much larger area of the central and eastern Pacific. It may be construed that, while the impact of the 2023 El Nino may be strong in tropics, its global reach may be little diluted and not as stern as in 1997 & 2015.
IOD: The positive Indian Ocean event continues. The IOD index for the week ending 10 Dec 2023 was 1.21°C, nearly the same as the previous week. The highest weekly IOD index for the current event was +1.92°C for the week ending 15 Oct 2023. IOD event typically starts evolving around April and last for about 6-8 months. With the monsoon trough shifting in the Southern Hemisphere, the index starts narrowing in December and phases out during winters of the Northern Hemisphere. The strong episode of positive IOD may take a little longer than normal to break down. It is also known to have symmetric interaction during the active El Nino episode and therefore prolongs its stay over the Indian Ocean.
MJO: A moderately strong Madden – Julian Oscillation pulse is currently over Western Pacific. It is likely to move over the Western Hemisphere & Africa during the last days of December. Therefore, MJO pulse may enhance convection over the Western Pacific and adjoining Maritime Continent. Simultaneously, it is likely to suppress convection over the Indian Ocean. It may become inconsequential in the remaining period of northeast monsoon over South Peninsula. Also, it may be less supportive of a tropical cyclone spinning up in the Indian basins.
With the likely scale down of IOD in the Indian Ocean and stifling role of MJO, the Indian seas may lie mostly dormant during 2nd half of December. The less energetic equatorial systems may restrain active phase of northeast monsoon. The transitory weather systems may just be gentle enough to trickle monsoon showers in the remaining phase of the season.