La Niña Is Here, Why Did The BOM Take So Long?
Why did the Bureau of Meteorology (BOM) take so long to call the current La Niña event?
Various other meteorological agencies called it many months ago. In all recent updates, the USA's National Oceanographic and Atmospheric Association (NOAA) has stated 'La Niña is present', similarly, the Japanese Meteorological Association (JMA) said, 'Atmospheric and oceanic indicators suggest ongoing La Niña conditions in the equatorial Pacific', and even the World Meteorological Organisation (WMO) said 'La Niña conditions in the tropical Pacific have persisted and strengthened as trade winds intensified during mid-July to mid-August 2022.'
So why did we sit on 'La Niña Alert' for so long when the indicators throughout the Pacific Ocean all sit above the appropriate thresholds.
It comes back to the baseline periods the signals are being compared to from agency to agency.
Firstly, the monitoring of the state of the El Niño Southern Oscillation (La Niña, El Niño or Neutral) is done by observing the difference in the surface temperature from 'normal' across the equatorial Pacific Ocean. That 'normal' being the thirty year climate average.
The region spanning the central Pacific Ocean gives the greatest indication of whether we're in a cool or warm water phase, and this is known as the Niño 3.4 region. The average temperature across this section of ocean is compared to the long-term climatic mean and this is where the crux of the issue falls.
The BOM is comparing the temperatures across the Pacific Ocean to the 1960-1989 baseline, but thanks to climate change, sea surface temperatures have warmed slightly during the years following.
This has to be accounted for, as any drop in the surface temperature away from the current 'normal' won't be as great if using the older baseline.
The warming is only small and about 0.3°C through the Niño 3.4 region but the threshold for an El Niño/La Niña event is of a similar scale and +/- 0.5C respectively for global agencies, a touch higher and +/-0.8°C for the BOM.
This has an effect on the classification of such an event throughout the Pacific Ocean depending on which baseline you use.
The average temperature across the Niño 3.4 region from 1960-1989 was 26.79°C, but for the period 1990-2019 it was 27.1°C - a warming of 0.3°C.
Now, the Niño 3.4 reading for July 2022 was 26.59°C. That's -0.51°C below the 1990-2019 baseline, and falling into La Niña thresholds for NOAA and other agencies, but comparing it to the 1960-1989 baseline (which the BOM uses) it only comes in at -0.2°C, under the threshold for La Niña.
This is why the BOM's current classification differs from all other agencies. It's using an old baseline (along with a slightly higher threshold) which in the current ocean climate becomes irrelevant. On the other side it also makes the El Niño threshold much smaller, with the Niño 3.4 region only needing to warm up 0.5°C, which is inaccurate.
The August values for the Niño 3.4 region have just come in at 25.89°C, pushing us deep into La Niña territory, regardless of the baseline and threshold used. Using the older BOM baseline we've got a difference of -0.9°C, and with the more relevant climate baseline, a difference of -1.21°C.
With this latest data expect the BOM to issue a La Niña update any day, coming into line with most of the agencies around the world (they did just that on publishing this article).
Whether we see the BOM updating their baseline to a more current and relevant thirty year period is unknown, though my thoughts are these adjustments will need to be made in the near future for the alert system to remain relevant.