This post first appeared on the climate.gov ENSO blog and was written by Emily Becker
La NiƱa is still here, but forecasters estimate about a 60% chance that neutral conditions will return this spring. By the fall, the chance that La NiƱa will return is approximately equal to the chance that it will not. Letās take a stroll around ENSOās eclectic bookshelf and see what we can learn
Romance novels
The temperature of the ocean surface in the NiƱo 3.4 region of the tropical Pacific, our primary El NiƱo/Southern Oscillation (ENSO) measurement, was just about 1.0Ā°C (1.9Ā°F) below the long-term average in January, according to the ERSSTv5 dataset. As of this month, the long-term average is calculated over 1991ā2020. (More on this page-turner later!)
Of course, ENSO wouldnāt be ENSO without the coupling between the ocean and the atmosphere. In January, La NiƱaās expected relationshipācooler-than-average ocean surface and a stronger-than-average Walker circulationāwas clearly going hot and heavy. Evidence of this was provided by more rain and clouds over Indonesia and less over the central Pacific, stronger west-to-east winds aloft, and stronger east-to-west winds near the surface of the equatorial Pacific.
Speculative futurism
Although comfortably exceeding the La NiƱa threshold of more than 0.5Ā°C cooler than average, the January NiƱo 3.4 index moved closer to average from December, as you can see in the graph above. Forecasters think this tendency will continue, and they estimate thereās about a 60% chance the index will move into neutral territory by AprilāJune. The next most likely scenarioāthe remaining 40%āis a continuation of La NiƱa through the spring.
What happens next fall is less clear. There is a wide range of possible outcomes shown by the computer model forecasts, but weāre getting into the spring predictability barrier, a time of year when model forecasts are less reliable. By the fall, the chances of La NiƱa are about 50%, with approximately 40% chance of neutral and 10% chance of El NiƱo.
Historical investigation
For the past five years, our ālong-term averageā in the ENSO world has been defined as the average over the 30-year period 1986-2015. Many weather and climate organizations, including NOAA, use a 30-year period to define āaverageā or ānormalā conditions, also known as climatology, in accordance with the World Meteorological Organization (WMO). Most organizations update their 30-year climatology once per decadeāagain, including NOAAāso now they are turning the page from 1981ā2010 to 1991Āā2020. ENSO is a bit specialāIāll get to that in a minute.
Defining average conditions is helpful for a couple of reasons. First, it provides useful information about local climate, such as āthe average January high temperature in Baltimore is 41Ā°F.ā Also, knowing the average helps us to place current events in a historical perspective, and to quickly understand patterns. For example, if we were to just say āFebruary in Minnesota was cold,ā it wouldnāt tell us much, because February is always cold in Minnesota! But saying it was ācolder than averageā is more informative.
This 30-year convention dates to the early 20th century. Why 30 years? To quote the WMOās Guidelines on the Calculation of Climate Normals, 2017 edition, āThe 30-year period of reference was set as a standard mainly because only 30 years of data were available for summarization when the recommendation was first made.ā That may seem a bit arbitrary, but also, it seems to me that 30 years happens to be a reasonable memory span for an adult, making it a little easier to understand ācolder than averageā and so on. At any rate, 30-year normals are deeply ingrained in weather and climate science now.
Contemporary thriller
El NiƱo and La NiƱa are defined as above- and below-average tropical Pacific sea surfaces, respectively. Because the tropical Pacific is warming along with the rest of the planet, ENSO researchers realized that if they continued to use the previous three-decade average to define ENSO events, it might look like El NiƱo was happening more often than it really was, and La NiƱa could appear scarcer than it should be. They decided it would be a better picture of reality if the 30-year climatology was updated every five years.
Historical ENSO events are evaluated based on centered 30-year averages. For example, the individual years in 1986Āā1990 are compared to the 30-year average 1971ā2000, and the years in 1991ā1995 are compared to 1976ā2005. Check out this climate.gov article, written by our illustrious editor, Rebecca Lindsey, for more details on this ārollingā climatology approach.
Since we donāt know how the next 15 years will turn out (our jobs would certainly be quite easy if we did!), more recent years in the record are compared to the latest 30-year period. This means that, up until now, 2001ā2020 were compared to the 1986ā2015 climatology. As of this month, those years starting in 2006 are now compared to 1991ā2020.
This has had the effect of slightly changing the historical ENSO event table. One effect is that the winter of 2019ā2020, which featured above-normal NiƱo 3.4 index values, is no longer colored redāindicating an El NiƱo eventāin the table. At the time, forecasters thought that these warmer NiƱo 3.4 index values were more likely due to temperature trends than to El NiƱo, especially since there was no consistent atmospheric response, and this has been borne out.
Epic saga
I could go on about the implications of trends and ENSO for many pages, but Iām running out of column inches here! Thanks for sticking around through another chapter in our serial ENSO novel.
Great article! It explains a lot. As for this season, it looks as if predictions (well, the probability of results) became true; as a result of La NiƱa, the northern resorts have gotten pounded, with Central Rockies benefitting, too. The prediction for New England was listed as “inconclusive”, and I think that’s what they got, with snow, not as bad as it could have been with a balmy Winter.