July La Niña Update: Comic Timing

[Sea surface temperatures around the equator in the central and eastern Pacific were mostly cooler than average (blue) in June 2022. A few warm pockets (orange) dotted the far eastern Pacific. NOAA Climate map from our Data Snapshots collection.] [From NOAA Climate written by Emily Becker] I’m in San Diego this week, looking across the Pacific to the cool tropical ocean surface of La Niña. (I’m not here for Comic-Con, but there are plenty of posters around town bringing this event to the fore.) Just above my horizon, La Niña, the cool phase of The El Niño-Southern Oscillation (“ENSO” for short) – remains in effect, despite some warming in sea surface temperature over the past month or so. Forecasters expect La Niña to continue through summer, fall and early winter.

Ka-Pow!

In terms of numbers, there is about a 60% chance of La Niña occurring during the summer, reaching a bit of the mid-60% around 66% by October to December 2022. The second most likely outcome is neutral ENSO conditions. El Niño is a distant third, with chances in the single digits only at the start of winter. This forecast is not very different from that of the last two months.

[The official CPC/IRI ENSO probability forecast. The bars show the seasonal chances for each possible ENSO state—El Niño (red), La Niña (blue), and neutral (gray)—from spring 2022 through winter 2022–23. The forecast is based on a consensus of CPC and IRI forecasters, and it is updated during the first half of the month, in association with the official CPC/IRI ENSO Diagnostic Discussion. It is based on observational and predictive information from early in the month and from the previous month. Image from IRI.]

While we do the numbers, let’s see how La Niña measured up last month. As I mentioned above, the cool sea surface temperature anomaly weakened a bit in June, but remained in La Niña territory. (Anomaly = difference from the long-term average, the long-term here being 1991 to 2020, and the La Niña threshold is -0.5°Celsius, or just under 1 degree Fahrenheit.) According to the ‘ERSSTv5, our most consistent sea surface temperature dataset, the June sea surface temperature anomaly in the Niño-3.4 region was -0.8°C. It’s the 7e– the strongest negative June anomaly in our 1950-present record.

[Three-year history of sea surface temperatures in the Niño-3.4 region of the tropical Pacific for 8 previous double-dip La Niña events. The color of the line shows the ENSO state in the third winter (red: El Niño, darker blue: La Niña, lighter blue: neutral). The black line shows the current event. Monthly Niño-3.4 index is from CPC using ERSSTv5. Time series comparison was created by Michelle L’Heureux, and modified by NOAA Climate.]

This recent weakening of the Niño-3.4 anomaly – it was -1.1°C in May – is partly due to a slight decrease in the trade winds, the prevailing winds from east to west near the equator, in the first half of June. When the trade winds weaken, wind-driven evaporative cooling slows and the surface heats up. Additionally, a fairly weak downgoing Kelvin wave, a region of warmer than average water below the surface, has been moving west to east over the past few months, gradually rising towards the surface.

The trade winds strengthened during the second half of June and remain stronger than average as of press time. This will likely help cool the surface and could contribute to an upwelling Kelvin wave, a cooler-than-average region of groundwater that moves west to east. As well as being a sign that the amplified La Niña Walker Circulation – the colder sea surface atmospheric response of La Niña – is still present, the stronger exchanges are a source of confidence in forecasts that La Niña continues throughout the summer.

[Sea surface temperatures the week of July 9, 2022, showing the warm-to-cool gradient in temperatures across the tropical Pacific Ocean from west to east. Temperatures in the West Pacific Warm Pool, around the Maritime Continent, are above 80 degrees Fahrenheit (yellow-orange), while a cooler tongue of water (blue) extends from the coast of South America to the central Pacific. The prevailing east-to-west trade winds near the equator create this temperature contrast by pushing warm water west and allowing deeper, cooler water to well up to the surface. NOAA Climate image from our Data Snapshots collection.]

Zowie!

That said, there may well be short-term fluctuations in sea surface temperature in the Niño-3.4 region that flirt with the La Niña threshold. For example, the current weekly index Niño-3.4 is -0.5°C. (This uses another sea surface temperature monitoring dataset, OISST.) However, as Michelle detailed a few years ago, ENSO is a seasonal phenomenon, which means that we assess it using monthly and seasonal averages, not weekly ones. Most climate models predict that the three-month average Niño-3.4 will remain below -0.5°C, another source of confidence in the forecast.

Snap!

As I mentioned above, the sea surface temperature anomaly in June was the 7e– the most negative June anomaly ever recorded. Where is the atmospheric response? Let’s take a look at the Equatorial Southern Oscillation Index (EQSOI), an index that measures the relative surface atmospheric pressure in the equatorial eastern Pacific compared to that of the western Pacific.

[The Equatorial Southern Oscillation Index compares pressure anomalies across a broad region of the eastern tropical Pacific (5 degrees North and South latitude, 80–130 degrees West longitude) to pressure anomalies on the other side of the basin (5 degrees North and South latitude, 90–140 degrees East longitude). NOAA Climate image by Fiona Martin.]

When this index is positive, it means that the pressure is relatively higher in the east and weaker in the west, indicating a stronger Walker circulation. June 2022 is tied for third in all records (from 1950 to present), which made me wonder about the relationship between the strength of the Walker circulation in the summer and the Niño-3 index, 4 at the beginning of the following winter.

It turns out that the June EQSOI has a correlation of about 0.7 with the Niño-3.4 index during the following period from November to January. This is a fairly strong correlation, but it in no way guarantees a particular outcome from November to January. Another tied third June 2013 was followed by a neutral ENSO winter. However, most of the other Junes in the range of the 2022 value were followed by La Niña winters.

[Each dot on this scatterplot shows the atmospheric ENSO conditions each June (horizontal axis) since 1950 versus the oceanic ENSO conditions the following November–January (vertical axis). When the June Equatorial Southern Oscillation Index (ESOI) is negative, winter Oceanic Niño Index conditions are frequently in the El Niño range (red dots), sometimes neutral (gray dots), but rarely in the La Niña range (blue dots).  When June ESOI is positive, the winter is usually in the La Niña range, sometimes in the neutral range, but rarely in the El Niño range.  The June 2022 EQSOI—shown as an open circle on the horizontal axis—was the third-highest June SOI on record. Data from CPC, image by NOAA Climate.]

Blammo!

Given all these trusted sources, why isn’t the likelihood of La Niña higher? First, while the majority of climate models predict the continuation of La Niña, there is still a fairly wide range of potential outcomes. Also, as I mentioned above, the subsurface temperature in the eastern half of the tropical Pacific is a bit warmer than average. If this anomaly is large – much colder than average or much warmer – it has a stronger relationship with the possible Niño-3.4 index. However, when it is quite small, as it was in June 2022, the results are more varied. Finally, as we’ve covered extensively, a triple plunge (three laps!) La Niña is quite an unusual event.

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