June El Nino Update: Pet Projects

[May 2020 sea surface temperature departure from the 1981-2010 average. Image from Data Snapshots on NOAA Climate.] [From NOAA written by Emily Becker]  After several months of hovering above average, sea surface temperatures in the tropical Pacific have dropped rapidly in recent weeks. The current CPC/IRI ENSO forecast estimates about a 60% chance that ENSO-neutral conditions will remain through the summer. Chances for the fall and winter are about equally split between neutral continuing and La Niña developing, at around 45% each. El Niño isn’t completely ruled out, but it is less likely, with a chance of about 10%. June finds us starting to emerge from the spring predictability barrier, when computer models are less successful at predicting how ENSO will behave. However, the models and nature aren’t showing strong signs, which is reflected in the approximately even odds between neutral and La Niña.

Puppies and kittens

Before I get into the forecast, let’s take a look at how conditions have evolved in the tropical Pacific over the past few months.   The weekly Niño3.4 index, which measures the sea surface temperature departure from the long-term average in the east-central equatorial Pacific, cooled by a full degree Celsius (1.8°F) since mid-April. Over one week in May, the Niño3.4 index dropped by half a degree Celsius. Water has a high heat capacity, meaning it takes a lot of heat to change the temperature, so rapid changes are uncommon. According to this post by Michelle, a half-degree week-to-week change is quite rare, happening about 1% of the time. Since March, the low-level winds along the equatorial Pacific (the trade winds) have been generally stronger than average. These winds helped cool the surface and support an upwelling Kelvin wave, an area of cooler-than-average water under the surface that moves gradually toward the east (see animation above). The subsurface heat content has also dropped substantially over the past several weeks, reflecting that upwelling Kelvin wave, and the May departure from average in the central-eastern equatorial Pacific was -0.92°C. Why should you care? I’ll cover that a little bit later! [Area-averaged upper-ocean heat content anomaly (°C) in the equatorial Pacific (5°N-5°S, 180º-100ºW). The heat content anomaly is computed as the departure from the 1981-2010 base period pentad (5-day) means. Heat content increased has recently decreased. NOAA Climate figure from CPC data.]

Hamsters and gerbils

Most of the dynamical computer models are predicting that the Niño3.4 index will remain slightly cooler than average for the next few months, but not strongly into the La Niña range (more than 0.5°C below average). It’s likely that ENSO will remain in neutral through the summer. As for next fall and winter, though, there is less agreement. Some of the models predict a weak La Niña, while some predict otherwise. Statistical computer models, which make predictions based on how historical events have evolved, are mostly predicting that the Niño3.4 index will be a bit warmer than average. This may in part be related to the fact that we have not observed a La Niña developing following a neutral winter (like we just had) in our 1950–present record. Predicting the future based on past cases is very tricky, though, as even small differences can lead to very different outcomes.