September La Nina Update: Feeling Groovy

[From NOAA Climate written by Emily Becker]   It’s likely that the tropical Pacific is on the verge of getting into the La Niña groove, and forecasters estimate a 70-80% chance of La Niña this winter. NOAA’s forecast discussion says “the ocean-atmosphere system reflected ENSO-neutral [in August], but is edging toward La Niña.” Let’s dig into that!

The age of Aquarius

First things first: a reminder of why we’re here every month, rain or shine, talking about ENSO (El Niño-Southern Oscillation, the entire El Niño/La Niña system of the tropical Pacific). We do it because when it comes to the global climate, the tropical Pacific has all the influence of Jimi Hendrix playing guitar in the middle of a concert crowd. Sure, those closest by are experiencing the main effects, but the flow of people all around the field gets shifted, too, as people move toward or away from the performance—depending on their individual feelings about electric guitar. ENSO does the same to the global climate. So, let’s tune in to the sea surface temperatures in the Niño-3.4 region, our primary monitoring index for ENSO. [Monthly sea surface temperature anomalies (difference from average) in the Niño 3.4 region of the tropical Pacific for 2020-21 (purple line) and all other years starting from first-year La Niña winters since 1950. NOAA Climate graph based on ERSSTv5 temperature data.] The first step on our “Is it La Niña?” flowchart is a one-month average sea surface temperature anomaly in the Niño-3.4 region that is 0.5° C below the long-term average. A couple of definitions: here long-term means 1991–2020, and I’ll use anomaly to describe the difference from the long-term temperature average. The La Niña threshold is an anomaly of -0.5° C. [Flowchart showing decision process for determining La Niña conditions. Figure by Fiona Martin, adapted by NOAA Climate.] The decision tree doesn’t specify which ocean temperature dataset, and some months—like this one—the various datasets will have different values. While ERSSTv5 says the August anomaly was -0.45° C, which rounds to -0.5° C (at the La Niña threshold), the OISST says the August anomaly was -0.44° C, which rounds to -0.4° C!  And there are additional SST datasets too!  So have we checked off the first box of the flowchart, or not? Such is the dilemma of the ENSO forecaster. Fortunately, we don’t have to make decisions based solely on one month of the Niño-3.4 Index. (And yes, of course there’s the atmosphere, but I’ll get to that in a minute.) We also have weekly monitoring of the sea surface temperature. While we definitely do not make decisions based solely on the weekly value, it can provide a clue about the next step on the flowchart—do we expect that monthly anomaly to stay more than 0.5° C below average for the next several seasons? A season here is any three-month period, e.g., September–November. ENSO is a seasonal pattern, meaning the sea surface temperature anomaly needs to stay in La Niña territory for an extended period, indicating that the ENSO feedback between the ocean and the atmosphere is present. The weekly Niño-3.4 index as of September 1^st was about -0.3°C. This suggests that, while we are close to La Niña, we’re still seeing enough wobble in sea surface temperatures that we can’t be sure that La Niña conditions have locked in.