Following is a summary of what's happening -- or not happening -- with the Pacific Ocean's turn toward cooler temperatures and the formation (or not) of La Nina over the balance of the next few months. NOAA climate scientist Emily Becker is the author of these comments. -- Bryce
"The task of a climate forecaster is to see the forest, and not get hung up on the individual trees. Especially that extra tall one over there, with the gnarl that looks like a face, and the low branches that would be so easy to climb, and -- uh, right. My point is that we try to look beyond shorter-term weather to see longer-term monthly and seasonal patterns. After all, a particular winter can have several colder-than-average days and still be warmer than average overall.
"Which brings me to the current situation in the tropical Pacific! The October ENSO forecast says La Nina conditions are favored during the fall and winter 2017-18, but at press time (early October) the ocean-atmosphere system didn't quite meet the criteria for a La Nina Advisory. Specifically, while the atmosphere is generally consistent with La Nina, the sea surface temperature in the Nino 3.4 region has been volatile, recently edging up close to average following several weeks near or below the La Nina threshold (0.5°C colder than average).
"Is the overall pattern truly La Nina, with some short-term fluctuations temporarily obscuring the pattern? Or has the atmosphere-ocean system really not settled down into a consistent pattern at all? The difference between these two scenarios is subtle, and the ENSO forecast team is maintaining the La Nina Watch as we wait for a clearer picture. The forecast is for that picture to become clearer soon, with La Nina conditions 55-65 percent likely during this fall and winter.
"The average wind and cloud pattern over the equatorial Pacific during September looked a fair bit like what we'd expect during La Nina, including stronger near-surface east-to-west winds and upper-level west-to-east winds over the western tropical Pacific. The pattern of greater-than-average clouds and rain over Indonesia and less over the central Pacific is also typical during La Nina. The Southern Oscillation Index and Equatorial Southern Oscillation Index were both weakly positive during September, another indicator of a stronger-than-average Walker Circulation.
"All those atmospheric bits sound like La Nina. But the first criteria for La Nina is a monthly average Nino 3.4 index more than half a degree Celsius below the long-term average. Which we had! The September Nino 3.4 average sea surface temperature anomaly (departure from long-term average) was -0.5 degrees Celsius. For NOAA to declare La Nina, though, we need the expectation that it will stay there for five overlapping 3-month periods.
"Instead, we saw the surface temperatures warm substantially over the last half of September. The warming wasn't enough to wipe out the whole-month cool anomaly, but it was enough to undermine -- at least a little -- forecasters' confidence that the pattern is solidly established and will last for the required five overlapping seasons. So, no La Nina yet.
"Diagnosing exactly why the surface temperatures warmed over a few weeks is difficult. One possibility is the tropical instability waves that have been active in the east-central tropical Pacific over the past month. These waves can mix warmer surrounding water into the cooler waters right along the equator and cause sea surface temperatures in the Nino 3.4 region to rise.
"It seems likely that the surface waters will trend cooler again, as those stronger-than-average surface winds I mentioned above work to cool the surface and keep the warmest Pacific waters trapped in the far western Pacific. Also, we still have a substantial amount of cooler-than-average water below the surface. The cool subsurface waters increased during September, and will likely provide a source of cooler waters to the surface over the next few months.
"The dynamical computer models are not very clear on which way sea surface temperatures will go for the next month or so. There is a very large range of potential outcomes for October, including well above and below the La Nina threshold. However, the models are more consistent after October, predicting that the most likely outcome for the late fall and early winter is sea surface temperatures below the threshold for La Nina.
"Usually, we wouldn't focus that much on a short-term change in one of the physical signs we monitor. (For example, we've cautioned against using a single weekly anomaly to conclude that the El Nino of 2015-16 was the Most Powerful Ever.) But the sea surface temperature anomaly is essential to the ocean-atmosphere system that is ENSO. If it doesn't return to cooler-than-average territory soon, it could leave the whole system looking more like a bunch of random trees than a forest; that is, more like random climate variability than La Nina.
"Fun fact time! My blog-brother Tom has spent a lot of time studying the historical record of ENSO. He discovered that if La Nina develops this year, it will be the only La Nina on record where the Nino 3.4 sea surface anomaly increased for several months, nearing the El Nino threshold, before diving into La Nina territory. Said another way, all the other 21 La Nina events followed a steady decline from warmer-than-average or followed consistently below-average sea surface temperatures. This year would follow cooler, then warmer, then cooler again sea surface conditions."
Bryce Anderson can be reached at Bryce.firstname.lastname@example.org
Follow him on Twitter @BAndersonDTN
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