LA NINA 2016/17 BACK STORY:
La Nina called for, then canceled, then called for again, then 50/50 chance, now it’s here… kinda…
NOAA first called for La Nina this Fall/Winter for North America last spring.
They issued a La Nina Watch, said La Nina had a 75% chance of happening, then they canceled the La Nina Watch on September 8th, 2016 and said that La Nina wasn’t favored to happen.
Then, they brought back La Nina on October 13th, 2016 saying it had a 70% chance of happening.
On November 22nd, 2016, NOAA said there was only a 55% chance of La Nina happening this 2016/17 fall/winter and they dropped the La Nina forecast from a La Nina Watch to a La Nina Advisory.
Now, NOAA is now saying that La Nina conditions currently exist but that La Nina may exit the scene as early as next month…
“Synopsis: La Niña conditions are present, with a transition to ENSO-neutral favored during January-March 2017.” – NOAA, today
NOAA is saying that we’re currently in a weak La Nina but it’s so weak that Bureau of Meteorology in Australia is current saying that there is no current La Nina and that La Nina is not likely to develop.
“Although some very weak La Niña-like patterns continue (such as cooler than normal ocean temperatures and reduced cloudiness in the central and eastern Pacific), La Niña thresholds have not been met. Climate models and current observations suggest these patterns will not persist.” – Bureau of Meteorology in Australia on December 6th, 2016
NOAA has issued an advisory level of La Nina Advisory.
Below are two NOAA La Nina update articles. The first one is readable, the second is pretty thick.
December 2016 ENSO Update: Weeble wobbles
La Niña’s clinging on by her fingernails! If last year’s big El Niño was likened by some (not us) to a certain monster lizard, this La Niña is more like a gecko. Weak La Niña conditions were present during November, and are favored to continue through the mid-winter. It’s looking more likely that the tropical Pacific will transition to neutral conditions by the January – March period.
The temperature of the ocean surface in the Niño3.4 region in the east-central tropical Pacific was about 0.9°C below average during November using the ERSSTv4 data set, and the September – November period was about 0.8°C below average. This is the third three-month period in a row below the La Niña threshold of -0.5°C—this has to last for at least five consecutive three-month periods in order to qualify as a La Niña event by this indicator. Forecasters think we’ll just barely make it.
The atmospheric conditions that we expect with La Niña—stronger-than-average winds, both near the surface and high up in the atmosphere, and more clouds and rain over Indonesia with less over the central and eastern Pacific—have persisted over the past couple of months, although they aren’t particularly impressive. The atmospheric changes are a response to the changes in sea surface temperature, and also act to reinforce them.
The forest and the trees
Over the past few years, we’ve spent some column inches on “seasonal” versus “subseasonal” climate phenomena. El Niño and La Niña are seasonal, meaning when you average over a period of several months, the signs we look for in the tropical Pacific are consistent. If the overall pattern doesn’t persist for several months, it’s not La Niña.
There is always going to be some variation within a season—even a warmer-than-average winter will have some cold snaps mixed in. Overall, November showed La Niña conditions were present. However, there was also some subseasonal activity in the tropics that interfered with the La Niña signals. The Madden-Julian Oscillation, or MJO, is essentially an area of storminess that travels from west to east along the Equator. It’s not always active, but it was during November, with the area of clouds and rain moving through Indonesia and the central Pacific.
When this area of clouds, rain, and storms moves through Indonesia and the tropical Pacific, it can enhance or reduce the La Niña patterns. (All this applies to El Niño, too, but I’m going to dance with the one that brought me.) The area of storms is flanked by drier-than-average spots ahead and behind it.
At the beginning of November, the pattern of cloudiness over the tropical Pacific looked clearly like La Niña conditions, with more cloudiness than average over Indonesia (responding to warmer ocean waters there) and less over the central Pacific (as La Niña’s cooler waters led to less rising air.)
In mid-November, though, the pattern is weaker, as you can see in the middle panel above. It’s still there, but since the MJO was moving through the central Pacific, it interfered with La Niña’s patterns. Specifically, the stormy area of the MJO weakened the drier area associated with La Niña in the central Pacific. Also, the drier area trailing the MJO reduced the rain over Indonesia.
The MJO can circle the globe in 1-2 months, and especially speeds through the Western hemisphere. By late November, the stormy area was back in the Indian Ocean. The drier area ahead of the MJO was weakening the La Niña-associated rain over Indonesia. Since the MJO was far away, the La Niña dry area in the middle of the Pacific had reasserted itself. As of early December, the MJO has gone to sleep, and the forecast for MJO through December is uncertain. If you’d like to read about the current MJO status in detail, check here.
The animals and the zoo
That area of storms has a wind signature, too, as the rising air causes low-level winds to rush in toward the storms. Changes in the near-surface winds can lead to changes in the ocean temperatures, both through cooling or warming the surface and by kicking off Kelvin waves—areas of cooler or warmer subsurface waters that travel eastward under the surface of the Pacific over a few months.
My point here is that the effect of subseasonal systems like the MJO can last beyond the immediate impact. It’s too soon to tell if this MJO has had a longer-lasting effect, but we’ll certainly keep our weather eye on the tropical Pacific for the next few months. (We would do it anyway, because that’s our job!)
The kangaroos and the outback?
If you’re a real ENSO enthusiast, you may have noticed the Bureau of Meteorology, our counterpart in Australia, has updated their forecast, saying that they do not think that La Niña conditions are present, and are unlikely to develop in the coming months. This isn’t as contradictory to what we’re saying as it sounds. Mostly, it’s a good indication of how puny the La Niña conditions are. The Bureau of Meteorology uses a different threshold, requiring the Nino3.4 Index to be cooler than 0.8°C below average (our threshold is 0.5°C below average). Neither threshold is wrong or right, and we’re looking at the same weak atmospheric conditions.
The weak La Niña is just one player in the US winter forecast. Check out CPC’s long-range predictions for the outlooks over the next several months.
|EL NIÑO/SOUTHERN OSCILLATION (ENSO)|
CLIMATE PREDICTION CENTER/NCEP/NWS
and the International Research Institute for Climate and Society
|08 December 2016|
ENSO Alert System Status: La Niña Advisory
Synopsis: La Niña conditions are present, with a transition to ENSO-neutral favored during January-March 2017.
La Niña conditions persisted during November, with negative sea surface temperature (SST) anomalies present across most of the central and eastern equatorial Pacific [Fig. 1]. The Niño indices remained negative during November, except for the Niño1+2 index which reflected near-average SSTs in the extreme eastern Pacific late in the month [Fig. 2]. Also, the upper-ocean heat content remained below average [Fig. 3] in association with cooler temperatures at depth [Fig. 4], although this cooling lessened somewhat during the month. Atmospheric convection remained suppressed over the central tropical Pacific and enhanced over part of Indonesia [Fig. 5]. The low-level easterly winds remained enhanced in the west-central tropical Pacific, and upper-level westerly winds persisted across the tropical Pacific. However, these signals were masked at times by intra-seasonal activity. Overall, the ocean and atmosphere system during November reflected a continuation of weak La Niña conditions.
The multi-model averages favor La Niña (3-month average Niño-3.4 index ≤ -0.5°C) to continue through December – February (DJF) 2016-17 [Fig. 6]. Given the current conditions and the model forecasts, the forecaster consensus also favors the continuation of weak La Niña conditions through DJF 2016-17. In summary, La Niña conditions are present, with a transition to ENSO-neutral favored during January – March 2017 (click CPC/IRI consensus forecast for the chance of each outcome for each 3-month period).
La Niña is anticipated to affect temperature and precipitation across the United States during the upcoming months (NOAA’s 3-month seasonal outlook will be updated on Thursday December 15th). The current seasonal outlook for DJF 2016-17 favors above-average temperatures and below-median precipitation across much of the southern tier of the U.S., and below-average temperatures and above-median precipitation in portions of the northern tier of the U.S.