La Niña 2020: the bitch is back again?
Haas™ op 27 septembre 2020 · 13:14
La Niña conditions are present and are likely to continue through the Northern Hemisphere winter (~75% chance).
In August, La Niña conditions were present, with below-average sea surface temperatures (SSTs) extending across the central and eastern equatorial Pacific Ocean [Fig. 1]. In the last week, all Nino indices were negative, with the Nino-3.4 index at -0.9°C and the Nino-1+2 and Nino-3 indices cooler than -1.0°C [Fig. 2]. Equatorial subsurface temperature anomalies averaged across 180°-100°W were negative [Fig. 3], with the largest departures observed in the east-central Pacific from the surface to 200m depth [Fig. 4]. Atmospheric circulation anomalies over the tropical Pacific were also generally consistent with La Niña, despite sub-seasonal variability during the month. The low-level and upper-level winds were near average for the month as a whole, but enhanced low-level easterly winds were prominent across the equatorial Pacific Ocean during early and late August. Tropical convection remained suppressed over the western and central Pacific, and was near average over Indonesia [Fig. 5]. Both the Southern Oscillation and Equatorial Southern Oscillation indices were positive. Overall, the coupled ocean-atmosphere system was consistent with La Niña conditions.
A majority of the models in the IRI/CPC plume predict the continuation of La Nina (Niño-3.4 index less than -0.5°C) through the Northern Hemisphere winter 2020-21 [Fig. 6]. The forecaster consensus supports that view, and favors a borderline moderate event (Niño-3.4 index near -1.0°C) during the peak November-January season. In summary, La Niña conditions are present and are likely to continue through the Northern Hemisphere winter (~75% chance; click CPC/IRI consensus forecast for the chance of each outcome for each 3-month period).
This discussion is a consolidated effort of the National Oceanic and Atmospheric Administration (NOAA), NOAA's National Weather Service, and their funded institutions. Oceanic and atmospheric conditions are updated weekly on the Climate Prediction Center web site (El Niño/La Niña Current Conditions and Expert Discussions). Forecasts are also updated monthly in the Forecast Forum of CPCs Climate Diagnostics Bulletin. Additional perspectives and analysis are also available in an ENSO blog. The next ENSO Diagnostics Discussion is scheduled for 8 October 2020.
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Het effect van La Niña op de winter:
La Niña = Skiers Delight over the Northern United States
In a nutshell, La Niña is associated with a retracted jet stream over the North Pacific Ocean. The retreat of the jet stream results in more blocking high pressure systems that allow colder air to spill into western and central Canada and parts of the northern contiguous U.S. At the same time, storm track activity across the southern tier of the U.S. is diminished under upper-level high pressure, which also favors milder-than-normal temperatures. The storm track is in turn shifted northward across parts of the Ohio Valley and Great Lakes (2).
Based on climate analysis (3) from this new snow dataset, we see that La Niña favors increased snowfall over the Northwest and northern Rockies, as well as in the upper Midwest Great Lakes region. Reduced snowfall is observed over parts of the central-southern Plains, Southwest, and mid-Atlantic.https://www.climate.gov/news-features/blogs/enso/what-about-snow-durin...
*message édité par Haas™ op 27 sept. 2020 13:17
Haas™ op 11 octobre 2020 · 18:53
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