Special Update: El Niño –Part II

By: Steve Gregory , 1:07AM,GMT on August 19,2015

TUESDAY 18-AUG-15

SPECIAL NOTE: Assuming Tropical Storm DANNY continues to develop into a Hurricane and remains on a track towards the CARIB - Daily Updates on it will begin with the FRIDAY Update. (When Tropical Cyclone Updates are provided, the ‘normal’ WX and Temp forecasts will be limited to the Temp Anomaly Forecast charts with minimal ‘Discussion’.)

EL NIÑO AND THE 2015/16 WINTER

So exactly what kind of Weather can we expect this winter as a result of the strongest El Niño in nearly 20 years?

To be clear - the 3 major climate models referenced below are all calling for a ‘classic’ El Niño winter Temperature and storm pattern - similar to that observed during the 1997/98 ‘Super El Niño’. And to be sure, the ‘odds’ are STRONGLY in favor of it. However, there are several other potentially mitigating factors that MAY play a role in at least ‘modifying’ current expectations. Some of these factors will be briefly discussed below.

Because of these other factors, I want to stress that I am not making a Winter forecast at this time since it is simply too soon to know how strong of an influence these ‘other’ factors will play on the winter pattern (including some that are NOT covered at all in todays discussion). And while the strong El Niño will be THE major forcing mechanism on the upcoming winter pattern, I will be issuing my own forecast at the same time that I normally do every year – in November.

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Fig 1: Large Scale ‘composite’ Weather Pattern during a Strong El Niño event.During a major El Niño episode like we have now, sustained convection in the tropical EPAC strengthens the sub-tropical jet stream which in turn, tends to create a strong, west-to-east storm track from California to the SE US. This also tends to create more frequent ‘Pineapple Express’ events in which deep tropical moisture streams east-northeast from near Hawaii to the west coast in association with strong Low pressure storm systems – producing extremely heavy Precip in many portions of the west coast. In addition, because of the stronger sub-tropical jet stream, the polar jet stream to the north that 'controls' where arctic air masses and most winter storms track, also tends to take on a more zonal-like orientation and a highly progressive overall flow pattern. This type of flow pattern typically prevents prolonged periods of extreme cold. The southerly storm track generally results in a wetter and colder than normal winter across the southern tier of states, even as Temps tend to be above normal in the northern portions of the nation.


Fig 2: The latest NMME and CFS Forecasts for SST anomalies in the Niño 3.4 Region. Consistent with all major forecast models of the past 6 months, the latest NMME and CFSv2 model suite forecasts call for SST anomalies to peak around +2.2˚C this fall. The latest reading of +2.0˚C(!) is among the highest ever recorded during the summer season, and during well-developed El Nino’s, peak values are typically reached towards the end of the year – making the +2.2˚ forecast look overly ‘conservative’. The peak value during the 1997/98 Super El Niño event in the benchmark 3.4 region reached +3˚C. To reach that level, at least 1 more MAJOR Westerly Wind Burst (WWB) will likely be needed during the fall – typically associated with a strong MJO. However, there are no forecast models capable of forecasting such an event more than a couple weeks ahead of time.


Fig 3: Hovmoller diagram for the past month and 7-day forecast for 850MB (~5,000’) zonal wind (the ‘u’portion of the u-v vector) anomalies. For the last 5 months (since the ocean and atmospheric pattern became ‘coupled’) a strong westerly wind anomaly became established in the CPAC (between 150˚E and 130˚W longitude). During the past 4 weeks alone, there have been 2 WWB’s. Another ‘surge’ in westerly anomalies is expected during the next few days (even without a major MJO forcing mechanism). This sustained westerly wind anomaly is often referred to as part of the El Niño ‘Base state’. It is quite possible that the surge in westerlies this week may lead to another uptick in SST anomalies to the +2.1˚C level by the end of the month (even as 'normal' SST's tick up during the next 6 weeks).


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Fig 4: NMME Winter Temp anomaly forecast for North America. The projected Temp anomaly for DEC-FEB is a ‘text-book’ classic pattern during a strong El Niño event. The projection of much above normal Temps is primarily a result of a strong westerly (zonal- like) wind flow across the continent - preventing the development of extremely cold arctic air masses over the Canadian arctic (the Yukon), or from air masses originating in Siberia that often times cross the high arctic and plunge southeastward into the US.. What makes this forecast ‘different’ from most other years with El Niños' is the unusually high probability assigned to it – due almost entirely from the strong El Niño.


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Fig 5: The CFSv2 Model forecast for SST anomalies during the Fall and (Left) Winter (Right) Seasons. The +PDO and very strong El Niño anomaly pattern is no surprise, but the ‘weaker’ positive SSTA’s in the Gulf of Alaska (GOA) and the ‘Tripole’ SST anomaly pattern in the Atlantic basin could prove to be quite significant to the winter Temp pattern across the nation. A +PDO, and in particular, the warm SST’s in the GOA, COULD result in an especially strong positive PNA Teleconnection like we have had during the past 2 winters (though it is weakening). While its unlikely to overwhelm the strong El Niño influence, it could ‘modify’ it towards a colder outcome – especially in the eastern and southeastern US. (To be discussed at a later time.) The tripole SST anomaly pattern in the Atlantic basin actually tends to support a positive NAO teleconnection, (A +NAO usually means a warmer weather pattern over the eastern US.) Indeed, as shown below, the last 2 winters did have a +NAO. However, the very strong +PNA and EPO last winter ultimately ‘overwhelmed’ the +NAO, resulting in the very cold winters. In the context of this coming winter, if we again have a generally +NAO, it, along with El Niño could mitigate the impact of a +PNA (assuming that happens again). Overall - another 'positive' for a 'warm' winter.


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Fig 6: The CFSv2 Temperature and Precip anomaly forecast for the Winter. The above forecast is quite similar to the CFS Model forecasts, though somewhat more ‘muted’ in the forecast anomaly magnitudes. Still, 3 month Temp anomalies of +2˚ to +3˚C is EXTREMELY warm. The most noteworthy Precip anomalies of course are in California and the SE US. (1 mm/day is roughly equivalent to an average anomaly of about 4” above normal for the winter. However, it would not be surprising to see +10” above normal totals in parts of California.)

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Fig 7: The IRI (International Research Institute for Climate & Society) Forecast for SST anomalies for 4, ‘3 month’ seasons. Per the associated text above, the most important ‘take away’ is the declining SSTA’s in the far NE Pacific (including the GOA). Even without an El Nino – the weakening and potential reversal of anomalies in the GOA by mid-winter would negate the forcing mechanism for a +PNA which prevailed during the last 2 winters, and this fact alone would argue for a far more ‘normal’ winter Temp pattern vs. a cold one..


Fig 8: SSTA Changes during the past month. Aside from the remarkable +2.0˚C anomaly now in the Niño 3.4 region – SSTA changes in the GOA and across the ATL basin have been substantial over the past month. Most importantly (beside the warm-up in the tropical Atlantic) is the large decline in the GOA. This lends support to what I’ve been mentioning for the past 2 months – as SSTA’s drop in the equatorial WPAC – they tend to lead a decline in the GOA as well. This appears to be well under way now, and if it continues as forecast by the IRI – the likelihood of a strong +PNA (and colder Temp pattern in the eastern US like last year) declines as well.


Fig 9: 'Close-up' of declining area of positive SST anomalies in the GOA.. Not only have actual anomalies decreased - the areal extent of warm anomalies has decreased and shifted towards the south over the past month..


Fig 10: The ‘idealized’ NAO pattern and historical values for the past 165 years. A Negative NAO COULD result in a colder temp pattern than currently forecast by all the major models. However, as shown above, the last 2 winters overall were +NAO seasons – and this ‘indicator’ completely failed. (Most likely due to the strongly +PNA and EPO teleconnections.) However, one of the major forcing variables for a +PNA has been the very warm SST’s in the GOA – something that appears to be weakening – if not vanishing.


Fig 11: The ‘PNA’ Teleconnection For those unfamiliar with the ‘PNA’ teleconnection – suffice to say, a strongly positive PNA usually results in the depicted upper air flow pattern – which in fact dominated last winter.



Fig 12: The ‘TRIPOLE’ SSTA Pattern and the it’s possible correlation with the NAO teleconnection. While not a strong correlation (and very difficult to ‘tease out’ with available data sets, it does appear that once a Tripole pattern is in place – it tends to support a positive NAO – which ‘normally’ leads to a warmer Temp pattern over the eastern half of the nation. As mentioned above, however, this indicator failed miserably during the last 2 winters when a strongly positive PNA overwhelmed the effects of a +NAO.

THE NEXT REGULAR WX UPDATE WILL BE POSTED TOMORROW (WEDNESDAY, AUG 19)
WHICH WILL PRIMARILY FOCUS ON TROPICAL STORM DANNY IN THE ATLANTIC.

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Steve