An Update on El Nino and its Long-Term Trends
The general theme for the month of October is trending closer to the strong El Nino analogs. After modeling over the past several months (ECMWF, CFS, NMME, IMME, etc.) was quite aggressive with a warm East Coast ridge, we have seen more Canadian "banana high pressure systems" arching down the Appalachian spine. Now, this certainly has not resulted in extreme cold, but in terms of departure from normal, the East Coast has been the 'coolest' place in the nation. THAT is a typical reflection of a strong El Nino in October. It may turn out that many places east of the Appalachians will end up right around normal for the month (or a degree or two above/below).
Certainly, the October analog is MUCH cooler across the nation on the whole; however, an El Nino reflection (i.e. coolest East) can be argued. The excessive September-October warmth can likely be attributed to the warmest sea-surface temperatures on either coast in the satellite era.
With the +PDO regime continuing to have a major influence on western United States temperatures, I suspect that the November temperature departure map remains "red" Rockies and West. The emergence of a strong subtropical jet stream should spell cool and wet for the south central. Finally, I suspect that the East will see larger departures from normal, on the positive side, in November (than October).
We continue to observe full-basin warmth across the central and eastern equatorial Pacific, indicative of a strong El Nino. The atmospheric El Nino reflection has been strong -- perhaps to record levels -- at times from the summer into the fall and we expect this to continue into the winter.
Such a strong signal often means that model success rates are higher than normal (i.e. the modeling has a firm idea to grasp onto). Traditionally, a strong El Nino would produce a cool to cold October Midwest and East with near- to above-average warmth in the Rockies and West. While the October pattern has mostly produced the coolest departures in the East, the nation has been warmer on the whole than it typically is in a strong El Nino.
This likely means that we can follow the strong El Nino November analog package as a guide to place cool and warmth, but perhaps use the idea that October has turned out warmer than the typical strong El Nino October in just about all areas.
We use the Multivariate ENSO Index (MEI) to track the strength of the El Nino. The MEI is especially useful because it "integrates more information than other indices; it reflects the nature of the coupled ocean-atmosphere system better than the [Southern Oscillation Index (SOI), Sea-Surface Temperature Indices]." The MEI value, through early October, is second highest to just 1997 since 1950.
The mid-October sea-surface temperature anomaly pattern in the equatorial Pacific is strikingly similar to that of 1997-1998. The Nino 3.4 Index anomaly has steadily climbed in recent weeks and is now +2.5 C (the highest it has been in this particular El Nino regime). Meanwhile, the Nino 1+2 Index has leveled out near +2.0 C.
The location of the warmest water will play a large role in the location of the most concentrated convection during the winter months. An El Nino with the most concentrated warmth in the East (1+2 region) will have different impacts than one with the warmest waters near or west of the 3.4 region. Make no mistake, this is a strong, east-based El Nino. Some comparisons have been made to 2009-2010, but a glance at sea-surface temperatures alone reveals vast differences. This particular El Nino may slowly revert toward a central-based El Nino at the tail end of winter (February) or March, but, by that time, the core of the winter will have already passed.
As of mid-October, we see a strong MJO pulse emerging in the Indian Ocean. The convection will round the globe and likely emerge in the Pacific Ocean early in December. At that point, one more El Nino-strengthening westerly wind burst may occur across the equatorial Pacific. This would give the strong El Nino one last push around the start of meteorological winter, sending some of the warmest waters across the basin from the 3.4 region and toward the 1+2 region.
Back to the MEI Index line plot above, you'll notice that in the case of the strongest El Nino events (1982-83, 1997-98), the Nino quickly fades and La Nina conditions take over in late spring or early summer. Huug Van Den Dool's sea-surface temperature analog long-range forecast along with the JAMSTEC see a transition from an El Nino to a La Nina during the summer of 2016. A mean of the analog package shows a warm to hot summer in 2016, from the Plains on eastward; however, the summer of 1998 was quite cool in this same region. Anomalously warm sea-surface temperatures across the western Atlantic could play a more significant role next summer as the El Nino fades. Long-term sea surface temperature trends also paint an active tropical cyclone picture in the Atlantic next summer-fall.
