I’ve seen some debate on various blogs and forums regarding whether the current El Nino is East- or West-based. In recent months, we saw a warming trend begin in the far equatorial Pacific, in particular the Nino 1+2 and Nino 3 basins. As time has progressed, however, the warming trend has spread westward and now encompasses much of the equatorial Pacific from east of the Dateline to the South American coastline. This can be visualized in the image below, which is a cross-sectional cut along the Equator in the Pacific Ocean region. Although the general trend is warming in all basins, it is worth noting that the most recent frame or two has displayed some tendency for cooling along the surface in the western Nino regions.
To determine whether this is, in fact, an East- or West-based El Nino event, it is necessary to first define what is meant by East- or West-based. The image below from shows the Nino 4, Nino 3, Nino 3.4, and Nino 1+2 regions, as defined by the CPC. I have extended the lines dividing each region to the corresponding meridian and line of latitude for easier viewing. The Nino 4 basin is the farthest west, extending beyond the International Dateline from 160E to 150W. Nino 3 lies to the east of this region, extending from 90W to 150W. The Nino 3.4 basin, which is the quintessential ENSO region used by CPC and others to identify the strength of an ENSO event, encompasses parts of both the Nino 3 and Nino 4 basins. It extends from 165W to 120W longitude. All of these basins are centered along the Equator from 5N to 5S latitude. The one exception is the Nino 1+2 basin, which lies somewhat to the south of the Equator. From west to east, it extends from 90W and the South American coast, and it lies just south of the Equator to 10S latitude.
Now let’s take a look at how current SST anomalies stack up in comparison to these Nino subregions. First, let’s take a look at an overhead map of current anomalies overlaid with the various Nino regions. In the map below, which shows the average SST anomalies for the period August 5 through September 1, 2012, I have outlined the Nino 4 region in blue, the Nino 3 region in red, the Nino 3.4 region in purple, and the Nino 1+2 region in black. As you can see the two largest and most important ENSO regions are Nino 3 and Nino 4.
From the image, we can that the largest positive anomalies generally lie along and west of 150W, which is the boundary between the Nino 3 and 4 subregions. This also means that the largest positive anomalies are generally in the eastern parts of the Nino 3.4 basin. However, the largest positive anomalies are fairly centrally located in the equatorial Pacific, with cooler waters in the far eastern parts of the Nino 3 regions and into the Nino 1+2 basin. On the basis of this, I would define this event to be central-based (or east-central based). In any event, the signal is not particularly strong one way or another. This can also be seen on the Equatorial temperature anomaly from the September 5 cross-sectional cut (extending from the surface to a depth of 450 m). Again, I have overlaid the various regions. From the subsurface anomalies, we can easily see that the warmest anomalies lie generally along and east of 150W. This warm pool has been propagating eastward with time, as shown by the .gif above. The warm subsurface temperatures in the eastern Nino 3 basin should help to reinforce the warm surface anomalies in this region.
You might be wondering what all the fuss is about. As displayed in the animation below, East-Based El Nino are generally warmer for most of the country, whereas West-Based El Nino events typically bring colder weather in the Eastern U.S. Nevertheless, the strength of the El Nino event is also an important factor to consider. Many of the notable East-Based Ninos were strong events with basin-wide warmth, such as 1982-83 and 1997-98. Neither of these are suitable analogs for the upcoming winter, because the ENSO signature was so much stronger than the present event.
The best scenario for fans of the cold & snow would be a Weak, West-Based El Nino (although even stronger events, such as 2009-10, can result in cold & snow if West-Based). With the upcoming winter, however, we should see more of a Central-Based, or even basin-wide event. This makes 2006-07 a good analog for the present Nino. Even a West-Based Nino, however, does not guarantee cold & snow with an uncooperative Pacific & Atlantic signature. There have been relatively few Nino events that have coincided with a -PDO (generally a +PDO favors El Nino and vice versa), but two notable analogs that I have pointed out are 1994-95 and 2006-07. I have seen 1976-77 and 2009-10 trotted out as analogs elsewhere, but both of these occurred during +PDO events. Moreover, they are not ideal matches with respect to projected MEI values. I believe this Nino will feature peak MEI values somewhat higher than the 1976-77 event, and much lower than the Strong Nino of 2009-10. I still favor a low-end Moderate Event, peaking during the late Fall and early Winter. And I as also pointed out, one should be aware of the general trends when making a winter forecast. 1976-77, in particular, was a very extreme result, that occurred in a cooler global temperature regime. It also occurred coincidental to an extreme blocking pattern related to a strongly negative NAO/AO configuration (this also occurred during 2009-10, although U.S. temperatures were not as cold in that more recent winter).