Sunday, September 9, 2012

Why We Aren't Truly In An El Nino, And What It Means For This Winter

Sea surface temperature anomalies have consistently been displaying an El Nino is in place. Anomalies of as high as 1 degree above normal are being spotted in the waters of the equatorial Pacific. However, the atmosphere is not acknowledging the presence of an El Nino, and this could be a problem going into winter.

As the regional SST anomalies from the Climate Prediction Center are showing, above normal water temperatures are present across the central ENSO regions (Region 3 and 3.4). These anomalies are inching up towards the 1 degree mark, which would make this a borderline moderate El Nino.

Animations of the water anomalies show a progression west of these warmer waters, and this is reflected by a sharp drop-off in the Nino 1+2 region, known as the most eastern region in the ENSO monitoring area.

Here, we see observed 850mb wind anomalies across the ENSO monitoring area. Looking at this chart, we do not see any significant anomalies suggesting any phenomena is ongoing in the area. Now, if we look at the typical El Nino 850mb anomalies, we see an enhanced area of winds in the central portion of the monitoring area in the September timeframe (image below).

This lack of 850mb winds is a curious feature, and would seem to confirm the proposition that the atmosphere is not on the same page as the sea surface temperatures. However, 850mb winds are only one card in the deck of factors we use to monitor the ENSO phases, so let's continue digging for information.

Something else we can use to watch for an El Nino or La Nina is the Southern Oscillation Index, or SOI. When the SOI is above 8, the atmosphere is indicating the presence of a La Nina. On the other hand, a -8 on the SOI shows that an El Nino is shown. Looking at this screenshot of recent SOI observations from the Australian government, we can see monthly average SOI values. In June, the average was -10.2, indicative of an El Nino. However, in July, the SOI was basically in neutral, only at 0.1- an oddity for El Nino-like SST waters. Then, in August, only -6.2 was registered on the SOI, further confusing forecasters on what the atmosphere is doing at the moment.

Only adding to the confusion is the lack of enhanced negative OLR values. Outgoing Longwave Radiation, or OLR, measures convection in the equatorial Pacific waters. During an El Nino, the enhanced convection brings about a negative OLR, and a positive OLR indicates a La Nina.

However, looking at observations from the NCDC and CPC (respectively) in the 2 images above, there is no definitive anomaly across where the El Nino would be showing up. More-so, a positive OLR is showing up over the 140W-160W region, where the El Nino has the most influence at the moment. This further confirms the potential that the atmosphere is not fully prepared to handle an El Nino.

In times like these, we turn to analogs to see what would happen in similar situations of a 'fake' El Nino, where it technically is an El Nino, but the atmosphere is not confirming the suspicion. The best analog going around at the moment is the winter of 2006-2007. Let's take a look.

Precipitation anomalies for 2006-2007 were much more La Nina-like than that of an El Nino, with both the Northwest and Southwest drier than normal. Given, the Northwest is typically wetter than normal in a La Nina, but remember, this winter was a 'fake' El Nino- therefore, it still will show some signs of an El Nino. The Plains, Midwest and Appalachians were all wetter than normal, another telltale sign of a La Nina, while the Eastern US was dry as a bone. Even though I am talking a lot about La Ninas, it is only for comparison. I am not indicating a La Nina is going to be present this winter.
As for temperatures, the western US was slightly cooler than normal overall, but much of the region was normal. The northern Plains and East Coast were warmer than normal, showing that an El Nino is influencing the weather patterns.

Looking at that year, one can definitely see telltale signs of an El Nino, such as a warmer northern Plains and dry Northwest, but some odd La Nina patterns are also observed, like a warm East Coast and wet Central US.

It will have to come down to the atmosphere getting its act together and saying that an El Nino is present, because without that, forecasters like myself may as well be shooting blind.