Archive for September, 2012

El Nino: 2009 vs. 2012

I have seen quite a few weather blogs and forum posts about this years winter, being in the 09-10 style. Most of this claims were based solely on the El Nino. I will share my opinion on this topic, about the El Nino being similar to the last one in 2009, backing it up with some re-analyse charts I made, using NOAA tools.
This year, there are positive anomalies across ENSO 3.4 region, but we still cant really talk about an El Nino. The overall atmosphere circulation and patterns are not really reflecting real El Nino effects just yet. I will compare the two years with the same period (from around July – Sept), so some differences can be expected by default, since ENSO anomalies can develop differently with each event.
Here are the current sea surface temperatures across the equatorial Pacific. Marked is the ENSO 3 region, where lately, there was a negative trend of the positive SST anomaly.

Lets look at the current SST anomalies, compared with the same date in 2009. The difference is quite obvious. The huge difference is the negative PDO area this year. This is really limiting the proper development of the El Nino, among other factors.

Here are two reanalysis of the SST anomaly, for the Aug – Sept period, for 2009 and 2012. The most obvious difference is again the cold PDO area.

PDO has a huge impact on the Pacific patterns. Usually when moving into the winter period it is really affecting the PNA pattern. Negative PDO favors negative PNA, while positive PDO favors  positive PNA.

And we can also compare the two years on this graph, where we can see that the El Nino in 2009 was quite strong and had a normal development with an upward trend all the way to the peak intensity.

The Southern Oscillation Index tells us part of the story.  With a clear drop in 2009, this year we have a constant “in and out” movement.  SOI must be -8 or lower to be favorable for an El Nino development, and opposite for La Nina.

Looking at the  GWO (Global Wind Oscillation) and AAM (Atmospheric Angular Momentum), gives us the idea why the El Nino cant really strengthen, beside the limiting cold PDO. Area around 1 favors La Nina, while area around 4 favors El Nino. In the past few months, it was really going back and forth, not really staying around the El Nino favoring stage 4.

The global AAM anomaly was/is negative, which is not favorable for real EL Nino strengthening, unless it starts to move into positive values which favor El Nino development.

The ENSO forecast is quite interesting.  The dynamical models are pushing forward a real El Nino, while the statistical models are holding back and promoting a strong neutral warm phase. In both cases, we are not that close to a 09-10 El Nino repeat, when looking at the forecasts.

Lets look at September pattern  compared to 2009. Keep in mind that the 2012 composite features only half of the month. The composite pattern over Europe and pacific wont really change that drastically but the USA pattern will get a bit more defined, with negative anomaly on the E CONUS and positive on W CONUS, featuring a positive PNA. 2009 on another hand had a similar EU pattern like 2012, but quite different over pacific and with a Rex block over N America.

Looking at the 925hpa zonal wind anomaly in the tropical Pacific, in the July-Sept period, also shows some important differences. There is a zonal flow present in the ENSO region this year like in 2009,  but there is also a lot more easterly anomalies, especially E and NE of the date line.

925hpa wind anomalies are also affected by MJO.  In 09, the amplitude and propagation was different/weaker than this year. Keeping in mind that in 2009, El Nino was quickly strengthening in this period.

The OLR (Outgoing Longwave Radiation) reanalysis (uninterpolated), shows how the convection is distributed differently this year than in 2009. I have to mention that the convection placement is affected by other factors, not just ENSO, with some effects of cold PDO being visible in the reanalysis.

I have pointed out some differences between El Nino development between 2009 and 2012. Patterns can theoretically still be the same down the line, but at least the current ENSO configuration is not really favorable for an exact or very similar winter pattern, when comparing to the effects of the 2009 El Nino.

Lets see how the 09-10 winter pattern actually looked like. Polar circle was under strong blocking, zonal flow was reversed, AO and NAO were both strongly negative, featuring favorable configuration for an intense winter.

Strong negative Arctic Oscillation.

To see just how strong the blocking was, lets look at the stratosphere, at the 20mb level. I have to point out that there were two SSW events in the 09-10 winter. One small in December and a major SSW event in January. Here we see blocking at this level, zonal reversal and a split polar vortex.

The polar front was pushed southward.

My next post will be published somewhere in the first half of October, when I will look at some seasonal forecasts and compare that with 2009 patterns and setup. I will also try to find a similar pattern in the past years, using the NOAA reanalysis tools.

So stay tuned!