El Nino-ish La Nina

[#NoPoles]

SST's are indicative of La Nina...resulting weather patterns in the desert southwest are the opposite of what you would expect...the STJ is very active...precipitation totals are 150%-200% above normal

I was wondering what is different about this La Nina compared to the average La Nina?

[usedtobe]

SST's are indicative of La Nina...resulting weather patterns in the desert southwest are the opposite of what you would expect...the STJ is very active...precipitation totals are 150%-200% above normal

I was wondering what is different about this La Nina compared to the average La Nina?

Diane, that's a good question. I don't know the answer.

[isohume]

Well for one thing, the current La Nina isn't very strong...it's at -0.7 C in the 3.4 region, which is decent for one month...but to classify as a La Nina episode...you need 5 consecutive months of -0.5 C or less departures from normal and this hasn't been the case. The current ENSO state is classified as La Nina conditions, which are expected to remain weak or nearing neutral through the remainder of the winter.

Therefore, other global blocking patterns can and do have more influence. In the past couple weeks, the PNA has been around neutral (mostly weakly negative), which in turn can displace the Aleutian Low and allow for the teleconection pattern of periodic stong upper ridges over the far NW leading to the development of closed off lows across the desert SW and an ensuing split flow pattern across the rest of the CONUS...such as we've been seeing. Also, the sub-trop high has been stronger than normal helping to block the lower lat flow and developing these closed lows as well.

There hasn't been much precip over the desert SW...and mainly across the higher terrain, but then again it doesn't take much rain to reach 150-200% of normal in that region where normal values range from a few hundreths to a few tenths so far for Dec.

[kylemacr]

ENSO is a complicated fellow because we often describe it as either El Nino (positive) or La Nina (negative), but in reality it is so much more complex. First of all, there's no such thing as a typical La Nina (or El Nino). An "average" La Nina exists, because you can easily average together all of the La Nina events that we've been able to record data for. But each one acts a little differently.

To illustrate this, I have uploaded 3 images from the TAO buoy array averaged over November of 1997, 2007, and 2011. The top panel on each image is the average field and the bottom panel is the anomaly field.

Note that in each event, the core of the anomalously cold water is different. Part of this has to do with the fact that La Nina is in a different stage during November of each event, but I don't think this is the main reason. The core of the cold water in 1998 is clearly much further West than in either 2007 or 2011. This is very important. If we assume that one of the main reasons that ENSO affects midlatitude weather is through the Walker Circulation, the anomalous suppression of convection (over the cold pool) will be shifted in 1998 relative to 2007 and 2011, and the anomalous convection itself (near Maritime continent) will also be shifted. If you shift the convection, you shift the source of Ridge building (through diabatic heating) and you switch the entire orientation of the PNA-like pattern. (Note that many people argue that the PNA as we know it is not a direct result of ENSO, I can discuss this in more detail if there is interest).

Also, the subtropical jet tends to shift Westward during La Nina, but the location of the convection plays a huge role in this. If the Walker circulation shifts, then the upper level westerlies which enhance the jet also move, and this results in a slightly different Jet placement which changes weather patterns, of course.

I could go on and on about this topic, but the tl;dr of the situation is that every La Nina event is its own character, and has its own personality. It's not nearly a simple as the 3 mode model that we often use (El Nino, Neutral, La Nina). One, of many reasons, is the activity of the MJO and other equatorial waves. The MJO is likely critical in moving the warm water East to ignite an El Nino event, and vice-versa for La Nina. Again, a story for another day.

Edit: Okay, so it won't let me upload .png files (:-().

[#NoPoles]

thanks for the explanations...please feel free to go into more detail...i enjoy this stuff

[kylemacr]

Now that I have some more time, let's take a look at the extratropical response this year versus the "average La Nina", which I define, literally, as the average of the La Nina (SST 3.4 <= -1sigma) from Aug-Nov 1950-2010 (didn't want to include this year).

First map is 300 hPa vector wind, or jet during the average La Nina, and the second map is this year's jet. Notice that this year, the core of the subtropical jet is about 20 degrees East of average. This is consistent with my previous post showing that the core of the cold SST anomalies are further East... entire Walker Circulation moves a bit.

Secondly, look at the 500 hPa height anomaly maps. First map is average La Nina, second map is this year. This year's extratropical wave train appears stronger than the average. The position of the Aleutian low is also different. I suspect some of this is real, and some is due to the fact that in different individual years, the wave moves East and West, and therefore the average of them is somewhat washed out... That said, the entire orientation of this wave is different this year than in the average.

Thirdly, take a look at the surface temperature anomaly maps (these show SSTs too). This year is much less extreme than average... warmer in the Pacific, not as warm in the U.S., etc.

Again, this is not ideal because I'm not looking at individual events, I will compare this year to 1998 in my next post...