Friday, 8 June 2012

The importance of the PDO flip in winter 2012-13

I think we all understand that next winter has the potential to produce weather that will make up for last winter. What I want to break down for you is what factors are involved and which ones are most important in the development of next winter. Firstly I am using two sets of analog years showing the two  most possible outcomes for next winter. You will notice these sets have huge differences in temperatures based on a few key factors. In the first set I am using el nino years coming off of a la nina or multiple ninas, with a cold PDO and a warm AMO in one  year, negative AMO in the other. I am using 1921-52 and 1972-73 for this analog set which looks like this:

You notice warm anomalies in the east while the west stays cool. This would not be the winter that we are looking for in terms of cold and snow.

 My second analog set would be el nino years following a double or triple la nina, with a positive PDO in the winter months although still negative in a decadal sense. Every year except for 2002-03 is in a cold PDO phase overall but the coming El Nino causes the PDO to flip to positive for the winter and spring months. I also chose years with a warm AMO, expect for 1976-77. My years are 1957-58, 1976-77, 2009-10 and 2002-03. Now I know that some people do not want to include years following a triple nina but I believe that those years should be included in the analog set and share many similarities to double nina's anyways. Here is what these years looked like.
So there we have it, its the PDO thats driving the winter pattern! The PDO flipping to warm is one of if not the number one driver in producing a cold winter in my opinion. Notice cold in the east and warmth in the west. Also notice the very warm anomalies across greenland showing a lot of blocking in these years. In my other set there is little to no blocking across the north due to very warm anomalies there. This year is favoured for blocking because of the negative AAO values showing up now, as well as the NAO and also the warm, cold then warm AMO trifecta which favour blocking. Solar activity has been running pretty low lately as well.  Now here are the keys we should be watching very closely for next winter with the knowledge that we are heading into a weak to moderate el nino after a double la nina (shown strongly by the modelling and SST's) . 1) The warmest water must be found in nino 3.4 detached from the coast. 2) The PDO has to flip to positive in time for next winter as a result of the coming el nino 3) Blocking should be found across North America due to the PDO spike which I will show later in the post

 So in order to produce an analog set you want to include all the possible variables, so it is important as I had shown before with my MEI analogs that all the players are on the table here. As we can see most of the time when the AMO is positive, and an el nino is coming off a la nina or two la nines/ three la nines you get a positive PDO as well even though its overall negative. For example 57-58, 09-10, 76-77. 51-52 went the other way with the same positive AMO, cold PDO overall and nino off of la nina. Notice 1951-52 is the only year that is following just a single la nina. This is good news for us because this may be showing that if you are coming off of a multiple nina in a warm AMO your chances of a warm PDO spike is better.

Now what if the AMO stays colder then what will happen. There is a cold signal here as well with three years that I found were 1965-66, 1976-77 and 1972-73. This scenario probably would be cold as well.  72-73  ended up with a cold PDO winter and a warm winter overall and 1965-66 and 1976-77 had cold winters across the east. This cold AMO scenario seems unfavourable at the moment so until we see signs this may occur it probably won't and all modelling I've seen is hinting the other way.



As I mentioned before there was a lot of blocking found in the winters of 1957-58, 1976-77 and 2009-10. 1951-52 and 1972-73 on the other hand had a positive NAO throughout most of  the winter. I will compare 1957-58, 2009-10 and 1951-52 separate from 1976-77 and 1972-73 because of the AMO which was warm in the first set. You can see that the correlation is found in the PDO which went warm for the winters even thought the PDO phase was cold overall. Its not the AMO because blocking is still shown in 76-77 with a cold AMO! Also you can't say the double nina before is the driver because 1972-73 had that and look what happened.

NAO values for dec-feb:


1951   12   1.13
1952    1   0.57
1952    2  -1.38

Compared Too:

1957   12   0.02
1958    1  -1.14
1958    2  -1.64
2009   12  -1.88
2010    1  -1.80
2010    2  -2.69
Cold AMO years:
1972   12   0.09
1973    1  -0.46
1973    2   0.52
Compared Too:
1976   12  -1.57
1977    1  -1.72
1977    2  -1.00
Somebody could argue that the QBO would be the cause of blocking or no blocking in these winters (similar to the upcoming one). The winter of 1972-73 had a negative QBO but could still not develop a block due to the PDO which remained negative. QBO values from dec-feb 1972-73.
-16.78 -4.40 0.08

Same case in 1951-52:

-3.98   -6.80   -7.71 
You could argue the warm water detached from the south american coast is the driver but all these years have this and we still see the differences in blocking and cold.



Now the only difference between these years is the PDO which spiked in all the years except for 1951-52 and 1972-73. I think we are seeing a strong correlation between the NAO going negative and the PDO flipping to warm giving just another reason why the flip of the PDO is the most important driver with regards to next winter. This will occur by the warmth shifting into nino 3.4 which its forecasted to do.


Taking a look at the current AAO which is already strongly negative and is forecast to dip below -3. A negative AAO in the summer is usually an indication that we will see blocking during our winter. We are already seeing a pattern that is capable of producing strong blocking(shown this week) and we are seeing a much different pattern then last winter where no blocking was found.

I hope you appreciate the important links I showed you and the work it took to dig this up. I am trying to show you why the NAO was so negative in these years and how they match up to our current pattern. I'm not just saying 1951-52 and 1972-73 were warm winters because the NAO was positive, it is why the NAO was positive which controlled  the pattern in these winters. Please feel free to leave comments!





2 comments:

  1. It sure shows you put a lot of hard work and time into it. Great article!

    ReplyDelete