What happened to winter 2018 2019?

Winter 2018-2019 was a winter of stark contrast.  But going back and reviewing what My outlooks said. I didn't get everything right, but the overall pattern was close to what I said it would be like.    I was closer to what actually happened than what many of y'all think. 

This post will try to show and explain the things that caused winter 2018-2019 to act the way it did.   

This winter I've had to deal with the reality of the situation VS the precipitation of the situation. There is a wide gulf between the two.  Why was winter 2018-2019 so close to my outlook, yet so far? As I said, my chosen analog years did come close to the overall pattern. But as I said in my three part pre outlooks released in September and the final ones released in October, an outlook is based on an average of all those analogies; so different parts of those analogies will show up at some point. But more

 

So what happened?

The Madden-Julian Oscillation (MJO):

The MJO and the Southern Oscillation Index (SOI) didn't work together very well during winter 2018-2019.  As a rule of thumb, during the winter Phases 1&2 are cold phases, while phases 5 &6 are warm phases. The other phases tend to be more transitional, going from warmer to colder and vice versa. The farther the MJO is from the center Null circle in the center of the diagram, the stronger the MJO is considered .  The MJO went through the warm phases twice with a very high amplitude.  The first one was in December of 2018. But then we went through the Sudden Stratospheric Warming; which lead to the Polar Vortex split in early January. During with 2017-2018 the split in the Polar Vortex happened in Mid February. That was the major reason April 2018 saw near record cold. With the PV split in 2019 occurring in January, we saw the cold for February and extend through March 2019.  For much of winter 2018-2019 the temperature and precipitation anomalies didn't match up that well to the pattern.  The MJO and SOI not really phasing this winter, is undoubtedly the major reason the models had so much volatility during winter 2018-2019.

Figures 13 and 14 show how the SOI behaved.

Figure 13

 

Figure 14 

 

Subtle Nuances:

When I went back and placed the winter of 2018-2019 under my microscope; at first things looked like they should. But under higher magnification, subtle differences came into focus.  there were several little things that stood out as strange. Taken by themselves none of these variables were enough to have any real impact; at least to the degree and scale of what happened.  The power of these nuances was in their cumulative effect on the overall pattern.  

 

The El Nino Southern Oscillation  (ENSO):

For much of the Fall of 2018 the Pacific equatorial region was warmer than average. It was clear we were going into an El Nino. By late fall the Pacific looked like we were in a Modoki (central based) El Nino.

When we deal with the ENSO we're dealing with how  warm sea surface temperatures are, and where those warm SST are sitting.  Depending on this affects wind speed, and direction, due to a temperature gradient that develops. This in turn determines where convection and precipitation will set up. 

Figure 1

 

The temperature gradient is called the Walker circulation.  During El Nino neutral conditions, we have an east-west temperature gradient; where the warm SST in the Western Pacific form a huge contrast to the cooler SST in the Eastern Pacific. The direction and strength of the Pacific trade winds, depends on the strength of the Walker circulation.  So, when we have neutral EL Nino conditions, the trade winds blow from East to West. During La Nina, the Walker Circulation is very enhanced. So we have very strong trade winds blowing from East to West.  Now, during an El Nino the Eastern Pacific warms and the western Pacific cools; this causes the walker circulation to normally reverse. So, the trade winds blow from West to East. This typically causes the main area of convection and rainfall to setup in the Central Pacific.  But this reversal in the gradient doesn't  always happen the same way.

It wasn't until February 14th, 2019,  that a weak El Nino was finely able to be declared by NOAA. During the Fall and into Winter 2018-2019, we had a weak El Nino Modoki signal. In fact at times we exceeded  the El Nino 0.5 ⁰ C threshold.  But the atmosphere didn't respond  as it was expected to. The reason for this lack of and very delayed coupling with the atmosphere and ocean, very likely has to do with the behavior of the walker circulation.

In the Fall of 2018 the expected enhanced convection and rainfall in the Central and Eastern Pacific never materialized.  Here are some images that show how Fall 2018 compared to what we would have expected under weak El Nino conditions.

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

 

Figure 2 shows how the equatorial Pacific rainfall pattern looked like for Fall of 2018. Figure 3 shows how 2018 stacked up in a scatter plot.  When we look at the figure 5, We compare the SST pattern in 2018 to what would be expected from a typical El Nino of the same strength; we see a pattern that looks very La Nina like. This is roughly the opposite of what was expected. When I looked back at pass years that developed El Nino conditions by winter, the only match I could find was in 2014. The winter of 2014-2015 had a similar pattern. NOAA didn't issue an El Nino advisory until March of 2015 for that season.  The Fall of 2014 had the same lack luster convection and lack of rainfall in the Central and Eastern Pacific as Fall of 2018.  Figure 4 shows what the SST pattern would look like during a neutral El Nino would look like. Figure 6 shows  how the SST pattern and the convective rainfall pattern from the temperature gradient combined. The result of which was quite similar to what we actually ended up with.

The 500 mb during February 2019 showed above average heights over the Gulf of Alaska and the Eastern U.S.  At the same time we had below average heights over the Central and Western US.  These anomalies are the exact opposite to those typically seen during an El Nino.  The height anomalies show the westward retraction of the Pacific Jet stream and the downstream amplification in the circulation.

Clearly the investigation has found a major culprit for what happened. But it's not the only one.

 

The Tropical/Northern Hemisphere pattern (TNH):

I've been reading some post by Thomas E. Downs and Joe D'Aleo  at WeatherBELL Analytics LLC.  They raised the idea that the TNH teleconnection was one of the reasons for the trough/ridge temperature pattern this past winter.  

The TNH is only calculated during the Months of December, January, and February. Normally during an El Nino the TNH is negative. But during the Fall 2018 into Winter the TNH index was positive. Not just positive but very positive.  Given the nature of the weak Modoki El Nino signal this is quite puzzling.

Figure 7

 

Figure 8

 

Looking at figures 7 and 8 we can see the positive THN footprint and the resulting temperature pattern that was the overall thyme for this past winter. Like the PNA, the THN plays a big role in the placement and eastward extent of the Pacific Jet.  For most of the time during  December through February we had a split Pacific Jet pattern. The southern split brought storms into the West Coast with a southern storm track. While the northern split blocked the storms away from the East Coast. We also had a weaker trough centered over the Hudson Bay than was expected.   

Looking back at past winters that had a weak El Nino. We find 1958-1959, 1976-1977, and 2006-2007 also saw very positive TNH index readings. Figures 8,9,and 10 show a composite of how those Februarys turned out.  February 1959 matches up very well with how February 2019 turned out.

Figure 9

Figure 10

Figure 11

The rest of these figures show how the temperature, precipitation, and teleconnections stacked up during February.

Figure 12

Figure 13

 

Figure 16

 

Figure 17

 

Figure 18

 

Figure 19

 

Figure 20

 

 

The overall storm track for winter 2018-2019 can be laid at the feet of the rare positive phase of the THN.

 

The North American Oscillation (NAO) and the Arctic Oscillation (AO): 

The NAO and the AO were at odds for most of the winter. But ended up being basically zero to positive for much of Winter 2018-2019. 

The Atlantic Multi-decadal Oscillation (AMO) and the Pacific North American :

Just like winter 2017-2018 the AMO was negative for winter 2018-2019. As for the PNA is was positive for the first two thirds of January, then for the last 10 days of January the PNA was weakly positive to negative. We also, had a negative PNA during February 2019.

All four of these teleconnections had a part to play in the lack of Upstream Greenland blocking and lack of any major Nor'easter storms along the Middle Atlantic and Coast of New England.

 

Well that's about it. This shows my analysis of the two primary reasons and a few negligible reasons, for winter 2018-2019 temperament issues.