What caused winter 2019-2020 to behave the way it did?

November 2019 saw the western  contiguous United States (CONUS) and much of the Northern Hemisphere quite warm. But for the eastern CONUS into Southeast Canada, winter started early for most of us.  November into a little before Christmas was cold and wintery. The chill made it all the way down to the Gulf of Mexico.

The first 2/3rds of December saw 3 snowstorms that brought accumulating snow to northern Pennsylvania, New York State, and New England. I and many others thought this was a sign of what was to come for 2019-2020.  But once we approached Christmas, all of that Changed. After Christmas through the first half of February 2020, temperatures became very warm, with overall surface temperature anomalies running well above average. While northern parts of New York State and northern New England saw snow in-between warm shots. All of the Major cities in the Northeast and Middle-Atlantic from Washington DC to Boston have seen much less snow than average.  The rest of the Northeast and Mid-Atlantic saw mainly just rain.

January 2020 temperatures were above average for nearly every U.S. climate division of the Lower 48 states. The first half of February has been nearly as bad over the CONUS.  Ice cover on the Great Lakes is well below average. 

Why has the heart of winter 2019-2020 been so warm?

The most noteworthy difference between the winters of 2009-2010 and 2019-2020 isn’t so much the bare ground in much of the Northeast as the upper atmosphere over the North Atlantic and over the Pacific.

The Jet Stream (JS) has been pulling moisture and warmer air from the Gulf of Mexico, since the end of December into Mid-February, the jet has twisted to our north. Usually the Jet Stream is over the Northeast or just to our south. That is what brings the arctic air into the Northeast during our winter. 

The Arctic Oscillation (AO) has been mostly positive since early December 2019. The Eastern Pacific Oscillation (EPO) has been in a primarily warm phase this winter as well.  These two teleconnections are two of the main reasons for the mild conditions this winter. They are the reasons for the behavior and placement of the jet streams.  The Polar Vortex (PV) has been near record strong for the last several weeks.

As I’ve said many times, the PV is nothing new, it’s a permanent large area of low pressure that spins over the Arctic Circle.  It’s even in place during the Summer, only in a weaker state. During the winter, when the meandering JS causes the PV to be displaced and move south, we get these colder outbreaks. The stronger the PV the warmer our winters here in the CONUS.

All of these things are strongly interconnected. 

When we have a warm (by Arctic standards) atmosphere over the polar areas, the AO tends to become negative, and the colder Arctic air drops south. We saw this happen in November into December.

While there have been a few instances where the AO did flip negative, allowing for strong cold outbreaks; they were fairly short lived.  But the polar setups changed mid-December and the AO became strongly positive.  The AO has been mostly in a positive phase since early December, typically the AO is more variable, it’s unusual for it to stay stuck in place for longer than a month or two at a time.  Since the AO has been mostly positive, the JS has stayed to the north, helping keep the PV in check.  As long as the PV stays strong and the AO stays positive, keeping that winter cold bottled up well to our north, it's going to favor a continuation of milder than average conditions. But that is not a guarantee…

Something that is closely linked to the AO is the North Atlantic Oscillation (NAO).  Most of the time it mimics the phase the AO is in.  While there are multiple factors that affect where we see snow and where we don’t; the NAO is the major player, when it comes to Great Lakes, Northeast, and Middle Atlantic Region snowfall.

The NAO is an index that measures the pressure difference between the Arctic and lower latitudes closer to us (specifically between the subpolar low typically found near Iceland and subtropical high pressure near the Azores).  When the NAO is positive, we typically see lower pressures and heights over Greenland/Iceland, and higher pressures and heights in the eastern CONUS into the Central North Atlantic. The NAOs negative phase is the opposite. When we see a predominately negative NAO, the setup becomes favorable for cold air to descend from the higher latitudes, this is a prerequisite for snow in the Northeast and Middle Atlantic Region. The setup also allows for an interaction between the colder air and the warmer Sea Surface Temperatures (SST), a negative NAO also favors coastal snowstorms.

The EPO is similar to the NAO, but is located in the East Pacific.  When the EPO is positive the heights and pressures are lower over the northern latitudes and higher heights and pressures to the south.

When the EPO is in a positive phase, we typically see a faster zonal JS flow.  This allows for warmer Pacific air to dominate the pattern across the CONUS.  The EPO also tells us were the troughs and ridges are located over North America.  With a positive EPO we typically see a trough over Alaska. A negative EPO features a ridge over Alaska; this setup favors very cold are being forced south into the central and eastern CONUS. Which is why we typically see widespread cooling over the central and eastern CONUS. The EPO is a big part of why the eastern CONUS has been very warm since Christmas; Alaska has been very cold this winter.

Here are a couple of images that shows how this basically works. 

Something that works alongside the EPO is the Pacific/North American Pattern (PNA). The PNA deals with the orientation of troughs and ridges in the JS from south of the Aleutian Islands down to the West Coast.  When the PNA is in a positive phase, we typically see a trough over the Aleutians and a ridge over the West Coast, this allows for a predominate trough over the East CONUS. The negative phase normally has a ridge south of the Aleutians and a trough over the West Coast, which allows for a ridge to set over the East Coast.  When we have a positive PNA we tend to see colder temperatures east of the Rockies.

The reason the opposite of a cold winter is occurring, can be laid to the feet of the AO and EPO, with the NAO and PNA lending a big hand.

Why did I get so many things wrong with my mid-winter thoughts in the outlook?

Weather and climate are very complex. There are many interrelated factors that govern what occurs. Some of these teleconnections are well known, others not so much. Much of my outlook ideas came from how things looked last Fall.

A week ago, the MJO was in Phase 6; which is a very warm phase for January February and March. During January, the MJO stayed in phases 4,5, and 6, then it went into a null phase, before emerging back into phase 6 again for February.  I didn’t envision the MJO acting this way. My major forecasting problem was the initial Sea Surface Temperature profile and the forecast for the SSTs.

When we compare the conditions in November to what actually occurred, we see a subtle difference.  The ring of warmth ended up farther west. This drastically altered how the jet stream and the ridge/trough pattern behaved during the heart of winter.

The warm pool in the Gulf of Alaska has also weakened. Even with a warming Pacific.  With the exception of NINO Region 1 & 2 most of the Pacific is seeing warmer than average SST.

When I looked at all the other factors; these subtle changes were something I didn’t anticipate.

Moving forward

The warmer than average pattern for the last 8-9 weeks is fairly rare. As I said above, it is very atypical for a positive AO/NAO pattern to hang on this long. The nearly off the wall strong PV, allowed for a long lasting strong positive AO, resulting in a persistent zonal flow over the CONUS.  The question is if this winter is a fluke or if it is a sign that what we’ve seen so far is going to be more common in the years to come.   We know that the MJO can alter the expecting intersessional pattern.

There is no doubt that over the last three/four years; our Februaries have been quite warm. I made a post on my Facebook page that showed how warm the last three Februaries were. I also showed  how those same years saw cooling in March into April. This trend is something that has to be respected by forecasters.

I know many are saying this is a symptom of global warming.  I want to state (before the arguments start) yes, the globe is warming, but IMO, human kind is not the prime reason. But in the case of this winter, the warmer than average outcome is more directly linked to the Arctic weather pattern, than actual global warming.  I’ve noticed how some are saying this is the new normal.  I’m not smart enough or arrogant enough to say that is so! But there have been decades that were warm in the past and decades that were cold. Climate cycles are highly complex. These cycles can be millennium-scale, century-scale, interannual to decadal in nature. Could what we’re seeing be just part of a cyclical pattern, or something new? I tend to believe the former.  But the latter has to be considered as part of the equation as well.  

For what’s been going on this winter to change, we need some big changes. First the MJO is going to have to settle down. And as I showed, for the first time since November, there are signs that the MJO is finely going to head into the colder phases. We're also  going to need the AO to go negative, which should allow the NAO to go negative as well. But for that to happen the PV is going to have to weaken, allowing for high pressure to develop near Alaska, as well as high pressure near Greenland. For this to happen we will need to see the Quasi-biennial Oscillation (QBO) go negative, along with a negative EPO and positive Pacific Decadal Oscillation (PDO).  

There are signs that these changes are occurring or about to occur.  So there is hope that the winter like weather is on the horizon, time will tell.

Well that’s it…………hope you enjoyed reading this.