December - January Outlook: Colder Pattern Expected in North; Mild in East, South; Frigid Canada

As I discussed on this blog's Facebook page yesterday, this is my outlook for the end of December through the month of January. We'll be using four primary components in this outlook: the stratosphere, the Pacific Decadal Oscillation, the La Nina, and the Madden-Julian Oscillation. I'll describe each component below, closing this article with my forecast.

I. Stratosphere

As things stand at this writing, the stratospheric polar vortex (SPV) is a bit battered & bruised, but overall in good shape. Attached on the right is a look at the 70-millibar pattern over the Northern Hemisphere, exemplifying something we saw develop in the last couple days: a split in the SPV at the 50-millibar level and lower. Indeed, the two distinct closed-off height contours in this image reflect a split vortex at the lower levels, while at the upper-stratosphere (10-millibar level), where it really matters for sudden stratospheric warming (SSW) purposes, the vortex remains intact.

You may ask, what's the difference if the vortex splits at lower levels or upper levels? If were to see the SPV split up into two pieces throughout the entire stratosphere - not only the 70-millibar level but up through the 10-millibar level as well - it would signify that the stratospheric polar vortex is really in trouble, and greatly increase the probabilities of Arctic air being unleashed to lower latitudes of the Northern Hemisphere. Because the vortex is still intact at that key 10-millibar level, I would consider this something worth briefly discussing, but not something that will have a massive influence on the outlook.

In fact, the only influence I'm drawing from the stratosphere is quite a warm influence; model guidance over the next two weeks suggests the stratospheric polar vortex will remain around normal or above-normal strength for this time of year, which should help to lock that frigid Arctic air up in Canada. This is confirmed in the latest Arctic Oscillation (AO) forecast from the long-range ECMWF model, shown below, with the average of these 51 ensemble members having the AO stay almost entirely positive during the period, and may even be substantially positive for January:

For the month of January, then, the stratosphere adds warm risks across most of North America.

II. Pacific Decadal Oscillation

Another key component to my January outlook is an index called the Pacific Decadal Oscillation. This is a longer-term oscillation, on the order of months and years compared to regimes like the Arctic Oscillation and North Atlantic Oscillation (AO and NAO, respectively) which work on the order of days to weeks. Below I've attached a look at the PDO not using sea surface temperature anomalies, as is normally the case, but judging how the atmosphere is determining the PDO's phase:

The above image shows an index of the Pacific Decadal Oscillation using atmospheric measurements, as opposed to the alignment of sea surface temperature anomalies. This matters because, while one can see how certain water temperature anomalies line up, what really matters is if the atmosphere "picks up" on that signal and responds accordingly. After all, if water temperatures say our PDO should be negative but the atmosphere doesn't act like it's in a negative phase, is it *truly* negative?

In any event, the above image shows the Pacific Decadal Oscillation in its most negative state since the late 2000s, and at its second-most-negative reading since the late-1970s. This matters because, as the image on the left shows, the PDO materially impacts the Northern Hemisphere during boreal winter. For some more context, the image on the left shows the correlation between the mid-level pattern and the PDO index. If values are negative, it means a positive-PDO results in stormy/cold weather, or a negative-PDO results in warm/calm weather during the Dec-Feb period. Similarly, if correlation values are positive, it means a positive-PDO results in warm/calm weather over that area, while a negative-PDO results in cold/stormy weather over that area.

We can confirm this by looking at the correlation between the PDO index and surface temperatures during the winter months, as in the image directly on the left. Indeed, given that we're in a negative-PDO, the positive correlation values in the West suggest this negative-PDO results in colder weather, while negative correlation values in the East U.S. imply a warmer pattern from a negative-PDO.

Given how strong the PDO is currently seen in the atmosphere, and how strongly negative it is, my outlook anticipates persistent colder and stormier weather across the West U.S., equivalent to a negative-PNA index (resulting in milder risks across the East U.S.).

III. La Nina

We remain steadfastly in a La Nina environment, with water temperatures in the Equatorial Pacific comfortably colder than normal and the Southern Oscillation Index (SOI; a shorter-term indicator to identify whether we are in a La Nina or El Nino) staying in the positive-teens, indicative of a La Nina reflecting in the atmosphere as well (instead of just showing up in ocean water temperatures).

The probability-based forecast of the ENSO phenomenon from the Climate Prediction Center and Columbia University is shown on the right, and portrays very high confidence in the ongoing La Nina persisting through meteorological winter into meteorological spring. We see that the La Nina (negative-ENSO state) correlates positively to below-normal temps during Dec-Feb across the Northern U.S. as below...

... while the La Nina tends to provoke above-normal temperature anomalies in the South U.S. as the Pacific jet stream is shunted northwards to deliver storm systems to the Pacific Northwest, rather than the Desert Southwest. Combining the negative-PDO with the La Nina, and accounting for constructive influence between the La Nina and negative-PDO as discussed in Rao et al. 2019, below-normal temperatures seem quite likely across the West U.S. and especially into western Canada for January 2022.

IV. Madden-Julian Oscillation (MJO)

Perhaps the most important part of this outlook, the Madden-Julian Oscillation (MJO) looks to come back to life over the coming weeks, prompting a shift in the global weather pattern.

Before diving into this component of the outlook, let's review what the MJO is and why it matters. In a nutshell, the MJO is based on the positioning and intensity of enhanced thunderstorm activity across the world, along the Equator. The location of this enhanced thunderstorm activity along different parts of the Equatorial prompts different global weather regimes; indeed, the MJO is often seen as the most important sub-seasonal oscillation currently known.

The MJO is said to be in one of eight phases at any given time, depending on where enhanced thunderstorm activity along the Equator is located. If there is minimal thunderstorm activity, then the MJO is simply too weak to have a material impact on the global weather pattern. This is all summed up well in the phase-space diagram below:

This chart might look intimidating at first blush, but we can break it down to make it easier to understand. As I discussed, the MJO can be broken into eight different phases, depending on where thunderstorm activity is most focused along the Equator. Each of those eight phases is labeled in this phase-space diagram, and the rough location of where that enhanced thunderstorm activity is during any given phase is written on the axes of the chart. You'll also notice number ticks on both axes - that tells us the intensity of the MJO at any given time. For instance, if the jagged line goes further away from that middle circle and towards larger-magnitude numbers on the x- and y-axis (whether positive or negative), it means the MJO is becoming stronger, and the global weather pattern should increasingly reflect the 'typical' pattern that develops when the MJO is in that given phase.

Examining the chart further, we find a black jagged line which then turns into a multi-colored thick line, surrounded by a whole spaghetti mess of thin yellow lines. That solid black line shows the progression of the MJO over the last five weeks or so - we see that the MJO began November in that middle circle, meaning the MJO was too weak to be placed in a specific phase / too weak to influence the global weather pattern on a material level. The MJO then spent almost all of November in a weak Phase 4-5 state, bouncing in and out of that middle circle. Since December 1st, however, the MJO has woken up and surged into Phase 6, where we now find ourselves.

The red, blue and purple lines represent the average, bias-corrected forecasted state of the MJO from the latest long-range ECMWF model; here, it's the forecast from December 8th through January 9th. To that extent, each thin yellow line is one of the 51 ECMWF long-range ensembles. 

And now that we're through with all of that, we can finally get into the forecast part!

The ECMWF has been consistent in taking the MJO through Phase 6 until December 18th or so, remaining in a rather strong state through the entire timeframe. Referring to typical temperature anomalies in each MJO phase on the right, we see that a Phase 6 event in meteorological winter corresponds to much warmer than normal temperatures in the eastern half of the country, while the West U.S. experiences below-normal temperatures and generally stormier weather.

This does look to verify, with temperatures set to soar across the eastern half of the country in the coming days (not shown).

By December 20th, the MJO is projected to push into Phase 7, still at a rather strong level, and the index remains in Phase 7 through the first several days of the new year (albeit at a weaker magnitude). Phase 7 still encourages warmer weather in the East U.S., but we now also see colder weather pushing south and east from western Canada into the northern U.S. Rockies and north-central U.S. Put together, it seems likely that the remainder of December will be warm across the eastern half of the country, though with a shot of cold air between December 25 - 31 as the MJO's trip into Phase 7 inspires increased chances for cold Canadian air to develop in western Canada and eventually push into the U.S. for a brief period. A return to warmth is expected thereafter.

January 2022 Outlook

With our four components examined above and the end-December outlook written in the previous sentence, we can get to work on what really matters: the January 2022 outlook.

I am expecting the month of January to feature below-normal temperatures across the West U.S., extending into the Rockies and especially in western Canada. It seems likely that temperatures will be much colder than normal in western Canada, a consequence of the typical January La Nina pattern, negative-PDO and MJO in Phase 7 - all of which encourage colder weather in western North America. 

Temperatures should go through quite a bit of back-and-forth in the Plains, a consequence of creeping cold from Canada bumping up against formidable warmth in the East. A strong polar vortex should allow warm weather to win out over colder weather on average, and I expect most of the Central to be on the warmer side of normal.

The East U.S. is likely to lean firmly warmer than normal, with these warm signals showing up in the MJO, La Nina, negative-PDO and strong polar vortices at the tropospheric and stratospheric levels. Confidence in the East Coast being warmer than normal is higher than normal. Indeed, confidence in this forecast is higher than normal, and it's been some time since I've made an outlook with forecast confidence at this high of a level.

Andrew