Wednesday, March 11, 2020

Why Cold Now - Part 2

Back in late January I raised the question of why this winter produced persistent cold over Alaska; it's such a dramatic change and stark contrast to other recent winters that it begs for attempts to explain it.  After all, if we can't explain these things even in hindsight, what hope do we have (speaking personally) of predicting them?

To confirm the point that this winter was indeed remarkably cold in Alaska, the chart below shows the December-February statewide average temperature according to NOAA's climate division data.  In absolute terms, the winter was the coldest since 1998-99, and compared to the trailing 30-year average it was the coldest in 50 years (since 1969-70).  One could argue that this winter was about a once-in-a-generation cold winter relative to the modern warmer climate; and if background warming continues (as seems inevitable), it may be a very long time before it's this cold again.  Click to enlarge:

In my earlier post I discussed the apparent lack of connection of the winter pattern with tropical ocean temperature signals.  Tropical oceanic and atmospheric phenomena are often closely followed by long-range forecasters because of their slow and predictable progression, together with known (or supposed) mechanisms for influencing higher latitude weather.

In addition to sea surface temperature (SST) patterns, forecasters often track the behavior of tropical convection, i.e. regions of disturbed weather, including large clusters of thunderstorms.  One tool for such tracking is the so-called velocity potential of the upper troposphere; the velocity potential simply isolates the divergent part of the flow, which reveals where convection is relatively active (divergence aloft) or inactive (convergence aloft).

To illustrate, here's the departure from normal of this winter's velocity potential (VP) at 200mb.  Blue shading (negative VP) indicates unusual divergence aloft, implying more rain than normal (hence the locust outbreaks in East Africa), and yellow/orange indicates unusual convergence aloft (less rain than normal).  Note that VP is only useful in this sense in the tropics, because large-scale weather disturbances outside the tropics are not typically dominated by convective processes.

Now to the point of this discussion: it's worthwhile examining past cold winters in Alaska to see if there are similarities in the VP patterns.  If there are, then we might argue that the tropical convection was linked to the outcome in Alaska; and such a finding would not be controversial.

Here are the VP anomaly maps for Alaska's three coldest winters relative to trend since 1980; the lack of good satellite data prior to 1980 makes me reluctant to look at the VP analyses from earlier years.

Unfortunately there are no obvious similarities between the VP patterns in these years.  1989-90 perhaps comes closest to 2019-20, with enhanced rainfall in the Indian Ocean and suppressed convection over the Maritime Continent, but 1989-90 did not have enhanced activity over the central Pacific or over most of Africa.

Here's what a good match looks like: the winter of 2002-03 had similar tropical VP patterns to 2019-20, but it was one of the very warmest winters on record in Alaska.

To pursue a more objective approach, I calculated the similarity to 2019-20 of the VP anomalies along the equator, and none of the 3 cold years is in the top 10 matches among 40 years of data (although 1989-90 comes closest).  And just to put a nail in the coffin of this hypothesis, here is a map of winter patterns in those top 10 matches, i.e. the 10 winters with the most similar VP to 2019-20.

The pattern is about as opposite as it could be for northwestern North America; contrast the 2019-20 maps below.  So even if we had known the VP patterns perfectly in advance, an analog approach like this would have predicted another warm winter for Alaska.

In summary, this cursory analysis provides absolutely no evidence that Alaska's cold winter was directly linked to patterns of tropical rainfall activity, and this is a disappointment.  It's not entirely surprising, though, because the SST patterns provided no help either, and there are very close links between SST anomalies and tropical convective regimes.

In Part 3 of this analysis, I'll look at another aspect of the winter that became increasingly unusual as the winter progressed, and that's the strongly positive Arctic Oscillation and increasingly intense stratospheric polar vortex.  In this case there is definitely some demonstrable correlation to Alaska's winter weather, but I think we'll find that it still leaves many questions unresolved.


  1. From Fairbanks it appeared:
    Something caused a regime change in the Arctic between late Fall and early Winter 2019.
    The Jetstream circled the Polar Vortex but rarely disturbed the latter. It may have contained and contributed to its longevity.
    There were at times prolonged strong Stratosphere-Trophosphere coupling events with associated low Polar Cap Heights.
    Thee were few high altitude clouds observed. It was either clear or snowed then repeated. A shortwave event then back to cold.

    I wonder if looking at the regime change in latter 2019 might offer some clues? Beyond that.....what contributes to and maintains a strong Polar Vortex?


    1. Thanks for the comments, Gary. Yes, the strong stratosphere-troposphere coupling by late winter was a very striking aspect. There is a broad literature on stratospheric vortex forcing/disruption mechanisms, so I agree that would be a good line to pursue. And I'll try to identify the regime change you mention... an interesting suggestion.

  2. Here's a readable summary discussing both the tropospheric and stratospheric polar vortices (PV):

    The authors suggest a strong stratospheric PV event may be due to several factors including: Infra-red cooling at the Poles equalling or exceeding solar heating, and seasonally weak Rossby Wave flow from the troposphere up into the stratosphere supporting that radiative equilibrium and cold air. Also note in my post above this winter's stratosphere>troposphere downward coupling and cooling of the lower atmosphere.

    My takeaway at least. I'm sure there's more.


  3. One last question. Does the Climate Prediction Center that forecast a "warmer" winter for Interior Alaska as I recall have an unfogged rear view mirror? If so what changed? And particularly over the Aleutian Islands and Gulf of Alaska?