Monday, November 17, 2014

Continued Warmth Aloft

Warm air aloft continues to reside over interior Alaska, and the duration of the warm event has now reached record levels for the winter season.  Specifically, the 850 mb temperature has remained above freezing at Fairbanks for 13 consecutive soundings, or 6 full days (3 pm AKST November 11 through 3 pm today).  The previous record for the winter season (November though March) was 12 consecutive observations in three different events, including last January.

A similar record is also now tied at the 925 mb level (around 2000-2500 feet above sea level), where the temperature has remained above freezing since 3 am on November 10, or 16 consecutive soundings.

Another, perhaps more striking, record was also broken today: the week ending today was the warmest week on record for the months of November through April at both 850 mb and 700 mb at Fairbanks.  Twice-daily soundings began in Fairbanks in 1948, and since that time there's never been a warmer week in the cold half of the year at these levels aloft.

In view of this extraordinary event, I thought it would be interesting to look at the historical distribution of extremely warm or cold events in the Fairbanks upper-air history.  The best measure of the overall temperature of the lower half of the atmosphere is the 1000-500 mb thickness, which is the vertical distance between the 1000 mb and 500 mb pressure levels; this is directly proportional to the mean temperature in the layer.  The chart below shows the annual number of soundings in November through March that observed a thickness of 5450 m or greater (red columns), or 4800 m or less (blue columns).  Together these events account for about 1 percent of the total number of soundings, so these are the extreme tails of the distribution.

Last winter saw a record number (9) of extremely warm soundings, and this winter is already at 7 despite the early date.  There has been a relatively high number of these events since 2006, and the overall trend appears to be upward.  However, on the cold side, it is interesting to note that the number of extremely cold air masses has not diminished.  The total number of "extreme" thickness observations has increased over the past 65 years.

The warmth in the past two years is probably at least partially related to the very warm surface water in the North Pacific Ocean.  As shown in the charts below, the area-average sea surface temperature in the North Pacific has been far above normal since June 2013, and the 12-month trailing mean is higher than at any time in the 160-year history of this NOAA Extended Reconstruction SST (ERSST) dataset.


  1. What I find interesting is that the warmth above hasn't translated into warmth on the surface. I would think that radiative processes would allow the surface to warm up from the top. But the lack of winds and layer mixing has done little. This makes me wonder how much surface temp records are related to upper-air temps in the long run - cf that discussion we had 6 months ago about the Barrow, arctic, and Bering weather regime patterns.

    1. Eric, I agree it hasn't been nearly as warm at valley-level as aloft, but it has been continuously above normal since November 6. The normal high/low at the airport this time of year is 9/-8. The building high pressure aloft has favored a strengthening inversion.

      I don't have data to show this at present, but radiative warming from aloft is effective only in the presence of clouds. If it had been warm aloft with a heavy overcast, it would have been much, much warmer at the surface.

    2. I would have thought it was a warm airmass creating clouds. But what you said and what we are seeing suggests its much more complicated: there's a part where clouds create a warm airmass. Probably some type of positive feedback is present.

      How long is this persistent warm air suppose to last?

      I believe it was Brian who mentioned that he found no consistent patterns with how tropical storms affect Alaska. But this event suggests that a warm fall/winter has happened in a like manner before. Has it?

    3. Eric, I would suggest that warm southerly flow aloft is generally more cloudy because warm "advection" is associated with ascent and therefore increased relative humidity. However, the subsidence from a big ridge nearby will cause major drying. The distance from the ridge axis is probably a good predictor of humidity aloft, and therefore of surface inversion strength. If the high is overhead, then it will be clear and cold at the surface even though warm aloft.

      The extreme warmth aloft ends tomorrow, but 850 mb temperatures appear likely to stay above normal for another week or more.

      It would be interesting to look at the tropical storm connection in more detail. Perhaps Brian can brief us on his knowledge in this area, and suggest directions for future work.

    4. Richard, the tropical connection analysis that I looked at earlier was based on season totals for Accumulated Cyclone Energy (ACE) – not so much from specific storms. If I have time I'll dig into it a little deeper.

  2. Not to worry...clouds are moving in from the SE and the E-W jet contrails are spreading nicely from south to north.

    Clouds require water vapor, the amount and vertical distribution of which, plus temps at altitude, determine their formation. Warm and relatively dry air may not create clouds unless the saturation point is reached. All I know about them except they are my friend in winter.