Sunday, December 17, 2017

Warm Air Aloft

In pondering the nature of winter warm-ups over Alaska, I find it interesting to consider how much of the atmospheric column rises above freezing, even while surface temperatures usually remain below freezing, at least for most interior locations.  In the most recent case, for example, the Thursday afternoon sounding from Fairbanks reported that nearly 20% of the atmosphere (by mass) was above freezing above the Golden Heart City.  The deep layer of above-freezing air is evident in the diagram below where the red line (the temperature trace) extends to the right of the diagonal blue 0°C line from the surface up to about 800 mb.

If we look at historical Fairbanks sounding data back to 1958, we find that the deepest layer of above-freezing air observed in December was in 1985, when the freezing level was way up around 8000 feet above ground (above 750 mb) on the 11th of the month.  The most extreme case to occur at any time in winter was the amazing warm spell of late January 2014, as documented here.  In that case over 30% of the atmosphere by mass was above freezing, and that's higher than normal for the height of summer.

The chart below shows a box-and-whisker depiction of the history of monthly means of the above-freezing fraction.  The numbers are calculated by adding up the pressure depth of each above-freezing layer in each sounding, after interpolating the freezing level(s) between reported heights.  Note that the record values labeled on the chart are for the monthly means, not for individual events.  It's interesting to note the sustained extreme warmth observed in some recent years in spring and summer, but not so much in winter.

A quick look at trends by month reveals that April is easily the month with the strongest warming trend based on this metric; the trend is actually quite remarkable (see below).  The mean value for June-August also shows a statistically significant upward trend, but I'll refrain from making a similar statement about the winter trend as a highly non-Gaussian distribution makes the statistics a bit more tricky.

Thursday, December 14, 2017

Exceptional Warmth

Unusual warmth is prevailing across Alaska with remarkable persistence so far this month, and yesterday was the warmest day yet for the interior; McGrath reached 39°F, Fairbanks made it to 41°F, and Bettles (at nearly 67°N latitude) hit 38°F.  The Bettles high temperature tied the record for the month of December, with data back to 1951.

Looking at the joint history of climate data from these three interior sites (1951-present), there is only a handful of days when all 3 locations reached 38°F or higher on the same day in the depths of winter (December through February).  It has happened only 6 other times, with the most recent occurrence being nearly 8 years ago (Jan 17, 2009).  January 2009 was also the last time that the daily mean temperature was at least 2 standard deviations above normal for all 3 of these sites on the same day in winter (November-March).

With respect to persistence, we are currently at 11 straight days with a combined mean daily temperature (for the 3 sites) of more than 1 standard deviation above normal.  The record for November-March is 13 days, so by this metric we're closing in on a record warm spell for the interior.

The chart below shows the kind of warmth that's been observed by the soundings from Fairbanks; for the first half of this week, temperatures were around 40°F or higher at about 600' above the surface.

Here's the 500mb height analysis from 3pm yesterday.  If it looks oddly similar to the chart from last week... that's because it is.

Saturday, December 9, 2017

Warm Days in Winter

Warm chinook flow from the south lifted temperatures above the freezing point on Thursday night in Fairbanks for the first time in more than a month.  Thursday's high temperature, which occurred at 10pm, was 37°F, but this isn't close to a daily record; the extraordinary chinook of early December 1934 set all the high temperature records in Fairbanks from the 3rd to the 8th of the month (including 58°F on December 5).  Nevertheless, the flow anomaly from this week's event was impressive; the map below shows the 500mb height analysis from 3pm Thursday.

It's obviously unusual for temperatures to rise above freezing in the depths of winter in Fairbanks, but most years do see at least a brief thaw at some point in December through February; only 10 years since 1930 remained below freezing throughout the 3-month period (most recently 1998).

The chart below shows the number of days each winter on which the daily high temperature was above 32°F in Fairbanks, and the red shading indicates how many days the daily mean temperature was also above freezing - the latter being a much rarer event.  The relative abundance of warm days prior to about 1945 may have something to do with the fact that measurements were taken downtown from 1930-1942, rather than at the international airport as they have been since 1951.

The asterisk annotations indicate winters when La Niña conditions were present in the tropical Pacific Ocean, as they are this year.  Despite the fact that La Niña winters tend to be colder on average in Fairbanks, the frequency of warm days is not much reduced, and this is because the temperature variability is heightened during La Niña winters.

The scatter plot below shows how the number of above-freezing days varies with an index of the El Niño/La Niña (ENSO) phenomenon.  A clear peak is evident for near-neutral ENSO conditions, and this is an interesting example of non-linearity in climate behavior; while El Niño winters tend to be warm on average in Fairbanks, the number of very warm days is no higher than normal, because temperature variability is low.  I've discussed this kind of thing before.

The same chart for the PDO rather than ENSO shows a hint of the same behavior, although there is more asymmetry between the positive and negative phases.  There is a notable reduction in warm day frequency when the PDO is significantly negative, but so far this winter the PDO is near-neutral.

Monday, December 4, 2017

Warm North, Cold Southeast

November brought a sharp contrast in temperature anomalies across the state of Alaska, with extreme warmth (relative to normal) in the north and below-normal temperatures in the southeast interior, south-central, and southeast Alaska.  Here's Rick Thoman's graphic from the other day:

As an example of the extremes, Utqiaġvik (Barrow) saw their warmest November on record, but the notorious cold spot of Chicken reached -30°F on 12 consecutive days, which is the most on record prior to mid-late December (but the period of record only goes back to 1997).  The chart below shows how the temperatures at the two sites diverged in November; note that Utqiaġvik had several days of missing data.

As it turned out this year, Utqiaġvik had "only" their 8th warmest October since 1930, but the warmth in November was unprecedented at 16°F above the 1981-2010 normal.  The early November peak in anomalous warmth is completely consistent with the absence of sea ice nearby; the chart below (taken from this 2014 post) shows an early November peak in the recent warming of surface air relative to air aloft at Utqiaġvik.

As for the weather pattern that led to the statewide contrast last month, we can blame La Niña for a persistent ridge of high pressure that allowed cold conditions to prevail downstream over southeast Alaska.  The first map below shows the November mean 500mb height - note the ridge over the Bering Sea - and the second map shows the departure from normal.  This is a classic La Niña pattern; but the warmth in the far north has more to do with the sea ice situation.

Friday, December 1, 2017

Sea Ice Update

Freeze-up is progressing at an extremely slow pace this year in the Arctic and near-Arctic ocean waters surrounding Alaska.  The latest sea ice analysis from the National Weather Service continues to show a very large region of open water in the Chukchi Sea, and it's only in the past couple of days that the coastal water has frozen up at Utqiaġvik (Barrow).

The absence of ice in the Chukchi and Bering Seas is unprecedented for the time of year in the period of record since satellite monitoring began.  According to the NSIDC's regional sea ice index, the Chukchi Sea's ice extent was 25% below the previous record (in 2006) as of yesterday.  Another way of looking at it is that yesterday's ice extent was equal to the 1981-2010 normal for October 13; so in comparison to that reference period, we could say that freeze-up is running about 7 weeks late.

Here's a chart of the historical November average ice extent for the Chukchi and Bering Seas.  This year's November average of 320,000 km2 is very nearly the same as the 1981-2010 normal for September, when seasonal melt-out reaches its peak.

And here's a simple animation of the last 10 days of ice analysis.  From 1979-present, the Chukchi Sea has never failed to freeze up completely before the end of the year (last year it got there on December 30); but this year it still has a long way to go.

Wednesday, November 29, 2017

November Ridge Follow-Up

Following up briefly on last week's post about high pressure over the Bering Sea, here are maps showing the sea-level pressure and temperature anomalies for November in the 8 analog years I identified.  A couple of the years (1988 and 1999) don't show much of a ridge at all for the full month of November, but the others have more similarity in the pressure patterns.

It's interesting to note the wide variety of temperature patterns in the analog years, and especially for the western and northern parts of the state.  Whether or not cold air prevails near and to the east of the ridge axis depends on the orientation of the ridge and the pressure gradient to its north; it is not uncommon for plentiful warm air to be swept over the ridge by a fast westerly flow.  This month has certainly been an example of strong westerly and southwesterly flow bringing warmth to the west and north - see the MSLP and temperature maps below.

Now take a look at the MSLP and temperature maps for January of the analog years (i.e. two months later):

In contrast to November, low pressure to the northeast of the ridge (rather than the northwest) allows a more northerly component to the flow east of the ridge, and consequently there is a notable cold signal across western and southern Alaska.

Saturday, November 25, 2017

Warming from Clouds

The past couple of days have seen an interesting example of the winter-time effect of clouds on temperatures in Fairbanks.  With the noon sun now less than 5° above the horizon, solar heating is very small and there is a more-or-less permanent inversion of temperature near the surface.  When clouds are present, the infrared radiation they emit provides a warming influence, especially when the clouds are thick.  However, when clouds are thin, high, broken, or absent, then radiation from above is reduced, and surface temperatures can drop - sometimes dramatically.

The chart below shows the ups and downs of temperature in the past three days at Fairbanks airport, and the gray squares show the amount of cloud cover as reported by the ASOS; the cloud cover scale is on the right.  I've also added a darker gray shade to denote hours when snow was falling, indicating that the clouds were relatively thick; the light gray squares indicate cloud with no snowfall.

Note the rapid drop in temperature on Thursday afternoon (Nov 23) when snow ended and breaks appeared in the clouds; but then the temperature began a steady climb back up to around 0°F as clouds thickened and snow occurred again on Thursday night and Friday morning.

A similar evolution then happened yesterday afternoon/evening, with thinning and breaking cloud allowing the temperature to drop back to -15°F, but then again snow and clouds pulled the temperature back up to 0°F by mid-morning today.  Until this morning winds were calm or light throughout this period, so the temperature changes were not related to changes of air mass.

And for a third time, and even more dramatically, dissipating clouds have once again allowed the temperature to drop very sharply in the past couple of hours.  It's a simple energy balance: the surface loses heat by upward radiation at a fairly constant rate, and as soon as the return flow of energy diminishes as clouds disappear, the temperature responds.  Without the beneficent presence of clouds, the cold of interior Alaska's winter would be much, much more severe.