Saturday, May 28, 2016

Arctic Ice Update

There's been considerable news coverage of the woeful state of Arctic sea ice this year, but it's worth posting an update on this anyway.  First off, a caveat: a critical U.S. satellite sensor failed last month, forcing a hasty, unplanned transition to a newer satellite to maintain the U.S. sea ice monitoring program.  Read about it here.  The NSIDC is showing current data from the new satellite, but it has not undergone a careful calibration process and is considered provisional.  That said, here's the latest time series of total Arctic sea ice extent; the sensor transition occurred on April 1, but there's no obvious discontinuity at that time.

If the new satellite data are to be trusted, sea ice has retreated rapidly this month and now sits far below the 1981-2010 normal; it looks like the anomaly is between 3 and 4 standard deviations, which is huge.  The previous record lowest extent was observed in (September) 2012 and this year's ice is far less extensive than 2012 at this date.  This doesn't mean 2016 is guaranteed to see a new record minimum, but I'd guess the seasonal rate of melt would have to be remarkably slow from here on out to avoid a new record.

Here's a similar figure from the Danish Meteorological Institute; they are using the same satellite data as NSIDC, so it's not surprising the charts look similar.
The latest map of Arctic sea ice shows big deficits compared to normal in the Barents Sea and along the east coast of Greenland, and of course sea ice is still well below normal in the Bering Sea.  Remarkably there is a large open region in the Beaufort Sea between Banks Island and Alaska, and another open area is evident in the northern part of Baffin Bay.

The PIOMAS model of Arctic sea ice shows that estimated ice volume grew uncharacteristically slowly in the first 4 months of the year and was far below normal at the end of April.

There won't be any prizes for guessing why ice growth was so anemic this winter; it was undoubtedly related to the extreme warmth around the Arctic basin.  To illustrate this, I created maps of monthly temperature anomaly from November through April, using both the station data and the gridded reanalysis (model) data - see below, and note that the color scales are the same (click to enlarge the maps).  I like to think of the left-hand maps as the "ground truth", as they're based on station measurements, but the reanalysis gives a generally credible picture of conditions over the Arctic ocean itself. 

Note the amazingly strong and persistent warm anomaly in the northern Barents Sea, a direct result of the abnormal lack of sea ice.  This anomaly might be a bit overdone by the reanalysis model, but perhaps not by much.  There is an observatory on one of the islands in Franz Josef Land, and their mean November-March temperature was 12.4°F, compared to an estimated 1981-2010 normal of -6.6°F, for a 5-month mean anomaly of +19°F.  (This station doesn't show up on the maps because I required 28 years of data to calculate the normals, but this station only had 24.)

Finally here's the ECMWF model-estimated 2m temperature north of 80°N so far this year, courtesy of the Danish Meteorological Institute.  It would appear that not a single day has been cooler than the 1958-2002 normal.

Sunday, May 22, 2016

Stratospheric Connection

A couple of weeks ago Rick Thoman pointed out that Barrow had their windiest April on record, by a large margin - see the chart below.  According to the GHCN data, the average wind speed was above 20 mph on 15 days, which is a record for any month.  Interestingly the all-time windiest calendar month in Barrow (1984-present, GHCN) was just 2 months earlier, in February of this year.

The map below shows why it was so windy in April on the North Slope (graphic courtesy of Brian Brettschneider).  The sea-level pressure was the highest on record for April over the Beaufort Sea, and yet the SLP was below normal over nearly all of Alaska, and well below normal over the west and southwest.  Consequently the pressure gradient across the North Slope was intense.

The record high pressure north of Alaska in April is intriguing, because it occurred in the wake of a remarkably strong stratospheric warming event back in March.  It is well-known that sudden stratospheric warming events create high pressure anomalies that propagate down to the troposphere over the course of several weeks, and so I suggest that there was a chain of causality between the upper-level warming event and the April high pressure.

The following figure illustrates the downward propagation of the flow disturbance in the recent event; this is a time-height cross-section of geopotential height anomaly over the Arctic (a height anomaly is equivalent to a pressure anomaly).  Beginning in early March, a pronounced positive height anomaly emerged high in the stratosphere, reflecting the complete breakdown of the polar vortex that occurred in the dramatic "sudden warming" event.  Note that earlier in the winter the stratospheric vortex was extremely strong (i.e. lower than normal heights), as we noted in December.

It's pretty clear from the figure that the height anomaly propagated down from the stratosphere to the troposphere and eventually to the surface between mid-March and late April.  Again, this is a typical course of events after a sudden stratospheric warming.  As an aside, this provides an unusually extended and useful window of predictability for high-latitude weather patterns in the troposphere.

The map below shows the standardized height anomaly at 850mb during the first 3 weeks of April, when the wind was particularly strong in Barrow.  The high pressure was centered about 500 miles north of Prudhoe Bay and produced a strong easterly flow along the Arctic coast of Alaska.

As we look at pressure levels higher up in the troposphere and stratosphere, the high pressure anomaly becomes more expansive and - by the time we get to 100mb - very intense.

Finally, it's revealing to look at the evolution of 850mb anomalies over consecutive 10-day periods from late March to early May - see the maps below.  In the last 10 days of March there was no high pressure evident in the Arctic at 850mb, although the stratospheric anomalies were already very intense at that time.  High pressure then emerged north of Alaska in early April and intensified and expanded in mid-April; and by late April high pressure was very widespread north of the Arctic Circle, and the Arctic Oscillation was strongly negative. 

In summary, a quick look at the reanalysis data suggests that the Beaufort Sea high pressure in early-mid April was the first manifestation of a "surge" of high pressure anomalies originating high in the stratosphere about a month earlier.  The lower-tropospheric anomaly subsequently expanded throughout the Arctic and set up a notable blocking pattern over the North Atlantic, which led to some remarkably cold conditions in Europe in late April - and I can attest to that, having seen snow fall on multiple days while in England and Wales at that time.  From the North Slope of Alaska to Europe it was a most unusual month, and I think it's clear that the stratospheric disruption was at least partially to blame.

Thursday, May 19, 2016

Seasonal Forecast

NOAA's Climate Prediction Center released their new forecast for summer (June through August) today, and the temperature outlook looks much as expected for Alaska: a warm tilt throughout, with the probability of significantly above-normal (upper tercile) temperatures greater than 50% for approximately the southern half of the state.

There is also an enhanced probability of a wet summer in the northwestern half; the CPC forecasters cite "anomalously open sea ice and warmer-than average ocean temperatures" as the likely cause.

The forecast for June temperatures is nearly the same as the summer forecast, except with a higher chance of warmth in the south; the continued strongly positive PDO phase is no doubt the main rationale for this forecast.  Wet conditions appear relatively likely across southwestern Alaska, and this is based on a consensus of computer models.

Here's a recent sea surface temperature map, showing both the positive PDO pattern and also the demise of El Niño - note the thin band of below-normal temperatures now evident along the equator in the eastern Pacific Ocean.

Let's compare these forecasts to the latest CFSv2 model forecasts (which of course do contribute to the subjective CPC forecast).  The model agrees with widespread warmth for June, but the probabilities are now lower than in recent months' forecasts across southern Alaska; the really high probabilities are now confined to the ocean areas.

There's virtually no signal for June precipitation anomalies across the state:

The summer (JJA) forecast looks quite similar to CPC's forecast, except that CPC has wisely (in my opinion) ignored the Arctic cold anomaly that appears to be related to the CFSv2 sea ice forecast.

There's a little more of a wet signal for the three-month period:

The other day I showed an analog forecast based on the late winter PDO phase, suggesting that summer might not be all that warm across interior and southern Alaska.  Reader Eric asked if the phase of ENSO was related to the patterns that emerged in the analog years, and it appears the answer is yes.  The map below shows the 5 (out of 10) years that had the strongest El Niño conditions in February through April (although El Niño was only truly strong in 3 of the years); there is only a weak cool signal over central Alaska.

Contrast this result with the map below for the 5 years with the weakest El Niño conditions in February through April; 3 of these years had a near-neutral ENSO phase (the other 2 were weak El Niño's).  In this case the summer cold signal is much stronger; so it appears that the cool summer scenario is more likely when a strongly positive PDO occurs in late winter without a strong El Niño; and that is certainly not what has happened this year.

Tuesday, May 17, 2016

Reality Check

In a reminder that summer isn't quite here yet, a dramatically cooler air mass has pushed into the northern and eastern interior in the past couple of days.  Yesterday was the first day since March 19 that the daily mean temperature was below normal in Fairbanks; 57 straight days were warmer than normal.

Saturday's high temperature of 82°F was the second earliest on record for such warmth, as only the great May heat wave of 1995 was more anomalous (88°F on May 11).  Ten years ago, in 2006, Fairbanks never even reached 82°F all summer long.

The sudden flip to chilly conditions today probably shouldn't be too surprising, as we noted last year that extreme warmth at this time of year seems to invite a strong reversal of temperatures.  In 1995, the high of 88°F on May 11 was followed by a low of 32°F on May 15, and then it turned even cooler, with a high of only 49°F on May 21 and a low of 28°F the next day (one of the latest occurrences of a 28°F freeze).

If today's high temperature in Fairbanks is in the 40s, as appears likely, then this will be a rare instance of the temperature exceeding 80°F before mid-summer and then a subsequent day (prior to autumn) failing to reach 50°F.  This has only happened before in 1963 and 1995.  Today is certainly quite cool, with 5pm temperatures of 35°F at the Chatanika RAWS (1626') and on the Parks Highway at Nenana Hills (1421'); and it's below freezing at Eagle Summit.  [Update: the temperature at the airport reached 51°F after light rain ended.]

Webcam images from Ester Dome above Fairbanks captured some snowflakes earlier this afternoon, and the Central webcam is showing fresh snow cover in the hills.  Farther afield, Eagle and Northway both reported some light snow earlier in the day.

The 500mb analysis from 4am AKDT today (courtesy of Environment Canada) shows the upper-level flow pattern responsible for the change: note the mid-tropospheric low right over the central interior.

Friday, May 13, 2016

Possible End to Warmth

While making a routine seasonal forecast the other day I stumbled on something surprising and, I think, quite interesting.  Here's what I came across: the map below shows the temperature pattern during summers (June through August) following a strongly positive PDO phase in late winter (February through April).

This year, as we all know, the PDO phase has been extremely positive so far; the February-April average PDO index was second only to 1941.  With above-normal temperatures continuing in apparently unending fashion across Alaska this spring, it is interesting to see that summer tends to be cooler than normal when the PDO is strongly positive in late winter.  Note that this is not because the PDO suddenly turns negative in these years; out of the 10 years shown above, the June-August PDO remained strongly positive in 4 of 10 years.  Two more of the years had a significantly positive summer PDO, and it was close to neutral on average in 3 years and became negative in only 1 year.

The map above is derived from the NCEP/NCAR reanalysis, which as we've seen before is not very good at all for Fairbanks temperatures in summer, so I checked the result with station data from Fairbanks.  Here's what the analog years looked like in Fairbanks; I've plotted the monthly temperature anomaly normalized by the standard deviation.  The black line shows the median of the 10 years for each month.

As expected the late winter and spring tend to be warmer than normal when the PDO is strongly positive, but the warm anomaly goes away by June, and each month from June through September is more likely to be cooler than normal.  Remarkably, August temperatures were nearly 1 standard deviation below normal on average in the 10 analog years.  It's fascinating also to see the warmth return in October and November.

Here's the corresponding chart for precipitation, expressed as a percentage of the 1981-2010 median for each month.  Dry conditions are typical in January through March in these positive PDO years, but the normally dry month of April is often relatively moist - as happened this year.  There is not much signal for precipitation in the rest of the year; May, June, and September show a tendency to be dry, but July and August show no marked departure from normal.

We can get a sense of the progression of the temperature signal by looking at month-by-month patterns from the reanalysis for the same analog years:

The expansion of the cool anomaly in August and September is pretty remarkable.  The maps below show the sequence of events from the perspective of 500mb height, with yellows showing a tendency for above-normal height (ridging or high pressure) and blues showing below-normal heights.  It appears that the interior's cool weather in June and July tends to be associated with high pressure to the north and cool northeasterly flow.  However, in August there is a strong tendency for a trough over the Beaufort Sea, which would bring a cool airmass to interior Alaska from the west or northwest; and in September there is often troughing over southeast Alaska, which is certainly a cool pattern.

Despite the lack of an obvious signal in the Fairbanks precipitation data, a broader look at precipitation in the analog years suggests that July is more likely to be wet than dry.

The other summer months don't show a notable precipitation signal, but the reanalysis solar radiation data suggests that relatively sunny conditions are favored in August and especially September (see below).  So if we look all the way ahead to early autumn, the PDO analog suggests that relatively chilly and clear conditions may be likely.  It will certainly be interesting to see if 2016 matches the historical pattern.

Update: for comparison, here's NOAA's forecast for June-August, issued last month.  NOAA will update their forecast on Thursday, but I doubt it will change much.