Wednesday, September 20, 2017

Autumn Cooling

There's a slight chill in the air today across interior Alaska, with afternoon temperatures around Fairbanks only in the mid-40s at valley-level thanks to clouds and some light rain.  Temperatures are in the mid-upper 30s in the higher hills; and a glance at the calendar reveals the reason.  Here are some of the observations as of about 6pm (click to enlarge).

Yesterday was the first day with a high temperature below 50°F in Fairbanks, and this is about a week late compared to normal.  Fairbanks usually sees its first sub-50°F day more than a week earlier than Anchorage, which illustrates the contrast between the rapid cooling of the interior and the slower cooling that occurs closer to the waters of the North Pacific.  Anchorage has never seen a sub-50°F day in August during the modern historical record, but it's not too uncommon in Fairbanks and happened just two years ago.

Here are a couple of webcam views of Fairbanks; autumn colors look to be a little past peak.

At this time of year it's always interesting to be reminded of the rapid drop-off in solar radiation at northern latitudes during autumn.  Thanks to 15 years of high-quality data from the Fairbanks CRN site (actually 11 miles northeast of town), we know what the normal incoming solar radiation curve looks like - see below.  I've added the normal cloud cover from Fairbanks airport to give an idea of relative changes in cloudiness through the year, although cloudiness is presumably a bit greater at the CRN site.

Notice how quickly the solar radiation diminishes in mid to late September; the average amount of incoming solar energy drops by 50% in the last 3 weeks of the month.  Of course, at the beginning of September it's already down by about 50% from the peak in June.

Here's the equivalent chart for Barrow, also from 15 years of CRN data; there's little available solar energy by this date in Barrow, and the very high cloud cover in late summer and autumn only exacerbates the rate of decline.

Just for fun, here's the equivalent solar data from the most southerly CRN site in the continental U.S., at Everglades City in Florida, along with the normal cloud cover from nearby Fort Myers.  By September 20 the normal solar radiation has decreased by only about 20% from the solstice, although the annual peak in south Florida occurs in May prior to the wet season.  The slow decline of the solar input, along with the maritime environment, explains why September is basically still high summer in Florida.

But the most interesting aspect of the comparison may be that Barrow receives more solar energy on average in June than south Florida - even though Barrow is very much more cloudy!  This is the result of 24-hour daylight; the noon sun in Barrow is nothing like the noon sun in Florida, but the hours of sunshine really add up in the far north.

Monday, September 18, 2017

La Niña Emerging

While the attention of many meteorologists (this one included) has been drawn to the remarkable storminess in the tropical Atlantic Ocean lately, significant events have been unfolding at the same time in the tropical Pacific.  Here's a sequence of maps showing the departure from normal of the sea surface temperature (SST) at 4-week intervals; notice the expanding area of below-normal SSTs along the equator in the central and eastern Pacific.

In tandem with the clear trend towards more La Niña-like conditions, the leading seasonal computer models have shifted quite dramatically towards a La Niña outlook for winter, and in response the most recent IRI/CPC forecast showed a sudden change to a much higher probability of La Niña.  Compare the two forecasts below, issued only a few weeks apart.  This is as dramatic a shift as I can recall in what is usually a slowly-evolving assessment of long-range forecast possibilities.

So what does the prospect of La Niña mean for Alaska?  One could be forgiven for having a hazy recollection of the last La Niña episode - the last time a modest La Niña occurred in winter was 2011-2012, and the last strong episode occurred the year before that.

The key feature of a La Niña winter, from the Alaskan viewpoint, is that the normal trough of low pressure over the Bering Sea and Aleutians tends to be relatively weak, and episodes of high pressure in this area are more common than normal.  A ridge over the Bering Sea is a cold set-up for most of the state, so it follows that most of Alaska is usually colder than normal during La Niña, although the signal is strongest in the southern part of the state.

The propensity for high pressure to Alaska's west and southwest reduces the frequency of storm systems moving into southern Alaska, so the amount of snow and rain is typically lower than normal in the south.  However, the west and north of mainland Alaska tend to see above-normal snowfall.

Here are maps of the November-March pressure, temperature, and precipitation patterns that were associated with the 10 strongest La Niña episodes since 1950, based on the Multivariate ENSO Index of Klaus Wolter.

Another aspect of the La Niña influence is that the flow pattern and associated temperatures tend to be more variable than normal over Alaska, so while the mean is below normal, fluctuations from week to week tend to be large.  This means that very cold conditions (much below normal) are considerably more likely than normal, but the chance of occasional very warm conditions is not much reduced except in southern and southeast Alaska.  For more reading on this, search "ENSO variance" on this blog.

Finally, the charts below provide another viewpoint on Fairbanks winter temperatures as they relate to the ENSO phenomenon. For this analysis I've calculated the November-March temperature anomaly relative to two different climatology periods in an attempt to remove the 4.5°F systematic difference between the early (1950-1975) and later (1976-2016) periods.  The vertical colored lines show the ENSO tercile boundaries.

The cold signal associated with La Niña is evident in the lower left, but it's clear that marginal La Niña winters are not always colder than normal, and the overall correlation between the ENSO index and Nov-Mar temperatures is not particularly good.  This partly reflects the fact that the PDO phase is not always aligned with the ENSO phase, and of course the PDO is more closely connected to Alaska winter temperatures than ENSO - see the corresponding chart below for the PDO correlation.

If we re-do the ENSO chart after excluding winters with an out-of-phase PDO, then the correlation improves quite a bit as we would expect, and especially for El Niño winters.

So in conclusion, if La Niña continues to develop into a strong episode this winter, then the usual La Niña patterns will become quite likely in Alaska; but if we see a less robust La Niña, then the outcome is much less certain.  Another way of saying this is that the PDO is more important; but unfortunately it is much more difficult to predict the PDO phase.  The PDO has been bouncing around near neutral recently and shows no indication of becoming significantly negative - and indeed I would say a significantly negative phase is probably unlikely because of the lingering subsurface effects of the strongly positive PDO phase that we've experienced in recent years.

Friday, September 8, 2017

Cooler Arctic Summer

Arctic sea ice extent is now close to its seasonal minimum, but in the past few weeks there has been less ice retreat than in some recent years.  Consequently a new record minimum extent is not likely to occur this year, although the National Snow and Ice Data Center notes that the ice edge in the Beaufort Sea is currently farther north than at any other time in the satellite era.  Here's the latest map of ice extent (defined as areas with greater than 15% ice concentration).

The summer circulation pattern was quite unsettled over the Arctic Ocean, with persistently below-normal sea-level pressure, especially to the north of Siberia.  The Arctic Oscillation was generally positive, corresponding to lower than normal pressure in the high latitudes.  Here are monthly maps of sea-level pressure anomaly from June through August.

Based on the 19 Arctic coastal observing sites that I've used before on this blog (e.g. here), temperatures were quite close to the 1981-2010 normal this summer on average around the Arctic basin, and this is a marked contrast from the persistent and extreme warmth of 2016.  The first chart below shows this year's June-August mean temperature in the context of recent decades, and the second chart shows the rather sudden cooling (relative to normal) that has occurred since late winter.

Given that Arctic sea ice extent is still far below normal, it's very likely that unusual warmth will return around the Arctic basin this autumn as the wide expanse of open water provides a direct heating influence; this post from last year shows the extraordinary warming trend for the month of October.  It will be interesting to see how the winter turns out, but in light of recent years it would be surprising to see anything other than significantly above-normal temperatures again for the seasonal mean over the Arctic basin.

Saturday, September 2, 2017

Brooks Range Chill in Context

Following up briefly on the last post, temperature data from the Atigun Pass SNOTEL provide a bit more context for the rather chilly conditions observed at this high-elevation site in recent days (down to 17°F on Wednesday).  Historical temperature data go back to 1999, although prior to 2006 there is only one data point provided per day - the temperature at midnight.

The chart below shows the accumulation of freezing degree days through August and September for each year since 1999 (calculated with only the midnight temperature throughout, for consistency).  The black line, indicating this year's pace of freeze-up, is a little ahead of the 18-year normal, but only by a few days, and the early chill this year has been much less severe than in 2000 and 2002.

Looking just at August total FDDs, the data from the early years confirm what I suspected - that the apparently cooler August conditions of the past few years are not unusual relative to a longer history.  From this perspective, the relative absence of chill in a number of years from 2004-2012 seems more unusual.

A longer and higher-quality history of temperature data is available from Toolik Lake Research Station, just 40 miles farther north up the haul road - and while this site is not quite in the mountains, the elevation at Toolik is still a respectable 2500'.  The chart below shows August FDDs at Toolik, confirming that recent years have brought freezes closer to the long-term normal, although not as severe as in some of the earlier years.

What could explain the early arrival of autumn chill in the Brooks Range in recent years?  The 500mb height chart from Wednesday provides a clue: notice the strong, cold upper-level trough over the North Slope.  More analysis would be needed to be sure, but it's likely that recent years have seen this kind of feature relatively more often at about the same time on the calendar.

Finally, lest we be tempted to infer that Arctic-wide conditions are cooler now than in recent years, the latest Alaska-centric sea ice analysis shows open water for more than 300 miles north of Barrow.  Arctic sea ice extent won't set a new record low this year, but it is still far below the long-term normal, and estimated ice volume is tracking near record lows.  More on that another day.