Monday, June 10, 2024

May Climate Data

The month of May was the coldest since 2013 for Alaska as a whole, according to NOAA/NCEI data, and it was also wetter and cloudier than normal across most of the state - a rather unpleasant end to spring, and problematic for breakup flooding, especially for Kuskokwim River communities.  The only somewhat dry region was the Arctic northwest and Brooks Range, according to ERA5 data.  Also notable is the fact that it was the first wetter-than-normal month of the year for the southern Panhandle.

The far southwestern mainland and upper Alaska Peninsula saw the most anomalous cold for the month, with the "Bristol Bay" climate division having the coldest May since 2001, and before that 1985.  Checking in on a few sites in the region, Iliamna stands out as particularly unusual: the monthly mean temperature of 37.1°F was the coldest since 1971.

The cold and wet pattern was a direct reflection of a cold upper-level trough over southwestern Alaska.  The jet stream was well to the south, riding the strong north-south temperature gradient between extreme warmth in the central North Pacific and chilly conditions in the Bering Sea region:

By my calculations, the monthly PDO index of -1.8 was the most negative for May since 1950.  The 3-year smoothed PDO index is also the most negative since the early 1950s, illustrating that we're in one of the most entrenched and significant negative PDO regimes in the modern climate history.

As for wind, both northwestern Alaska and the eastern Aleutians were much windier than normal in May, and so was the southern Panhandle, according to ERA5.  However, the Arctic coast was relatively calm after a rather windy start to the year.

Wednesday, June 5, 2024

Bering Sea Cold

Alaska's west coast and southwestern peninsula have been remarkably chilly in the past couple of days - the coldest in many years for this date on the calendar.

But before I get into details on that, it's worth noting that the Fairbanks NWS office has issued 4 severe thunderstorm warnings (so far) today: one early this afternoon to the east of Fairbanks, and then 3 more for severe storms west of Nenana and up towards Manley Hot Springs.  Historically, many years pass with less than 4 severe thunderstorm warnings issued for the whole summer in the Fairbanks office.  Here's a photo taken around 4:20pm from UAF, courtesy of the NWS via X/Twitter:

A very strong and cold upper-level trough to the west and south provides part of the explanation for the stormy weather: there is unusually strong convective instability caused by the cold air aloft. 

The trough has also produced extremely chilly conditions for the Bering Sea and southwestern Alaska yesterday and today.  Bethel's high temperature was only 40°F yesterday, and snow was observed briefly in the morning.  Bethel hasn't seen a daily high temperature that low in June since 1972.  Similarly, yesterday's high in Cold Bay was only 39°F - again with some snow - the coldest this late in the spring since 1985.

Out in the Bering Sea, St Paul Island reported several lengthy periods of light snow earlier today, with visibility down to 1.5 miles and the temperature hovering at or just below freezing.  The FAA webcam showed some accumulation, as illustrated below:

Remarkably, this morning's balloon sounding from St Paul Island reported a temperature of -9.7°C at 850mb, which is the lowest this late in the season since 1985.  This is about the same as the normal 850mb temperature in January.

The unseasonable cold is related to a suddenly very intense negative phase of the PDO.  Check out the SST anomaly pattern across the North Pacific: extreme warmth extends eastward from Japan, but the Bering Sea has become much colder than normal.

The daily PDO index managed to reach -3.0 for the first time since September 2012, and it's only the second time it's happened in the history of daily data since 1982.

What's driving the anomaly?  Well, the negative PDO regime has been more or less locked in for several years now; but the current amplification is happening as the perennial North Pacific ridge has joined forces with Arctic high pressure via a ridge over eastern Russia, allowing the Bering Sea trough to deepen and intensify.  As for the Arctic ridging, it's a "blocking pattern" that's conducive to extremes at lower latitudes - for example, flooding in central Europe a couple of days ago, and snow in upland Scotland yesterday.

Thursday, May 30, 2024

What Will Fire Season Bring?

Alaska's wildfire season traditionally gets under way in June, and it's an interesting question as to what this year will bring.  Two years ago the fire season was very active (3.1 million acres) despite an entrenched La Niña, and that was an unusual combination; see my early season comments from 2022:

In contrast, last year we had a strongly developing El Niño, and Canada had its worst fire season on record, but remarkably Alaska's fire acreage was less than 300,000 acres (and most of that was in August).  The 1979-2023 statewide annual median acreage is 482,000 acres.

The chart below highlights that a majority of very active years have occurred with a warm tropical Pacific phase (positive ENSO or El Niño) in recent decades, but the last two years illustrate how fragile the statistical connection is.

In previous comments on this topic, I suggested that the unusual ocean warmth close to Alaska probably had a lot to do with the 2022 season: this too was highly unusual for La Niña.  If we look at conditions in May leading into the 10 most active fire seasons (since 1979), we see a strong connection to unusual warmth across the Bering Sea and western Alaska - see below.  Bear in mind that fires in May rarely contribute significant acreage to the season's total, so this warm signal is a precursor to the vast majority of the total fire activity in big years.

In years with very low fire acreage, May tends to be cool and damp, with a notable cold signal over the Chukchi Sea, no doubt partly related to higher sea ice in the 1980s.

May 2023 was indeed a lot cooler over the Bering Sea and western Alaska than May 2022.

And how are we looking this year?  May 2024 has been quite cool in Alaska, and notably so in western and southern areas, so it seems there may be a good chance of another subdued fire season like last year.  This would also be quite consistent with the CPC's summer forecast, which points to an elevated probability of significantly above-normal rainfall across interior Alaska, albeit with above-normal temperatures for central and eastern regions:

The May 1 wildfire outlook from NIFC called for "normal wildfire potential statewide from May through August", but given how May has unfolded, I wouldn't be surprised to see an adjustment toward below-normal risk in the upcoming June 1 outlook.

Thursday, May 23, 2024

Early Spring Wet Pattern

I promised to follow up with a few remarks about the wet pattern that has become a recurring theme of early spring (March-April) for western Alaska since 2018.  Rick Thoman pointed out this phenomenon on his blog:

As I commented briefly before, it seems pretty obvious that the immediate cause of the wet weather is the vigorous jet stream and storm track into western Alaska caused by a persistent ridge over the North Pacific.  Here's the March-April 500mb height departure from trend, averaged over the last 7 years.

And here's the average 500mb vector wind anomaly (departure from normal): note the stronger than normal westerly flow into southwestern Alaska. 

I decided to look back prior to 2018 for an independent assessment of the relationship between west coast precipitation and North Pacific 500mb height patterns.  Here's the correlation of March-April 500mb height (detrended) with March-April precipitation in Nome:

This certainly supports the idea that a ridge axis from the Sea of Japan to the Gulf of Alaska is favorable for enhanced precipitation in Nome at this time of year.  A trough along the eastern Arctic coast of Russia also tends to be in the mix.  The pattern in the historical analysis implies a southerly wind component across western Alaska, and that corresponds to widespread warmth for western and northern Alaska.

The March-April pattern was slightly different this year, with a more westerly regime and cooler conditions in southwestern Alaska. 

How about sea surface temperatures?  Surprisingly, there's not much historical correlation between Nome precipitation and North Pacific SSTs at this time of year; it seems that early spring precipitation has historically been controlled by the vagaries of the weather pattern much more than by ocean temperatures.

In contrast, however, I think the recurring pattern of the last 7 years is in fact linked to North Pacific SSTs, and strongly so.  Check out the sequence of March-April SST maps below, starting with 2014, when the so-called "blob" of excessive warmth was very much evident in the northeastern North Pacific.  This anomaly was linked to the "ridiculously resilient ridge" over western North America, leading to California drought and other things.  The "blob" pattern evolved into a strongly positive PDO pattern by 2016 in response to the intense 2015-16 El Niño, but then a regime change occurred in 2017-2018 as warmth transitioned to the northwestern North Pacific:

It is remarkable to see how persistent the pattern has become since... yes, 2018.  Here's a summary graphic showing the March-April detrended SST anomaly over the 7 years:

The alignment with the 500mb height anomaly is undeniable:

A really interesting aspect of this is that the warmth from Japan to the waters south of Alaska has persisted more or less unchanged regardless of El Niño and La Niña.  In the sequence of SST maps above, notice the major swings in tropical Pacific SST anomalies.  Normally we expect extratropical SST patterns to respond to ENSO - as in 2015/2016 - but the northwestern North Pacific seems to be marching to a different tune.

As an aside, it's worth noting that the emergence of warmth in the NW North Pacific in 2018 was quite striking at the time, and I commented on it in the North Pacific "blob blog":

Is this just the negative PDO phase that we're seeing in the North Pacific pattern?  Not entirely.  The PDO has indeed been persistently and significantly negative since 2021 - but not in 2018-2019.

A classical negative PDO phase tends to be linked to a trough over Alaska and cool waters from the Bering Sea and Gulf of Alaska to the northeastern Pacific, but the Bering Sea has been generally warmer than normal in recent years (especially in 2018-19), and Alaska hasn't been unusually cold.  Also, a negative PDO is not linked to above-normal precipitation in Nome in early spring.  The following maps show the average March-April pattern when the PDO was strongly negative over the decades.

So what can we take away from this discussion?  I think we can conclude rather confidently that the early spring wet pattern in (especially) western Alaska is linked to the persistent and exceptionally unusual North Pacific warm zone stretching east from Japan that emerged in 2018.  This developed in tandem with atmospheric ridging that both reinforced and was reinforced by the SST anomaly, and the atmospheric circulation anomaly has produced a storm track into western Alaska.

As for the deeper cause of the North Pacific anomaly, we can attribute some of it to the negative PDO phase (i.e. natural multiannual/decadal variability), but the excess warmth from the Bering Sea to the Gulf of Alaska is unexplained.  It feels like a new angle on the multi-year northeastern Pacific warm episode that developed in winter 2013-14, although the new warm zone now has a much greater duration.  I also have a sense that the new regime is related to "Arctic amplification" of the global warming trend, but this is speculation; presumably at some point the North Pacific pattern will shift again.

Finally, a question for next time: how else has the 2018 regime shift affected Alaska climate?  It will be interesting to look for other climate anomalies that developed at the same time, but for other locations and at other times of year.

Saturday, May 18, 2024

Strong Bering Sea Storm

Rick Thoman pointed out that an unusually strong storm made its way across the northwestern Bering Sea earlier this week.  Here's the surface analysis from 4pm AKDT Wednesday, courtesy of Environment Canada, with a red circle indicating the location of St Lawrence Island.  Click to enlarge.

The storm's minimum MSLP was estimated to be 974mb as it passed west of St Lawrence Island, and it "made landfall" in eastern Chukotka at 977mb.  This is unusually low for the time of year - in fact, it's the lowest MSLP on record (1950-present) anywhere in the northern Bering Sea for this late in the season.

To illustrate this, the map below shows the 1950-2023 minimum MSLP for May 11th through the 31st, based on ERA5 data.  The record low values from the northern Gulf of Anadyr across to the northeastern Bering Sea and Norton Sound are 980mb or above; so this was a record strong storm for this late in the season (the June record values are higher). 

Are strong storms becoming more common at this time of year in the Bering Sea?  Not at all.  Interestingly, the trend is the opposite: here's the annual series of May minimum MSLP in a box that approximates the northern Bering Sea.  Note that this is for all of May: there have been stronger storms than this week's event in the first week of the month.

The trend is statistically significant at a fairly high level (R=+0.28, 74 years), and it makes this week's event all the more notable.  But admittedly there is some uncertainty in the reanalysis data, especially for the earliest years (pre-1958, in particular).

Looking at trends in monthly minima through the year, spring stands out as a time with positive trends, i.e. less extreme storms over time, whereas February shows a trend for increasingly strong storms.

The time series for February again has a significant trend, and indeed the change around 1990 does look real.  I don't have an explanation for these trends.