More on Persistence

I suspect that not everyone finds this as interesting as I do, but nevertheless here's a follow-up on seasonal temperature persistence in and around Alaska, this time from a map perspective.  Using ERA5 reanalysis data, I calculated the correlation of consecutive monthly temperature anomalies from 1950 through 2020, with the linear trend (specific to each month) removed.  A positive correlation means that the sign of the anomaly (i.e. above or below trend) tends to persist from one month to the next, but a negative correlation indicates that it tends to reverse.

It is usually a safe rule of thumb that weather and climate anomalies tend to be "persistent" even over land - there is a positive autocorrelation - so it's a surprise to see that the overall correlation is slightly negative for a portion of east-central Alaska and an adjacent zone in northwestern Canada.

This is actually the only place on the planet that has a negative month-to-month correlation, according to ERA5 data for this particular historical period.  There are plenty of regions with very low correlations, but this small region just to the north of Eagle is the only place with inverse persistence.  Here's a map for the Northern Hemisphere extratropics.

The tendency for temperature anomalies to reverse sign is mostly found in the winter for interior and eastern Alaska.  Here's the December-January correlation:

The rest of the year is added below.  The maps confirm the observation I made in the previous post: for the state as a whole, persistence is very much heightened in April-May and in July-August.  One might say those pairs of months are temperature twins: they tend to resemble each other in terms of departure from normal.

The other striking point is how much higher persistence is in summer than in winter for the maritime southern regions, and for late summer and early autumn near the Arctic Ocean.  Clearly the warm season temperature anomalies are dominated by slowly-varying ocean temperature regimes in locations close to open water, whereas relatively chaotic atmospheric flow patterns tend to control the month-to-month temperature changes during winter or over ice-covered ocean.

Seasonal Persistence

As spring gets under way in Alaska, temperature anomalies tend to become more persistent from week to week and from month to month.  What I mean is that colder than average - or warmer than average - weather tends to stick around more in spring than in winter; but winter temperatures are more variable over the course of weeks and months.

The chart below shows some evidence to back up this claim.  I've taken the NOAA/NCEI monthly temperatures for Alaska as a whole, removed the 1950-2024 trend, and calculated the month-to-month similarity of departures from normal.  The orange columns show the traditional correlation coefficient of adjacent monthly anomalies, and the blue columns show a "persistence index" that I defined here: the index takes a value of 1 if the adjacent monthly anomalies always have the same sign (perfectly persistent), and a value of 0 if the anomalies always reverse sign from month to month (perfectly anti-persistent).

The only pairs of months with fairly strong month-to-month persistence of statewide temperature anomalies are April-May and July-August, although there's a secondary peak of modest correlation in October-November.  Month-to-month persistence from November though March is remarkably low.

Looking at weekly data from Fairbanks shows a more prominent peak of persistence in the autumn (early October at a one-week lag), and there's also a clear peak in mid-April.  These two peaks are undoubtedly related to the persistent impact of snow cover anomalies: if there's more snow than usual at those transition times, it tends to remain cold, but if snow is lacking, it tends to remain warm.

It's interesting to note a pronounced dip in persistence in early June for Fairbanks, but I can't immediately think of an explanation for that.

In Anchorage we see much more of the late summer persistence that characterizes the statewide temperatures.  It's tempting to attribute this to the persistent effect of sea surface temperature anomalies around the western and southern parts of the state, but I'm not sure why the effect would show up more prominently at just that time of year (late July, early August).

The NCEI data for the Cook Inlet climate division confirms July-August as peak season for temperature persistence in the South-Central region, but the correlations are also significantly positive throughout spring and early summer.

Hopefully readers agree that "secondary" climate statistics like this provide interesting nuance and subtle insight into seasonal climate: there's a lot more to climatology than just the progression of normals and averages.  The next step in this analysis will be to calculate persistence from the gridded data so that we can examine the spatial distribution for each time of the year.

Late Winter Chill

Clear skies and a cold air mass have produced very chilly overnight temperatures in the past several days across interior and northern Alaska.  Some of the coldest readings have come in from the remote (but high quality) CRN sites, including the Selawik NWR (-38°F this morning), the Koyukuk NWR (-35°F), and the Toolik Lake site (-31°F).  It was -36°F this morning at the Beaver RAWS on the Yukon Flats, and Anaktuvuk Pass dropped to -33°F at the other end of the elevation spectrum.

Fairbanks airport dropped to -7°F, but this isn't at all noteworthy from a historical standpoint; it is typical to see -20°F at some point in the second half of March.

Here are this morning's lows across the central and northern interior (click to enlarge):

Interestingly, the change of weather pattern has brought significant snow to Anchorage for the first time since autumn.  Anchorage saw its least snowy winter (December-February) on record, with a measly 4.6" of snow in those three months, but now they've managed to accumulate 14.6" in the past week.  Adding this month's total to October's heavy snow gives a total of 36.1", well over 3 times the amount that fell in November through February.

The following chart illustrates the contribution of November-February to the total seasonal snowfall in the modern Anchorage climate history.  Typically about 30% of the seasonal snowfall occurs outside of Nov-Feb, and this ratio has been as low as 11% (1986-87) and as high as 62% (2001-02) in past years.  This year the ratio is 78% and the snow season isn't over yet.  Note that there's zero correlation between the two sub-totals over the years.

It's a topic for another day, but there is actually a statistically significant downward trend in the contribution of autumn and spring snow to the seasonal total in Anchorage.  This is what we would expect if the climate warms, because the autumn and spring temperatures are more marginal for snow.  So this winter very much counters the trend, because of the extremely dry winter (from the standpoint of snow - there was rain instead).  Last winter was the polar opposite, with only 12% of snowfall outside of Nov-Feb.

Fairbanks sees only slightly more of its seasonal total snowfall in months other than November-February; the average is about 33%.  The ratio has ranged from only 7% (2016-17) to 63% (1947-48).  Unlike in Anchorage, there is a slight positive correlation between the two sub-totals; but there is no significant trend in the contribution of autumn and spring snowfall in Fairbanks.

February Climate Data

NCEI and ECMWF have updated their climate data for February, so we can have a look back at last month and also the traditional climatological winter season of December-February.

February was the "least warm" month of winter for Alaska as a whole, but it was still warmer than the 1991-2020 average across most of the state's land area.  It was especially warm in the west and southwest, and quite warm on the North Slope too:

The obvious difference from December and January was in the Southeast, where February was colder than normal, and quite significantly so in the southern Panhandle.  This change occurred because instead of a ridge over western Canada, February saw a trough from the north-central Pacific to southern Canada, and the flow orientation brought cold interior air to southeastern Alaska.  November was somewhat similar, with Southeast being colder (relative to normal) than the rest of the state.

For Alaska as a whole, December-February was the 3rd warmest such period in the NCEI climate data history (1925-present); only 2000-01 and 2015-16 were warmer.  While unusual warmth was universal across Alaska, it was most pronounced for the Alaska Peninsula and the northwestern Gulf coast.  Both Cold Bay and Kodiak had their second warmest Dec-Feb on record (2013-14 and 2014-15 were the warmest at these two sites respectively).

While February didn't mark a dramatic break with earlier warmth in most of the state, the precipitation contrast between January and February could hardly have been greater, as nearly all of the state was drier than normal in February.  In southwest and south-central areas the deficit was really unusual; Anchorage saw its driest February on record, and according to Rick Thoman several locations in the Palmer area reported no measurable snow at all.  This is really remarkable, following January's onslaught of moisture.

January was so wet that despite February's dryness, large parts of the state were significantly wetter than normal for Dec-Feb overall.  According to ERA5 model data, only the North Slope was notably dry, but the NCEI February estimate strongly disagrees on this.  Given the paucity of reliable ground-truth winter precipitation data, I would tend to trust ERA5 more, but Utqiaġvik did report over an inch of liquid equivalent, almost enough to be in the top 10 for Dec-Feb precipitation.

As of the end of February, snowpack was very healthy for monitored areas in the western and northern interior, but major deficits are evident in South-Central.  The Iditarod start was moved to Fairbanks for the 4th time (other years 2003, 2015, 2017).

ERA5 snow data shows a similar north-south contrast in snowpack fortunes:

The lack of snow in southern areas increases the risk of early season wildfires, and that has been reflected in the fire potential outlook:

https://akfireinfo.com/2025/02/28/low-snow-high-risk-what-alaskans-need-to-know/

Burn permits will be required beginning just one week from now across southern Alaska:

https://akfireinfo.com/2025/03/06/early-wildland-fire-season-expected-in-alaska/

Finally, I'll close with ERA5's estimate that the Dec-Feb season was much windier than normal across the northern half of Alaska (see below).  This is broadly consistent with the warmth, as winds tend to mix out the surface-based inversion that is associated with valley-level cold; but the relationship is quite weak and is very elevation-dependent.  See here for some previous results on the temperature-wind correlation in the model data:

https://ak-wx.blogspot.com/2020/12/wind-and-cold.html

Winter Warmth and the PDO

Blog posts have been few and far between lately, owing to personal circumstances, but hopefully the pace will improve in the near future.  For now I think it's worth highlighting a particularly interesting aspect of this winter in Alaska: the remarkable juxtaposition of far above normal temperatures with a negative PDO phase.

And warm it certainly has been.  While February climate data isn't all available yet, Fairbanks and McGrath both had their 3rd warmest winter (Dec-Feb) on record.  Winter 2000-01 holds the record at both sites, and they differ on the 2nd warmest (1976-77 in Fairbanks, 2017-18 in McGrath), but both came in at #3 this winter.  Here's the time series for Fairbanks:

We (or at least I) tend to think of 1976 as the great climate shift to multi-decadal warmth in Alaska, and that change certainly had a lot to do with the PDO phase shift at the time; here's a paper on that topic:

https://journals.ametsoc.org/view/journals/clim/18/22/jcli3532.1.xml

If we look at Fairbanks DJF temperatures since 1950 with the PDO phase indicated, we see the predominance of a positive PDO in the 1980s, and we also see that virtually all of the very warmest winters have occurred with a positive PDO phase - until this winter:

In terms of correlation, the PDO index has historically shown a moderately strong relationship with Fairbanks winter temperatures, and therefore again winter 2024-25 is a pretty significant outlier:

What seems to be happening is that the PDO-temperature correlation is diminishing over time; it was quite strong before 1990, much less significant in more recent decades, and now it seems this winter has lost the relationship altogether.

There are a few different hypotheses one could pursue on this.  One might be that while the PDO ocean temperature index continue to oscillate (it's just a mathematical construct), the atmospheric circulation patterns that are occurring in tandem with those ocean anomalies have changed in such a way that the PDO now relates differently to Alaska climate.  This winter, for instance, there was an atmospheric ridge over Alaska even though the anomalous warmth in the northwestern North Pacific (and consequent negative PDO) might have been expected to deliver a trough near Alaska.

Another perspective might be that high-latitude warming ("Arctic amplification" of global warming) has become so pronounced that a negative PDO can't be expected to produce historically cool temperatures; even a "cold" pattern is now warm by historical standards.  I think there's some of this going on - broader Arctic temperature trends are partly responsible for this winter's warmth - but in fact the circulation pattern was favorable for warmth in Alaska this winter; it wasn't a "cold" pattern except in terms of the raw PDO index.

It's "interesting" to consider what might happen if all the signals line up on the side of warmth one of these years; it seems a strongly positive PDO index with a classically strong Aleutian low and rampant southerly flow could produce a winter far warmer than any observed in modern climate history.  It may just be that this winter is actually quite "normal" in terms of the climate of the next few decades.

Freezing Rain Trends

Back in December I commented on a significant ice storm in Haines, and I noticed that there seems to have been an increase in frequency of freezing rain events there.  I've been meaning to take a closer look at this.

First, here are some charts showing the number of hours each winter with freezing rain reported by the airport ASOS instruments in Haines, nearby Skagway, Juneau, and also Yakutat (not in the same climate zone but not too far away).  The two winters before this one (2022-23 and 2023-24) both brought a lot of freezing rain to Haines, and 2021-22 saw far above normal freezing rain in Juneau, but unfortunately the data is missing for Haines for most of that winter.

At Yakutat, the data shows a sustained higher frequency of freezing rain since 2016-17; but interestingly this winter is not following suit, with only a couple of brief instances (not included on the chart).

If we average the data from these 4 sites together (using the annual departure from normal at each site), we get an interesting result that does suggest an uptick in frequency since 2021-22.

Does this have any support from the ERA5 reanalysis data?  There is a modest correlation between ERA5 and "ground truth" data for Haines, but surprisingly ERA5 does not show any increase in the last couple of years:

A map view of the ERA5 data confirms no widespread increase in freezing rain across the northern Panhandle region, although an increase shows up in a few places for 2023-24.  Of course the region has extremely complex topography and we would expect huge variations in local conditions; there's no doubt the ~30 km grid spacing of ERA5 is far too coarse to handle this properly, but nevertheless it apparently does have some modest ability to reproduce conditions in Haines, at least prior to 2022.

Here's the average since 2013-14, the beginning of the recent sustained warmth in the North Pacific and Alaska region:

There's no signal for the northern Southeast, so it is difficult to draw a firm conclusion as to whether freezing rain is increasing there or not.

Of course it's a different matter in the southwestern interior, Y-K Delta region, and the northern Bristol Bay coast, where freezing rain has certainly increased in recent years.  Here's a chart of observed frequency from Bethel:

And a longer-term view, bearing in mind that pre-1998 data comes from manual observations, not ASOS, and so there might be (probably is) a bias/discrepancy in how frequently freezing rain is reported.

See this previous post for additional results on winter rain and freezing rain in Alaska and the Arctic:

https://ak-wx.blogspot.com/2022/12/winter-rain.html

Finally, for good measure, here are charts for Anchorage and Fairbanks.  Note that this is for freezing rain, not plain rain (such as occurred last month in many places).