Ocean Temperatures and Halong

In an earlier comparison of ex-typhoon Halong versus other historic autumn Bering Sea storms, I noted an intriguing similarity: Halong, Merbok, and the major storms of 2011 and 1974 all occurred during prolonged La Niña episodes in the tropical Pacific.  It's worth digging into this a bit more to see what might be going on.

Looking back at historical data since 1950, there have been 11 years - including this year - when La Niña occurred in late autumn for the second or third consecutive year.  (It's not uncommon, by the way, for major La Niña episodes to extend over multiple years, in contrast to El Niño, which tends to be "one and done".)  Here's the average sea surface temperature (SST) anomaly pattern in September through November of those years (relative to the long-term trend):

For comparison, here's the September-October SST anomaly this year:

There's a lot more warmth in the northeastern Pacific this year compared to the historical pattern, but there is a similar contrast between a cool tropical Pacific and warmth extending eastward from Japan.  The warmth from Japan eastward is very characteristic of a negative PDO phase, and indeed the PDO has been strongly negative in recent months.

Zooming in on the North Pacific, it's interesting to see that Halong and Merbok both passed over very warm ocean water (relative to normal) to the south of the Aleutians before moving up into the Bering Sea.  Based on the analysis above, that region of warm water is typical of persistent La Niña and negative PDO regimes.

The storm of 2011 also originated over a region of warm water, although the North Pacific as a whole was much cooler back then.

It's tempting to speculate that the extra warmth and moisture available from the unusually warm ocean to the south of the Aleutians provided more fuel for these storms than would be derived from cooler oceans.  Rick Thoman mentioned to me that modeling research is already under way to quantify the role of the unusually warm SSTs for Halong, with preliminary results showing a significant impact.

Another aspect of the persistent La Niña/negative PDO SST pattern is that the north-south ocean temperature gradient is greater than normal near the Aleutians, and this gradient tends to enhance the North Pacific jet stream, providing more upper-atmosphere support for strong extratropical cyclones.  Here's the average 500mb height anomaly in the same "persistent La Niña" years; notice the strong trough from the East Siberian Sea to Alaska.

Again, this is not too dissimilar to what we've seen this autumn so far:

It's worth noting that autumns in which La Niña is just developing do not have the same magnitude of warmth to the east of Japan, and they lack an upper-level trough near the Bering Sea that would support (and reflect) strong storm activity.  The PDO tends to be less negative.

Part of the reason for the difference is that La Niña favors high pressure ridging over the central North Pacific that gradually produces warmer ocean temperatures over time, and so the warm anomaly east of Japan becomes more amplified by the second or third consecutive La Niña winter.  In contrast, an initial La Niña is often developing after El Niño, and so the mid-latitude North Pacific SSTs tend to be cooler owing to the lingering El Niño influence (lower pressure with more wind and cooler conditions to the south of the Aleutians).

An interesting corollary to this discussion is that the long-term trend seems to be favoring more frequent and persistent La Niña and negative PDO conditions in the Pacific; and this therefore seems to raise the risk of more frequent severe Bering Sea storms in autumn.  Last year's UAF ACCAP report "Alaska's Changing Environment" indeed documented a recent increase in Bering Sea storms, but also indicated that no clear long-term trend has yet emerged.  It will be interesting to see how this assessment evolves in the coming years.

https://uaf-accap.org/alaskas-changing-environment/

Summer Review

I've been meaning to post a few comments on the climate patterns and anomalies during summer - better late than never, I trust.  On a statewide basis it was a fairly warm and fairly dry summer, but neither aspect was highly unusual in terms of the state averages: according to NOAA it was the 10th warmest summer in the last 30 years (warmer than 2024 but cooler than 2023), and statewide precipitation was the fifth lowest in 30 years (driest since 2019).

However, regional differences were significant, especially for precipitation.  The summer was much drier than normal along the central and eastern Gulf Coast, but much wetter than normal for most of western Alaska as well as the northern interior and western Brooks Range.  Here are the ERA5 (top) and NOAA (bottom) percentile rank graphics for the three-month period:

Here's a chart of accumulated precipitation at Yakutat, to illustrate the dryness (click to enlarge).  The June through August period was the second driest on record, following 2007, with total rainfall amounting to only 37% of the long-term median (9.6" versus 25.8").

Precipitation deficits were very pronounced in July along the northern Gulf Coast, but August really exacerbated the drought (relative to normal) and also set up the big contrast with excessive rainfall in northwestern Alaska:

We covered the late August stormy pattern for western and northern Alaska here, and the August 500mb height anomaly highlights the pressure gradient across western Alaska: the ridge over southern Alaska deflected the jet stream northward into western and northern Alaska.

In terms of sunshine and wind, it was a very poor summer for the Y-K Delta region, with all three months bearing part of the blame.  However, only June was cooler than normal in the southwest.  Eastern and northern Alaska were generally sunnier and less windy than normal, although not in August.

Looking back at lightning and wildfire activity, both were above the long-term median, but lightning much more so.  For a time in late June and early July, lightning strike counts were running second only to 2015 (with data back to 2012), and the seasonal total ended up in 4th place, similar to 2019 and 2022.  In contrast, wildfire acreage ended very close to 1 million acres, which is about twice the long-term median but well below 2019, 2022, and of course 2015.

It seems likely that Alaska's very wet spring was largely responsible for the relatively subdued fire season.  The statewide average for the April-May total precipitation was easily the highest on record, with both April and May ranking in second place statewide; and importantly, the wet anomaly included large parts of the fire-prone interior.

 

May was also cooler than normal, and June was only a little warmer than normal in the interior.  This is a big contrast with, say, the enormous warmth of May 2015 that dried out the state in advance of that huge fire season.

Here's a map to illustrate the distribution of lightning activity relative to normal.  There was far more lightning than normal in northern Alaska, and the more typically active southern interior also had more lightning than normal.

A plot of actual strike count density shows that the bulk of the activity occurred across the interior, as always, but it was very unusual to see the high strike counts extending so far north into the Brooks Range.

I suspect that the combination of fairly high temperatures and high humidity in the northern interior was part of the reason for the very active lightning season there (warm, humid air is fuel for thunderstorms).  Here's a map of dewpoint percentile relative to the 30-year climate:

From a broader climate perspective, widespread and extremely unusual warmth in the North Pacific this summer is one of the biggest stories in climate science at the moment.  June-August sea surface temperature anomalies were as much as 3°C above normal in the northwestern Pacific, leading Japan to have its third record-hot summer in a row.

Remarkably, the August area-average SST reached 20°C for the Pacific basin north of 30°N, according to NOAA's ERSST data.  The acceleration of warming in the past few years is quite something:

I'm out of time for this post, but I'll append below the rest of my August climate percentile maps for the sake of completeness.