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/
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