El Niño and Bering Sea Storms

With memories of last year's ex-typhoon Halong still fresh in many minds, it's worth asking the question, "What about this year?"  Does the major El Niño that is now occurring affect the risk of a similar outcome during this autumn's storm season on Alaska's Bering Sea coast?

It's well-known that El Niño tends to strengthen the "Aleutian Low" in winter as a response to major alterations in tropical patterns of convection/rainfall.  Looking at the strongest El Niño events of the past, this effect is evident from November on, but not in September and October.  The following maps show the average 500mb height (pressure) departure from normal during the strongest El Niño conditions for each month in autumn and early winter:

Looking at historical storm track data from ERA5 reanalysis (processed data courtesy of the University of Manitoba), there are some modest differences between El Niño and La Niña.  The maps below show the number of autumn storm tracks passing within each grid cell, for storms with a central pressure of 980mb or lower (a modest threshold - many of these are run-of-the-mill storms, not intense storms).  Note that here I'm defining El Niño and La Niña based on tercile thresholds from the Multivariate ENSO Index.  Click to enlarge:

The main feature that stands out to me is an indication that the Seward Peninsula and Norton Sound region tends to see more autumn storms during El Niño than during La Niña.  Farther south, the storm density is generally similar for the southern Y-K Delta and around Bristol Bay, and the Gulf of Alaska clearly sees more autumn storms during La Niña.

The classic El Niño enhancement of storm activity emerges in December through February, but by then the typical storm track is south of the Y-K Delta region, so the winter storm frequency is not notably enhanced for Alaska's west coast.

Looking more closely at the autumn results from the Seward Peninsula to the northern Y-K Delta, the increase in storm frequency during El Niño is mostly found in November and then continues into December, as we would expect from the 500mb maps.  Here's a table showing the number of 980mb or lower storms that passed through a box over the region:

Here's the box I used for the analysis:

Another way of looking at the ENSO sensitivity is that this box has seen 11 storms below 970mb during the El Niño tercile, but only 3 such storms during La Niña (since 1950).  The three strongest autumn storms in the El Niño set dropped below 960mb in this box and occurred in the Novembers of 1979, 2009, and 2015 (the latter being a very strong El Niño), whereas the lowest MSLP in a La Niña autumn (963mb) was last year's ex-typhoon Halong.  Ex-typhoon Merbok (September 2022) is in 3rd place (964mb) for a La Niña autumn storm.

It's interesting to observe that 3 of the 4 strongest storms in the Norton Sound box during La Niña have occurred in the last few years: Halong, Merbok, and another strong storm in mid-November last year.  In this context, it's worth bringing up again the remarkable warmth that has prevailed in the northwestern North Pacific in recent years, as discussed post-Halong here.  It's not unusual to see a warm northwest Pacific during La Niña, and especially after a multi-year La Niña, but the warmth of recent years has been exceptionally pronounced.

Checking in on current North Pacific SSTs, extreme warmth is still persisting from northern Japan to the waters south of the Aleutians.  This is uncommon during El Niño, when the circulation pattern tends to drive cooling to the east of Japan, i.e. a more positive PDO phase.  The combination of a strong El Niño with a significantly negative PDO phase is highly unusual.

I don't want to draw any strong conclusions from this brief analysis, but it does seem there are a couple of reasons to suggest the odds of west coast storm activity may be higher than usual this autumn.  In the first place, if extremely warm ocean temperatures persist to the south of the Aleutians, then decaying typhoons would be expected to be more vigorous as they enter the Bering Sea in general.  September and October are the peak months for decaying typhoon potential.

And then looking ahead to November and December, historical data suggests that El Niño does favor an increased risk of strong early winter storms for the central west coast of Alaska, i.e. around the Norton Sound region.  However, I haven't attempted to assess the statistical significance of this result, and certainly the last few years demonstrate amply that you don't need an El Niño to get a damaging west coast storm.