Wednesday, January 4, 2023

Anatomy of a Coastal Storm: December 27th

Photo by David Barker, submitted to the King Tides program via MyCoast, on the morning of December 27th.  Gig Harbor area

Well somehow I went a whole year without a post...not exactly sure what happened to be honest, except that I was very focused this year on a few projects that are now wrapping up. But an event that hit the Washington shoreline in the last week of December was more than enough to snap me out of it.  The long and the short of it is that this event broke high water records at all but one of the tide gauges in the Puget Sound basin, some of them set 40 years ago.  It was a doozy.    

Photo by Joan Schrammeck, submitted to the King Tides program via MyCoast, on the morning of December 27th.  Camano Island.

Most of the record-breaking for this event happened in the Salish Sea, and really in the Puget Sound basin, even though it was no walk in the park for Pacific Coast shorelines either.  The photo below, for example, was taken at the trailhead on Rialto Beach in Olympic National Park on December 26th.  The crest of the berm here is at an elevation of roughly 18 feet above MLLW, and if the water pushes much higher under the influence of tides, surge or wave run-up it spills down into the parking lot just behind.  

Photo by Kim Sager-Fradkin, posted here with permission.  Rialto Beach, Olympic National Park, December 26th.  

But back to the Puget Sound basin, and here are the breakdowns of coastal water level records for the five tide gauges within the Puget Sound basin that set records during the event, specifically on the morning of December 27th, from north to south:

A few things of note here.  First, the max water levels recorded in the table above are unverified as of yet, so may change. The Bremerton tide gauge is new, so not a surprise that a record was set there, but records were absolutely shattered in both Friday Harbor and Seattle, where tide gauges have been recording water level for 89 and 125 years, respectively. If the water levels are verified as they are reported in the table above, then Seattle's water level during this event was a full 7 inches above the record set just last year. To try to put that into context, that record was set last year on January 7th, 2022, by just barely exceeding the previous record (that one set in 1977) by less than half an inch. 

Another surprise for me is just how wide-ranging this event was have records set at tide stations in north and south Puget Sound on the same tide is notable to me. The various photos, and the astonishing video posted above, from the Point No Point area on the northern Kitsap Peninsula also illustrate this - they are coming in from all over Puget Sound.

What made it happen? The most obvious answer is that we rolled snake eyes in the annual game of chance that we play along the shoreline. The plot above is of water level (top panel), pressure (middle panel) and the "non-tidal residual" (storm surge, or the difference between the predicted and measured water level) for both coastal (left column) and Puget Sound (right panel) tide gauges. Essentially a high astronomical tide coincided with the peak of the low pressure passing through the area on December 27th, and that low pressure drove a very large (for our area) non-tidal residual that peaked at exactly the same time as the high tide. That same "perfect storm" didn't happen on the coast.  Instead, the peak of the low pressure and non-tidal residual coincided with a falling tide, so while water levels were very high, they didn't shatter records.  

But there are at least two other likely contributors that I'm not prepared to really rigorously attribute, but almost certainly played some role in driving water to record heights on December 27th. The first is sea level rise. At most of the locations in the table above there is a documented relative sea level rise trend. While the rate associated with that trend vary across those locations, in general they equate to something along the lines of a few inches over the four decades that have passed since many of our previous coastal water level records were set during the 1982-1983 El Nino winter. These few inches didn't "cause" this event, but they did contribute, and this on-going sea level rise makes it very likely that the records set in this December 27 event will be broken again. The second is the lunar nodal cycle, which is illustrated in the plot above this paragraph and was the subject of some media reporting last year. This astronomical cycle operates over a ~19 year period, and influences the range of the tides, or the difference between the average high and low tides. In other words, at the peak of this cycle, on average, high tides are a bit higher, and low tides are a bit lower.  The data I show above are from the Friday Harbor tide gauge, and what I've done here is to average all of the highest tides for each year going back to the 1960's (to capture a few of these cycles)...and during the peak of the cycle the high tide is, on average, about 6-7 inches higher than it is during the bottom of the cycle. And we are, as you can see, heading towards a peak in this cycle. Again, this didn't cause this event, but may have contributed a bit to it.  

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