The non-tidal residuals of December

Flooding in Westport, Washington on December 14, 2024

This past December has been notably stormy, including a few events, especially on the coast (photo above) and along the Strait of Juan de Fuca (check out these great aerials from Three Crabs Rd near the mouth of the Dungeness River by John Gussman at DC Productions), that led to some flooding and other damage. The highest water levels of the month occurred on December 14th and were associated with a storm coinciding with one of this year's winter "king tides", But overall, the month has been characterized by a lot of positive "non-tidal residual" (referred to hereafter as NTRs), which is the part of coastal water level controlled by weather and not by the astronomical tides. NTRs are recorded by tide gauges, and for our purposes here are simply defined as the difference between the predictable astronomical tide, and the actual measured water level at a tide gauge.  

The non-tidal residuals recorded by the Friday Harbor tide gauge in December 2012 and December 2024

Back in 2012 I wrote here about a similar December, during which NTRs were generally high for a good bit of the month.  To illustrate this the plot above compares the NTRs from December 2012 and December 2024 as recorded at the Friday Harbor tide gauge. The things to note here are 1) that those NTRs were positive for long stretches of both months - in other words the measured water level at the tide gauge was higher than predicted most of the time and 2) that the maximum NTRs exceeded 0.50 m, or roughly 1.5 feet, on multiple occasions in each month.  For our region those are relatively large NTRs.

The manual staff gauge used by coast watchers on Puget Sound to measure water level

But what I want to focus on a bit here is a bit of a rabbit hole I went down because of an email discussion I had the good fortune to be part of with a group of property owners on Puget Sound. They reached out to discuss some of their water level forecasts and observations, and I absolutely love these kinds of notes, and the thought that there are people out there that really dig into this kind of stuff. This particular group has devised a really savvy system for both measuring (using a manual staff gauge, photo above) and predicting extreme water levels on the coastline so they can prepare for flooding along their waterfronts. Amongst other things they were looking to discuss a few days in November during which their observations and predictions didn't really line up because of the difficulty in predicting NTRs.

NTRs (top) and air pressure (bottom) measured at three tide gauges in Washington over December 2024.  The weird measure early in the month from Friday Harbor is a low-quality measurement that will eventually be removed from these preliminary data, downloaded from NOAA.

The thing about NTRs in Washington is that they generally are well correlated with atmospheric pressure - so high pressure generally leads to low, or negative NTRs (i.e. measured water level is near, or slightly below predicted), and low pressure generally leads to large NTRs, such that coastal water level is higher than predicted.  Above and below, for example, are the NTRs and pressure measurements from three tide gauges in Washington, that show this relationship, referred to as an "inverse barometer" effect. The group that I was emailing with used this relationship in their forecasts, to predict the water elevation for their area by combining the predicted tide with atmospheric pressure forecasts.

NTR vs. Pressure for data collected at three gauges in Washington during December of 2024

But what this group pointed out in our email exchange were time periods when this relationship sort of broke down.  In particular they pointed to a day in November...November 21st, 2024, when this general relationship between pressure and the NTR didn't hold, and diving into the data I think I'm starting to see what they are talking about.  Here, for example, are the pressure vs NTR data for three tide gauges in Washington for November 2024, with the measurements from November 21 highlighted in red:

What is interesting here is that on this day there really is no relationship between pressure and the NTR...the pressure more or less stays the same whereas the NTR at each of these tide gauges is varying by over 0.3 m, or about a foot, over the course of the day. I don't have a great, clear way to explain this, except that it does point to two important lessons.  First, the ocean is messy, and clean relationships almost never hold all the time. While there clearly is some relationship between atmospheric pressure and NTRs when looking at the data in aggregate, when we focus in on this particular day (and there are others like it), the relationship isn't so clear. Second, there are all sorts of other processes that can lead to variations in NTRs. My friend and colleague Eric Grossman at the USGS, for instance, has explored this problem in a paper, describing what they call remote sea level anomalies that are generated in the Pacific Ocean and can propagate into the Salish Sea. While it is not completely clear if the processes that are alluded to in this paper explain these observations, at least its a place to start.