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| El Nino intensity forecast from here. |
I've been getting a lot of questions about this winter's El Nino forecast, and in particular what it may mean for coastal flooding and erosion in Washington State. I don't have fantastic answers to that question, but if a strong El Nino does develop (generally associated with
the Oceanic Nino Index exceeding 2 degrees C), and
past is prologue, it probably does mean something for those hazards in Washington. Those questions also sent me down a bit of a rabbit hole focused on coastal water level here in Washington in particular, and specifically what is referred to as the "non-tidal residual", or NTR, which is
the difference between the astronomical tide, and the water level actually measured on the coast by tide gauges.
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| The non-tidal residual, or NTR, measured at the tide gauge in La Push |
For us in Washington, the NTR is due generally to weather - coastal water level tends to be higher during storms, and lower, and even negative, during nice weather with high pressure. What this means on the ground is that the actual tide is a higher than predicted by your tide chart when the NTR is positive, and a lower than predicted when the NTR is negative. The NTR as measured at our tide gauges is therefore a useful metric for generally evaluating the potential impacts of an El Nino on coastal flooding and coastal erosion.
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Winter (Nov-March) NTR distributions from Seattle (top panel) and the ONI index for the same winter months in the bottom panel. The dashed line in the bottom panel is the 2 degree Celsius threshold that generally is used to characterize a "strong" El Nino
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I grabbed a few datasets I had previously compiled of hourly water level from some of our tide gauges here in Washington, specifically La Push, Cherry Point, Seattle and Port Angeles, and
also the ONI index compiled by NOAA to just quickly evaluate the relationship between the ONI and non-tidal residuals for those gauges. I started by generating a summary figure like the one shown above, to sort of visually see the distributions of both datasets for winter months (I chose November - March). I then plotted the relationship for each gauge for the mean of both of those datasets for each winter in the record. I will focus on those.
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| Average winter NTRs from Seattle versus the average ONI index |
Generally, the story from all of the locations is similar. Three strong El Ninos are in the ONI index record: the winter of 1982-1983, 1997-1998 and 2015-2016. All three of these winters cluster at the top of these plots with the largest mean NTRs on record, hovering between 15 - 20 cm. In practical terms: On average during those winters, the actual measured coastal water level in Washington was 6-8 inches above what is predicted in the tide chart. That doesn't seem like a lot, but a few additional inches of average coastal water level
can have a considerable influence on the likelihood of flooding.
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| Average winter NTRs from Port Angeles versus the ONI index |
But there are some nuances that make it hard to know exactly what is coming this winter. There isn't a perfect relationship between the ONI index and the mean NTR...the 2016-2017 El Nino winter was the strongest on record based on the ONI index but has the lowest mean NTR of the three we can see in our tide gauges. The opposite is true of the 1982-1983 El Nino, which has a lower ONI index but the highest NTR. There is also spatial variability - the NTRs in Port Angeles associated with the 1982-1983 El Nino and the 1997-1998 event are a bit higher than those in Seattle. Some of this might be explainable, and it is worth noting here that I haven't removed sea level rise from the tide gauge data I've used here, and also didn't use
the new Relative ONI, but I think the point still stands...for us here in Washington, a strong El Nino is likely to increase our odds for higher than average coastal water levels, and the coastal flooding and erosion associated with it, but it is hard to know exactly what to expect. Furthermore, El Ninos
influences far more than just coastal water level - I'm focused here on just a tiny slice of the potential impact "pie".
