Thursday, September 17, 2015

El Nino and the Washington Coast

Coastal erosion along the California coast associated with the 1997-1998 El Nino winter

We are experiencing El Nino conditions now, which are very likely to continue...with forecasts pointing towards a strong peak in late fall or early winter. What does a strong El Nino mean for the Washington coast?

The scientific literature exploring the coastal impacts of El Ninos on the west coast of the US suggest that we should expect above-average erosion and property damage. BUT, most of this literature focuses on impacts in California...and in California there is no doubt that El Nino winters are a big deal. In California the cost of coastal damage from both the 1982-1983 El Nino and the 1997-1998 El Nino was measured in the millions of dollars. There is quite a bit less available regarding how previous El Nino winters have played out on Washington's Pacific coast. The one analysis that I could find (which includes as a co-author Washington State Department of Ecology's own George Kaminsky) looking at erosion along the west coast during El Nino winters that INCLUDED shorelines in SW Washington was unclear. El Nino years WERE erosive on the SW Washington coast...but so were lots of other years not associated with El Nino conditions.

This paper, though, shows how multiple mechanisms exist that can conspire to create potentially erosive conditions on Washington's Pacific coast during El Nino winters. At play are three primary factors: Elevated average winter sea level, above-average wave size, and a change in the average wave direction over the winter season.

Average sea level is definitely elevated during strong El Nino winters on Washington's coast. Here are monthly average sea level data from Toke Point, Washington (on the coast in the mouth of Willapa Bay):

Note in particular the "spikes" in the plot above, marking months in which average water level measured at this tide gauge was >0.3 m (~ 1 foot) above the long-term average for that month. The first is centered on Feb 1983, and the other on February 1998.

But its not just about water level. On the open ocean coast above-average waves and changes in mean wave direction play a major role in driving patterns of erosion and damage to infrastructure during El Nino winters (see references here and also this paper). What to expect in the inland waters of Washington where ocean waves aren't much of a factor? Here there is virtually no documentation that I could find regarding any unusual coastal impacts associated with the 1997-1998 winter. In fact, I spoke to Hugh Shipman, Coastal Geologist with the Washington Department of Ecology regarding his recollections from 1997-1998...and mostly he talked about other years, especially 1996-1997 when heavy precipitation drove coastal bluff erosion and failure around Puget Sound:

A home in Seattle after heavy precipitation in the winter of 1996-1997.

All that being said, El Nino years definitely do drive elevated average sea level in Puget Sound. Here is the monthly average sea level for Seattle:

again, with those large "spikes" in average water level during the winter of 1982-1983 and 1997-1998, identical to those observed in Tokeland. In fact, the highest water level ever recorded in Seattle happened on January 27, 1983 - during a strong El Nino winter.

But unless those elevated water levels are associated with some heavy winds/waves, or maybe some heavy precipitation, they may not lead to much in the way of damage. Instead, it can lead to some coastal "nuisance" flooding - definitely a problem, but typically not extraordinary in terms of damage. But should we expect stormier conditions this winter in the inland waters of Washington associated with a strong El Nino, that could couple up with elevated water level and really cause some damage?

There is an apparent relationship between El Nino winters and a reduced frequency of storms specifically associated with north wind and colder than average temperatures (Nick Bond, personal communication). This is perhaps consistent with the observation from past El Nino winters of higher than average winter temperature:

Temperature anomalies during El Nino winters versus a long-term average. Courtesy of Nick Bond.

But what about general storminess? Based on a very preliminary analysis I am going to go out on a limb and say that there likely isn't much connection between El Ninos and heavier-than-usual wind or lower pressure (associated with storms) in the inland waters of Washington State. To arrive at this conclusion I collated hourly observations from the Seattle tide gauge from 1991-2001, and looked at patterns for the winter months (October - March). Here are those data for wind speed expressed as a box plot for each winter, where the red line is the median value, the edges of the box represent the 25th and 75th percentiles, and the "whiskers" cover approximately between the 1st and 99th percentiles of the distributed data. I didn't include outliers here, since my interest was in average winter condition:

So as you can see the winter of 97-98 really doesn't look too different than the 3 or 4 winters that preceded it (though it looks like those winters were windier, on average, than those at the beginning and end of the decade).

Wind direction tells a similar story:

with the winter of 97-98 really not looking all that different as compared to the 3 or 4 winters before, as well as the winter of 98-99...though all of those winters collectively look to have had a bit more south wind than the winters at the beginning of the decade (and the beginning of the 00's).

and finally, pressure is generally a good indicator of storminess, with lower pressure equating to a stormier winter. In seattle, there isn't much distinction, on average, across the winters I looked at:

And what about precipitation? Generally for our area El Nino winters are associated with less precipitation than normal. Here, for example, is a look (courtesy of Nick Bond) at how precipitation has varied nationally during El Nino winters since 1957 (strong and weak), as compared to the long-term average:

Indeed, that is the forecast for this winter (20 August 2015 update).

So all in all, what to expect? There seem to be two stories - one for the Pacific Ocean coast favoring an increased chance for erosion and damage due to the combination of higher-than-average water level, higher-than-average wave heights, and a shift in the average wave direction. And another for the inland waters, favoring some elevated water level and perhaps enhanced nuisance flooding...but possibly not much else, at least that would be unusual as compared to any other winter.

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