Thursday, May 29, 2014

A new pulse of tsunami debris?

A piece by King 5 news from 26 May 2014 investigating reports of another pulse of debris on SW Washington beaches

Recent media reports suggest that a new pulse of debris - some of it probably from the Tohoku tsunami, is littering beaches in Washington. There is the implication in these reports that this, finally, could be the leading edge of the massive wave that has been feared all along...but for the reasons below I am going to argue that this probably isn't the case.

Animation, based on numerical ocean modelling, of debris transport in the North Pacific following the March 2011 Tohoku Tsunami. Courtesy of the International Pacific Research Center

First off, if you watch the model results above (get the full suite of model animations here), you note that they suggest that as of right now, the highest concentration of debris appears to be far off the coast of California and Oregon, but that there are episodic "tongues" that are advected up towards the coast of Washington, B.C. and even Alaska. This is perhaps most obvious if you break out individual windage classes shown combined in the video above. Here, for example, are results from the "2% windage class" alone (see this white paper on how the modellers defined the various windage classes":

These model results are consistent with observations from our coast. Debris has seemed to arrive in pulses, and my overall impression based on the last three years is that late spring/early summer is a very likely time to see pulses of debris. Here is a media report, for example, from June 2012 investigating a general increase in the debris load, and June 2012 was when a large dock washed up on Oregon's Agate Beach. I reported on a June 2012 pulse in a talk given as part of the Olympic National Park's Perspective series based on the monitoring and clean-up work done by Russ Lewis on the SW WA is the slide:

Early summer of 2013 was quieter, but not without some apparent pulses. Here is the report from Russ from June 12 2013, "There was an uptick in long range debris overnight as there was a noticeable number of plastic bottles, small chunks of s-foam, some light bulbs, a few small fishing floats, larger plastics and also some local stuff in the mix such as rope, plastic bags". Why might late spring/early summer be associated with pulses of debris? It likely relates to the seasonal variation in near coastal winds, and its influence on currents along the west coast of the U.S.

Part of what motivated me to write this blog was my own brush with a suspected debris item, found May 14th banging around on the rocks in the high intertidal zone at San Juan County Park on the west side of San Juan Island. Its not clear to me what this was back before it was marine debris:

but when I flipped it over it was carrying the signature of a long ocean voyage - a heavy load of Lepas anatifera, also known as the Pelagic Gooseneck Barnacle:

and the evidence that it came from Asia? Clusters of large mussels that I have tentatively identified as being Mytilus galloprovincialis:

This species is native to the Mediterranean but is raised widely in Asia for food. This species occurred on the dock that washed up on Washington's coast a year and a half ago, and while it does occur as an invasive in Puget Sound, in combination with the Pelagic Gooseneck barnacles it suggests the possibility of Asian origin.

Thursday, May 22, 2014

Message in a bottle

Walking to the waters edge with messages-in-a-bottle

Christine and I were married on July 15, 2006 at Crescent Beach on the Strait of Juan de Fuca. As part of our ceremony we asked those in attendance to write short messages about the day, or our marriage, or really whatever they wanted. We stuck all of those messages in two wine bottles, corked them, and sealed them with melted wax. Then, at the end of the ceremony, we walked to the waters edge and flung them into Crescent Bay:

I recall that some of our friends and loved ones were a bit surprised: We weren't going to read them first? We didn't, and I think in our minds, we just wanted the ocean to hold all of those messages of love and inspiration for us. But now we have a chance to read at least some of them, 8 years later...

One of the bottles lying on Arcadia Beach in Oregon. Photo by Christine Webber

On April 28th we received an email from Christine Webber, who was travelling through Oregon and happened to stop at the beach. And, incredibly, she found one of our bottles on the beach (see her wonderful blog post about the bottle here). Unbelievable! I am a big fan of messages in bottles and have sent many of them into the ocean throughout my life. When I was young and my dad would go to sea with the Navy I would sometimes send bottles with him to throw overboard once they were clear of the coast. But never has one actually come back.

The bottle, freshly tossed ashore by Neptune. Photo by Christine Webber

What journey did that bottle take? In the past I may have let that question rest and perhaps even might have chalked it up to some mythic intent of the ocean...but now I am all about lets dig into this a bit. We tossed the two bottles into Crescent Bay it was the middle of July, and during most of the summer the Strait of Juan de Fuca is characterized by an outward directed residual flow. This is characteristic of estuaries, and the Strait of Juan de Fuca is indeed part of the massive estuary that we know as the Salish Sea:

Map of the Salish Sea & Surrounding Basin, Stefan Freelan, WWU, 2009

The residual flow is the mean longer-term (i.e. over weeks) flow, which averages out the back-and-forth currents associated with tidally-generated currents. Here, for example, is a beautiful animation of surface salinity in the Salish Sea derived from a numerical hydrodynamic model...and if you watch this a few times you can start to see the residual flow laid over top of the fact this animation is sped up so much that the oscillating tidal currents are barely even perceptible:

Numerical model by the MoSSea Program

Work by Thomson and others (2007) estimated that those outward directed residual surface currents in the Strait of Juan de Fuca can average an astonishing 0.5 m/s...or just shy of 1 knot. This suggests the possibility that it only took the bottle a few days to make its way out of the Strait of Juan de Fuca, since it is ~200 km from Crescent Bay to the shelf edge off the coast of Washington or Vancouver Island.

How about from there? This is where things get fun...I turned to a tool called OSCURS (Ocean Surface Current Simulator). Oceanographic models are generally fairly elaborate constructs, and may require both considerable expertise and computing power to run. I view OSCURS as something like an oceanographic model for the people, since it is very simple and runs online. And its got some chops - its driven by measured surface pressure, and has been used to understand and model the movement of everything from fish larvae to debris in the ocean. And when I plugged in a starting point outside the mouth of the Strait of Juan de Fuca, set it up with the appropriate start and end date, and played a bit with the parameters (which let you sort of "tune" the model for the type of item drifting on the ocean's surface), I got the following:

OSCURS trajectory for a drifting object released from the mouth of the Strait of Juan de Fuca on 20 July 2006.

Now, you can see that OSCURS is far from perfect. Somehow this bottle is supposed to end up deep in the Yukon Territory? Furthermore, by tweaking the parameters I can make this bottle end up inside China:

But at the very least OSCURS allows one to dream of the things that this bottle may have experienced on its 8-year journey. Even if (and this is a possible, and even probable, path) it washed up on Crescent Beach a few hours after our wedding, sat in the high intertidal undisturbed for almost 8 years, and was finally dislodged and sailed down the coast by the California Current just the spring, its still amazing that it made the journey at all and is now back in our possession.