Matthew Alford

Ocean dynamics (including internal waves, the mixed layer, abyssal overflows and turbulence) and their impact on the global circulation and coastal ecosystems

Personal Web-site: Applied Physics lab

Research Web-site: Wavechasers.uw.edu


Wavechasers

Friday, April 05, 2013

Tracking skyscraper-high waves across the globe

The sea is full of “internal waves,” subsea cousins of surface waves you’ve seen on beaches. Internal waves move the ocean’s layers up and down tens to hundreds of meters – as high as several Space Needles stacked on top of each other. These waves arise when the wind blows on the ocean’s surface and when tidal currents flow over seafloor bumps. Once created, they can travel across the ocean’s basins, just like surface swells – as seen in the movie above. And – also like surface waves – they can break, which they sometimes do in dramatic turbulent events that mix cold water below with the warm water above it.

This internal-wave-driven mixing turns out to be a vital aspect of the ocean's circulation. We currently believe that without breaking internal waves, the deep sea would be a stagnant, homogenous deep pool of cold water with a very thin warm layer atop it. Since we instead observe a much more gradual decrease in temperature, we conclude that there is mixing in the abyss – and that breaking internal waves lead to much of it. Therefore, internal wave mixing is part of the ``bloodstream” of the ocean, enabling the upward part of the “conveyor belt” circulation by moving cold water upward. And that means that our predictions of climate change have significant uncertainty because we do not fully understand the sources, travel pathways and eventual breaking locations of the internal waves in the sea.

At their most dramatic, internal waves can carry ships horizontally along with them at speeds of a few knots, and potentially even cause submarines to be swept beneath their crush depth or to surface unintentionally. They can also carry nutrient-rich fluid along with them, helping to fuel biological production by bringing nutrients into the shallow sun-lit ocean where photosynthesis can occur, and onto continental shelves. They can therefore play a key (but still poorly understood) role in coastal ecosystems.

Their importance to climate, coastal ecosystems, sound propagation and navigation necessitates a better understanding of where internal waves are generated, how they travel over the globe, and how and where they break. Enter Wavechasers! Our mission is to understand all three of these aspects of internal waves by using a variety of techniques to study them and the turbulence they create. One of our favorites is to deploy arrays of moorings, each of which has a "moored profiler," an instrumented robot crawling up and down a moored wire, as shown above. We also detect the waves with instruments towed from ships. We care a lot about ocean technology, and so are always specializing and improving these methods. 

Stay Connected to Wavechasers. Email wavechasers@apl.washington.edu to get on our mailing list, or check our website to stay in touch.  We have all sorts of cruise and lab volunteering opportunities and do regular outreach, so we'd love to hear from you.
 

An animation showing internal waves traveling over 3500 km away from Hawaii and Alaska. Space-borne satellites detect the surface signature of the waves, which can then be separated into their northbound (red/orange) and southbound components (blue). The inset shows a cross section of the waves traveling across the sea. We use a combination of satellites, moorings and ship-towed vehicles to track these waves from their birthplaces to their breaking locations.