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Current position of the SSV Robert C. Seamans. Click on the vessel to view position history. Use the tools, top right, to change the map style or view data layers. Dates and times use GMT (Greenwich Mean Time).

SEA Currents: SSV Robert C. Seamans

May 28, 2020

Seamans in the North Pacific Subtropical Gyre

Jan Witting, SEA Professor of Oceanography


Ocean surface circulation as often shown in textbooks.

As the Robert C. Seamans sails from Hawai’i to California, we’re telling the story of the Pacific through ArcGIS Storymaps. We’re reposting some of the entries here, in our SEA Currents blog, but we encourage you to follow regular updates by going directly to Pacific Crossing 2020 StoryMaps.

As Captain Rick Miller described two days ago, there is an atmospheric high pressure located over the Seamans. He also described a pattern of winds rotating clockwise around the center of this high, resulting in a pattern of northeast trade winds in the low latitudes (equator to about 20˚ N), and the common northerly winds on the west coast of the US. Since the Seamans is a sailing vessel, we pay close attention to winds, but winds do far more than move the ship – they move the whole ocean!

As air moves across the surface of the ocean, it starts dragging water with it. The blue arrows on Rick’s weather map show wind direction, and this circular wind pattern is powering a similar circular current system in the ocean that we call a subtropical gyre. In the weather map there are two centers of high pressure, but typically those centers are actually connected as one vast feature. Underneath, the surface ocean is similarly turning in one giant circle known as the North Pacific Subtropical Gyre, an important part of the world ocean circulation system.

There is another important process happening at the same time, though. As it happens, wind is not the only force involved – there is also the Coriolis effect, which begins to tug at the water molecules as soon as they are set in motion. This tug results in a process we call the Ekman Drift, and causes the top of the surface waters in the gyre to slowly spiral toward the center where they converge and, as since there is no other way to go, they end up sinking (downwelling).

Any object that floats and drifted in with the surface water is not going sink, though. They remain floating at the surface, and slowly accumulate there. There are bits of driftwood, feathers from seabirds, pumice from underwater volcanic eruptions, sea life that had adapted to a floating lifestyle, and… plastic. Lots and lots of plastic.

In textbooks the gyre is often shown as a ring of arrows, creating a picture of narrow, fast jets of water. In some places that is true, the equatorial currents can be like that, as are currents found on the western edges of the large ocean basins – like Gulf Stream in the Atlantic, or the Kuroshio Current off Japan. But in the interior of the vast gyres, the currents are weak, meandering flows (see the visualization on the right). Here, asking which way the ocean is moving can be a hard question to answer!

We use different tools to measure ocean circulation. Drifters are buoys that just drift while broadcasting their position to researchers via an on-board satellite transmitter. Data from drifters has allowed us to visualize the grand pattern of surface ocean circulation. Another way to study currents is via ship-mounted Acoustic Doppler Current Profilers. These instruments bounce sound off small planktonic animals in the water column and, based on the doppler shift in the frequency of sound echoed back to the ship, calculate the speed and direction of the currents under the ship, as far down as 2000 feet!

Eventually the Seamans will cross the convergence and enter the California current. This current is almost it’s own remarkable ecosystem with much higher productivity and more abundant sea life – as you can see from the whaling map that Dr. Rich King wrote about yesterday!

- Jan Witting, Professor of Oceanography.

Categories: Robert C. Seamans, • Topics: pacificcrossing2020 • (0) Comments
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