Staying Current

Slackjaw Sally here with reflections on water’s wishy-washy ways. Being on the ocean surface has me thinking about how water moves and mixes. Tides cause the water in the ocean to slosh back and forth in its basin, winds push energy through water in the form of waves, and density stratifies water and leads to the slow creep of arctic water toward the equator along the ocean floor, but the most energetic mixing of oceanic water comes from surface currents!

It’s all a result of the unequal heating of Earth’s surface! Earth is roughly spherical so sunlight hits the surface at a more perpendicular angle near the equator than farther north or south. This causes more warming near the equator. Cooler air and water is denser and moves toward and underneath warmer air and water which is less dense.

Here’s how the water circulation gets started. During winter at the north and south poles ocean water cools and freezes. When water freezes, the salts remain in solution below the ice making the unfrozen ocean water saltier. The combination of increased salinity and decreased temperature makes the water denser and it sinks. Sinking water pushes the water beneath it toward the equator and leaves space above it for warmer water to take its place. The warmer water cools, freezes and sinks continuing the cycle. That cold, salty dense water moves slowly along the ocean floor until it is gradually warmed.

But there’s more going on at the surface than just water warming in the sunshine. Winds transfer their energy to the water dragging the water across the ocean in increasingly deeper layers. Due to the rotation of the planet the water doesn’t go directly in the direction of the prevailing wind; it turns to the right in the northern hemisphere and to the left in the southern hemisphere. The water is pushed in large somewhat circular currents called gyres (rhymes with hires). The gyres north of the equator rotate clockwise and those south of the equator rotate counterclockwise.

We’re launching the Sacred Heart Star of the Seas close to the yellow dot above.

We’re launching the Sacred Heart Star of the Seas close to the yellow dot above.

The biggest current in the world is just south of us in the Southern Ocean. It’s called the Antarctic Circumpolar Current (ACC) and is the only current that flows completely around the globe. All that energetic Roaring 40’s and Furious 50’s wind drives it. A balance between kinetic energy added by wind and energy removed by friction with the seafloor keeps its speed in check. It’s fast, it’s wide and it’s deep! Bounding the ACC to the north is the Agulhas Current. It flows along the eastern side of southern Africa to the tip of Africa where it turns to flow east parallel to the ACC.

We’re in the Agulhas Current where the cooler water is flowing eastward. Later today, we’ll launch a small sailboat, the Sacred Heart Star of the Seas, with a GPS onboard to track its path around the ocean. The data it broadcasts to NOAA will help to improve ocean surface current maps. From here, the little boat could follow the subtropical gyre and end up back in South Africa, Indonesia, or Australia, or it might join the ACC and sail around the world! Students in Kingston, Massachusetts built the Star of the Seas as a school project to learn about the ocean currents in the Indian and Southern Oceans. They’ll be following Star of the Sea’s adventure and hoping to connect with other students on far away shores.

Catchin’ a current,