Staying Current

By Theresa Williams

Slackjaw Sally here with reflections on water’s wishy-washy ways.  There’s very little water movement in my part of the ocean, but being up here on the 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.  On the surface, there is wind caused by the uneven heating of Earth’s surface.  Air flows from areas of dense, cool, dry air (high pressure) toward areas of less dense, moist, warm air (low pressure).   Air can blow in any direction but the average direction is called the prevailing wind.  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.  The South Pacific gyre moves warm water down past Australia and cool water up the west coast of South America.  There are also smaller, less prominent currents moving water around near the surface.

The ocean near the equator doesn’t experience much water movement from surface currents because warm, humid air rises there instead of generating current-driving winds.  The Atlantis is floating at about 23⁰ south of the equator on the northern edge of the South Pacific gyre.  At the end of our research in this area, we’ll launch a small sailboat, the Jackalope, with a GPS onboard to track its path around the Pacific.  The data it broadcasts to NOAA will help to improve ocean surface current maps.  Students in Laramie, Wyoming built the Jackalope as a school project to learn about the ocean currents in the Pacific Ocean.  They’ll be following Jackalope’s adventure and hoping to connect with other students on far away shores.

Catchin’ a current,

Sally