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Currents: Water on
I S S U E 2 0 • S U M M E R 0 1
an you name Earth’s
most important rivers? Would
you guess the Nile, the
Amazon, the Mississippi, or
maybe the Yangtze? Did you
know there are rivers that
flow through the world’s
oceans? And that they are
more important in shaping our
environment than the major
rivers on land? These mighty
bodies of moving water are
called currents.
Ocean currents move water
continuously along specific
pathways, often over very
great distances. This happens
both on the surface and in the
deep ocean.
Currents driven by the wind
travel thousands of miles
across the ocean’s surface. In
the process, they move heat from warmer to cooler areas. Ocean waters also
move vertically, mixing waters of different temperatures and salinities
(amounts of salt).
Currents influence temperature, climate, plants, and animals in the ocean and
C
Ocean in Motion
Most of the currents in the upper half-mile of the ocean are caused by winds that blow
year-round in one direction across Earth. Wind pushes directly on the ocean surface, causing
the ocean’s top layer to move. Surface winds also set in motion other currents that extend
to the ocean bottom, far below the direct influence of the wind.
However, most ocean currents do not occur at the surface and are not directly caused by
wind. Deep ocean currents may move across great horizontal distances in the same way as
surface currents. They also may run through the ocean vertically, although vertical currents
OCEAN CUR- RENTS MAP When wind-driven surface currents run into conti- nents, the water must turn and flow toward either the poles or the equa- tor (Earth’s mid- section). The effect of Earth’s rotation causes moving objects on Earth to follow curved paths (a scientific prin- ciple known as the Coriolis effect),
Scientists Studying Currents
Surface currents have been studied by sailors for hundreds of years. Early ships depend-
ed on winds and currents. Understanding currents was important to people who used the
ocean for fishing, trade, and travel.
Scientific studies of currents date back more than 100 years. In early studies, researchers
set afloat drift bottles and other types of floating markers from ships or into offshore cur-
rents. Their movement was then charted with the help of other scientists, sailors, or even
beachcombers who would report finding them.
For the past 30 years scientists at Scripps, and around the world, have been involved in
intensive efforts to understand currents in relation to world ocean circulation patterns. Most
recently, the relationship between the ocean and the atmosphere and its effect on climate
has become a major environmental issue.
The development of a worldwide network of communication satellites, and the invention
of computers able to process huge amounts of information, set the stage for the current
generation of instruments.
Drifters were invented that recorded the speed and direction of currents from the motion
of the drifter within the current. Drifters able to take many different direct measurements
simultaneously were also developed. They could record ocean temperature, salinity, pres-
Temperature
Temperature can vary a lot in the ocean. Shallow, tropical waters can reach close to 9 0°F (31° C), while water at the poles will be close to freezing (28.4° F, - 2° C). Seawater freezes at a lower temperature than fresh water (32° F, 0° C) because of its salt content. The warmest water in any area is at the surface, and the coldest water is at the bot- tom. This is partly because the sun warms surface water. However, cold water is heavier
Pressure
Pressure is a measure of the weight of the water and atmosphere pushing down- ward at any given point. There is 15 pounds per square inch of pressure at sea level. In the deepest parts of the ocean, pressure is around 15,000 pounds per square inch, an
Density
Density is a measure of weight, technically the weight of a liquid divided by how much space it takes up. Salinity, temperature, and pressure work together to create density. Movement happens when two opposing forces are not in balance. When one part of the ocean is denser than the surrounding area, water movement will take place. This creates currents. Because of the many fac-
Properties of Water
In general, currents in the deeper ocean are caused by the mixing of bodies of water at
different temperatures, salinity levels, pressures, and densities, although this may not be the
Of Toys and Tennis Shoes
Would you believe that tennis shoes, rubber duckies, and dolls’ heads have all helped in the
tracking of ocean currents? All of these things have fallen into the ocean as the result of spills
from cargo ships during storms at sea.
Eighty thousand tennis shoes were washed overboard near the Alaskan Peninsula in
1990. Beachcombers from Alaska to northern California collected hundreds of them. Over
the next few years shoes washed up in Hawaii, the Philippines, and Japan.
Yellow duckies and green frogs ended up on beaches from Alaska to Oregon from a spill
of thousands of plastic bathtub toys in the North Pacific Ocean in 1992. Last year tens of
thousands of Rugrats™ dolls’ heads went overboard in the same area and are currently wash-
ing ashore.
What does this have to do with science? Some oceanographers have used the floating
toys to plot ocean surface currents. Studies have shown that about 1 percent of a spilled
cargo eventually washes ashore. The oceanographers have kept records of where the toys
Salinity
Salinity is the scientific term for the amount of saltiness in water. All water has some dissolved salts in it, but the ocean has much more salt than fresh water. Ocean water is always evaporating into the atmosphere, but the salt remains in the ocean. Thus, the ocean remains salty even though fresh water is continually being added by rainfall and by rivers and runoff from land. Density is the scientific term for the weight of water. Salt water weighs more than fresh
Make Your Own SOLO Float b y K e v i n H a r d y
What You’ll Need 1 clear 2-liter soda bottle filled with water to within 2 inches of the top 1 soda straw 1 small ball of clay 1 toothpick 3 to 4 pieces of clear tape 1 tall glass of water
Building Your Float
- Cut the straw to 4 inches long.
- Cut the toothpick to 1 inch long.
- Roll the clay flat to about 1/ inch thick.
- Plug one end of the straw by pushing it into the clay. Twirl the straw and remove it. There now should be a plug inside the straw. The clay represents the vehicle’s endcap and the straw represents the hull.
- Tape the toothpick against the side of the straw on the end with the clay plug. The toothpick should extend about 1/4 inch past the end. This is the antenna.
Ballasting
- Place the model drifter in a tall glass of water to see how it floats.
- The top of the straw should float no more than 1/8 inch above the water surface. If it sinks, remove some clay. If it floats too high, add more clay.
- Watch for air bubbles sneaking out the top past the clay plug. It must be airtight.
What’s Happening When conducting both of these experiments, you’re changing the drifter’s density by chang- ing its displacement, which is how much water is pushed out of the way for it to be there. The air inside the straw is part of the drifter. When you squeeze the soda bottle, you raise the pres- sure inside the soda bottle enough to compress the air inside the straw. Because the air now takes up less space, the drifter becomes a little heavier and sinks. You can adjust how much you compress the air inside the straw and make the drifter sink, float, or stay at one depth. A real Scripps SOLO float changes its displacement by pumping oil
Experiment 1 Step 1: Squeeze the sides of the bottle. What does the drifter do? Why? Step 2: Watch the air inside the straw as you or a friend squeeze the soda bottle. Step 3: Your drifter should sink when you squeeze the bottle and rise when you release it. Practice making the drifter
Experiment 2 Step 1: Hold the bottle with both hands on the table top, and give it one quick swirl so that the water swirls around inside. Your drifter will be riding along on the surface. Step 2: Now squeeze the bottle and send the drifter down about halfway to the bot- tom. The drifter should be riding along below with the “current.” Step 3: Release the soda bottle and the drifter will rise to the surface, but now in a
