Exercises: Surface Ocean Circulation

The North Atlantic Subtropical Gyre includes the Gulf Stream along the east coast of the United States. State whether this gyre rotates clockwise or counterclockwise, and name the hemisphere effect (Coriolis effect) that is mainly responsible for this direction.

Define the Coriolis effect. Then state the direction in which moving air or water is deflected in the Northern Hemisphere and in the Southern Hemisphere.

Near the equator, the trade winds blow from east to west and push the surface water of the Pacific and Atlantic Oceans with them. In which direction does the resulting equatorial current flow? Explain what happens to this water when it reaches the western edge of an ocean basin, such as the Caribbean or the coast of Asia.

Along the coast of California, winds often blow from the north toward the south, parallel to the shore (the coast runs roughly north-south, with the ocean to the west). Ekman transport moves the surface layer of water at about $90^\circ$ to the right of the wind direction in the Northern Hemisphere.

1. Determine the direction of the net surface water movement.

2. Explain what must replace the water that moves away from the coast.

3. Describe one effect of this process on water temperature and on marine life.

The Gulf Stream, on the western side of the North Atlantic, flows at about $1.5\ \text{m/s}$ and is relatively narrow. The Canary Current, on the eastern side, flows at about $0.1\ \text{m/s}$ and is wide and diffuse.

1. Calculate how many days water takes to travel $1{,}000\ \text{km}$ in each current.

2. Explain how the shape of the ocean basin, together with the Coriolis effect and the winds, helps produce strong western boundary currents.

In the Bay of Fundy, on the Atlantic coast of Canada, the tidal range can reach about $16\ \text{m}$. In the Gulf of Mexico, the tidal range is often only about $0.5\ \text{m}$.

1. Calculate how many times larger the Bay of Fundy range is.

2. Explain how the shape of a basin (a long, narrowing bay versus a wide, nearly enclosed gulf) changes tidal currents.

3. Explain why tides drive coastal currents rather than the large gyres.

The Antarctic Circumpolar Current flows eastward around Antarctica. No continents block it, except where it squeezes through the Drake Passage, which is about $800\ \text{km}$ wide. In contrast, the North Atlantic has closed gyres bounded by continents.

Analyze why the Southern Ocean has a continuous circumpolar current while the North Atlantic has a gyre. In your answer, discuss the roles of (a) prevailing winds, (b) the Coriolis effect, (c) the shape of the ocean basin, and (d) water density differences between cold polar water and warmer water to the north.

A shipping container with 10,000 plastic toys falls overboard in the North Pacific at $45^\circ\text{N}$, $170^\circ\text{E}$ (a hypothetical event). The toys float at the surface and travel with the current.

1. Using the North Pacific gyre, the westerlies, and the Coriolis effect, describe the path the toys would most likely follow and where some might wash ashore.

2. If the toys travel an average speed of $0.1\ \text{m/s}$ along a total path of $20{,}000\ \text{km}$, calculate how many years the trip takes (use $1\ \text{year} \approx 3.15 \times 10^{7}\ \text{s}$).

3. Explain one reason the actual travel time might be longer or shorter than your estimate.