Tides and Currents: Motion in the Ocean

Author: NOAA (National Oceanic and Atmospheric Administration)
Subject: Earth Science

What This Lesson Is About

This lesson helps students learn what causes ocean currents (moving water in the ocean) and ocean waves.

Helpful Websites

• A lesson about the Coriolis force (a force caused by Earth's spin that bends the path of moving water and air)
• A tutorial about tidal, coastal, and ocean currents
• NOAA's "Tides and Water Levels" tutorial
• NOAA's Center for Operational Oceanographic Products and Services (CO-OPS) website, which has data about tides, currents, and weather
• Lesson plans about currents from the University of Southern Maine
• An online tutorial with more details about ocean currents

Time Needed

One 45-minute class period, plus extra time for students to do research.

Goal

Students will explore what causes ocean currents and waves.

What Students Will Learn

• The main causes of ocean currents and waves
• Why ocean currents are different depending on latitude (how far north or south a place is from the equator)
• What causes the Coriolis Effect, and how it changes ocean currents
• How to figure out the strength of ocean currents using data from floating trackers called drifters

Background Information

More than 98% of all cargo (goods) shipped to and from the United States travels by water. To keep ships safe, sailors need accurate maps of the coast. They also need up-to-date information about weather and sea conditions. These conditions can change quickly and be very different from place to place. That's why sailors need real-time information to avoid crashes and getting stuck.

NOAA's Center for Operational Oceanographic Products and Services (called CO-OPS) collects and shares information about the ocean. This helps keep ships safe, efficient, and good for the environment. CO-OPS does the following:

• Shares information about water levels, coastal currents, and tides
• Sets rules for how to collect and study this data
• Studies new and better ways to observe the ocean
• Builds computer programs to study the data better
• Checks the data regularly to make sure it's correct
• Shares this information with the public

CO-OPS also runs a system called PORTS® (Physical Oceanographic Real-Time Systems) in major U.S. harbors. PORTS® gives sailors real-time information, including:

• Water levels
• Currents
• Air gap (the space between the water and the bottom of a bridge)
• Weather data

This helps ships avoid running aground or crashing.

CO-OPS mostly studies currents near the coast and in estuaries (places where rivers meet the sea). Other parts of NOAA study currents way out in the open ocean. NOAA's National Oceanographic Data Center (NODC) collects information from tools like current meters and drifters, which measure how water moves. Their website has data about currents, beach temperatures, ocean temperature and saltiness, and more. NOAA also has a website called Ocean Surface Current Analyses that uses satellites to study currents all over the world.

In this lesson, students will explore how currents, wind, and ocean waves are connected.

Steps for Teachers

1. Get ready. Look over the background information about tides and currents, the Currents Subject Review, and the "Problems on Winds, Waves, and Currents" worksheet. You may also want to check out the "Tides and Water Levels" tutorial, which explains how tides and water levels work and gives extra lesson plans.

1. Start a discussion. Ask students why ocean travel still matters today, even though we have airplanes. Students should realize that oceans are still very important for shipping goods, making energy, and having fun (like boating). Talk about why real-time information keeps sailors safe. Have students think of what kinds of information a sailor today would find useful. Tell students their job is to learn basic facts about "ocean motion" and use that information to solve problems about wind, waves, and currents.

1. Complete the Subject Review. Have students read the Currents Subject Review and answer its questions. You can have different students or groups study different parts. If there's no internet access, print copies for everyone. This can be homework to save class time. Afterward, discuss the answers as a class, and talk about why knowing about tides is useful.

1. Do the worksheet. Give each student or group a copy of the "Problems on Winds, Waves, and Currents" worksheet. Have them answer the questions.

1. Review the answers. Discuss the worksheet answers together. Make sure students understand how wind, ocean waves, and currents are connected.

Correct Answers

1. 3 feet
2. Raising the wind speed by 60 knots would make the wave grow to about 12 feet. Raising the fetch (the distance wind blows over water) by 60 nautical miles would only make the wave less than 6 feet.
3. A 60-knot wind would need to blow over about 9 miles of water to make a wave 10 feet tall.
4. The distance between the two points is 524.6 nautical miles. The total time was 6 days and 10.25 hours, which equals 154.25 hours. So the current's speed is:

524.6 nautical miles ÷ 154.25 hours = 3.40 knots

The current flows toward the northeast.

1. The distance between the two points is 1,443.68 kilometers, which equals 1.444 x 10⁸ centimeters. The total time was 14 days and 2.92 hours, which equals 338.92 hours, or 1.220 x 10⁶ seconds. So the current's speed is:

1.444 x 10⁸ cm ÷ 1.220 x 10⁶ sec = 118.4 cm/sec

The current flows just slightly east of due south.

1. Since the latitude at the equator is zero, the math shows that the Coriolis acceleration (the push caused by Earth's spin) is zero right at the equator.

1. Tijuana's latitude is about 32.5° N. A speed of 10 meters per second equals 1,000 centimeters per second. So the Coriolis acceleration there is:

(sin 32.5° • 1.5 x 10⁻⁴ • 1,000) cm/sec² = 0.081 cm/sec²

This is a very tiny effect.

1. Even though the Coriolis effect barely changes the path of something small, like a soccer ball or a walking person, it becomes very noticeable when it acts on huge amounts of water or air moving very long distances.

More Resources: The Bridge

The Bridge is a growing online collection of ocean science resources for teachers. Experts check these resources to make sure they are correct and current. On the website, click "Ocean Science Topics," then "Physics," to find links about tides, waves, and currents.

Make It Personal

Have students write a short essay about how the Coriolis force affects them personally, even though it's usually too small to notice in everyday life.

How to Check Student Learning

Students will complete the worksheet.

Extra Activities

1. Visit the "Tides and Water Levels" learning kit for more resources and lesson plans.
2. Visit the University of Southern Maine's website for more lesson plans and activities about currents.
3. Visit the Multimedia Learning Objects website. Click on Lessons 8 and 9 for interactive presentations and activities about Ocean Currents and Ocean Waves — including an activity where you land safely on an aircraft carrier by planning for the Coriolis Effect!

What You'll Need

• Copies of the "Problems on Winds, Waves, and Currents" worksheet (one per student or group)
• Copies of the Currents Subject Review
• Computers with internet access

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Student Worksheet: Problems on Winds, Waves, and Currents

Ocean waves at the surface are made by wind. How tall a wave gets depends on three things: how fast the wind blows (speed), how long the wind blows (duration), and how far the wind blows across the water (fetch).

In 1952, a scientist named Charles Bretschneider made a diagram that shows how these three things work together. It helps people predict how tall a wave will be, based on the wind conditions. This kind of chart is called a "Sverdrup-Munk-Bretschneider nomogram" (say: NO-mo-gram). Figure 1 shows an example.

On the chart:

• The up-and-down side (y-axis) shows Wind Speed
• The side-to-side line (x-axis) shows Fetch Length
• The curved lines in the middle show Wave Height in feet

When you use the chart, make sure you match up the right lines with the right labels!

Figure 1: Sverdrup-Munk-Bretschneider Nomogram

1. If wind blows over a 10 nautical mile fetch at 21 knots, how tall would the wave be?

1. Which would make the wave grow taller: increasing the wind speed, or increasing the fetch length? (See the problem above for comparison.)