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Diving Deeper: Episode 15 (August 12, 2009) —
What are Currents?

HOST: Welcome to Diving Deeper where we interview National Ocean Service scientists on the ocean topics and information that are important to you! I’m your host Kate Nielsen.

Today’s question is….What are currents?

Currents describe the motion of the water. The speed and direction of currents can be measured and recorded. Currents are essential for maintaining the balance of life on Earth, but they can also cause many problems.

To help us dive a little deeper into this question, we will talk with Laura Rear on currents – what they are, what causes them, and how we measure them. Laura is an oceanographer with the Center for Operational Oceanographic Products and Services where she manages the National Current Observation Program. Hi Laura, welcome to our show.

LAURA REAR: Hi Kate, thanks for inviting me here today to talk about currents. I’ve been studying and measuring currents for the past ten years and I’m glad to have the opportunity to speak to your listeners.

HOST: Laura, we touched on this question before in an earlier episode of Diving Deeper called what are tides, but can you remind us about the difference between tides and currents?

LAURA REAR: Sure thing Kate. As my colleague, Steve Gill, mentioned in that episode, the word tides is a general term used to define the alternating rise and fall in sea level with respect to the land. This means that tides move up and down during the day. Currents are different from tides in that they move horizontally rather than vertically. So currents describe the horizontal motion of the water and they’re driven by several different factors.

HOST: You mentioned that there are several different factors that drive or even can cause currents. Can you elaborate on these a little bit more?

LAURA REAR: Absolutely, there are several different factors that drive currents. Let me highlight a few of these different types of currents for you. There’s something that we call ocean currents, tidal currents, and coastal currents. The factors that cause these do vary.

HOST: OK, well Laura, can you first explain tidal currents to us and what causes these?

LAURA REAR: Tidal currents are a type of current that’s associated with the rise and fall of the tides. As you may remember from that earlier tides podcast, the rise and fall of tides is driven by the gravitational attraction of the sun and the moon on our oceans. The vertical motion of the tides near the shore causes the water to move horizontally, creating currents. When a tidal current moves towards the land and away from the sea, it’s something that we call flooding. When it moves toward the sea away from the land, it’s what we call ebbing. These tidal currents that ebb and flood in opposite directions are called rectilinear or reversing currents.

So tidal currents, just like tides, are affected by the different phases of the moon. So, if the moon is at a full or new phase, the tidal current speeds are strong and are called spring currents. When the moon is at another phase such as first or third quarter, the speed is weaker and the tidal current is referred to as a neap current.

HOST: Thanks Laura. What are coastal currents?

LAURA REAR: Kate, coastal currents are driven by winds, waves, and the shape of the land, so they’re not affected by the sun and the moon at all. First, let me give you a little bit more background on how winds and waves operate. So, wave height is affected by water depth, wind speed, how long the wind blows, and the distance over the water the wind is blowing, or something that we call fetch. So, if wind speed is slow and fetch is a small distance, then only small waves are built. Think about your bathtub. The fetch is only a few feet from one side of the tub to the other. The fetch in the ocean on the other hand is thousands of miles. If you were to blow a fan across the bathtub, only small ripples would form, however if you blow wind across the ocean, large waves would form. 

HOST: Thanks Laura for that great example, that really helps us visualize coastal currents a little bit better. Are there different types of coastal currents depending on the speed of the wind or the shape of the land, some of the factors that you talked about here earlier?

LAURA REAR: Yeah Kate, there really are. Let me just highlight a few here so that your listeners can get a better idea of what we’re talking about.

First, there’s often strong surface river currents at the upper ends of tidal estuaries where rivers meet the sea. River currents sometimes dominate all other currents during high river flow seasons.

There’s also something that we call longshore currents. Waves move towards the shore at different speeds depending on the shoreline shape and what the wave encounters before reaching the shoreline. Because of this, waves can bend to match the general shape of the coastline and waves typically arrive at a slight angle rather than perfectly parallel to the shoreline, so the wave bends as it reaches the shoreline. When a wave reaches the beach or the coastline, it releases a burst of energy that creates a current and that’s what we call the longshore current, and this current runs parallel to the shoreline.

Sometimes the longshore current moves on and off the beach, rip currents can form around breaks in sandbars or structures like piers. A rip current is a local current that flows away from the shoreline toward the ocean and this can happen at an angle or even perpendicular to the shoreline. Rip currents are something that many people are familiar with and you need to be careful because even the strongest swimmers can be pulled away from the shore very quickly. If you are caught in a rip current, remember not to panic, try to keep your head above the water, don’t exhaust yourself fighting against the current. Did you know that the best way to escape a rip current is by swimming parallel to the shore instead of towards it?

HOST: Thanks Laura. As you mentioned rip currents are something that many of us are aware of and this is really helpful advice to remember during our summer vacations and trips to the beach. Finally, can you explain open ocean currents to us?

LAURA REAR: Absolutely Kate, open ocean currents are similar to coastal currents in that they’re driven by the wind. Coastal currents are affected by winds near the coast, so they have more of a local impact. Surface ocean currents happen out in the large open ocean and are driven by a complex global wind system.

HOST: Laura, have we covered the most common types of currents with these three that you’ve outlined so far and what causes them?

LAURA REAR: For the most part we have, but there’s one more important type of motion to talk about and that’s what we call the global ocean conveyor belt. The ocean is not a still body of water as we have already talked about with the rise and fall of tides and the back and forth motion of currents. The global ocean conveyor belt is a generalized way of describing the constantly moving system of deep-ocean circulation that is driven by temperature and salinity differences throughout the ocean. We also call this global ocean conveyor belt, thermohaline circulation.  The word thermohaline can be broken down into two parts – first, thermo which means temperature and then haline which means salinity or the saltiness of the water. Temperature and salinity determine the density or how heavy the water is.

The process is very slow. Water moves around the deep ocean at only a few centimeters per second. It takes approximately 1,000 years for the conveyor belt to complete its cycle. Although the conveyor belts are very slow, they move massive amounts of ocean water. Scientists also believe that the conveyor belt system is affected by changes in global climate.

HOST: Thanks. That’s very interesting and I didn’t actually know about that or that it would take so much time for this conveyor belt to complete through one cycle.

LAURA REAR: It’s amazing, isn’t it?

HOST: It sounds like there are many factors and different levels to each of these factors that affects currents and their intensity. Are we able to monitor currents?

LAURA REAR: We are Kate. As a matter of fact, mariners have been studying and measuring currents for hundreds, if not thousands of years. The two main components of currents are speed and direction. The easiest way to describe how measuring is done is to say that the basic tools that you need are an observer, a floating object or a drifter, and a timing device. So an observer would stand on say the bow of a ship that’s anchored, throw something into the water like something that floats – a piece of wood or a cork or a bottle even – and then they would measure the time that it takes that object to move along the side of a ship. And that’s the easiest way to understand currents.

As technology improved over time, oceanographers began using mechanical current meters. A ship would deploy a meter and usually some sort of rotor would turn and measure the currents. This is still the basic process today; however we use more accurate and sophisticated instruments.

HOST: Thanks, it’s really interesting to hear how we use to do it, but still how that basic concept is behind what we’re using today. What tools and technology do we use today to measure and predict currents?

LAURA REAR: Today, in the open ocean a drifter as I mentioned earlier, would be more like a buoy in the water that may be equipped with global positioning system technology or satellite communications that would relay data and information. It’s also possible that a drifter of some sort would submerge for long periods of time to measure ocean currents at a particular depth. The drifter would then resurface occasionally to send a signal with its data and position to observers on the land.

HOST: Is a buoy the only tool we use to measure and monitor currents?

LAURA REAR: Absolutely not. There’s many other tools that we can use. Let me provide you with another example or two. The Acoustic Doppler Current Profiler has been around for about the last 20 years or so and is commonly used in the oceanographic community to measure currents. The meter is normally deployed on the sea floor or attached to the bottom of a boat. It sends an acoustic signal into the water column and that sound bounces off particles in the water. The signal is then returned to the instrument and using the Doppler shift theory – and knowing the frequency of the return signal, the distance it traveled, and the time it took for the signal to travel – the instrument can calculate the speed and direction of the current. It’s very similar to what we were talking about earlier with knowing the speed of the floating particle that came by. You track the distance and the time and you can calculate the speed.

It’s normally assumed that the speed of that particle in the water, that that acoustic signal it’s bouncing off of, is the same speed that the water is moving. So we can determine the speed of the water that way.

Many oceanographers also use radio antennas and high frequency Radio Detecting and Ranging systems, known as radar, to measure surface ocean currents. Similar to the Acoustic Doppler Current Profiler, these shore-based instruments also use the Doppler effect to determine when currents are moving toward or away from the shore. Scientists also use these measurements to determine the velocity of the current. When two or more radar antennas are used, a scientist can calculate surface current velocities for a large area at thousands of points. Using this data, a scientist can then produce a map of surface currents for that large coastal area.

HOST: That sounds like rather sophisticated technology actually to provide us with good data on currents that we need. What are the measurement units or how do we measure for currents?

LAURA REAR: Well Kate, currents are normally reported as distance over time as we’ve talked about. At NOAA we use knots to measure current speed. The term knot is defined as one nautical mile per hour. One nautical mile is equal to 1.15 standard miles. One knot is also 51.44 centimeters per second or 3.281 feet per minute.

HOST: Laura, what types of problems or complications can you run into when you’re collecting current data?

LAURA REAR: This is a really great question, Kate. There’s so many issues with collecting current data. In the coastal areas when we are collecting data, we always run the risk of our equipment being hit by ships with large drafts, meaning how much of that ship is underneath the water. For example, some ships have a draft of 30 feet. Our equipment might be deployed at say 35 to 40 feet. So we have to make sure that we let mariners know that our equipment is out there. We do this by issuing something called a Notice to Mariners through the United States Coast Guard.

Other problems that we may run into are when oceanographers deploy a current meter and sometimes it never comes back. Perhaps it was buried in the sand, or maybe it broke loose and might be floating around the ocean. It’s happened before. Sometimes we actually deal with vandalized and stolen equipment.

There’s also problems with the instruments themselves if they happen to malfunction in the harsh environment. There are many parts to the electronics and sometimes something can go wrong while they’re deployed.

One last thing that we run into is weather problems. When deploying our instruments, oceanographers hope for light wind and calm seas, however, even near shore this is not always the case. We’ve had many delays in deploying instruments due to high winds and rough seas anywhere from the Alaskan waters to even the Hudson River in New York.

HOST: Thanks Laura, it does sound like there are a lot of things that oceanographers can face and have to take into consideration with each time you’re out there collecting this data. Why is it so important to measure, monitor, and predict currents?

LAURA REAR: Kate, I’m really glad that you’re asking this question. There’s so many reasons why it’s important for us to measure currents. These measurements support safe and efficient shipping and marine transportation. In addition, current measurements are important for commercial fishing, recreational boating and safety, swimming, and even search and rescue operations. With predicted, real-time, and forecasted currents, people can safely dock and undock ships, maneuver them in confined waterways, and safely navigate through coastal waters. This all helps to avoid ship collisions or actually delayed arrival of goods.

When supporting search and rescue operations, understanding the speed and direction of the currents in an area helps to narrow down the rescue and recovery effort. Current prediction information can even help scientists clean up after a hazardous oil spill by helping them understand the direction and movement of the oil. Engineers also use currents information to help build marine structures such as bridges or docks and piers. Current observations are also used to develop and evaluate coastal nowcast or forecast model products that are now being provided online.

As you can see, there are really so many ways that the ability to track, measure, and predict currents is important to all of us in our everyday lives, whether you live along the coast or in the non-coastal areas of the United States.

HOST: Thanks for these examples, what is the role of the National Ocean Service in studying currents?

LAURA REAR: Kate, the office that I work for, the Center for Operational Oceanographic Products and Services is part of the National Ocean Service. Our office is responsible for managing a program called the National Current Observation Program which collects, analyzes, and distributes observations and predictions of currents. We support many of the efforts that I mentioned earlier including safe marine transportation and response efforts for hazardous events such as an oil spill. 

HOST: Are these observations and predictions available for the public as well?

LAURA REAR: They are indeed. It’s all available on the Center for Operational Oceanographic Products and Services Web site. We also produce annual tidal current tables which have the predictions in them.

HOST: Thanks Laura. Do you have any final closing words for our listeners today?

LAURA REAR: First Kate, I’d like to say thank you again for inviting me here today to talk about currents with your listeners. I’d like to make sure that your listeners today understand that predicting and measuring currents is important not only for getting cargo ships safely into and out of ports and harbors, but for determining the extent of an oil spill, building bridges and piers, and even determining the best fishing spots. I hope I have helped to clarify some of your listeners’ questions about currents and why they are important.

HOST: Thank you Laura for joining us on today’s episode of Diving Deeper and talking more about currents and why they are important to us. To learn more about currents or access some of the products that Laura talked about today, please visit

That’s all for this week’s show. Please tune in on August 26th for our next episode on maritime heritage.