Exploring Satellite Oceanography

(A set of lesson plans for high school science students)

Lesson Plan #1: Introduction to Satellite Oceanography

Graduate School of Oceanography / University of Rhode Island

Amy Schubert and Peter Cornillon

Abstract

This unit is an introduction to infrared satellite imagery and is designed to be used as a classroom exercise. It begins with an introduction to infared satellite imagery and explains two sample images. It then gives some uses of satellite imagery and suggests questions that can be answered with the images provided at the end. You can see blow-ups of the small images by clicking on them and see the images listed at the end by clicking on their date.

An Introduction to Infared Satellite Imagery

The satellite images shown here are of sea surface temperature. The data used to construct these images were obtained with an AVHRR sensor (Advanced Very High Resolution Radiometer) carried on a NOAA satellite (Nation Oceanic and Atmospheric Administration). This sensor measures the amount of thermal infared radiation given off by the surface of the ocean. It is called 'thermal' radiation because radiation of this frequency warms our skin. A fire feels warm because it gives off lots of radiation at this frequency. The amount of thermal infared radiation given off by an object is related to its temperature (dying embers give off less radiation than a hot fire and a person gives off even less). Thus by measuring the amount of radiation given off by the ocean we can calculate its temperature.

NOAA has been flying satellites that carry AVHRR sensors since 1978. At any one time there are two operational satellites carrying AVHRR sensors, one passing overhead in the early morning, around 2:30 AM, and in the early afternoon, about 2:30 PM; and the other in mid-morning, about 6:30 AM, and late afternoon, about 6:30 PM. Currently NOAA 13 and NOAA 14 are the two operational satellites. These polar orbiting satellites collect data over nearly the entire globe; however, this unit contains only images from the North Atlantic focusing on the Gulf Stream. In order to make the information easily understandable it is color coded and an 'image' (a sort of picture) of the ocean is made.

A few introductory images

Here is an image of the North Atlantic from June of 1984. Blue and purple represent cold water while green represents warmer water. Just as Canada is cooler than Florida, water off the coast of Canada is cooler than water off the coast of Florida. This north-south gradient is seen clearly in the image. This image also shows a current that brings warm water from the south up to the north. This current is called the Gulf Stream; it moves north along the coast of Florida and then turns eastward off of North Carolina flowing to the north-east across the Atlantic. It is one of the strongest currents in the ocean with an average velocity of 1 m/s (3 ft/s).

This image also shows that the Gulf Stream does not follow a straight path. It has many twists and turns called meanders. A meander is charecterised by its wavelength (the distance along the stream from one wave crest to the next), and its amplitude (the distance perpendicular to the stream between the wave crest and trough). If a meander becomes really sharp, it may pinch off and form what is called a ring. This is much like the formation of an oxbow lake by a river. Rings can be formed either to the north or to the south of the stream. For those rings formed to the north, the water in the center of the ring is warmer than the surrounding water and thus such rings are called warm core rings. For those rings formed to the south of the stream, the center contains water that is cooler than the surrounding water and they are called cold core rings. In this image you can see one warm core ring and two cold core rings. If you are unsure what a ring looks like, look at the next image from a different time in which the rings are labeled.

This image is a close up of part of the Gulf Stream. The rest of the images will cover this same region and will be color-coded in the same way. In this image the core of the Gulf Stream ranges between 25 and 28 deg C (77 and 82 F). The yellow water below the stream is about 23 deg C (73 F) and the green water off Long Island is about 14 deg C (57 F). The blue water around Nova Scotia is about 5 deg C (41 F)! The black line shows where the ocean is 1000m deep, (water shoreward of this line is less than 1000 meters deep and water seaward of this line is more than 1000 meters deep). This is usualy taken to be the edge of the continental shelf.

In most of the images to follow we will see a problem with collecting data from a satellite. A thick cloud blocks all the the radiation and we get no data (shown on these images as white). But perhaps more confusing, a thin cloud only blocks part of the radiation, making the water appear cooler than it actually is. Since there are usually clouds in the sky, we usually lose some data. One way of dealing with this problem is to combine 2 images taken close together in time. Since clouds always make the water appear cooler, a third image is made using the warmer of the two values at each point. If the two images contain clouds at different spots, the third image will contain few clouds. More than two images may be composited in the same fashion. The process of compositing images has been used to create most of the images shown here.

Uses of satellite images

These images are used for many purposes. Oceanographers use them to obtain an idea of how the Gulf Stream actually looks: how wide it is, how large rings are, how many rings and meanders there normally are, etc. Some oceanographers use these images to see where the Gulf Stream is at a certain time. For instance if a scientist wants to take a ship out to study a meander, they can use these images from the past few days to locate a meander. Since the Gulf Stream is such a strong current, it affects the speed of a boat; therefore, many people sailing in the North Atlantic also want to know where the current is and can use these images to find out.

Here are some questions that scientists have answered with these images, see if you can answer them as well.

  1. What is the average width of the Gulf Stream and how much does it vary at a given location over time? How much does it vary downstream at a given time?
  2. What is the average size of a cold core ring? A warm core ring? How much do they vary in size?
  3. How many meanders are there normally?
  4. How big is the largest meander you can find? What is the wavelength of this meander? Can you find one with a larger wavelength?
  5. What is the average location of the stream and how much does it vary?
  6. Is there a difference in temperature between day and night images? Between summer and winter images?
  7. There is a sailboat race from Rhode Island to Bermuda every two years and racers often come to me to ask for satellite images of the Gulf Stream. If you were to sail in this race this year (June 1994) which path would you choose?
Other scientists use these images to study how the Gulf Stream moves and changes in an attempt to know what physical forces control it. Here are some questions that they have answered using these images. Can you answer these questions too?

  1. In what direction do meanders move?
  2. In what direction do rings move?
  3. How fast do meanders move?
  4. How fast do rings move?

Some images

The AVHRR Archive Home
Amy Schubert: amy@uri.gso.uri.edu
Peter Cornillon: pete@uri.gso.uri.edu