Instructions: First print copies of Exercise 4, Lag-time questions, and Real-time questions. Read the text and examine the images. Write answers to the questions on your copies. When you are satisfied with your answers, type them into the computer and submit. Keep the copies as a study guide.
The ocean is not a homogeneous body of water. Rather, there are layers with distinct temperatures, salinities, and dissolved gas and nutrient content, and currents with distinct characteristics that circulate water up and down within the ocean and along the ocean surface. Because these currents are large-scale features, global views such as those obtained from satellite measurements are needed to study them. In this voyage we will examine some of the techniques used to investigate ocean currents, and will begin to look at the interactions of ocean and atmosphere that create climate.
Large-scale currents operate in the world ocean to redistribute heat. Warm-water currents from the equator travel toward the poles, and cold-water currents travel from the poles toward the equator. Two examples of this large-scale circulation are the North Pacific and North Atlantic gyres. Click here to obtain a diagram of the major oceanic currents. Clicking will bring up a separate window that you can keep open while you are working on the exercise. (You can also print the diagram, if you like.) If the current names are not clearly visible, you can view a similar diagram in the textbook on page 149 (Figure 8.8).
How can we observe these currents? Here are three techniques.
For centuries, people have thrown objects into the ocean to see where the currents will send them. Today, the technique is still used, but with a high level of technological sophistication. Buoys are dropped by ships and airplanes into the ocean, where they drift with the currents and directly measure water characteristics with built-in instruments. They are tracked by satellites in orbits far above Earth and transmit data several times a day.
Figure 1. This map of the North Pacific Ocean contains data from three buoys. The data were recorded during a 10-month period between February and December 1995. The arrow in the middle of each plotted line shows the direction of each buoy's motion. The beginning of the line shows where the buoy was located in February and the end of the line shows where the buoy was located in December. I obtained the data from a web site that provides buoy latitude and longitude information and used them to plot positions of the buoys through time.
NOTE: Longitudes 80-180 degrees on the left (west) side of the diagram are East Longitudes. Longitudes 180 to 80 on the right (east) side of the diagram are West Longitudes. Latitudes north and south of the equator are also shown.
Because currents often have different temperatures then the surrounding water, measurements of sea-surface temperatures (SST) can be used to map currents. SSTs are easily measured from satellites.
Figure 2. This image shows sea-surface temperatures (SSTs) measured by instruments on satellites. Different temperatures were assigned different colors (see color bar along right side of image; numbers are degrees Celsius). The SSTs show two currents --- the Gulf Stream (yellow to red colors) and the Labrador Current (blue colors) --- that flow parallel to the western edge of the North Atlantic Ocean, along the East Coast of the United States. The gray color is land, extending from Cuba (just south of Florida) to Cape Cod (just north of New York). SSTs range from a cold 4 degrees Celsius (40 degrees F; dark blue color) to a moderate 17 degrees Celsius (63 degrees F; green color) to a warm 29 degrees Celsius (84 degrees F; red color).
Major currents such as these can be thought of as rivers in the ocean that can transport incredibly huge amounts of water from place to place. For example, the Gulf Stream transports more than 150 cubic meters of water per second, compared to a flow of 0.6 cubic meters per second for all of the rivers that flow into the Atlantic Ocean.
Figure 3. This image shows SSTs for the entire world ocean in August 1995, also measured from a satellite. Temperatures are in degrees Celsius. The temperature range and colors are similar to those in Figure 2.
Different currents contain different amounts of life-supporting nutrients such as nitrogen and phosphorus. Because the water in some currents is more biologically productive than in other currents, we can use productivity measurements to investigate the location and shape of the currents.
Figure 4. This image shows worldwide variations in primary productivity (plant growth) based on satellite measurements of the concentration of chlorophyll pigment in the water near the oceans' surface. Red, orange, and yellow colors indicate high values; purple and blue colors indicate low values (see color bar scale). Black areas are parts of the ocean where insufficient data were collected. The images incorporate data that were gathered from July through September (Northern Hemisphere summer months).
Two factors affect the amount of primary productivity in the ocean: light and nutrients. Light changes seasonally, particularly at high latitudes. High nutrient levels are often observed along the edges of oceans, where upwelling currents bring nutrient-rich water to the surface. Low nutrient levels are often observed in the centers of the large oceanic gyres.
Currents are just one component of the ocean that influences our climate. It is part of the system of ocean and atmosphere circulation that acts to redistribute heat on earth and to keep our planet a habitable environment for living things. Many people are concerned that human actions are upsetting the heat balance of earth. Of particular concern is our burning of fossil fuels (oil, gas, coal, etc.), which inputs large amounts of Carbon Dioxide (CO2) into the atmosphere. Pages 113-114 in the textbook (see Figure 6.12) explain the heat budget on earth. Pages 313-315 in the textbook (see especially Figure 15.22) explain how the addition of greenhouse gases (including CO2) could upset the balance and cause overall warming on the planet.
Article 1. Click here to read an article ("solid evidence 'greenhouse gas' heating up earth") from the San Francisco Chronicle earlier this year that describes some evidence that the planet is warming and also some reactions to this evidence. Note that this link will bring up a new window that will connect you to the SF Gate web site.
Article 2. Click here to read an article ("Greenland ice cap is melting, raising sea level) about some potential impacts of global warming.