Engagement

• Earth's obliquity -- the tilt of its rotational axis relative to its orbit -- causes us to experience four seasons each year: summer, fall, winter, and spring. Timing of the seasons in the northern and southern hemispheres are reversed. For example, summer in the northern hemisphere occurs at the same time as winter in the southern hemisphere.
  • Click here to see an animation about Earth's seasons.
  • Be aware that this animation is 3.1 MB in size!
• Figure 1 shows how sea surface height varies from its annual average from season to season. In this activity you will analyze four satellite data maps and match them to the season they represent. The images are derived from sea level measurements made by Earth-orbiting TOPEX/Poseidon satellite from September 1992 to August 1993.
• A clue for interpreting the images is that water has a very high heat capacity, meaning that it takes a lot of energy transfer to change its temperature, i.e., heat it up or cool it down. Thus, the ocean heats and cools more slowly than land. You will use this concept and your knowledge of the amount of solar heating that occurs in each season, to determine which map is from fall, winter, spring, and summer.

Activity

1. Observe all four images together. First, locate the land masses on each image. You will need a map with latitude markings for reference. Use this map to roughly sketch the equator and relevant lines of latitude on each of the four images in Fig. 1. For this discussion, focus on the mid-latitudes in the northern hemisphere (20°N - 50°N). You will need a map with latitude markings for reference.
2. Make sure that you are familiar with the color-coded sea surface height scale for the images. In which image do you find the highest sea surface heights? In which image do you find the lowest sea surface heights? Explain your answers.
3. Use your sea surface height observations to determine which of the four images represents the sea levels from fall in the northern hemisphere (spring in the southern hemisphere). Why did you choose that image? How might the timing of the seasonal sea level variations relate to the heat capacity of water?
4. Once you have determined which image represents the fall in the northern hemisphere, try to find the image that represents summer. Why did you choose that image? How does sea surface height compare between summer and fall? Finally, try to identify the maps of north-ern hemisphere winter and spring. Explain your answers.
5. Compare sea level responses in mid-latitudes in the northern hemisphere with those in the southern hemisphere. Do they vary in the same way? Which hemisphere has larger variations in sea surface height? Can you guess why?

Explanation

• Regional sea levels vary with the seasons. At mid-latitudes, 20° to 50° north and south, sea level change is controlled by heating (which causes higher sea levels due to thermal expansion of the water) and cooling in the upper ocean. In the northern hemisphere fall, the temperature of the upper ocean layer and sea level are high after being warmed throughout the summer. The lowest sea levels occur in the northern hemisphere's spring.
• The southern hemisphere also experiences seasonal changes, but to a lesser extent. One reason for this is that there is less land in the southern hemisphere. This limits the extent of cold winter air that blows out from continents to cool the oceans. Moreover, the southern hemisphere has a greater proportion of ocean to land, which results in a more moderate climate with less seasonal change.