Not all oceanographers have their perspectives on the ocean shaped by experiences at sea. Take Kathie Kelly. She does not venture out on a ship, dive in a submarine, or even go to the beach to conduct her research. Instead, she experiences the ocean through satellite images and numbers on her computer. While she may miss out on the excitement of a life on the open ocean, the copious amounts of data at her fingertips more than make up for it. For Kathie, having enough data to pursue important questions about the ocean is what research is all about.
Observed variations in sea surface height (SSH) measured by the TOPEX/Poseidon altimeter (left). Two different model results of SSH (center and right).
These satellite data are helping Kathie gain insights into how the ocean influences the climate. At the simplest level, the ocean acts as a heat reservoir. It absorbs heat from the atmosphere when the atmosphere is warmer and releases heat to the atmosphere when the atmosphere is cooler. This transfer of heat between the ocean and atmosphere is called heat flux. The reality, however, is more complicated. Ocean currents redistribute the heat around the world by carrying warm water from the equator towards the poles. For example, the Gulf Stream moves huge amounts of warm water from the tropics up along the east coast of North America and across to northern Europe. The presence of this warm water in the North Atlantic helps explain why Scotland has a relatively mild climate when compared to places at similar latitudes in North America such as Churchill, Manitoba—a Canadian town famous for its seasonal polar bear population.
Kathie in her office at the Applied Physics Lab, University of Washington looking at NASA scatterometer.
An understanding of these interactions between air and water is critical to understanding the climate and predicting climate change. It requires sophisticated models both of the ocean and the atmosphere. Kathie is focusing her efforts on the ocean end. Specifically, she is trying to determine the most important factors influencing sea surface temperatures. These factors include heat flux from the atmosphere, ocean currents, winds, and the upwelling of water from beneath the surface.
Up until a decade ago, oceanographers had to go out to sea to measure parameters such as temperature, wind, and currents. These measurements were time-consuming, expensive, and infrequently repeated; therefore it was impossible to compare these parameters from year to year. Now Kathie receives satellite images from NASA containing this information every ten days. One satellite instrument called an altimeter detects currents by measuring horizontal differences in sea surface height. Changes in sea surface height can indicate the amount of heat in the ocean because the ocean expands when it is heated and contracts when it is cooled. So if you look downstream along a warm-water current, such as the Gulf Stream, the sea height on the right is higher (warm water) and the sea height on the left is lower (cool water). The altimeter measures sea surface height by bouncing radar signals off the ocean and timing their echo. They have an accuracy of three centimeters—an amazing number considering that the satellites are 1,300 kilometers high.
Tracks (or paths) of buoys drifting at sea (green). Tracks of the TOPEX/Poseidon satellite (blue). Study region is marked by red rectangle.
A second satellite instrument called a scatterometer enables Kathie and her colleagues to calculate wind stress. The satellite sends down a pulse and measures how much it is scattered by ripples and waves created by wind stress. (Think about how wind creates ripples on a pond). Unlike instruments that just measure air speed, the scatterometer measures the movement in air relative to the movement of water. For example, if the wind was blowing at the same speed and direction as a current, it would create no stress on the water and there would be no ripples. Wind stress is an important factor in determining the transfer of heat between the ocean and atmosphere.
Kathie and her colleagues are working with an eight-year record of the currents. These records show changes in some of the major currents, including a change in the heat content of the Kuroshio Extension, a warm-water current off of the coast of Japan. It is unclear what is causing these changes and whether it is a cyclical event or represents a long-term trend.
The larger question is how changes in sea surface temperature affect the atmosphere, particularly surrounding the major currents. Some studies suggest that it is very important, but others suggest that the effect is small. Kathie’s studies of the factors affecting sea surface temperature will contribute to answering this question.
- Principal Oceanographer, Applied Physics Laboratory
- Professor (Affiliate), School of Oceanography University of Washington
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