The oceans and atmosphere are inextricably linked. The sun heats the oceans. Rain, river runoff, and evaporation add and remove freshwater. Winds help drive the currents. These currents then redistribute the heat from the tropics to the poles, moderating air temperatures and shaping precipitation and evaporation patterns.
Scientists have developed complex models of how this coupling of the ocean and atmosphere shapes Earth’s climate. These models incorporate a wealth of data including ocean temperature, surface winds, evaporation, precipitation, and variations in sea level. From these models, scientists hope to predict how human-induced climate change might alter ocean temperatures and circulation. These models are works in progress. They need to be constantly refined to better represent the real world. According to Gary Lagerloef of Earth and Space Research, a nonprofit oceanographic institute in Seattle, these models will soon get a major boost through the addition of new data on a key ingredient: Salt.
Gary has spent a lot of time on the ocean, first with the Coast Guard, then with the officers corps of the National Oceanographic and Atmospheric Administration (NOAA). While with NOAA, he sailed on a research ship in the Bering Sea off the Alaskan coast where he gathered data on everything from ocean circulation patterns to marine mammals. He then came to Seattle, spent five years working in a marine lab and on his doctorate at the University of Washington before returning to sea for another two-and-a-half years, this time as an operations officer in charge of the ship’s scientific mission. Finally after a total of five-and-a-half years at sea, Gary decided to settle on shore full time.
Gary is now the principal investigator of the Aquarius/SAC-D mission, a joint satellite project of NASA and the Argentine space agency Comision Nacional de Actividades Espaciales. Once in orbit in 2009, this satellite will provide pioneering weekly measurements of sea surface salinity across the entire ocean for three years. These data will enable scientists to study the role that salinity plays in ocean circulation and help them develop better models linking the ocean and atmosphere. According to Gary, it will be the most important new measurement in oceanography in over a decade.
All oceans contain salt. The average salinity in the open ocean is 35 parts per thousand, but it varies from place to place and from season to season by as much as five parts per thousand. It is lowest in areas such as the tropics where rainfall is high, and highest in dry climates where there is high evaporation.
Scientists are interested in salinity because along with water temperature, it determines water density. As water cools or salinity increases, the water becomes denser and therefore less buoyant.
Variations in water density help drive ocean circulation. For example, the Gulf Stream carries warm salty water from the tropics to the far north Atlantic. These warm waters modify the climate of northern Europe, making Great Britain and Scandinavia more habitable. As these salty waters reach the far northern Atlantic, they cool and sink due to their increased density. Deep-water currents carry these cold waters throughout the abyss where they gradually warm and return to the surface. Scientists speculate that if the sub-polar waters of the North Atlantic warm due to human-induced climate change, melting ice caps could dilute the water by 0.5 to 1.0 parts per thousand. The water might no longer be dense enough to sink, and the ocean circulation responsible for driving the Gulf Stream could grind to a halt.
Although salinity measurements date back over 125 years, the data currently available are spotty, coming mostly over the summer months from ships and buoys. Seventy-three percent of the ocean has been sampled fewer than ten times or has never been sampled at all. The Aquarius satellite will measure the salinity of the top centimeter of all oceans every seven days. It will do so by sensing low-frequency microwaves that respond to the ocean’s electrical conductivity. This conductivity is a function of salinity and water temperature. Scientists will be able to correct for water temperature based on temperature readings other satellites provide. A radar instrument on the Aquarius/SAC-D satellite will correct for wind-caused disturbances along the ocean surface. A separate higher frequency microwave sensor will provide data on rainfall, wind, and sea ice.
As the principal investigator, Gary is working with the engineers at NASA to make sure that the satellite is designed to meet the scientific objectives of the mission. He also must make sure that the project remains on budget and on time.
Gary says that data the Aquarius satellite provides will help ocean and climate scientists on a number of fronts. For the first time, scientists will be able measure how sea-surface salinity patterns vary from season to season and from year to year. They can observe how salt is transported across the ocean and gain insights into why, for example, the Atlantic Basin is saltier than other ocean basins. Scientists will be able to refine their measurements of rainfall, runoff, and evaporation by observing changes in sea-surface salinity. These improved measurements will strengthen the models linking the ocean and atmosphere.
Scientists also hope to learn the role salinity plays in climate events such as El Nino. El Nino occurs when unusually warm water pools in the eastern Pacific Ocean off the coast of Peru and Ecuador due to the disruption of the normal east to west current. The climatic consequences are far-reaching, ranging from severe droughts in the western Pacific and eastern Africa to unusually heavy rainfall in Peru, Ecuador, and the southern United States. A 2003 study concluded that changes in salinity preceded past El Nino events, and that by measuring salinity, scientists can improve their forecasts of future El Nino events by six to 12 months.
Gary is also a principal investigator of OSCAR (Ocean Surface Currents Analysis Real-time). He and Fabrice Bonjean of Earth Space Research have developed equations to map ocean currents based on satellite measurements of sea surface elevation and wind speed and direction. The goal of OSCAR is to compile and analyze these data then post real-time information on ocean currents on a web site. Gary says that ocean scientists log onto the site most often, including those who are studying El Nino. But they are also trying to attract other users, including ship operators and fishery researchers studying how changes in ocean circulation affects the distribution of commercially important fish.
Both Aquarius/SAC-D and OSCAR will provide scientists with more complete information on which to build their models of sea-surface currents and ocean circulation. These models will in turn provide scientists with better insights into the interplay between atmosphere and ocean and how these interactions shape our climate.