January 2013 Issue
NASA's Aquarius Mission Provides New Ocean View
By Gary Lagerloef
NASA's Aquarius mission, launched in June 2011, has been measuring sea surface salinity from space for more than a year. The mission is designed to map sea surface salinity and its changes over the globe to study the links between ocean circulation, the global water cycle and climate.
These data help scientists better understand the far-reaching effects of changing ocean circulation and rainfall patterns, and events such as El Niño, floods, droughts and longer-term climate change. For example, recent multidecadal salinity trends provide a strong indication of changes in the global water cycle, with wetter areas gaining in rainfall and low rainfall regions becoming more arid.
In 2012, the team analyzed and corrected many aspects of the data processing and calibration, and a new validated data release is planned for late January 2013. Two scientific studies were published using Aquarius data in 2012, one that showed new discoveries about long-wave current variations in the tropical Pacific and the other identifying the effect of the passage of a hurricane over the freshwater river plume of the Amazon.
Both of these studies highlight the fact that Aquarius is revealing smaller spatial scale features in the surface salinity that was anticipated before launch. In September 2012, an oceanographic expedition Salinity Processes in the Upper Ocean Regional Study (SPURS) studied the arid subtropical Atlantic region, where the highest surface salinities in the world ocean are observed. Many autonomous sensors will remain in place for a year to investigate what ocean physical processes maintain these high salinity values and complement the large-scale measurements provided by Aquarius.
Sea surface salinity data from Aquarius provide a fundamentally new ocean remote sensing capability. The Aquarius microwave sensor, built by NASA's Jet Propulsion Laboratory and Goddard Space Flight Center, is the heart of the salinity measurement system and is the prime instrument on the joint United States and Argentinan Aquarius/Satélite de Aplicaciones Científicas-D (SAC-D) satellite observatory. The Argentine space agency, Comisión Nacional de Actividades Espaciales, developed the SAC-D spacecraft, which carries Aquarius and other scientific sensors developed by Argentina, Italy, France and Canada.
The Aquarius microwave sensor includes a 2.5-by-3-meter offset reflector and three radiometer feed horns (circular openings). Because of the angle these make with the reflector, Aquarius observes the surface along three parallel tracks (so-called pushbroom design) as the satellite progresses along a near-polar orbit.
Aquarius senses the ocean's surface microwave emission at 1.413 gigahertz (in the L-band portion of the spectrum protected for radio astronomy). The emissivity (measured as a parameter called brightness temperature) is modulated by the electrical conductivity of seawater, hence salinity. (This is somewhat akin to conventional oceanographic salinity measurements made in-water with CTD sensors.) The seawater microwave signature comes from the surface layer of approximately 1-centimeter thickness.
Salinity varies in the open ocean from about 32 to 38 grams dissolved salt per kilogram of seawater, measured with an international standard practical salinity scale based on electrical conductivity. To measure such small differences, the Aquarius microwave radiometers were designed and built to achieve unprecedented accuracy by including temperature control to within 0.1° C in the instrument design.
Many environmental factors affect the measurement, the most important one being surface roughness due to wind and waves. Aquarius is adapted to this as well, by including an L-band radar backscatter sensor that is fully integrated with the electronics and antenna, one of the first such passive and active instruments ever flown in space for Earth science.
The three Aquarius surface footprints range in size between 90 kilometers and 150 kilometers, and form a 390-kilometer-wide swath that provides global coverage every seven days. The mission is designed to achieve a monthly average global root-mean-square measurement error less than 0.2 (practical salinity scale—approximately parts per thousand by mass) at 150-kilometer resolution. The size of the Aquarius footprint limits the accuracy for measurements close to coastal boundaries because the warmer land brightness temperature adds a negative bias to the Aquarius sea surface salinity measurement. A nearshore land correction was developed for this before launch and continues to be improved.
Findings on Salinity, Wind Forecasts
The global map generated by Aquarius shows the predominant and well-known climatological ocean salinity features, such as higher salinity in the subtropics, higher average surface salinity in the Atlantic Ocean compared to the Pacific and Indian Oceans, and lower salinity in rainy belts near the equator, in the northernmost Pacific Ocean and elsewhere, as well as the striking salty-fresh contrast between the Arabian Sea and the Bay of Bengal. These features are related to large-scale patterns of rainfall and evaporation over the ocean, river outflow and ocean circulation.
The low-salinity waters associated with the Amazon River outflow are clearly visible, extending eastward from Brazil across the equatorial Atlantic. Superimposed are surface current vectors that illustrate the relationship between the meanders in the surface flow and the variations of the surface salinity patterns. These currents are derived from satellite sea level and surface wind measurements via www.oscar.noaa.gov. Combining these data sets in this way allows scientists to study the direct link between salinity patterns and ocean currents.
As noted above, another important element of the Aquarius microwave sensor is the scatterometer. This sensor measures the radar backscatter at L-band (1.26-gigahertz) frequency from the sea surface simultaneously with the radiometer emissivity measurements. This backscatter signal is proportional to the strength of the surface wind and waves (roughness). The Aquarius scatterometer has had exceptionally stable performance, with calibration changes less than 0.1 decibels over a year, allowing highly sensitive detection of sea surface roughness.
Areas of higher wind speeds are evident, such as the tropical trade wind zones and the storm belts in the sub-polar latitudes, and lower winds in the subtropics. Tropical or extra-tropical storms can also be identified, such as Prapiroon in the western Pacific. Without some proxy surface roughness, accurate salinity retrievals would not be possible. Numerical wind forecasts have limitations because of their lack of accuracy and coincidence in time with the satellite measurements. Aquarius solves this problem with simultaneous measurements that significantly improve the accuracy of the salinity data.
This year is expected to yield significant scientific discoveries for satellite salinity measurements. A landmark collection of research papers will address early scientific results of the mission and include many aspects of how rainfall, evaporation, river outflows and melting ice are linked to salinity, ocean current and climate variations.
The satellite salinity data are now being tested in numerical ocean models that are used for ocean now-casting and forecasting and will soon be used to enhance the skill of long-term climate prediction models.
The SPURS measurements will conclude in September, and the science teams will focus on data analysis, studying the relationships between the in-situ observations and the satellite salinity data.
In the future, a second SPURS may be carried out in an area that is dominated by intense annual rainfall, in direct contrast to the arid region of the first survey, in order to fully understand ocean processes that regulate the reduced surface salinities and climatic changes in those regions as well.
At the end of 2014, Aquarius will complete a baseline three-year mission designed to measure the seasonal cycle and inter-annual climate variability. Provided that the onboard systems continue to work as needed, NASA and Comisión Nacional de Actividades Espaciales plan to maintain the mission operations into the future and continue to gather this important new data set for studying ocean dynamics, climate change and the global water cycle.
Gary Lagerloef, of the Earth and Space Research institute in Seattle, Washington, is principal investigator of the NASA Aquarius satellite mission. He earned his Ph.D. in oceanography at the University of Washington. His research focuses on ocean physics and climatology, emphasizing new applications of remote sensing, with more than four dozen scientific publications.
Simon Yueh, of the NASA Jet Propulsion Laboratory, is project scientist of the Aquarius satellite mission. He received his Ph.D. in electrical engineering at the Massachusetts Institute of Technology. He has been conducting research in remote sensing of ocean surface salinity, ocean wind, terrestrial snow and soil moisture. He is an IEEE fellow.
Jeffrey Piepmeier, of the NASA Goddard Space Flight Center, is lead calibration engineer for the Aquarius microwave radiometers. He earned his Ph.D. in electrical engineering at Georgia Institute of Technology. His research focuses on technology development for airborne and spaceflight microwave radiometers enabling the next generation of Earth system science measurements.