4.11: Satellite Oceanography

Space-based observations of the ocean have revolutionized oceanography. Though physical oceanographers had long suspected the chaotic nature of ocean circulation, the first satellite images of ocean color in 1978 drew a collective “oh, my.” The surface of the ocean and the phytoplankton suspended within it resembled the sky in Vincent van Gogh’s painting Starry Night (1889): variable, complex, and organized in loose, rotating structures. The ocean’s mesoscale structure—the highly variable eddies that characterize nearly all of the ocean’s flows—suddenly became visible. Seemingly overnight, satellite oceanography, the study of the world ocean using sensors aboard space-based platforms, changed the way we thought about and studied the ocean. Barber and Hilting (2000) likened it to “seeing the ocean for the first time.”

Prior to this grand awakening, weather and military satellites had routinely observed the Earth, ice caps, and ocean. But on June 26, 1978, the National Aeronautics and Space Administration (NASA) launched the very first satellite dedicated solely to oceanography, the Seafaring Satellite, or Seasat. Four months later, NASA launched the Nimbus 7, which carried the Coastal Zone Color Scanner (CZCS), the first sensor designed to measure ocean color. Its success drew greater attention to the optical properties below the ocean’s surface and gave birth to the field of bio-optical oceanography, the study of the submarine light field and its variations due to biological processes (Smith and Baker 1978). This extraordinary technology continues to make significant contributions to our understanding of the world ocean in the 21st century.

Sea Surface Temperature

Covering 71 percent of Earth’s surface, the world ocean dominates temperature regulation on our planet. As NOAA puts it, “the ocean is the largest solar energy collector on Earth” (Lindsey and Dahlman 2020). Through heat exchange across the air–sea interface—the boundary between the ocean and the atmosphere—the world ocean adds or removes heat from the atmosphere and vice versa. These exchanges of heat drive the motions of the atmosphere and the ocean and prove important for a whole heckuva lot of other things we’ll learn about in the chapters ahead.

Satellite oceanography owes much of its early development to sensors used in aerial oceanography. One group of sensors, collectively known as radiometers, detects different kinds of electromagnetic radiation, the energy of light. Because the ocean and the stuff it contains remove, reflect, and scatter different wavelengths of electromagnetic radiation, we can learn about the ocean by measuring the wavelengths of light emitted by its surface. Sensors that detect different types of surface-emitted radiation are known as passive sensors.

On April 1, 1960, NASA launched the world’s first weather satellite, the TIROS-1 (Television Infrared Observation Satellite), one of a series of satellites carrying television cameras into space. Images of cloud and weather patterns ushered in an era of space-based weather forecasting. TIROS set the stage for the Earth-observing satellite systems that now deliver weather information every hour of every day across the globe.

Scientists soon developed additional tools for space-based observations of Earth. Measurements of the temperature of the sea surface—a property known as sea surface temperature (SST)—began with the Nimbus satellites launched between 1964 and 1978. These satellites carried high-resolution infrared radiometers designed to measure the temperatures of cloud tops and the sea surface at night (when sunlight wouldn’t interfere with the measurement). In 1978 NASA launched TIROS-N (N for NOAA), which carried the first of a series of Advanced Very-High-Resolution Radiometers (AVHRRs), multichannel, infrared-sensing instruments for measuring SST. The data from AVHRRs flown on TIROS-N and subsequent satellites represent the longest near-continuous measurements of SST from space, a period of more than 40 years (e.g., Casey et al. 2010; O’Carroll et al. 2019; Minnett et al. 2019).

One limitation of AVHRR sensors is their inability to “see” through clouds, which block surface-emitted infrared radiation. To overcome this limitation, NASA and the Japanese Aerospace Exploration Agency launched satellites that carried passive microwave sensors. These sensors detect microwave radiation which passes through clouds and from which estimates of SST and other properties can be derived. Unfortunately, microwave sensors lack the resolution of infrared sensors and tend to be less accurate (Castro et al. 2008). So oceanographers take the best of both worlds and blend data from infrared and microwave sensors into a combined product, the Multi-scale Ultra-high-Resolution SSTs (MUR-SSTs). Global and regional maps of MUR-SSTs adhere to standards developed by the Group for High Resolution Sea Surface Temperature, which promotes best practices for monitoring, processing, and reporting satellite-derived SST (Minnett et al. 2019).

SST measurements serve operational oceanography, climate change research, fisheries, marine heatwave tracking, and other interests (e.g., Beggs 2010; O’Carroll et al. 2019). NOAA’s Coral Reef Watch uses satellite SST data to inform resource managers, scientists, and the public about the potential for coral bleaching, the phenomenon whereby coral animals expel their algal “helpers” in response to prolonged elevated seawater temperatures. In recent decades, coral bleaching has devastated corals around the world (e.g., Hughes et al. 2018). Satellite-based forecasts of coral bleaching allow scientists to “rescue” rare coral species and house them in a laboratory. When the warm waters subside, the corals can be returned to their natural environment. It’s not the ideal solution, but coral rescues play an important role in coral conservation and management (e.g., Vardi et al. 2021).

Sea Ice Extent

Soon after the Titanic sank following its collision with an iceberg in 1912, the governments of the US and Europe formed the International Ice Patrol. Tasked with monitoring the whereabouts of icebergs, seaborne chunks of fractured glaciers, the Ice Patrol was the first to regularly monitor ice conditions at polar latitudes. Interest in polar ocean ecosystems and climate change led to efforts in the 1960s to develop tools for observing icebergs and sea ice, the frozen surface of polar oceans (Zwally et al. 1983). Fortunately, passive microwave sensors proved ideal for the job.

Observations of sea ice using passive microwave sensors aboard aircraft began in the Arctic in 1967 (Gloersen et al. 1992). The first passive microwave instruments were flown on Russian satellites in 1968 (Tikhonov et al. 2016). But the Electrically Scanning Microwave Radiometer flown aboard the Nimbus 5 satellite in 1972 began the era of “all-weather, all-seasons” sea ice observations that has continued to this day (e.g., Parkinson et al. 1987). A 40-year record from passive microwave sensors has revealed striking and alarming retreats of sea ice in the Arctic and Antarctic Oceans (Parkinson 2019). Multiyear ice, sea ice that remains present for longer than one year—thought to be critical for maintaining sea ice volume—has shown similar declines (Haibo et al. 2020).

A desire for higher-resolution imagery led to the launch of active microwave sensors, instruments that produce their own electromagnetic signal and detect the reflected signal (like the radar gun used to catch speeders). By bouncing a beam of energy off the surface of the ocean, oceanographers can measure a number of ocean properties with very high resolution (e.g., Gens 2008). An active microwave system known as synthetic aperture radar (SAR) flew aboard Seasat and performed so well it inspired a generation of SAR-carrying satellites in the 1980s and 1990s (Tsatsoulis and Kwok 1998). SAR works by electronically mimicking a large antenna, enabling it to produce detailed images of sea ice and other properties. SAR remains an integral part of Earth-observing satellite systems in the 21st century.

Sea Surface Height

By bouncing a beam of energy off the surface of the ocean, oceanographers can measure the shape of the sea surface–the sea surface topography, the bumps and depressions of the ocean surface. The best-known devices, the satellite altimeters, measure the time it takes for a microwave radar signal to bounce off the ocean surface and return to the satellite. Based on the return time and precise knowledge of the height and path of the satellite, oceanographers can calculate the distance between the sea surface and the satellite.

Satellite altimeter measurements allow mapping of sea surface height, the height of the sea surface relative to a baseline, such as Earth’s ellipsoid or geoid (i.e., its equi-gravitational surface; see Chapter 20). Sea surface height measurements permit oceanographers to learn about ocean circulation and ocean tides. It also provides knowledge of sea level changes in response to a warming ocean and melting ice caps. As you might expect, these observations and the computations required to generate useful satellite products—the results provided by satellite measurements—involve very accurate measurements and lots of computing power.

Arguably the most successful oceanographic satellite altimeter mission to date is TOPEX/Poseidon, the Ocean Topography Experiment/Poseidon mission. Launched in 1992 in a collaboration between NASA and the French space agency, Centre National d’Etudes Spatiales, TOPEX/Poseidon allowed oceanographers to observe changes in world ocean circulation over seasonal and annual cycles, advanced our understanding of tides and sea level rise, and enabled significant refinement of general circulation models. It provided unprecedented observations of the 1997–1998 El Niño, among the strongest in 100 years. After more than 13 years and nearly 62,000 orbits of Earth, TOPEX/Poseidon ended operations in October 2005. Legendary oceanographer Walter Munk (1917–2019) called it “the most successful ocean experiment of all times” (Munk 2002).

The Jason satellite altimetry missions extended the data record begun by TOPEX/Poseidon. Jason-3 (launched in 2016) currently operates in tandem with the altimeter-carrying Sentinel-6 Michael Freilich satellite. This satellite was named after the former director of NASA’s Earth Science Division, Michael Freilich (1954–2020)—a “passionate advocate” of measurements of Earth from space (Cook 2020). This satellite can track sea level rise with an error of less than a tenth of an inch (<1 mm) per year (Donlon et al. 2021).

Ocean Color

If you’ve ever spent time near an ocean in middle latitudes, you’ve undoubtedly noticed that the color of the water changes from season to season. Here in Southern California, the winter ocean appears blue, turns green in spring, blues up in summer, and greens again slightly in fall. These changes in ocean color originate from changes in the abundance of phytoplankton and their primary light-absorbing pigment, chlorophyll. Think green trees in summer and fallen leaves in winter. It’s a similar phenomenon in the ocean. As the concentrations of phytoplankton vary, so does the color of the water: more phytoplankton, greener water. Viewed from space, these organisms give the ocean a palette of greenish hues in what might otherwise be a blue desert.

Oceanographers define ocean color as the “relative amounts of water-leaving radiance in the various portions of the visible spectrum” (Austin 1993). In other words, ocean color is the color of the ocean. Pure ocean water absorbs mostly every color except blue. That’s why the open ocean appears blue. It’s the only color remaining after all others are absorbed. The CZCS (mentioned above) provided biological oceanographers their first global views of ocean color, whose hues varied with the concentration of phytoplankton in the water. Images from the CZCS revealed an intimate connection between ocean physics (i.e., seasonal stratification, upwelling, eddies) and phytoplankton.

The CZCS greatly exceeded expectations and outlived its life expectancy, but it ceased operations in 1986. A decade later, the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) lifted off from Vandenberg Air Force Base (August 1, 1997). By this time, oceanographers had developed in-water optical sensors and improved biochemical analyses of seawater. A new generation of oceanographers, the bio-optical oceanographers, stepped in. Their mission was to advance our understanding of biological oceanography through a study of ocean optics. Of them, Barale writes: “The space oceanographers . . . were like those pioneers who opened new trade routes, as what they discovered . . . surpassed many times what was expected in the beginning” (Barale 2010).

The design and execution of SeaWiFS built on the lessons learned from the CZCS, including the need for continuous ground truthing, the practice of verifying satellite observations with measurements obtained from within the water column. Though traditionally carried out on ships, ground truthing with AUVs is now under development (e.g., Bailey and Werdell 2006; Bennion et al. 2019). Buoys are also used, such as the Marine Optical Buoy system moored off the coast of Hawaii (e.g., Brown et al. 2007).

More than a dozen missions have been launched since SeaWiFS. The Moderate Resolution Imaging Spectroradiometer (MODIS) launched aboard the Terra satellite (December 1999) and the Aqua satellite (May 2002) brought improvements to sensor design and performance. The Visible and Infrared Imager/Radiometer Suite (VIIRS), the successor to MODIS, aboard the Suomi National Polar-orbiting Partnership, and a host of international sensors and satellites devoted to ocean color measurements ensure that global measurements of the ocean biosphere continue to this day.