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Earth Observations from Space: The First 50 Years of Scientific Achievements
8
Ocean Dynamics
Due to the remoteness of the vast open oceans, satellites provided the first truly global ocean-observing system. Presatellite observing platforms included ships, moorings, drifters, and other tools, none of which could provide ocean basin-scale coverage at the temporal and spatial scales required to resolve the dynamic nature of the ocean that has been revealed since. In fact, even a well-known and studied current such as the Gulf Stream was not fully characterized until satellite observations were available (Box 8.1, Figures 8.1 and 8.2). Satellite data from scatterometers, altimeters, infrared radiometers, and various ocean color sensors opened up a new window for observing and quantifying how and why water moves around in the ocean (ocean dynamics) and how energy is exchanged between the ocean and atmosphere (air-sea interaction).
As illustrated in more detail below, sea surface temperature (SST) measurements not only revealed important information about ocean circulations (e.g., the Gulf Stream) but also advanced climate research by providing detailed information on the heat input into the ocean. Ocean color combined with SST observations led to new discoveries about the physical-biological coupling in the ocean, with important implications for the ocean’s role in the carbon cycle (see also Chapter 9). Observations from altimeters have resulted in a slow revolution, as the accuracy of the sensors steadily increased, taking about a decade for their contributions to be broadly recognized. Altimetry embedded within other modern ocean measurements and models yielded a virtually complete description of first-order physical processes in the ocean. The measurements provided real surprises to physical oceanographers, including detection of the internal tide in the open ocean, of a highly variable surface ocean full of eddies, and of global sea-level trends at an accuracy of millimeters per year. Combining satellite data with in situ observations and models converted physical oceanography into a global science with actual predictive skill.
THE OCEAN’S ROLE IN CLIMATE CHANGE
Monitoring SST by the Advanced Very High Resolution Radiometer (AVHRR) is the marine remote sensing technique with the broadest impact on oceanography (Robinson 1985). SST is the earliest Earth-orbiting satellite measurement for oceanography and began with the launch of the Television Infrared Observation Satellite (TIROS-N) in 1978. SST measurements provide the longest continuous record of any oceanic property from space (Table 8.1). This long-term data set has been calibrated and validated by surface observations from drifters, buoys, and ships and is used for a broad range of oceanographic research questions, including studies of regional climate variability, most notably El Niño-Southern Oscillation (ENSO; see also Chapter 12), climate change, and ocean currents.
SST is one of the most important indicators of global climate change and a vital parameter for climate modeling (Hurrell and Trenberth 1999). Because of the large heat content of the ocean, more than 80 percent of the total heating of the Earth system is stored in the ocean, and ocean currents redistribute this heat across the globe. Consequently, “if [scientists] wish to understand and explain [global] warming, the oceans are clearly the place to look” (Barnett et al. 2005b). In addition, SST is central in coupling the ocean with the atmosphere and is a controlling factor in the heat and vapor exchange between the two (Johannessen et al. 2001). Trend analysis of SST provided evidence for global warming and the important climate-atmosphere feedback in the tropics that is also responsible for ENSO events (Cane et al. 1997). These SST observations, combined with in situ vertical temperature measurements of the ocean to a depth of 3,000 m provided evidence to detect anthropogenic global warming in the ocean (Barnett et al. 2001, 2005b).
Understanding the increase in SST and anthropogenic heat input to the surface ocean also has important
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BOX 8.1
Gulf Stream Path
Describing the path of the Gulf Stream and other major ocean currents was an early challenge to physical oceanographers who based their interpretations on very sparse data collected from oceanographic ships. For example, using data from a multiship survey, Fuglister and Worthington (1951) proposed the Multiple Current Hypothesis, which suggested that an instantaneous chart of the Gulf Stream would show a number of disconnected filaments of current that change in time (Figure 8.1). Furthermore, they concluded that three Gulf Stream configurations were possible: a single filament (Figure 8.1a), a branching current with two filaments (Figure 8.1b), or a number of irregular disconnected filaments (Figure 8.1c). In subsequent years, ship data could not distinguish between these three and other interpretations. However, in the mid-1970s the synoptic view provided by satellite thermal infrared imagery showed that the Gulf Stream was a single filament, albeit following a tortuous and time-changing path (Figure 8.2).
Over many years synoptic views of the Gulf Stream were obtained via satellite radiometers. These results showed considerable interannual variability in the path of the stream based on the position of the “North Wall”—the boundary at which strong temperature gradients (fronts) between warm Gulf Stream waters and colder waters of the Northwest Atlantic demarcate the northernmost extent of the stream (Lee and Cornillon 1995). These interannual motions were subsequently shown to be important to fisheries (Olson 2001) and to the productivity of the Slope Sea (Schollaert et al. 2004).
FIGURE 8.1 Fuglister’s multiple current hypothesis. SOURCE: Stommel (1965). Reprinted with permission from the University of California Press, copyright 1965.
FIGURE 8.2 SST image showing the Gulf Stream in the Atlantic Ocean. SOURCE: Provided by Otis Brown and Bob Evans.
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TABLE 8.1 TIROS-NOAA Satellites Carrying AVHRR Sensors Monitoring SST
Satellite
Dates of Operation
TIROS-N
Oct. 1978-Jan. 1980
NOAA-6
June 1979-March 1983
NOAA-7
Aug. 1981-Feb. 1985
NOAA-8
May 1983-Oct. 1985
NOAA-9
Feb. 1985-Nov. 1988
NOAA-10
Nov. 1986-Sept. 1991
NOAA-11
Nov. 1988-April 1995
NOAA-12
Sept. 1991-present
NOAA-14
Dec. 1994-present
NOAA-15
May 1998-present
NOAA-16
Sept. 2000-present
ramifications for quantifying and predicting sea-level rise (Cabanes et al. 2001). Recent work suggests that thermal expansion of the surface ocean (upper 500 m) can fully explain the sea-level rise of 3.2 (± 0.2) mm per year observed by the satellite TOPEX/Poseidon (Cabanes et al. 2001).
PREVALENCE OF DYNAMIC FEATURES
The ability to observe the ocean surface from space has profoundly altered the way the ocean is viewed. The Coastal Zone Color Scanner (CZCS), launched aboard the Nimbus 7 satellite in 1978, provided the first satellite observations used to quantify the chlorophyll concentration in the upper ocean (see Chapter 9, Box 9.3). The first images of surface chlorophyll distributions were truly astonishing, revealing a high degree of spatial variability never fully appreciated before satellites (Figure 8.3). The availability of global maps of chlorophyll, an estimate for marine plant biomass, has opened new avenues of research and changed the conduct of biological oceanography in many ways.
Mesoscale features such as vortices and jets, as well as tidal fronts and river plumes, had been seen previously in aerial photographs and thermal imagery from the TIROS satellites, but ocean color images revealed entirely new features. An example of this is the vast extent of the Amazon River plume stretching many thousands of kilometers across the Atlantic (Figure 8.4, Muller-Karger et al. 1988). The plume’s temperature does not provide sufficient contrast
FIGURE 8.3 CZCS image of phytoplankton pigments in the North Atlantic Ocean. CZCS was flown on the Nimbus 7 satellite launched in 1978. CZCS was the first multispectral imager designed specifically for satellite observations of ocean color variations. One of the primary determinants of ocean color is the concentration of chlorophyll pigments in the water. High concentrations of chlorophyll (red and brown areas in the image) are seen along the continental shelf (1) and above Georges Bank (2) where the biological productivity is high. Intermediate concentrations of chlorophyll pigments are shown in green, and the lowest levels are blue. Notice that the Gulf Stream (3) and the warm core eddy to the north (blue circle) have very low concentrations, reflecting the fact that the stream and the Sargasso Sea to the south are relatively nutrient poor. SOURCE: NASA.
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FIGURE 8.4 The vast extent of the Amazon River plume stretching thousands of kilometers into the Atlantic Ocean is an example of a new discovery that resulted from the first ocean color observations from space (Muller-Karger et al. 1988). The Amazon River plume is the green band extending across the Atlantic in this seasonally averaged CZCS pigment image for the months of September to November 1979. Bands of high pigment also mark the nutrient-rich upwelling along the equator in the Pacific and Atlantic, and the high latitudes and coastal regions are also seen as productive. Black areas over the ocean are missing data because CZCS operated only intermittently. SOURCE: SeaWiFS Project, NASA Goddard Space Flight Center, and GeoEye. Provided by the SeaWiFS Project, NASA/Goddard Space Flight Center and GeoEye.
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with the tropical Atlantic to be visible in thermal imagery, whereas its color makes it clearly visible. River discharge measurements from gauging stations have been shown to correlate with the temporal variability of plumes in satellite images of the Gulf of Mexico (Salisbury et al. 2001, 2004), thus offering a method for studying the influences of rivers on the coastal ocean.
As a result of satellite images, Earth scientists have gained a physical perspective and appreciation of the relationship of the ocean to land masses. Seaward-flowing jets and filaments associated with major fronts along the continental shelf off California and the Pacific Northwest were a focus of the Coastal Transition Zone Program during the CZCS era (Brink and Cowles 1991). These narrow filaments of productive water extending hundreds of kilometers seaward from the continental margin are now recognized as important pathways for the transport of materials from the continental shelves to the deep ocean (Strub et al. 1991). Other researchers used CZCS to look at the Columbia River plume (Fiedler and Laurs 1990) and to relate tuna catch to fronts and features seen in satellite images (Laurs et al. 1984).
UNDERSTANDING OCEAN TIDES: NEW SOLUTIONS TO AN OLD SCIENTIFIC QUESTION
Ocean tides have fascinated scientists since the early Greeks and were first explained by Newton to be caused by the gravitational attraction of the Moon and the Sun. A century later Newton’s theory was replaced by the dynamic response concept described by Laplace’s (1776) tidal equation. Because Laplace’s tidal equation strongly depends on the shape and bathymetry of the ocean basin and because oceans have clusters of natural resonances in the same frequency bands as the gravitational forcing function (Platzman 1981), analytical solutions to Laplace’s tidal equation cannot be found. Therefore, predicting ocean tides to some level of accuracy was made possible only by Darwin’s (1886) empirical method. The behavior of ocean tides, particularly in the open ocean, remained elusive until the advent of satellite altimetry (Le Provost 2001).
For the first time, satellite altimetry observations allowed synoptic measurements of ocean tides in the global open ocean. Although the first altimetry data were obtained from Geodynamics Experimental Ocean Satellite 3 (Geos3) in 1973, it was not until Seasat in 1978 that it became evident that a tidal signal could be retrieved from satellite altimetry (Le Provost 1983, Cartwright and Alcock 1983). Most of the advances in global ocean tide modeling have only been made since the launch of European Remote Sensing Satellite (ERS)-1 in 1991 and Topography Experiment (TOPEX)/ Poseidon (T/P) in 1992. Based on altimetry and tidal models, it is now possible to predict ocean tides globally, including in the deep ocean, with a precision of 2-4 cm over periods of several months to years (Le Provost 2001). Global information on ocean tides has resulted in an improved ability to model and predict them that would not have been possible without satellite information, due to the limitations of in situ tidal observations in the open ocean. This in itself is a major achievement. Consequently, the marine shipping sector has benefited from improved tidal predictions.
THE TURBULENT OCEAN
By providing the ability to measure the eddy variability globally, to determine its space-time variability, and to study its time evolution, altimetry led to a paradigm shift in oceanography in the late 1990s. The direct observations of the extensive eddy field by altimetry (see below) coupled with the recent focus on energy sources for internal wave mixing of the deep ocean (see next section), including those with tidal components, changed the way we think about the nature of the global ocean circulation (Wunsch and Ferrari 2004). Before altimetry the energy supply for the large-scale circulation was believed to be dominated by surface buoyancy forces related to changes in water temperature and salinity across and within the ocean basins leading to calculations and predictions of slowly changing large-scale and slow-moving features. Since the advent of altimetry, scientists know that energy is provided to the general circulation primarily by winds and tides. Perhaps the greatest single conceptual change (still not universally understood) is that the ocean is an extremely time-dependent, turbulent environment, with no steady-state patterns.
This new view of ocean dynamics has implications for understanding how the ocean has affected climate over geological time. Ocean dynamics are fundamental to understanding how heat is transferred between the ocean and atmosphere and how heat is moved from the tropics to the poles. New insights into the importance of tidal energy dissipation to ocean dynamics and to other characteristics of a turbulent ocean led to a new appreciation of the difficulty of trying to model paleoocean circulation based on proxies of scalar properties (e.g., temperature) inferred from measurements of ocean sediment cores (Wunsch 2007). A poor description of ocean circulation will lead to inaccurate models of climate change over geological time due to the high dependence of the Earth’s climate on ocean circulation (Wunsch 2007). Thus, the new knowledge gained from satellite observations has the potential to greatly improve the accuracy of ocean circulation models in the future.
Internal Tides and Their Contribution to Ocean Mixing
In addition to the impressive advances in ocean tide modeling, satellite altimetry revealed how ubiquitous and important internal tides were in the open ocean. Although the importance of internal tides to the continental shelf regions has long been known, satellite observations of the open ocean tidal signal allowed scientists to calculate their significant contribution to deep ocean mixing (Garrett 2003).
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This discovery not only transformed oceanography but also has major implications for climate change science.
Because internal tides result in a vertical surface displacement of only a few centimeters (a 1-cm surface elevation change corresponds to vertical displacements of isotherms of tens of meters) and are only on the order of 100 km long, early satellite altimetry measurements were not able to resolve such small variation in the sea surface height. However, since T/P, along-track analysis became possible with the availability of precise altimetry data leading to direct global measurements of internal tides (Tierney et al. 1998). Similar to tides at the ocean surface, internal tides spread as a wave within the ocean interior, and their amplitude has been shown to correlate well with features on the ocean bottom such as ridges and seamounts (Ray and Mitchum 1997). Internal tides are now considered equal to winds in generating energy for mixing.
Tides transfer 3.5 TW of energy from the Sun and the Moon to the ocean. The conventional view was that dissipation of this energy occurred on the continental shelves and was thus irrelevant to the general circulation of the ocean (Wunsch and Ferrari 2004). An unexpected finding from altimetry measurements was that internal waves of tidal period were much more prevalent and of higher amplitude than previously believed (Egbert and Ray 2000). Calculations showed that as much as 1 TW of the 3.5-TW tidal energy input could be available to mix the deep ocean (Munk and Wunsch 1998). Much of the tidal energy released in the deep ocean occurs in the presence of ocean ridges, seamounts, and other features of abyssal topography. Altimetric internal tide measurements led directly to the current physical oceanography focus on energy sources for the general circulation and the implication that both winds and tides control the circulation through mixing of the abyss. This had never even been discussed prior to about 1997.
Altimeter Measurements of Westward-Propagating Sea Surface Height Variability
From theoretical considerations, energy input to the ocean from wind and thermal forcing is expected to propagate westward in the form of Rossby waves. Rossby waves are large, slow-moving features that generally move across the ocean from east to west. Typical wavelengths are 1,000 km and longer with sea surface height (SSH) signatures of about 10 cm. While the existence of these waves had been accepted since the seminal studies by Rossby et al. (1939) and Rossby (1940), observational verification remained elusive until the accumulation of shipboard observations by the mid-1970s of a sufficiently long and spatially dense collection of vertical profiles of upper-ocean thermal structure in the North Pacific.
Satellite altimetry demonstrated the prevalence and thus the importance of Rossby wave-like variability of the ocean circulation—a central underpinning of all understanding of oceanic variability. The orbital configuration of the T/P altimeter was particularly well suited to study these features because this altimeter was specifically designed to avoid aliasing by tides. A global synthesis of T/P data by Chelton and Schlax (1996; updated by Fu and Chelton 2001) detected the expected westward propagation with latitudinally varying propagation speed in all ocean basins. Thus, T/P altimetry provided compelling evidence supporting the theory that Rossby waves are an important mechanism for moving energy from east to west in ocean basins.
A new view is evolving due to the availability of simultaneous measurements of SSH by the T/P and the European Remote Sensing Satellite (ERS) altimeters, which allows the construction of much higher-resolution SSH fields than can be obtained from a single altimeter (Chelton et al. 2007b). By merging the T/P and ERS altimeter data sets, SSH fields are obtained with approximately double the spatial resolution of SSH fields constructed from T/P alone (Ducet et al. 2000; Figure 8.5). The newly merged data set in the lower panel of Figure 8.5 shows the intricate structure of the ocean circulation. The observations of the time-dependent motions visible in this figure led to a much clearer understanding of the role such motions play in the time-varying ocean circulation. At latitudes equatorward of about 25 degrees, a Rossby wave-like character is still evident in the merged data. At higher latitudes, however, the doubling of resolution reveals that the SSH field is much more eddy-like in nature than suggested from maps constructed from only the T/P data (Chelton et al. 2007b).
Animations of the merged T/P-ERS data reveal that the resolved eddies propagate considerable distances westward. When an automated eddy-tracking procedure—developed for and applied to previous studies (Isern-Fontanet et al. 2003, 2006; Morrow et al. 2004)—is applied to the global data set, more than 8,300 eddies are trackable for 18 weeks or longer, and more than 500 eddies are trackable for more than a year (Chelton et al. 2007b). Although in a few regions there are preferences for eddy polarity, in most there is no significant difference between the numbers of cyclonic and anticyclonic eddies.
A striking characteristic of the eddy trajectories is the strong tendency for purely westward propagation. Globally, nearly 75 percent of the tracked eddies had mean propagation directions that deviated from due west by less than 10 degrees, with cyclonic and anticyclonic eddies having distinct preferences for, respectively, poleward and equatorward deflections. The fact that much of the extratropical SSH variability is attributable to nonlinear eddies rather than to linear Rossby waves (Chelton et al. 2007b)—as suggested by earlier analyses—may have significant implications for biological processes in the ocean because nonlinear eddies, in contrast to Rossby waves, transport properties vertically and horizontally.
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FIGURE 8.5 Global maps of SSH centered on August 28, 1996, constructed from T/P data alone (top) and from the merged T/P and ERS data (bottom). Based on the resolution limitations imposed by sampling errors (Chelton and Schlax 2003), the T/P data were smoothed with half-power filter cutoffs of 6° × 6° × 30 days, and the merged T/P-ERS data were smoothed with half-power filter cutoffs of 3° × 3° × 20 days. After filtering to remove large-scale heating and cooling effects unrelated to mesoscale variability, the anomaly SSH field consists of many isolated cyclonic and anticyclonic features (negative and positive SSH, respectively). SOURCE: Modified from Chelton et al. (2007b). Reprinted with permission by American Geophysical Union, copyright 2007.
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OCEAN WIND MEASUREMENTS REVEAL TWO-WAY OCEAN-ATMOSPHERE INTERACTION
Scatterometers have also made significant contributions to the study of ocean dynamics by providing a synoptic view (approximately 25 km spatial resolution) of vector winds over the ocean. The results showed new insights into the exchange of heat and momentum between the atmosphere and ocean. Weather forecasting has been significantly improved by incorporating scatterometer-derived winds into forecasts (see Chapter 3). In particular, scatterometer data are particularly useful for determining the location, strength, and movement of cyclones over the ocean. Furthermore, new insights as to the underlying physics affecting air-sea interaction have significant implications for ocean mixing, which is important for understanding the dynamics of ocean currents as well as the supply of nutrients supporting biological productivity.
Prior to the availability of scatterometer measurements of ocean vector winds, most of what was known about the space-time variability of the wind field over the ocean was based on 10-m wind analyses from the European Centre for Medium-Range Weather Forecasting (ECMWF) and the U.S. National Centers for Environmental Prediction (NCEP) global numerical weather prediction models. Feature resolution in these models is limited to wavelength scales longer than about 500-km (Milliff et al. 2004, Chelton et al. 2006), despite the fact that winds from the QuikScat scatterometer have been assimilated into both of these models since January 2002. The resolution is even worse in the reanalysis wind fields that are used in most models of ocean circulation and for most studies of climate variability. For example, the resolution limitation of the NCEP reanalysis winds is about 1,000 km (Milliff et al. 2004). As reviewed by Kushnir (2002), ocean-atmosphere interaction on the large scales resolvable by global atmospheric models is characterized by stronger winds over colder water.
An important satellite scatterometer contribution from QuikScat data revealed that ocean-atmosphere interaction is fundamentally different on scales shorter than about 1,000 km that are poorly resolved by global atmospheric models. As reviewed by Xie (2004), low-level winds are locally stronger over warm water and weaker over cold water throughout the oceans wherever strong SST fronts exist. This ocean-atmosphere interaction apparently arises from SST modifications of stability and vertical mixing in the marine atmospheric boundary layer (MABL). This is consistent with earlier in situ studies in the Gulf Stream (Sweet et al. 1981) and the Agulhas Current (Jury and Walker 1988) that observed enhanced vertical turbulent mixing as cold air passes over warm water, deepens the MABL, and mixes momentum downward from aloft to the sea surface, thus accelerating the surface winds. Decreased mixing over cold water stabilizes and thins the MABL, resulting in decreased surface winds. Wallace et al. (1989) hypothesized a similar SST influence on low-level winds in the eastern tropical Pacific based on historical observations of surface winds and SST from ships.
The SST influence on low-level winds has important implications for both the ocean and the atmosphere. The spatial variability of the SST field in the vicinity of meandering SST fronts induces curl and divergence in the surface wind stress field that are linearly proportional to, respectively, the crosswind and downwind components of the SST gradient (Chelton et al. 2004). An example of this SST influence on the curl and divergence of the wind stress is shown in Figure 8.6 for the California Current region.
For ocean applications the wind stress curl is of particular interest because it generates open-ocean upwelling and downwelling that drive the ocean circulation and bring cold water and nutrients to the sea surface. The SST influence on the wind stress curl field results in first-order perturbations of the large-scale background wind stress curl (O’Neill et al. 2003, Chelton et al. 2007a) with timescales on the order of a month. Therefore, this ocean-atmosphere interaction likely has strong effects on both the physics and the biology of the ocean. Moreover, the feedback effects of SST-induced wind mixing and wind stress curl on the ocean alter SST, thus resulting in two-way coupling between the ocean and the atmosphere.
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FIGURE 8.6 September 2004 averages of wind stress curl with contours of crosswind SST gradient (left) and wind stress divergence with contours of downwind SST gradient (right) over the California Current system. The wind stress fields were constructed from QuikScatdata. The SST fields were constructed from the U.S. Navy Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS). Satellite microwave measurements of SST are not well suited to studies in this region because of the coarse (~50 km) resolution and the inability to measure SST closer than ~75 km to land. SOURCE: Chelton et al. (2007a). Reprinted with permission from the American Meteorological Society, copyright 2007.