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8 Ocean Dynamics Due to the remoteness of the vast open oceans, satel- The Oceanâs Role in Climate Change lites provided the first truly global ocean-observing system. Monitoring SST by the Advanced Very High Resolu- Presatellite observing platforms included ships, moorings, tion Radiometer (AVHRR) is the marine remote sensing drifters, and other tools, none of which could provide ocean technique with the broadest impact on oceanography basin-scale coverage at the temporal and spatial scales (Robinson 1985). SST is the earliest Earth-orbiting satellite required to resolve the dynamic nature of the ocean that has measurement for oceanography and began with the launch been revealed since. In fact, even a well-known and studied of the Television Infrared Observation Satellite (TIROS-N) current such as the Gulf Stream was not fully characterized in 1978. SST measurements provide the longest continu- until satellite observations were available (Box 8.1, Figures ous record of any oceanic property from space (Table 8.1). 8.1 and 8.2). Satellite data from scatterometers, altimeters, This long-term data set has been calibrated and validated by infrared radiometers, and various ocean color sensors opened surface observations from drifters, buoys, and ships and is up a new window for observing and quantifying how and used for a broad range of oceanographic research questions, why water moves around in the ocean (ocean dynamics) and including studies of regional climate variability, most notably how energy is exchanged between the ocean and atmosphere El NiÃ±o-Southern Oscillation (ENSO; see also Chapter 12), (air-sea interaction). climate change, and ocean currents. As illustrated in more detail below, sea surface tem- SST is one of the most important indicators of global perature (SST) measurements not only revealed important climate change and a vital parameter for climate modeling information about ocean circulations (e.g., the Gulf Stream) (Hurrell and Trenberth 1999). Because of the large heat con- but also advanced climate research by providing detailed tent of the ocean, more than 80 percent of the total heating of information on the heat input into the ocean. Ocean color the Earth system is stored in the ocean, and ocean currents combined with SST observations led to new discoveries redistribute this heat across the globe. Consequently, âif about the physical-biological coupling in the ocean, with [scientists] wish to understand and explain [global] warm- important implications for the oceanâs role in the carbon ing, the oceans are clearly the place to lookâ (Barnett et al. cycle (see also Chapter 9). Observations from altimeters have 2005b). In addition, SST is central in coupling the ocean resulted in a slow revolution, as the accuracy of the sensors with the atmosphere and is a controlling factor in the heat steadily increased, taking about a decade for their contribu- and vapor exchange between the two (Johannessen et al. tions to be broadly recognized. Altimetry embedded within 2001). Trend analysis of SST provided evidence for global other modern ocean measurements and models yielded a vir- warming and the important climate-atmosphere feedback in tually complete description of first-order physical processes the tropics that is also responsible for ENSO events (Cane in the ocean. The measurements provided real surprises to et al. 1997). These SST observations, combined with in situ physical oceanographers, including detection of the internal vertical temperature measurements of the ocean to a depth tide in the open ocean, of a highly variable surface ocean of 3,000 m provided evidence to detect anthropogenic global full of eddies, and of global sea-level trends at an accuracy warming in the ocean (Barnett et al. 2001, 2005b). of millimeters per year. Combining satellite data with in situ Understanding the increase in SST and anthropo- observations and models converted physical oceanography genic heat input to the surface ocean also has important into a global science with actual predictive skill. 64
OCEAN DYNAMICS 65 BOX 8.1 Gulf Stream Path Describing the path of the Gulf Stream and other major ocean currents was an early challenge to physical ocean- ographers who based their interpretations on very sparse data collected from oceanographic ships. For example, us- ing 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 Âfilament (Figure 8.1a), a branching current with two filaments (Figure 8.1b), or a number of irregular discon- nected 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 âNorth Wallââthe 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). a b FIGURE 8.2â SST image showing the Gulf Stream in the Atlantic Ocean. SOURCE: Provided by Otis Brown and Bob Evans. c FIGURE 8.1â Fuglisterâs multiple current hypothesis. SOURCE: Stommel (1965). Reprinted with permission from the University of California Press, copyright 1965. one column width 8-1 a,b,c
66 EARTH OBSERVATIONS FROM SPACE: THE FIRST 50 YEARS OF SCIENTIFIC ACHIEVEMENTS TABLE 8.1â TIROS-NOAA Satellites Carrying AVHRR Prevalence of Dynamic Features Sensors Monitoring SST The ability to observe the ocean surface from space has Satellite Dates of Operation profoundly altered the way the ocean is viewed. The Coastal TIROS-N Oct. 1978-Jan. 1980 Zone Color Scanner (CZCS), launched aboard the Nimbus NOAA-6 June 1979-March 1983 7 satellite in 1978, provided the first satellite observations NOAA-7 Aug. 1981-Feb. 1985 used to quantify the chlorophyll concentration in the upper NOAA-8 May 1983-Oct. 1985 ocean (see Chapter 9, Box 9.3). The first images of surface NOAA-9 Feb. 1985-Nov. 1988 chlorophyll distributions were truly astonishing, revealing NOAA-10 Nov. 1986-Sept. 1991 NOAA-11 Nov. 1988-April 1995 a high degree of spatial variability never fully appreciated NOAA-12 Sept. 1991-present before satellites (Figure 8.3). The availability of global maps NOAA-14 Dec. 1994-present of chlorophyll, an estimate for marine plant biomass, has NOAA-15 May 1998-present opened new avenues of research and changed the conduct NOAA-16 Sept. 2000-present 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 ramifications for quantifying and predicting sea-level rise satellites, but ocean color images revealed entirely new fea- (Cabanes et al. 2001). Recent work suggests that thermal tures. An example of this is the vast extent of the Amazon expansion of the surface ocean (upper 500 m) can fully River plume stretching many thousands of kilometers across explain the sea-level rise of 3.2 (Â± 0.2) mm per year observed the Atlantic (Figure 8.4, Muller-Karger et al. 1988). The by the satellite TOPEX/Poseidon (Cabanes et al. 2001). plumeâs temperature does not provide sufficient contrast Figure 8.3â 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. ÂIntermediate 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.
OCEAN DYNAMICS 67 Figure 8.4â 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.
68 EARTH OBSERVATIONS FROM SPACE: THE FIRST 50 YEARS OF SCIENTIFIC ACHIEVEMENTS with the tropical Atlantic to be visible in thermal imagery, model and predict them that would not have been possible whereas its color makes it clearly visible. River discharge without satellite information, due to the limitations of in situ measurements from gauging stations have been shown to tidal observations in the open ocean. This in itself is a major correlate with the temporal variability of plumes in satellite achievement. Consequently, the marine shipping sector has images of the Gulf of Mexico (Salisbury et al. 2001, 2004), benefited from improved tidal predictions. thus offering a method for studying the influences of rivers on the coastal ocean. THE TURBULENT OCEAN As a result of satellite images, Earth scientists have gained a physical perspective and appreciation of the relation- By providing the ability to measure the eddy variabilityÂ ship of the ocean to land masses. Seaward-flowing jets and globally, to determine its space-time variability, and to filaments associated with major fronts along the continental study its time evolution, altimetry led to a paradigm shift in shelf off California and the Pacific Northwest were a focus oceanography in the late 1990s. The direct observations of of the Coastal Transition Zone Program during the CZCS the extensive eddy field by altimetry (see below) coupled era (Brink and Cowles 1991). These narrow filaments of with the recent focus on energy sources for internal wave productive water extending hundreds of kilometers seaward mixing of the deep ocean (see next section), including those from the continental margin are now recognized as important with tidal components, changed the way we think about the pathways for the transport of materials from the continental nature of the global ocean circulation (Wunsch and Ferrari shelves to the deep ocean (Strub et al. 1991). Other research- 2004). Before altimetry the energy supply for the large- ers used CZCS to look at the Columbia River plume (Fiedler scale circulation was believed to be dominated by surface and Laurs 1990) and to relate tuna catch to fronts and features buoyancy forces related to changes in water temperature seen in satellite images (Laurs et al. 1984). 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, Understanding Ocean Tides: scientists know that energy is provided to the general cir- New Solutions to an Old Scientific Question culation primarily by winds and tides. Perhaps the greatest Ocean tides have fascinated scientists since the early single conceptual change (still not universally understood) Greeks and were first explained by Newton to be caused is that the ocean is an extremely time-dependent, turbulent by the gravitational attraction of the Moon and the Sun. A environment, with no steady-state patterns. century later Newtonâs theory was replaced by the dynamic This new view of ocean dynamics has implications response concept described by Laplaceâs (1776) tidal equa- for understanding how the ocean has affected climate over tion. Because Laplaceâs tidal equation strongly depends on geological time. Ocean dynamics are fundamental to under- the shape and bathymetry of the ocean basin and because standing how heat is transferred between the ocean and atmo- oceans have clusters of natural resonances in the same fre- sphere and how heat is moved from the tropics to the poles. quency bands as the gravitational forcing function (Platzman New insights into the importance of tidal energy dissipation 1981), analytical solutions to Laplaceâs tidal equation cannot to ocean dynamics and to other characteristics of a turbulent be found. Therefore, predicting ocean tides to some level of ocean led to a new appreciation of the difficulty of trying accuracy was made possible only by Darwinâs (1886) empiri- to model paleoocean circulation based on proxies of scalar cal method. The behavior of ocean tides, particularly in the properties (e.g., temperature) inferred from measurements of open ocean, remained elusive until the advent of satellite ocean sediment cores (Wunsch 2007). A poor description of altimetry (Le Provost 2001). ocean circulation will lead to inaccurate models of climate For the first time, satellite altimetry observations allowed change over geological time due to the high dependence of synoptic measurements of ocean tides in the global open the Earthâs climate on ocean circulation (Wunsch 2007). ocean. Although the first altimetry data were obtained from Thus, the new knowledge gained from satellite observations Geodynamics Experimental Ocean Satellite 3 (Geos3) in has the potential to greatly improve the accuracy of ocean 1973, it was not until Seasat in 1978 that it became evident circulation models in the future. that a tidal signal could be retrieved from satellite altimetry (Le Provost 1983, Cartwright and Alcock 1983). Most of Internal Tides and Their Contribution to Ocean Mixing the advances in global ocean tide modeling have only been made since the launch of European Remote Sensing Satellite In addition to the impressive advances in ocean tide (ERS)-1 in 1991 and Topography Experiment (TOPEX)/ modeling, satellite altimetry revealed how ubiquitous and Poseidon (T/P) in 1992. Based on altimetry and tidal models, important internal tides were in the open ocean. Although it is now possible to predict ocean tides globally, including the importance of internal tides to the continental shelf in the deep ocean, with a precision of 2-4 cm over periods regions has long been known, satellite observations of the of several months to years (Le Provost 2001). Global infor- open ocean tidal signal allowed scientists to calculate their mation on ocean tides has resulted in an improved ability to significant contribution to deep ocean mixing (Garrett 2003).
OCEAN DYNAMICS 69 This discovery not only transformed oceanography but also circulationâa central underpinning of all understanding has major implications for climate change science. of oceanic variability. The orbital configuration of the T/P Because internal tides result in a vertical surface altimeter was particularly well suited to study these features displacement of only a few centimeters (a 1-cm surface because this altimeter was specifically designed to avoid elevation change corresponds to vertical displacements of aliasing by tides. A global synthesis of T/P data by Chelton isotherms of tens of meters) and are only on the order of and Schlax (1996; updated by Fu and Chelton 2001) detected 100 km long, early satellite altimetry measurements were the expected westward propagation with latitudinally varying not able to resolve such small variation in the sea surface propagation speed in all ocean basins. Thus, T/P altimetry height. However, since T/P, along-track analysis became provided compelling evidence supporting the theory that possible with the availability of precise altimetry data lead- Rossby waves are an important mechanism for moving ing to direct global measurements of internal tides (Tierney energy from east to west in ocean basins. et al. 1998). Similar to tides at the ocean surface, internal A new view is evolving due to the availability of simul- tides spread as a wave within the ocean interior, and their taneous measurements of SSH by the T/P and the European amplitude has been shown to correlate well with features Remote Sensing Satellite (ERS) altimeters, which allows the on the ocean bottom such as ridges and seamounts (Ray and construction of much higher-resolution SSH fields than can Mitchum 1997). Internal tides are now considered equal to be obtained from a single altimeter (Chelton et al. 2007b). winds in generating energy for mixing. By merging the T/P and ERS altimeter data sets, SSH fields Tides transfer 3.5 TW of energy from the Sun and the are obtained with approximately double the spatial resolution Moon to the ocean. The conventional view was that dissipa- of SSH fields constructed from T/P alone (Ducet et al. 2000; tion of this energy occurred on the continental shelves and Figure 8.5). The newly merged data set in the lower panel of was thus irrelevant to the general circulation of the ocean Figure 8.5 shows the intricate structure of the ocean circula- (Wunsch and Ferrari 2004). An unexpected finding from tion. The observations of the time-dependent motions visible altimetry measurements was that internal waves of tidal in this figure led to a much clearer understanding of the role period were much more prevalent and of higher amplitude such motions play in the time-varying ocean circulation. At than previously believed (Egbert and Ray 2000). Calculations latitudes equatorward of about 25 degrees, a Rossby wave- showed that as much as 1 TW of the 3.5-TW tidal energy like character is still evident in the merged data. At higher input could be available to mix the deep ocean (Munk and latitudes, however, the doubling of resolution reveals that the Wunsch 1998). Much of the tidal energy released in the deep SSH field is much more eddy-like in nature than suggested ocean occurs in the presence of ocean ridges, seamounts, and from maps constructed from only the T/P data (Chelton et other features of abyssal topography. Altimetric internal tide al. 2007b). measurements led directly to the current physical oceanogra- Animations of the merged T/P-ERS data reveal that the phy focus on energy sources for the general circulation and resolved eddies propagate considerable distances westward. the implication that both winds and tides control the circula- When an automated eddy-tracking procedureâdeveloped tion through mixing of the abyss. This had never even been for and applied to previous studies (Isern-Fontanet et al. discussed prior to about 1997. 2003, 2006; Morrow et al. 2004)âis 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 Altimeter Measurements of Westward-Propagating than a year (Chelton et al. 2007b). Although in a few regions Sea Surface Height Variability there are preferences for eddy polarity, in most there is no From theoretical considerations, energy input to the significant difference between the numbers of cyclonic and ocean from wind and thermal forcing is expected to propa- anticyclonic eddies. gate westward in the form of Rossby waves. Rossby waves A striking characteristic of the eddy trajectories is the are large, slow-moving features that generally move across strong tendency for purely westward propagation. Globally, the ocean from east to west. Typical wavelengths are 1,000 nearly 75 percent of the tracked eddies had mean propaga- km and longer with sea surface height (SSH) signatures of tion directions that deviated from due west by less than about 10 cm. While the existence of these waves had been 10Â degrees, with cyclonic and anticyclonic eddies having accepted since the seminal studies by Rossby et al. (1939) distinct preferences for, respectively, poleward and equatorÂ and Rossby (1940), observational verification remained ward deflections. The fact that much of the extratropical SSH elusive until the accumulation of shipboard observations by variability is attributable to nonlinear eddies rather than to the mid-1970s of a sufficiently long and spatially dense col- linear Rossby waves (Chelton et al. 2007b)âas suggested lection of vertical profiles of upper-ocean thermal structure by earlier analysesâmay have significant implications for in the North Pacific. biological processes in the ocean because nonlinear eddies, Satellite altimetry demonstrated the prevalence and thus in contrast to Rossby waves, transport properties vertically the importance of Rossby wave-like variability of the ocean and horizontally.
70 EARTH OBSERVATIONS FROM SPACE: THE FIRST 50 YEARS OF SCIENTIFIC ACHIEVEMENTS FIGURE 8.5â 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 6Âº Ã 6Âº Ã 30 days, and the merged T/P-ERS data were smoothed with half-power filter cutoffs of 3Âº Ã 3Âº Ã 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.
OCEAN DYNAMICS 71 Ocean Wind Measurements Reveal models. As reviewed by Xie (2004), low-level winds are Two-Way Ocean-Atmosphere Interaction locally stronger over warm water and weaker over cold water throughout the oceans wherever strong SST fronts exist. This Scatterometers have also made significant contributions ocean-atmosphere interaction apparently arises from SST to the study of ocean dynamics by providing a synoptic view modifications of stability and vertical mixing in the marine (approximately 25 km spatial resolution) of vector winds over atmospheric boundary layer (MABL). This is consistent the ocean. The results showed new insights into the exchange with earlier in situ studies in the Gulf Stream (Sweet et al. of heat and momentum between the atmosphere and ocean. 1981) and the Agulhas Current (Jury and Walker 1988) that Weather forecasting has been significantly improved by observed enhanced vertical turbulent mixing as cold air incorporating scatterometer-derived winds into forecasts (see passes over warm water, deepens the MABL, and mixes Chapter 3). In particular, scatterometer data are particularly momentum downward from aloft to the sea surface, thus useful for determining the location, strength, and movement accelerating the surface winds. Decreased mixing over cold of cyclones over the ocean. Furthermore, new insights as water stabilizes and thins the MABL, resulting in decreased to the underlying physics affecting air-sea interaction have surface winds. Wallace et al. (1989) hypothesized a similar significant implications for ocean mixing, which is important SST influence on low-level winds in the eastern tropical for understanding the dynamics of ocean currents as well as Pacific based on historical observations of surface winds the supply of nutrients supporting Âbiological productivity. and SST from ships. Prior to the availability of scatterometer measurements The SST influence on low-level winds has important of ocean vector winds, most of what was known about the implications for both the ocean and the atmosphere. The spa- space-time variability of the wind field over the ocean was tial variability of the SST field in the vicinity of meandering based on 10-m wind analyses from the European Centre for SST fronts induces curl and divergence in the surface wind Medium-Range Weather Forecasting (ECMWF) and the stress field that are linearly proportional to, respectively, the U.S. National Centers for Environmental Prediction (NCEP) crosswind and downwind components of the SST gradient global numerical weather prediction models. Feature resolu- (Chelton et al. 2004). An example of this SST influence on tion in these models is limited to wavelength scales longer the curl and divergence of the wind stress is shown in Fig- than about 500-km (Milliff et al. 2004, Chelton et al. 2006), ureÂ 8.6 for the California Current region. despite the fact that winds from the QuikScat scatterometer For ocean applications the wind stress curl is of particu- have been assimilated into both of these models since Janu- lar interest because it generates open-ocean upwelling and ary 2002. The resolution is even worse in the reanalysis wind downwelling that drive the ocean circulation and bring cold fields that are used in most models of ocean circulation and water and nutrients to the sea surface. The SST influence on for most studies of climate variability. For example, the reso- the wind stress curl field results in first-order perturbations lution limitation of the NCEP reanalysis winds is about 1,000 of the large-scale background wind stress curl (OâNeill et km (Milliff et al. 2004). As reviewed by Kushnir (2002), al. 2003, Chelton et al. 2007a) with timescales on the order ocean-atmosphere interaction on the large scales resolvable of a month. Therefore, this ocean-atmosphere interaction by global atmospheric models is characterized by stronger likely has strong effects on both the physics and the biology winds over colder water. of the ocean. Moreover, the feedback effects of SST-induced An important satellite scatterometer contribution from wind mixing and wind stress curl on the ocean alter SST, QuikScat data revealed that ocean-atmosphere Â interaction thus resulting in two-way coupling between the ocean and is fundamentally different on scales shorter than about the atmosphere. 1,000Â km that are poorly resolved by global atmospheric
72 EARTH OBSERVATIONS FROM SPACE: THE FIRST 50 YEARS OF SCIENTIFIC ACHIEVEMENTS FIGURE 8.6â 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 mi- crowave 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.