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CITATION Dohan, K., and N. Maximenko. 2010. Monitoring ocean currents with satellite sensors. Oceanography 23(4):94–103, doi:10.5670/oceanog.2010.08.

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downloaded from www.tos.org/oceanography The Future of Oceanography From Space

By Kathleen Dohan and Nikolai Maximenko Monitoring Ocean Currents with Satellite Sensors

Abstract. The interconnected ocean surface current system involves multiple 100-km wide), to submesoscale features scales, including basin-wide gyres, fast narrow boundary currents, eddies, and (1–100 km in scale), to turbulence (less turbulence. To understand the full system requires measuring a range of motions, than 1 m). The differences in scale result from thousands of kilometers to less than a meter, and time scales from those that from differences in the kinds of forcings are climate related (decades) to daily processes. Presently, satellite systems provide that induce the motions as well as from us with global and regional maps of the ocean surface’s mesoscale motion (larger differences in the underlying physics that than 100 km). Surface currents are measured indirectly from satellite systems. One define the motions’ characteristics. method involves using remotely sensed fields of sea surface height, surface winds, Prevailing global winds, such as the and sea surface temperature within a physical model to produce currents. Another trade winds and the westerlies, together involves determining surface velocity from paths of drifting surface buoys transmitted with Earth’s rotation and the restriction to satellite sensors. Additional methods include tracking of surface features of flow by continental boundaries, set and exploitation of the Doppler shift in radar fields. The challenges for progress up the general ocean surface circulation. include measuring small and fast processes, capturing the vertical variation, and These “gyres” have the basic form of a overcoming sensor limitations near coasts. Here, we detail the challenges as well as flow around a basin, with an intensified upcoming missions and advancements in satellite oceanography that will change our western boundary current and broader understanding of surface currents in the next 10 years. eastern boundary current, the theories of which were developed by Sverdrup The Surface motion. Other types span the continuous (1947) and Stommel (1948). This surface Current System spectra of spatial and temporal scales, motion description is far from complete, The ocean is unimaginable without from basin-wide motions, to mesoscale however. Large-scale surface motions motion. Breaking waves at its surface, a eddies (scales greater than 100 km) and consist of a complex interconnection of symbol of the sea, are only one kind of fast narrow currents (on the order of local currents, eddies, and turbulence.

94 Oceanography | Vol.23, No.4 Figure 1 shows a schematic of this to the Coriolis force. The extent of the La Niña (Philander, 1990). surface current hierarchy. Although rotation depends on length scales and Finally, the small scales (less than local currents have been observed for speed of motion, as well as on latitude, 100 km) and fast processes (less than a centuries for purposes such as navigation with large scales being more affected day to evolve), for example, small eddies, and fishing, the advent of satellite remote than small. The Coriolis force is zero at filaments, turbulence, and surface sensing has provided us with regular the equator and strongest at the poles. waves, are also important to the surface and global measurements of the complex Equatorial dynamics are, therefore, current system, not only for stirring and ocean surface motions. qualitatively different than the rest of the mixing, but also for strong associated Western boundary currents (WBC) ocean, with predominantly zonal flows vertical motions. are often less than 100-km wide. They (east-west) and large meridional (north- Because of the complexity of behavior, are also intense, at speeds on the order south) temperature gradients. Large- the range of temporal and spatial of 2 m s-1, or 175 km day-1, and are scale waves (1000–2000-km wavelength) scales, and the dependence on local responsible for significant poleward heat propagate along these temperature conditions for ocean surface currents, transport. For example, the Gulf Stream fronts. They play a crucial role in tropical satellite remote sensing is an ideal tool (the WBC along the eastern coast of dynamics, in particular El Niño and for studying ocean surface dynamics. North America) transports warm waters toward the western coast of Europe. In addition to heat, these boundary currents transport momentum, chemical components (e.g., CO2), nutrients, and marine life. WBCs eventually separate from the coasts and extend into the open ocean, forming large meanders that shed large eddies, and creating strong fronts and jets. A full discussion of the global peak in eddy kinetic energy in the areas of the extensions can be found in Fu et al. (2010). In the Southern Ocean however, the westerlies force a strong current that flows from east to west, called the Antarctic Circumpolar Current (ACC). With no continents to force boundary currents, the motion of this current is similar to that of wind, flowing entirely around Antarctica and inhibiting north- south heat transport. The ACC is the only current that connects the Pacific, Atlantic, and Indian oceans, and thus is crucial for the exchange of prop- Figure 1. Schematic of the hierarchy of near-surface currents focused on the Gulf Stream erties among basins. region in the North Atlantic Ocean. The gyre circulation consists of a broad eastern Earth’s rotation causes motions to component and a narrow, fast, western boundary current (WBC). The WBC is enlarged to highlight the mesoscale (100 km and larger) features and the submesoscale eddies, be deflected to the right (left) in the filaments, and turbulence. Global winds drive basin circulation, while local winds force Northern (Southern) Hemisphere, due local Ekman currents, surface waves, and turbulence.

Oceanography | December 2010 95 The regular, repeated coverage offered Satellite altimeters measure the height The Gravity field and steady-state through satellite systems is unachievable of the sea surface. One compromise Ocean Circulation Explorer (GOCE) through in situ measurements. However, that is made during the design of a satellite, launched in 2009 (http://www. the ability to capture the desired features satellite orbit is between the temporal esa.int/esaLP/LPgoce.html), is expected of the global surface current system and spatial resolution, that is, between to finally improve resolution to 100 km. becomes increasingly challenging for the frequency the satellite passes over Until that happens, various ocean satellite systems as the spatial scales an area and the distance between its observations and techniques are used to become smaller or the processes take track lines. Having a constellation of characterize ocean dynamics. Rio and place on faster time scales. satellites solves this compromise. The Hernandez (2004) and Maximenko et al. field of satellite altimetry thus benefits (2009) combine (in two different tech- Ma e suring Ocean Currents from 18 years of international coopera- niques) surface drifter trajectories and Using Satellite Sensors tion, when there have been at least two satellite sea level anomalies to compen- Today’s satellite sensors are not capable altimeters in orbit at any given time. The sate for the effect of eddies on the of measuring ocean currents directly. system of altimeters today captures the ocean’s mean level. Their mean dynamic However, remotely sensed data are used mesoscale, with real-time estimation topographies (e.g., in Figure 2a) reveal to assess current velocity with a variety (less than a day) of global SSH. More a complex structure of main oceanic of methods. The most direct method details on the satellite missions and frontal systems, such as the Gulf Stream uses satellite altimetry and ocean vector the SSH data produced can be found and the Kuroshio Extension, improving winds to estimate surface currents. through the Archiving, Validation and SSH calculations. Interpretation of Satellite Oceanographic C urrents from Satellite data (AVISO) data distribution site Ekman Currents Altimetry and Vector Winds (http://www.aviso.oceanobs.com). Geostrophic motions dominate the Geostrophic Currents To calculate SSH, several measure- mesoscale signal in many regions, but On scales of tens of kilometers and ments and calculations are required not all motions are geostrophic. Winds larger, horizontal motions are much along with altimetry data, including exert a stress on the ocean’s surface, larger than vertical motions, and the precise determination of the orbit and transferring momentum between the ocean is approximately hydrostatic (pres- highly accurate tide models. In addition, atmosphere and ocean, driving surface sure is determined by the height and because satellite altimeters measure SSH currents. In the classical Ekman balance density of the water column). At these relative to the geoid (the sea surface at (Ekman, 1905), the Coriolis force is scales, for approximately steady motions, resting state), accurate knowledge of the balanced by the vertical divergence and away from boundaries, the primary geoid is required. Gravity Recovery and of wind stress and has the shape of a balance of forces is between horizontal Climate Experiment (GRACE) satel- spiral decaying with depth. The Ekman pressure differences and the Coriolis lites (http://www.csr.utexas.edu/grace) velocity in the Northern (Southern) force, which drives currents that follow flying since 2002 have greatly improved Hemisphere is 45° to the right (left) lines of constant pressure, known as accuracy of the gravity model, enhanced from the wind direction, and the angle geostrophic currents. Because pressure spatial resolution of the geoid to some increases with depth. is related to sea surface height (SSH), hundred kilometers, and measured for Wind stress can be estimated from geostrophic currents can be calculated the first time long-term gravity changes winds using empirical relations. Ekman using horizontal gradients in SSH. due to redistribution of ocean mass. currents can then be derived from vector wind measurements (winds with both Kathleen Dohan ([email protected]) is Research Scientist, Earth and Space Research, direction and speed). The assumptions Seattle, WA, USA. Nikolai Maximenko is Senior Researcher, International Pacific are steady winds, a well-mixed surface Research Center, School of Ocean and Earth Science and Technology, University of Hawaii, layer, and a constant eddy viscosity. Honolulu, HI, USA. The eddy viscosity is the simplest

96 Oceanography | Vol.23, No.4 parameterization of turbulent processes. such as in the WBCs. Figure 3 shows a partly due to the ability to resolve sharp It is based on the concept that turbu- comparison between OSCAR’s currents gradients in SSH. The correlation is lent eddies act on the large-scale flow and those measured from the trajectories particularly high when considering that like a viscosity. of “drifters” (drifting buoys, see next the drifters will follow the small features There are several classes of instru- section) for two monthsfor two months and fast motions that are not captured ments that measure vector winds: scat- in the Agulhas region (the WBC along by the satellites. terometers, passive polarimetric sensors, the south tip of Africa). The figure shows and synthetic aperture radar (SAR). comparisons between one-day binned G lobal Drifting Buoy Array Each instrument has its own advantages drifter velocities and OSCAR veloci- Satellite technology has also revolu- and disadvantages, such as performance ties linearly interpolated to each drifter tionized measurements taken directly in high winds, in rain, or close to land. time/space point. OSCAR velocities are within the ocean. For example, drifters Bourassa et al. (2010) provide an over- systematically lower than those of the are buoys that drift on the ocean’s view of ocean vector wind sensors. drifters, although with high correlation. surface. These buoys measure surface The underestimation of amplitude is properties (e.g., temperature), but they Surface Current Products Ocean Surface Current Analyses Real- time (OSCAR; Bonjean and Lagerloef, a) 2002; http://www.oscar.noaa.gov, http://podaac.jpl.nasa.gov), Mercator/ SURCOUF (Larnicol et al., 2006; http://www.mercator-ocean.fr), and the Centre de Topographie des Océans et de l’Hydrosphère (CTOH; Sudre and Morrow, 2008; http://ctoh.legos. obs-mip.fr) all provide global surface current products directly calculated from satellite altimetry and ocean vector winds. The variations between the methods are most notably in the b) treatment of geostrophic currents at the equator (where geostrophy becomes invalid), treatment of wind-driven turbulence, and inclusion of an adjustment to the currents due to gradients in sea surface temperature. Although missing more complex physics, such as nonlin- earities, the advantage of these methods is in their lack of assumptions, producing as close to a direct satellite measurement Figure 2. (a) 1993–2002 mean dynamic topography consisting of fronts and eddies. Note the strong of surface currents on a fixed global grid signal from high-eddy WBCs such as the Gulf Stream (60°W, 30°N), the Kuroshio (140°E, 30°N), and at regular intervals as possible. the Agulhas (30°E, 40°S). (b) Velocities are calculated during the computation of (a) from simple rela- These models have shown great tions among drifter velocities, local winds, and sea level anomaly gradients. Mean streamlines shown here are calculated from 0.25° ensemble-mean velocities between February,15, 1979, and May 1, 2007, success in capturing currents in areas smoothed to 1°. Colors are magnitudes of mean drifter velocity and units are cm/s. From Maximenko strongly influenced by geostrophy, et al. (2009). Data are available at http://apdrc.soest.hawaii.edu/projects/DOT

Oceanography | December 2010 97 are attached to subsurface anchors sensors detect instantaneous positions phod/dac/gdp_drifter.php) have global (drogues) designed so that buoy motion of the drifters, and changes in the coor- coverage with more than 1350 drifters is dominated by water motion at drogue dinates can be converted into veloci- launched as of the fall of 2010. The depth. The data collected by the buoy are ties. The buoys of the Global Drifter drifters are drogued at 15-m depth, transmitted to passing satellites. Satellite Program (http://www.aoml.noaa.gov/ although the currents may vary with depth, and drifters suffer from some slip in high winds, which may affect actual drifter trajectories. The advantage of drifters is their high frequency of information transmitted (six hourly or better velocities) and their direct measurement of water properties. However, the data follow an irregular trajectory, resulting in irregular sampling of the ocean’s surface.

C ombining Drifters with Satellite Derived Currents Simple relations among drifter velocities, local winds, and sea level anomaly gradients recognized during computa- tions of the mean dynamic topography (e.g., Figure 2b; Maximenko et al., 2009) can be used to assess time-varying velocities. The Surface Currents from Diagnostic model (SCUD) data set is produced (Maximenko and Hafner, 2010) at the International Pacific Research Center (http://apdrc.soest. hawaii.edu/projects/SCUD). The data set uses satellite wind and altimetry, regressed locally to scarce drifter data, to expand the latter onto a global, 1/4-degree, daily grid. This product is particularly useful for studies of marine debris and other tracers with properties close to the ones of Figure 3. Comparison of velocity fields between drifting buoys and Ocean Surface Current Lagrangian surface drifters. Analyses Real-time (OSCAR) surface currents. (a) Snapshot of OSCAR currents in the Agulhas Current region. (b) One-day binned drifter velocities over two months. Red and blue vectors Velocities Through denote zonal direction. (c) OSCAR currents interpolated onto the drifter time and space posi- tion. Scales of vectors are set to be the same as in (b). Comparison statistics between the two are Feature Tracking given in the scatter plots of (d) and (e), with N = number of points, Cor = correlation coefficient, An alternative method for measuring Sk = 1-std(D-O)/std(D), RDS = (std(D)-std(O))/std(D), Slope = slope of the best-fit line, and where D = drifter and O = OSCAR signal. While overall the comparison is high between the two surface currents is to track features fields, the smoothed nature ofO SCAR compared to drifters is apparent in the vector fields. on the ocean surface. The Maximum

98 Oceanography | Vol.23, No.4 Cross-Correlation (MCC) technique data assimilation models are able to oriented in the right direction, push tracks the displacements of small-scale model the scales that remote sensing waters off the coast (due to the Coriolis features on high-resolution satellite cannot capture, with insights into three- force), forcing colder, nutrient-rich SST images and attributes them to dimensional dynamics, thus aiding the bottom waters to the surface. Much of advection by ocean currents (Emery understanding of physical processes and the biological production in the upper et al., 2003). With great possible poten- guiding scientific requirements for future ocean depends on the supply of nutrients tial, MCC today is limited to areas free satellite missions. from deep layers (Yoder et al., 2010). of clouds for periods long enough to Surface current trajectories also identify the displacements. In addition, Doppler-Based Surface impact marine life. Surface current the accuracy of the velocity estimate is Velocities from SAR patterns define marine migration impacted by the ability to distinguish An alternative method for calcu- (e.g., the migration of larvae), and “material” features from those associ- lating surface currents is through the knowledge of these paths is important ated with waves or processes such as exploitation of the Doppler shift in for efficient management of resources. heterogeneous mixing and local forcing, synthetic aperture radar data (Chapron, Shipping, commercial fishing, recre- which change the shape of the tracked 2005). This method has promise in ational boating, search and rescue, and feature. A strong advantage of a feature- particular for coastal zones, although pollutant dispersal (e.g., oil spills) all tracking technique is that it can be used there are many technical challenges, rely on surface current patterns. The on a variety of surface imaged fields. For including isolating the surface currents “Great Garbage Patch,” a huge cluster example, a similar technique has been from signals such as wave orbital of floating plastic debris formed in the used to track mesoscale features in the motion or tides. middle of the subtropical North Pacific surface height field measured with radar Ocean by converging Ekman currents, altimetry (Fu, 2006). Similarly, SAR I mportance of has the potential to damage marine life images (Liu, 2009) have been used to Surface Currents and alter the biological environment. map the surface drift. At all scales, currents are responsible for Figure 4 shows these convergent zones water exchange between different parts for surface tracers. G eneral Circulation Models of the ocean. The overall ocean circula- The surface branch of the ocean Some features of the mean circulation tion compensates for the effect of differ- general circulation is the location of are harder to diagnose exclusively from ential solar heating and air-sea fluxes all air-sea exchange. Although it is a surface observations. For example, by redistributing heat, momentum, and crucial component of the atmosphere/ rather than being constant, the Antarctic salt between the equator and the polar ocean system, the range of scales neces- Circumpolar Current is a circumpo- regions, and between the surface and sary to measure the complete system larly flowing sea of eddies, continually the deep ocean. The surface branch of is an insurmountable challenge for in focused and defocused into jets, and the circulation is a dominant term in situ sampling. Frequent global satel- interacting with the bottom, all of heat transport (Boccaletti et al., 2005). lite measurements provide invaluable which modify the mean circulation Surface currents play a particularly information that greatly increases our (e.g., Karsten et al., 2002; Williams et al., important role in the dynamics of such understanding of surface circulation (see 2007). Ocean general circulation models large-scale phenomena as the El Niño http://oceanmotion.org for a compre- (GCMs) and coupled atmosphere-ocean Southern Oscillation and the Pacific hensive overview of the surface system). GCMs have greatly benefitted from Decadal Oscillation (Lee et al., 2010). global satellite measurements through Wind variations create convergences Key Challenges and assimilation of the satellite data to and divergences of surface currents, Future Developments improve model performance. For an which in turn force vertical motion Satellite systems can now capture the overview of ocean state estimation from (downwelling and upwelling, respec- mesoscale global eddy field and western models, see Lee et al. (2010). In return, tively). Strong winds along coasts, if boundary currents, and with more

Oceanography | December 2010 99 (Madsen et al., 2007). These types of methods have been shown to accurately calculate circulation up to 5 km from land. All involve regional treatment of the problem, so that developing any complete global system of coastal currents will involve combining the results of all regional studies.

C apturing Small-Scale Features As resolution increases, both in numerical models and in satellite observations, the ocean looks more like a sea of inter- Figure 4. Tracer density distribution after 10 years of advection by surface currents, consistent with acting eddies than a system of gyres and statistics from drifters, starting from a uniform initial condition of unity. currents. Blended satellite products, such as those produced by AVISO, are showing than 18 years of altimetry data, we can near land, and strong tidal signals. In great success at capturing mesoscale now begin to observe changes at time addition, coastal processes are often high eddies, as in the rings of Figures 3 and 5a. scales relevant to climate. However, frequency, small scale, and controlled by These mesoscale eddies are an important many important physical processes the shape of the seafloor, which would mechanism for drawing nutrients up remain elusive to satellite remote sensing be missed by gaps between nadir satellite into the euphotic zone. Approximately because of their faster time scales, tracks. For example, storm surges can 20–30% of vertical nutrient transport in smaller spatial scales, and vertical varia- drastically affect coastal communities, the upper ocean is estimated to occur tion (see Figure 5). with potential increases in severity with within these eddies (McGillicuddy et al., climate change. These surges occur too 2003). The planned Surface Water and C oastal Regions quickly to be consistently captured by Ocean Topography (SWOT) mission Because the majority of recreational conventional altimetry. will provide an unprecedented global and commercial ocean use is along the Recent advances in coastal surface characterization of fine-scale features and coasts, it is of interest to understand current calculations have occurred fronts, with O(10 km) resolution, with coastal circulation patterns. Knowledge through several approaches. One area much closer coastal coverage. Fu et al. of these patterns is also important for of research is in waveform retracking— (2010, in this issue) include a detailed tracking land pollutants, river exchange, reprocessing individual satellite track description of the SWOT mission and and ecological components such as larval signals to recover the distorted wave- existing success at capturing near-real- migration. Coastal altimetry (applying form as it reaches land (Deng and time mesoscale eddies. satellite altimetry to coastal regions) is Featherstone, 2006). Another area of Between these eddies are submeso- an emerging field with a relatively recent development is application of regional scale features, such as small eddies and history because of limitations in the coastal physics to signal-processing high potential vorticity filaments that accuracy of the satellite system’s signals algorithms, such as customized atmo- are stretched and pulled by the eddies near land, which are only reliable at spheric physics or local tides (Volkov in ocean turbulence. Filament scales distances of 25–100 km offshore. These et al., 2007). A different approach is are kilometers thick and hundreds of limitations are due to several factors: size to combine satellite data with in situ kilometers long, with rapid changes of the altimeter footprint, atmospheric measurements, such as tide gauges on the order of a day, making them effects (wet tropospheric correction), (Saraceno et al., 2008), and to use difficult to observe by satellite. At wave- modulation of the altimeter waveform statistics to predict storm surges lengths shorter than 100 km, however,

100 Oceanography | Vol.23, No.4 measurement noise masks submeso- previously missed dynamics, validation from these fields use assumptions about scales (Fu and Ferrari, 2008). This is of both regional and global numerical the vertical structure of currents in order somewhat improved with multiple models, and validation for analytical to define a vertically averaged current. satellites, but filament structure has yet methods such as SQG and Lyapunov Vertical shear modifies calculations of to be observed globally. Filaments are exponents, as well as new insights into the transport of properties such as heat, ubiquitous features of the ocean that the physics of surface circulation. momentum, and salt. The trajectories are consistently seen in high-resolution of suspended pollutants (e.g., the Great SST images (Figure 5b, for example) but Vertical Shear Garbage Patch) depend on vertical shear. they have been studied mostly numeri- Satellite sensors measure surface proper- Additionally, shear affects biological cally and theoretically in the context of ties: SSH, winds at the ocean surface, and processes. For example, if a species geophysical turbulence. SST (the only exception is the GRACE moves vertically in the water column Modeling studies that account satellite pair, which measures ocean during the course of the day according for filaments are beginning to show mass, or ocean bottom pressure). The to the percentage of sunlight, it will be that motions at these small scales are models that calculate surface currents carried on a path that will rely heavily responsible for as much vertical transport as the large eddies (Klein and Lapeyre, 2009). Winds can also play a significant role in vertical transport, adding to the challenge of capturing these scales by satellite. Numerical studies suggest that fronts, the inter- face between water masses of different density, are also responsible for much of the vertical transport in the upper ocean (Mahadevan and Tandon, 2006). Using the principles behind the creation of tracer filaments from the stirring caused by mesoscale eddies, advances are being made in diagnosing submesoscale statistics from existing altimetry (d’Ovidio et al., 2009). Use of surface quasi-geostrophic theory (SQG) has also been successful (Held, 1995) in extracting three- dimensional dynamics from surface fields (Isern-Fontanet, 2008). This method requires high-resolution fields, and thus has relied to date more on SST fields, which have a much higher resolu- Figure 5. Issues in surface current measurements. (a) Time evolution of the breakdown of the Kuroshio tion than SSH fields. western boundary current into eddies, shown over 17 days on the OSCAR time spacing capability. The high-resolution SWOT mission (b) MODIS gridded SST at 4.63-km spacing, zoomed into the white box region in (a) (MODIS Terra Global will provide an unprecedented map of Level 3 Mapped ThermalIR SST from podaac.jpl.nasa.gov). (c) Drifter track overlaid on an OSCAR snapshot in the equatorial Pacific.A long with small-scale motions, near-inertial oscillations are prevalent in the the global submesoscale field, providing drifter signal. (d) Vertical variation of currents seen in mooring data (courtesy of TAO Project Office of the information needed for estimates on NOAA/PMEL http://www.pmel.noaa.gov/tao) on the same day as the OSCAR snapshot shown in (c).

Oceanography | December 2010 101 on the vertical structure of the currents, W ind-driven Fast Time Scales As we advance our understanding of both in strength and direction. The Ekman spiral is the response of ocean circulation, it is necessary to Many processes affect the vertical the ocean surface to a steady wind. In connect the surface to the subsurface. structure of the upper ocean. Diurnal general, however, winds are far from To this end, the Argo array of profiling fluctuations in solar radiation cause daily steady in either time or space. Variations floats (http://www.argo.ucsd.edu) has fluctuations in temperature structure, from the mean wind have an additional been providing a continuous global which in turn cause daily fluctuations in wave response, known as inertial waves, map of subsurface pressure, salinity, mixed-layer depth. Particularly in equa- trapped within the surface ocean mixed temperature, and velocity measurements. torial regions, the temperature difference layer. Inertial motions can be clearly seen Deployment of Argo floats began in can be large enough to limit wind-driven as the spiral modulation to the drifter 2000, and over 3000 floats are operating motions to the shallowest surface layer, trajectory plotted in Figure 5c. These at present, providing full global coverage. funneling motions into a shallow jet waves are an energy source for motions The floats range in depth of measure- for a few hours over the day. Equatorial that mix the ocean interior, comparable ment, typically down to either 2000 m regions also exhibit large subsurface to the energy from tides (Wunsch and or 1000 m. The floats descend to depth jets. An example of the vertical varia- Ferrari, 2004), and they consequently and then ascend to the surface, collecting tions in upper ocean currents is given in play a significant role in establishing data that are transmitted to satellites at the Tropical Atmosphere Ocean (TAO) the density structure of the ocean inte- the surface. Subsurface current plots mooring current plotted in Figure 5d. rior and hence the global overturning from Argo data are produced by the In addition, wind-driven waves and circulation. However, estimates of this YoMaHa project (http://apdrc.soest. currents are intensified at the surface. transfer, and its subsequent propaga- hawaii.edu/projects/yomaha). Ekman currents have an exponential tion into the ocean interior, rely on Ultimately, for a full understanding decay in depth on the order of tens accuracy of wind-field measurements of surface ocean circulation and its role of meters. The penetration depth of that capture both steady features and in thermohaline circulation, informa- turbulence and momentum transfer fast storms. Storm systems generally tion from long-term extensive in situ from breaking surface waves depends have scales of hundreds of kilometers, measurement campaigns in key locations on, among other factors, wind intensity, large enough to be captured by satel- of strong exchange between surface and with a qualitatively different regime lite systems. Whether a storm turns deep waters will also need to be coupled depending on whether conditions are clockwise or counterclockwise, though, to the global surface arrays. mild or stormy (Gargett, 1989). Surface can make an order of magnitude differ- waves carry an associated “Stokes drift,” ence in wave amplitude (Dohan and Summary known to surfers, which decays within Davis, in press), thereby requiring wind The view of the ocean’s surface move- the top few meters. measurements that include both speed ment has progressed far beyond the Surface current calculations, such as and direction. To capture these motions concept of its being simply the surface those from SSH and winds, will need to requires more than twice-daily measure- branch of the thermohaline circula- include a vertical component for prog- ments and therefore several vector wind tion that redistributes heat and salinity ress to be made. The notion of a surface measuring satellites. between the tropics and the poles. mixed layer with homogeneous turbu- Multiple satellite systems, measuring lence and constant density will need C onnecting Global Satellite SSH, SST, winds, and gravity, along with to be modified as applications demand Data to the Deep the in situ global drifting buoy array, more accurate and complete physics. The behavior of surface currents is a have provided us with a comprehensive These enhanced models will need in situ combination of short-term surface view of open-ocean surface motion, ocean mixed layer measurements in a forcing and long-term climatic changes. resolving scales on the order of hundreds variety of dynamical locations for valida- The ocean’s subsurface properties are of kilometers, and in nearly real time. tion and development. both affected by and affect the surface. Unprecedented understanding of upper

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