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Ocean Acoustic Tomography Has Heat Content, Velocity, and Vorticity in the North Pacific Thermohaline Circulation and Climate

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Ocean Acoustic Tomography Has Heat Content, Velocity, and Vorticity in the North Pacific Thermohaline Circulation and Climate B. Dushaw, G. Bold, C.-S. Chiu, J. Colosi, B. Cornuelle, Y. Desaubies, M. Dzieciuch, A. Forbes, F. Gaillard, Brian Dushaw, Bruce Howe A. Gavrilov, J. Gould, B. Howe, M. Lawrence, J. Lynch, D. Menemenlis, J. Mercer, P. Mikhalevsky, W. Munk, Applied Physics Laboratory and School of Oceanography I. Nakano, F. Schott, U. Send, R. Spindel, T. Terre, P. Worcester, C. Wunsch, Observing the Ocean in the 2000’s: College of Ocean and Fisheries Sciences A Strategy for the Role of Acoustic Tomography in Ocean Climate Observation. In: Observing the Ocean Ocean Acoustic Tomography: 1970–21st Century University of Washington Ocean Acoustic Tomography: 1970–21st Century http://staff.washington.edu/dushaw in the 21st Century, C.J. Koblinsky and N.R. Smith (eds), Bureau of Meteorology, Melbourne, Australia, 2001. ABSTRACT PROCESS EXPERIMENTS Deep Convection—Greenland and Labrador Seas ATOC—Acoustic Thermometry of Ocean Climate Oceanic convection connects the surface ocean to the deep ocean with important consequences for the global Since it was first proposed in the late 1970’s (Munk and Wunsch 1979, 1982), ocean acoustic tomography has Heat Content, Velocity, and Vorticity in the North Pacific thermohaline circulation and climate. Deep convection occurs in only a few locations in the world, and is difficult The goal of the ATOC project is to measure the ocean temperature on basin scales and to understand the evolved into a remote sensing technique employed in a wide variety of physical settings. In the context of to observe. Acoustic arrays provide both the spatial coverage and temporal resolution necessary to observe variability. The acoustic measurements inherently average out mesoscale and internal wave noise that long-term oceanic climate change, acoustic tomography provides integrals through the mesoscale and other The 1987 reciprocal acoustic tomography experiment (RTE87) obtained unique measurements of gyre-scale deep-water formation. (Worcester et al., 1993; Pawlowicz et al., 1995; Morawitz et al., 1996; Sutton et al., 1997; contaminate point measurements. (The ATOC consortium, 1998; Dushaw et al., 1999; Worcester et al., 1999) high-wavenumber noise over long distances. In addition, tomographic measurements can be made without temperature and currents. (Dushaw et al., 1993, 1994) Send and Kindler, 2001) risk of calibration drift; these measurements have the accuracy and precision required for large-scale ocean The 0–1000-m 80 80¡N˚ climate observation. The trans-basin acoustic measurements offer a signal-to-noise capability for observing 0.4 60° TOPEX ATOC California to l depth-averaged ATOC GEN ocean climate variability that is not readily attainable by an ensemble of point measurements. BER TS 0.2 SPI W temperature measurements 78¡ EST 0 CURRENT l On a regional scale, tomography has been employed for observing active convection, for measuring changes SPI 59˚N -0.2 WOA'94 (red, with error bars) are TSBER convection expected k Pioneer NLAND -0.4 1995 1996 1997 compared with historical data 76 E 30° in integrated heat content, for observing the mesoscale with high resolution, for measuring barotropic 76˚¡ E Seamount R G n G 1 EN N T d (WOA ’94) and the S C 0.4 currents in a unique way, and for directly observing oceanic relative vorticity. The remote sensing capability 5 U f California to k GREE EA R r NLAND R ENT 7 C) temperature derived from 4¡ 6 2 0.2 has proven effective for measurements under ice in the Arctic (in particular the recent well-documented o o 4 Kauai 0 TOPEX/POSEIDON (T/P) J AN 3 temperature increase in the Atlantic layer) and in regions such as the Strait of Gibraltar, where conventional vla1 -0.2 MA 72 57˚N altimetry (blue, assuming sea 72˚¡ YEN in-situ methods may fail. CURRE -0.4 1995 1996 1997 NT 0° surface height variations are K11/K21/K31 0.4 70¡ vla2 California to n caused only by thermal 150 As the community moves into an era of global-scale observations, the role for these acoustic techniques will JAN MA 90 20¡W YEN I. 180 120 E ° 0.2 10¡ 150 20˚ E ° expansion). The means have W 10 10˚ ¡ 0¡ be (1) to exploit the unique remote sensing capabilities for regional programs otherwise difficult to carry out, ° ° 0 10˚ 0˚ ° been removed from all time (2) to be a component of process-oriented experiments in regions where integral heat content or current data K12/K22/K32 -0.2 55˚N series. Note the smoothness are desired, and (3) to move toward deployment on basin scales as the acoustic technology becomes more 30° -0.4 1995 1996 1997 600 and small error bars of the ) Labrador 2 400 robust and simplified. 200 0.4 Kauai to n 0 acoustic data, especially for (W/m nz Heat Flux -200 0.2 100 60˚W56˚W52˚W 0 the long (5 Mm) paths. In the ° 50 60 (%) ) 0-1000 m Average Temperature ( -0.2 lower two panels, the annual Ice Cover -2 0 4 -0.4 1997 1998 1999 cycle has been removed from 0 Horizontal Integral from Tomography BASICS J m Local Estimate at Boundary Mooring the T/P data; the acoustic 9 Geometry of the RTE87 gyre-scale The low frequency (daily averaged) Comparisons of the acoustically 0.4 3 Local Estimate at Central Mooring Kauai to d 10 2 ATOC acoustic paths in the North rays on these particular paths reciprocal acoustic tomography barotropic currents for each leg and the determined heat content change Surface Heatflux Integral + 70 W/m 0.2 Sound travels faster in warm water than in cold x experiment, with acoustic transceivers areal averaged relative vorticity. The in the top 100 m (curves between 2 Pacific. The letters k, l, etc. are 0 sample below the seasonally water. By measuring the travel time of sound over a arbitrary names of receivers such as -0.2 varying surface layers, so at locations 1, 2, and 3. The ranges of vorticity is calculated by integrating the day 150 and 260) with other 1997 1998 1999 1 -0.4 known path, the sound speed and thus temperature the acoustic paths were 750, 1000, currents around the experiment triangle. estimates (triangles from XBTs, 1 the U.S. Navy SOSUS arrays. they do not observe the Depth (km) can be determined. Sound also travels faster with a and 1250 km. The calculation of vorticity cancels much points with error bars from CTDs). 0 annual cycle. current than against. By measuring the reciprocal of the variability of the currents. travel times in each direction along a path, the -1 2 -2 absolute water velocity can be determined. Each Oct Nov Dec Jan Feb Mar Apr May Jun 0 0.1 C) 1988 1989 (˚C) ° 11.5 0-1300 m Heat Content ( acoustic travel time represents the path integral of Barotropic and Baroclinic Tides -1.6 -1.2 -0.8 -0.4 0 Jul Oct Jan Apr Jul Oct Jan Apr Jul Oct Jan Apr Jul θ (˚C) the sound speed (temperature) and water velocity. 1997| 1998 | 1999 | 2000 Over the past several years a number of interesting observations of oceanic tidal variability have been made 26 As the sound travels along a ray path, it inherently 11 by long-range acoustic transmissions. These observations are examples of how line-integral observations N 24 averages these properties of the ocean, heavily (Top) Geometry of the tomography array in the (Top) In the Labrador Sea, the tomography mooring arrays were Acoustic paths going through eddies and fronts. provide significantly better signal-to-noise capability than point measurements for detecting large-scale 10.5 1997 1998HOT filtering along-path horizontal scales shorter than Greenland Sea during 1988–1989. A deep deployed in different years in the area where deep convection 22 In ocean acoustic tomography, data from a oceanic variability. This capability is a major advantage of acoustic tomography. Latitude 20 the path length. Over a 1000-km range, a depth- multitude of such paths crossing at many convective chimney was observed near the center activity is expected (shaded). 10 Hawaii averaged temperature change of 10 m° C is easily of the array during March 1989 (shaded region). different angles are used to reconstruct the Data from RTE87 and AMODE have shown that tomography can measure the harmonic constants of 164 162 160 158 156 154 (Bottom) A three-year time series of 0-1300-m heat content HOT TOPEX o measured as a 20-ms travel time change. sound speed (temperature) and velocity fields. barotropic tidal currents to within about 2% uncertainty. The uncertainty in the harmonic constants derived 9.5 Longitude W (Munk, Worcester, and Wunsch, 1995) (Bottom) Time-depth evolution of potential derived from tomographic data (cyan, dots) along the section 1996 1997 1998 from available current meter records appears to be about 20%, even when attempts have been made at temperature averaged over the chimney region. marked in the top panel and derived from data obtained at the ( Temperature 0-1000 Average separating the barotropic and baroclinic modes. (Dushaw et al. 1995, 1997; Dushaw and Worcester, 1998) Typical rms uncertainty as a function of depth is moorings at the ends of the section (blue: K11, red: K12). The shown to the right. Total surface heat flux and daily dashed line shows a three-year warming trend, equivalent to a 0 averaged ice cover are shown above. Convection heat flux of 10 Wm-2. The dashed green curve is the time-integral The travel time data from all the various Data from the Hawaii Ocean Time-series (HOT) site (about 100 km 21:00 45¡ N occurs in March, with cold water extending from the of the NCEP surface heat fluxes, corrected by the addition of acoustic paths can be used to reconstruct north of Oahu) are plotted with T/P data.
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