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Phase Speed Cross Spectra of Eddy Heat Fluxes in the Eastern Pacific

JOURNAL OF PHYSICAL OCEANOGRAPHY 2015年第5期45卷 1285-1301页

作者： Abernathey, Ryan Wortham, Cimarron Columbia Univ New York NY USA Univ Washington Appl Phys Lab Seattle WA 98105 USA

This study investigates the observed spectral character of eddy heat fluxes near the ocean surface, focusing on the distribution in wavenumber and phase speed space. Eddy heat fluxes in the eastern Pacific are calculated from concurrent satellite sea surface height and sea surface temperature data. A high-resolution coupled climate model is also analyzed in order to verify the physical mechanisms involved and to validate the model against observations. Wavenumber, frequency, and phase speed power spectra and cross spectra are constructed and presented as a function of latitude. These spectra reveal the dominance of coherent mesoscale eddies in both the length scale and phase speed of eddy heat fluxes. The breadths of the spectra are characterized via spectral moments;these moments show that the eddy fluxes are relatively concentrated around the dominant wavenumber and phase speed. Good agreement is found between the model and the observed spectra. The integrated heat transport and corresponding eddy diffusivity are shown to compare well with previous studies, but the results give a deeper insight into what determines the heat flux. Implications for eddy parameterization are discussed.

关键词： Geographic location/entity Pacific Ocean Circulation/ Dynamics Eddies Rossby waves Transport observational techniques and algorithms Remote sensing Mathematical and statistical techniques Fourier analysis

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Capturing the Transport Variability of a Western Boundary Jet: Results from the Agulhas Current Time-Series Experiment (ACT)

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JOURNAL OF PHYSICAL OCEANOGRAPHY 2015年第5期45卷 1302-1324页

作者： Beal, Lisa M. Elipot, Shane Houk, Adam Leber, Greta M. Univ Miami RSMAS Miami FL 33149 USA

The volume transport of the Agulhas Current was measured over a 3-yr period by an array of seven current meter moorings and four current-and pressure-recording inverted echo sounders (CPIES) deployed at 34 degrees S. CPIES extended the array farther offshore in order to capture, for the first time, the full Agulhas Current during meander events. Transports derived fromCPIES are well correlated with overlapping current meter transports (0.89). The Eulerian mean current is 219 km wide and 3000m deep, with peak surface speeds of 1.8ms 21 and a weak northward undercurrent on the continental slope below 1200m. A new algorithm to capture the western boundary jet transport at each time step T is defined as the poleward transport out to the first maximum of the vertically integrated velocity beyond the half-width of the mean jet. The mean transport of the Agulhas Current jet, so defined, is -84 Sverdrups (Sv;1 Sv equivalent to 10(6)m(3)s(-1)) with a standard error of 2 Sv. Sampling and instrumental errors are explicitly estimated and amount to an additional 9 Sv. A more traditional estimate, based on net transport integrated to a fixed distance offshore T-box, gives a mean transport of -77 +/- 5 Sv. This transport is 10 Sv greater than an equivalent transport at 32 degrees S, corresponding to a latitudinal increase equal to that predicted by Sverdrup dynamics. The time series of T and Tbox show important differences during solitary me ander events and at longer time scales. In terms of an annual cycle, the Agulhas Current appears strongest during austral summer, a similar phase to the Gulf Stream and Kuroshio.

关键词： Circulation/ Dynamics Boundary currents Transport observational techniques and algorithms In situ oceanic observations

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Detecting and Characterizing Ekman Currents in the Southern Ocean

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JOURNAL OF PHYSICAL OCEANOGRAPHY 2015年第5期45卷 1205-1223页

作者： Roach, Christopher J. Phillips, Helen E. Bindoff, Nathaniel L. Rintoul, Stephen R. Florida State Univ Inst Geophys Fluid Dynam Tallahassee FL 32306 USA Univ Tasmania Inst Marine & Antarctic Studies Hobart Tas Australia ARC Ctr Excellence Climate Syst Sci Hobart Tas Australia Antarctic Climate Ecosyst Cooperat Res Ctr Hobart Tas Australia CSIRO Marine & Atmospher Res Hobart Tas Australia Univ Tasmania Antarctic Climate Ecosyst Cooperat Res Ctr Hobart Tas Australia

This study presents a unique array of velocity profiles from Electromagnetic Autonomous Profiling Explorer (EM-APEX) profiling floats in the Antarctic Circumpolar Current (ACC) north of Kerguelen. The authors use these profiles to examine the nature of Ekman spirals, formed by the action of thewind on the ocean's surface, in light of Ekman's classical linear theory and more recent enhancements. Vertical decay scales of the Ekman spirals were estimated independently from current amplitude and rotation. Assuming a vertically uniform geostrophic current, decay scales from the Ekman current heading were twice as large as those from the current speed decay, indicating a compressed spiral, consistent with prior observations and violating the classical theory. However, if geostrophic shear is accurately removed, the observed Ekman spiral is as predicted by classical theory and decay scales estimated from amplitude decay and rotation converge toward a common value. No statistically robust relationship is found between stratification and Ekman decay scales. The results indicate that compressed spirals observed in the Southern Ocean arise from aliasing of depth-varying geostrophic currents into the Ekman spiral, as opposed to surface trapping of Ekman currents associated with stratification, and extends the geographical area of similar results from Drake Passage (Polton et al. 2013). Accounting for this effect, the authors find that constant viscosity Ekman models offer a reasonable description of momentum mixing into the upper ocean in the ACC north of Kerguelen. These results demonstrate the effectiveness of a new method and provide additional evidence that the same processes are active for the entire Southern Ocean.

关键词： Geographic location/entity Southern Ocean Circulation/ Dynamics Ekman pumping/transport Momentum Atm/Ocean Structure/ Phenomena Mixed layer observational techniques and algorithms Profilers oceanic

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Suppression of Internal Wave Breaking in the Antarctic Circumpolar Current near Topography

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JOURNAL OF PHYSICAL OCEANOGRAPHY 2014年第5期44卷 1466-1492页

作者： Waterman, Stephanie Polzin, Kurt L. Garabato, Alberto C. Naveira Sheen, Katy L. Forryan, Alexander Univ Southampton Natl Oceanog Ctr Southampton Hants England Woods Hole Oceanog Inst Woods Hole MA 02543 USA

Simultaneous full-depth microstructure measurements of turbulence and finestructure measurements of velocity and density are analyzed to investigate the relationship between turbulence and the internal wave field in the Antarctic Circumpolar Current. These data reveal a systematic near-bottom overprediction of the turbulent kinetic energy dissipation rate by finescale parameterization methods in select locations. Sites of near-bottom overprediction are typically characterized by large near-bottom flow speeds and elevated topographic roughness. Further, lower-than-average shear-to-strain ratios indicative of a less near-inertial wave field, rotary spectra suggesting a predominance of upward internal wave energy propagation, and enhanced narrowband variance at vertical wavelengths on the order of 100 m are found at these locations. Finally, finescale overprediction is typically associated with elevated Froude numbers based on the near-bottom shear of the background flow, and a background flow with a systematic backing tendency. Agreement of microstructure- and finestructure-based estimates within the expected uncertainty of the parameterization away from these special sites, the reproducibility of the overprediction signal across various parameterization implementations, and an absence of indications of atypical instrument noise at sites of parameterization overprediction, all suggest that physics not encapsulated by the parameterization play a role in the fate of bottom-generated waves at these locations. Several plausible underpinning mechanisms based on the limited available evidence are discussed that offer guidance for future studies.

关键词： Circulation/ Dynamics Turbulence Profilers, oceanic In situ oceanic observations observational techniques and algorithms Small scale processes Internal waves Diapycnal mixing

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Two Modes of Gulf Stream Variability Revealed in the Last Two Decades of Satellite Altimeter Data

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JOURNAL OF PHYSICAL OCEANOGRAPHY 2014年第1期44卷 149-163页

作者： Dolores Perez-Hernandez, M. Joyce, Terrence M. Univ Las Palmas Gran Canaria Inst Oceanog & Cambio Global Las Palmas Gran Canaria 35017 Spain Woods Hole Oceanog Inst Dept Phys Oceanog Woods Hole MA 02543 USA

Monthly mapped sea level anomalies (MSLAs) of the NW Atlantic in the region immediately downstream of the Gulf Stream (GS) separation point reveal a leading mode in which the path shifts approximately 100 km meridionally about a nominal latitude of 39 degrees N, producing coherent sea level anomaly (SLA) variability from 72 degrees to 50 degrees W. This mode can be captured by use of a simple 16-point index based on SLA data taken along the maximum of the observed variability in the region 33 degrees-46 degrees N and 45 degrees-75 degrees W. The GS shifts between 2010 and 2012 are the largest of the last decade and equal to the largest of the entire record. The second group of EOF modes of variability describes GS meanders, which propagate mainly westward interrupted by brief periods of eastward or stationary meanders. These meanders have wavelengths of approximately 400 km and can be seen in standard EOFs by spatial phase shifting of a standing meander pattern in the SLA data. The spectral properties of these modes indicate strong variability at interannual and longer periods for the first mode and periods of a few to several months for the meanders. While the former is quite similar to a previous use of the altimeter for GS path, the simple index is a useful measure of the large-scale shifts in the GS path that is quickly estimated and updated without changes in previous estimates. The time-scale separation allows a low-pass filtered 16-point index to be reflective of large-scale, coherent shifts in the GS path.

关键词： Atlantic Ocean Circulation Dynamics Boundary currents Indices Ocean dynamics observational techniques and algorithms Altimetry Mathematical and statistical techniques Empirical orthogonal functions

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Currents Associated with the Quasi-Stationary Jet Separated from the Kuroshio Extension

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JOURNAL OF PHYSICAL OCEANOGRAPHY 2014年第6期44卷 1636-1653页

作者： Wagawa, Taku Ito, Shin-Ichi Shimizu, Yugo Kakehi, Shigeho Ambe, Daisuke Tohoku Natl Fisheries Res Inst Shiogama Miyagi 985004 Japan Natl Res Inst Fisheries Sci Yokohama Kanagawa Japan

The hydrographic structure of a quasi-stationary jet separated from the Kuroshio Extension, a phenomenon that possibly leads to deepening of the winter mixed layer, is revealed via intensive observations using a conductivity-temperature-depth profiler and a ship-mounted acoustic Doppler current profiler (ADCP) carried out in September 2009. Data collected from profiling floats set to drift isopycnal layers and time series of sea surface buoys and satellite altimeters were also analyzed to understand the continuity, water characteristics, and time variation of the jet. Although the flow field was complex due to disturbances such as energetic eddies, the jet transported subtropical water to the downstream (northeastern) observation region in a layer shallower than 400 dbar and its highest velocity was concentrated in the mid- and deep layers (<= 0.30 m s(-1) at the sea surface and >= 0.05 m s(-1) at 800-m depth). The velocity axes of the jet detected from the satellite data corresponded to those detected through analysis of the ADCP data, and the intensity of the jet at the sea surface varies over interannual-to-decadal time scales. Part of the interannual-to-decadal variation in the velocity field of the jet is controlled by the dynamic state of the Kuroshio Extension and is correlated with that of the Kuroshio Extension latitude (linear correlation coefficient r similar to 0.67). The relationship between these variations can be interpreted qualitatively as being responsible for the inertial streamers that separate from the crests of meanders of the Kuroshio Extension to the jet region due to the large steering effect.

关键词： Geographic location/entity North Pacific Ocean Circulation/ Dynamics Currents Meridional overturning circulation observational techniques and algorithms Profilers oceanic Satellite observations Ship observations

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AUV Observations of the Diurnal Surface Layer in the North Atlantic Salinity Maximum

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JOURNAL OF PHYSICAL OCEANOGRAPHY 2014年第6期44卷 1595-1604页

作者： Hodges, Benjamin A. Fratantoni, David M. Woods Hole Oceanog Inst Woods Hole MA 02543 USA

Autonomous underwater vehicle (AUV) surveys of temperature, salinity, and velocity in the upper 10 m of the ocean were carried out in low-wind conditions near the North Atlantic surface salinity maximum as part of the Salinity Processes in the Upper Ocean Regional Study (SPURS) project. Starting from a well-mixed state, the development, deepening, and decay of a warm salty diurnal surface layer was observed at <1-h resolution. The evaporation rate deduced from the freshwater anomaly of the layer corroborates measurements at a nearby flux mooring. Profiles within a few hundred meters of the stationary research vessel showed evidence of mixing, highlighting the effectiveness of AUVs for collecting uncontaminated time series of near-surface thermohaline structure. A two-dimensional horizontal subsurface survey within the diurnal warm layer revealed coherent warm and cool bands, which are interpreted as internal waves on the diurnal thermocline.

关键词： Atm/Ocean Structure/ Phenomena Surface layer observational techniques and algorithms In situ oceanic observations

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A Note on the Phillips Spectral Framework for Ocean Whitecaps

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JOURNAL OF PHYSICAL OCEANOGRAPHY 2014年第7期44卷 1727-1734页

作者： Banner, Michael L. Zappa, Christopher J. Gemmrich, Johannes R. Univ New S Wales Sch Math & Stat Sydney NSW Australia Columbia Univ Lamont Doherty Earth Observ Ocean & Climate Phys Div Palisades NY 10964 USA Univ Victoria Victoria BC Canada

There has been a recent upsurge in interest in quantifying kinematic, dynamic, and energetic properties of wave breaking in the open ocean, especially in severe sea states. The underpinning observational and modeling framework is provided by the seminal paper of O. M. Phillips. In this note, a fundamental issue contributing to the scatter in results between investigators is highlighted. This issue relates to the choice of the independent variable used in the expression for the spectral density of the mean breaking crest length per unit area. This note investigates the consequences of the different choices of independent variable presently used by various investigators for validating Phillips model predictions for the spectral density of the breaking crest length per unit area and the associated spectral breaking strength coefficient. These spectral measures have a central role in inferring the associated turbulent kinetic energy dissipation rate and the momentum flux to the upper ocean from breaking wave observations.

关键词： Circulation/ Dynamics Turbulence Wave breaking Wind waves Physical Meteorology and Climatology Air-sea interaction observational techniques and algorithms Remote sensing

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Scale-Dependent Dispersion within the Stratified Interior on the Shelf of Northern Monterey Bay

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JOURNAL OF PHYSICAL OCEANOGRAPHY 2014年第4期44卷 1049-1064页

作者： Moniz, Ryan J. Fong, Derek A. Woodson, C. Brock Willis, Susan K. Stacey, Mark T. Monismith, Stephen G. Stanford Univ Dept Civil & Environm Engn Environm Fluid Mech Lab Stanford CA 94305 USA Univ Georgia COBIA Lab Coll Engn Athens GA 30602 USA Univ Calif Berkeley Dept Civil & Environm Engn Berkeley CA 94720 USA

Autonomous underwater vehicle measurements are used to quantify lateral dispersion of a continuously released Rhodamine WT dye plume within the stratified interior of shelf waters in northern Monterey Bay, California. The along-shelf evolution of the plume's cross-shelf (lateral) width provides evidence for scale-dependent dispersion following the 4/3 law, as previously observed in both surface and bottom layers. The lateral dispersion coefficient is observed to grow to 0.5 m(2) s(-1) at a distance of 700 m downstream of the dye source. The role of shear and associated intermittent turbulent mixing within the stratified interior is investigated as a driving mechanism for lateral dispersion. Using measurements of time-varying temperature and horizontal velocities, both an analytical shear-flow dispersion model and a particle-tracking model generate estimates of the lateral dispersion that agree with the field-measured 4/3 law of dispersion, without explicit appeal to any assumed turbulence structure.

关键词： Shear structure/flows Coastal flows Dispersion Circulation/ Dynamics In situ oceanic observations observational techniques and algorithms Internal waves Tracers

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Wave Breaking Dissipation in a Young Wind Sea

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JOURNAL OF PHYSICAL OCEANOGRAPHY 2014年第1期44卷 104-127页

作者： Schwendeman, Michael Thomson, Jim Gemmrich, Johannes R. Univ Washington Appl Phys Lab Seattle WA 98105 USA Univ Victoria Dept Phys & Astron Victoria BC Canada

Coupled in situ and remote sensing measurements of young, strongly forced wind waves are applied to assess the role of breaking in an evolving wave field. In situ measurements of turbulent energy dissipation from wave-following Surface Wave Instrument Float with Tracking (SWIFT) drifters and a tethered acoustic Doppler sonar system are consistent with wave evolution and wind input (as estimated using the radiative transfer equation). The Phillips breaking crest distribution (c) is calculated using stabilized shipboard video recordings and the Fourier-based method of Thomson and Jessup, with minor modifications. The resulting (c) are unimodal distributions centered around half of the phase speed of the dominant waves, consistent with several recent studies. Breaking rates from (c) increase with slope, similar to in situ dissipation. However, comparison of the breaking rate estimates from the shipboard video recordings with the SWIFT video recordings show that the breaking rate is likely underestimated in the shipboard video when wave conditions are calmer and breaking crests are small. The breaking strength parameter b is calculated by comparison of the fifth moment of (c) with the measured dissipation rates. Neglecting recordings with inconsistent breaking rates, the resulting b data do not display any clear trends and are in the range of other reported values. The (c) distributions are compared with the Phillips equilibrium range prediction and previous laboratory and field studies, leading to the identification of several inconsistencies.

关键词： Circulation Dynamics Gravity waves Wave breaking Atm Ocean Structure Phenomena Atmosphere-ocean interaction observational techniques and algorithms In situ oceanic observations Remote sensing Mathematical and statistical techniques Fourier analysis

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