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Variability of Southern Ocean Jets near Topography

JOURNAL OF PHYSICAL OCEANOGRAPHY 2014年第2期44卷 676-693页

作者： Chapman, Christopher C. Morrow, Rosemary Australian Natl Univ Res Sch Earth Sci Canberra ACT 0200 Australia Australian Natl Univ ARC Ctr Excellence Climate Syst Sci Canberra ACT 0200 Australia CSIRO Wealth Oceans Flagship Hobart Tas Australia Lab Etud Geophys & Oceanog Spatiales Toulouse France

The interaction of jets with topography in the Southern Ocean is investigated using 19 years of altimetry data. In particular, the "jet jumping" mode of variability, by which two or more jets passing close to the same topographic feature show strongly anticorrelated strengthening and weakening, is studied. Three regional case studies are described-the Southeast Indian Ridge south of Tasmania, the Macquarie Ridge south of New Zealand, and the Pacific-Antarctic Rise-where the jet jumping variability is found to occur. Using principal component analysis, the spatial patterns of variability show a vortex dipole forming on either side of a particular jet. For each regional study, it is found that the variability in strength of these vortices (as measured by the spatially averaged vorticity) is strongly correlated with time series of the principle component that describes the jet jumping variability. The observational analysis is complemented by a suite of idealized numerical experiments using a three-layer quasigeostrophic model with simple topography. The numerical results show similar spatial patterns of variability to those observed in the altimetric data. Internal variability is sufficient to generate jet jumping variability, as there is no time-varying external forcing applied in the model configuration. The simulations are used to investigate the effect of topographic scale and changing bottom friction. The authors find that both have a strong influence on the time scale of the variability, with larger topographic scales and higher bottom friction leading to faster time scales. This study shows that even in regions where the flow is strongly influenced by topography, Southern Ocean jet flow may exhibit low-frequency variability.

关键词： Interannual variability Models and modeling Geographic location/entity observational techniques and algorithms Jets Southern Ocean Satellite observations Variability Quasigeostrophic models Atm/Ocean Structure/ Phenomena

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Large-Amplitude Internal Solitary Waves Observed in the Northern South China Sea: Properties and Energetics

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JOURNAL OF PHYSICAL OCEANOGRAPHY 2014年第4期44卷 1095-1115页

作者： Lien, Ren-Chieh Henyey, Frank Ma, Barry Yang, Yiing Jang Univ Washington Appl Phys Lab Seattle WA 98105 USA Naval Acad Dept Marine Sci Kaohsiung Taiwan

Five large-amplitude internal solitary waves (ISWs) propagating westward on the upper continental slope in the northern South China Sea were observed in May-June 2011 with nearly full-depth measurements of velocity, temperature, salinity, and density. As they shoaled, at least three waves reached the convective breaking limit: along-wave current velocity exceeded the wave propagation speed C. Vertical overturns of ~100 m were observed within the wave cores;estimated turbulent kinetic energy was up to 1.5 x 10(-4) W kg(-1). In the cores and at the pycnocline, the gradient Richardson number was mostly <0.25. The maximum ISW vertical displacement was 173 m, 38% of the water depth. The normalized maximum vertical displacement was ~0.4 for three convective breaking ISWs, in agreement with laboratory results for shoaling ISWs. Observed ISWs had greater available potential energy (APE) than kinetic energy (KE). For one of the largest observed ISWs, the total wave energy per unit meter along the wave crest E was 553 MJ m(-1), more than three orders of magnitude greater than that observed on the Oregon Shelf. Pressure work contributed 77% and advection contributed 23% of the energy flux. The energy flux nearly equaled CE. The Dubriel-Jacotin-Long model with and without a background shear predicts neither the observed APE > KE nor the subsurface maximum of the along-wave velocity for shoaling ISWs, but does simulate the total energy and the wave shape. Including the background shear in the model results in the formation of a surface trapped core.

关键词： Ship observations Wave properties Internal waves observational techniques and algorithms Solitary waves Continental shelf/slope Geographic location/entity In situ oceanic observations Circulation/ Dynamics

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Vortical and Internal Wave Shear and Strain

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JOURNAL OF PHYSICAL OCEANOGRAPHY 2014年第8期44卷 2070-2092页

作者： Pinkel, Robert Univ Calif San Diego Scripps Inst Oceanog La Jolla CA 92093 USA

Depth-time records of isopycnal vertical strain have been collected from intensive CTD profiling programs on the research platform (R/P) Floating Instrument Platform (FLIP). The associated vertical wavenumber frequency spectrum of strain, when viewed in an isopycnal-following frame, displays a clear spectral gap at low vertical wavenumber, separating the quasigeostrophic (vortical) strain field and the superinertial internal wave continuum. This gap enables both model and linear-filter-based methods for separating the submesoscale and internal wave strain fields. These fields are examined independently in six field programs spanning the period 1983-2002. Vortical and internal wave strain variances are often comparable in the upper thermocline, of order 0.2. However, vortical strain tends to decrease with increasing depth (decreasing buoyancy frequency N-2 = -g/rho(d rho/dz) as similar to(N-2)(1/2), while internal wave strain variance increases as similar to(N-2)(-1/2), exceeding vortical variance by a factor of 5-10 at depths below 500 m. In contrast to strain, the low-frequency spectral gap in the shear spectrum is largely obscured by Doppler-smeared near-inertial motions. The vertical wavenumber spectrum of anticyclonic shear exceeds the cyclonic shear and strain spectra at all scales greater than 10 m. The frequency spectrum of anticyclonic shear exceeds that of both cyclonic shear and strain to frequencies of 0.5 cph, emphasizing the importance of lateral Doppler shifting of near-inertial shear. The limited Doppler shifting of the vortical strain field implies surprisingly small submesoscale aspect ratios: k(H)/k(z) similar to 0.001, Burger numbers Br = k(H)N/k(z)f similar to 0.1. Submesoscale potential vorticity is dominated by vertical straining rather than the vertical component of relative vorticity. The inferred rms fluctuation of fluid vorticity is far less for the vortical field than for the internal wavefield.

关键词： Article Circulation/ Dynamics Eddies Inertia-gravity waves Internal waves Shear structure/flows Small scale processes observational techniques and algorithms Profilers oceanic

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Near-Inertial Internal Wave Field in the Canada Basin from Ice-Tethered Profilers

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JOURNAL OF PHYSICAL OCEANOGRAPHY 2014年第2期44卷 413-426页

作者： Dosser, Hayley V. Rainville, Luc Toole, John M. Univ Washington Appl Phys Lab Seattle WA 98105 USA Woods Hole Oceanog Inst Dept Phys Oceanog Woods Hole MA 02543 USA

Salinity and temperature profiles from drifting ice-tethered profilers in the Beaufort gyre region of the Canada Basin are used to characterize and quantify the regional near-inertial internal wave field over one year. Vertical displacements of potential density surfaces from the surface to 750-m depth are tracked from fall 2006 to fall 2007. Because of the time resolution and irregular sampling of the ice-tethered profilers, near-inertial frequency signals are marginally resolved. Complex demodulation is used to determine variations with a time scale of several days in the amplitude and phase of waves at a specified near-inertial frequency. Characteristics and variability of the wave field over the course of the year are investigated quantitatively and related to changes in surface wind forcing and sea ice cover.

关键词： Circulation/ Dynamics Internal waves Geographic location/entity Profilers, oceanic observational techniques and algorithms Inertia-gravity waves Arctic

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Multiyear Volume, Liquid Freshwater, and Sea Ice Transports through Davis Strait, 2004-10*

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JOURNAL OF PHYSICAL OCEANOGRAPHY 2014年第4期44卷 1244-1266页

作者： Curry, B. Lee, C. M. Petrie, B. Moritz, R. E. Kwok, R. Univ Washington Appl Phys Lab Seattle WA 98105 USA Bedford Inst Oceanog Ocean Sci Div Dartmouth NS Canada CALTECH Jet Prop Lab Pasadena CA USA

Davis Strait is a primary gateway for freshwater exchange between the Arctic and North Atlantic Oceans including freshwater contributions from west Greenland and Canadian Arctic Archipelago glacial melt. Data from six years (2004-10) of continuous measurements collected by a full-strait moored array and concurrent high-resolution Seaglider surveys are used to estimate volume and liquid freshwater transports through Davis Strait, with respective annual averages of -1.6 +/- 0.5 Sverdrups (Sv;1 Sv equivalent to 10(6) m(3) s(-1)) and -93 +/- 6 mSv (negative sign indicates southward transport). Sea ice export contributes an additional -10 +/- 1 mSv of freshwater transport, estimated using satellite ice area transport and moored upward-looking sonar ice thickness measurements. Interannual and annual variability of the net transports are large, with average annual volume and liquid freshwater transport standard deviations of 0.7 Sv and 17 mSv and with interannual standard deviations of 0.3 Sv and 15 mSv. Moreover, there are no clear trends in the net transports over the 6-yr period. However, salinity in the upper 250 m between Baffin Island and midstrait decreases starting in September 2009 and remains below average through August 2010, but appears to return to normal by the end of 2010. This freshening event, likely caused by changes in arctic freshwater storage, is not apparent in the liquid freshwater transport time series due to a reduction in southward volume transport in 2009-10. Reanalysis of Davis Strait mooring data from the period 1987-90, compared to the 2004-10 measurements, reveals less arctic outflow and warmer, more saline North Atlantic inflow during the most recent period.

关键词： Geographic location/entity Sea ice Circulation/ Dynamics Arctic Transport Ocean circulation In situ oceanic observations observational techniques and algorithms Glaciers

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On the Warm Water Volume and Its Changing Relationship with ENSO

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JOURNAL OF PHYSICAL OCEANOGRAPHY 2014年第5期44卷 1372-1385页

作者： Bunge, Lucia Clarke, Allan J. Florida State Univ Dept Earth Ocean & Atmospher Sci Tallahassee FL 32306 USA

The interannual, equatorial Pacific, 20 degrees C isotherm depth variability since 1980 is dominated by two empirical orthogonal function (EOF) modes: the "tilt" mode, having opposite signs in the eastern and western equatorial Pacific and in phase with zonal wind forcing and El Nino-Southern Oscillation (ENSO) indices, and a second EOF mode of one sign across the Pacific. Because the tilt mode is of opposite sign in the eastern and western equatorial Pacific while the second EOF mode is of one sign, the second mode has been associated with the warm water volume (WWV), defined as the volume of water above the 20 degrees C isotherm from 5 degrees S to 5 degrees N, 120 degrees E to 80 degrees W. Past work suggested that the WWV led the tilt mode by about 2-3 seasons, making it an ENSO predictor. But after 1998 the lead has decreased and WWV-based predictions of ENSO have failed. The authors constructed a sea level-based WWV proxy back to 1955, and before 1973 it also exhibited a smaller lead. Analysis of data since 1980 showed that the decreased WWV lead is related to a marked increase in the tilt mode contribution to the WWV and a marked decrease in second-mode EOF amplitude and its contribution. Both pre-1973 and post-1998 periods of reduced lead were characterized by "mean" La Nina-like conditions, including a westward displacement of the anomalous wind forcing. According to recent theory, and consistent with observations, such westward displacement increases the tilt mode contribution to the WWV and decreases the second-mode amplitude and its WWV contribution.

关键词： Interannual variability ENSO Physical Meteorology and Climatology Climate variability Variability observational techniques and algorithms Climate records Decadal variability

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On the Transfer Function between Surface Fields and the Geostrophic Stream Function in the Mediterranean Sea

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JOURNAL OF PHYSICAL OCEANOGRAPHY 2014年第5期44卷 1406-1423页

作者： Isern-Fontanet, Jordi Shinde, Mahesh Gonzalez-Haro, Cristina Inst Catala Ciencies Clima Barcelona 08005 Spain

The real transfer function and the phase shift between sea surface height (SSH) and sea surface buoyancy (SSB) were estimated from the output of a realistic eddy-resolving model of the Mediterranean Sea circulation. The analysis of their temporal evolution unveiled the existence of a clear seasonal cycle closely related to that of the mixed layer depth. The phase shifts between SSH and SSB attain their minimum for deep mixed layers, which is different from zero. Besides, the spectral slope of the transfer function at scales shorter than 100 km fluctuates between k(-1) and k(-2). For deep mixed layers, it is close to k(-1), as predicted by the surface quasigeostrophic (SQG) solution. At longer wavelengths, it is approximately constant under the different environmental conditions in all of the subbasins analyzed with the exception of the Gulf of Lions. The capability to observe sea surface temperature (SST) from satellites motivated the extension of this analysis to SST and SSH. Results showed a similar qualitative behavior but with larger phase shifts. In spite of the presence of a phase shift, even for deep mixed layers, results revealed that it is still possible to reconstruct surface dynamics from SST using a transfer function, provided that the mixed layer is deep enough. For the present study, a threshold value of 70 m was enough to identify the appropriate environmental conditions. In addition, the results revealed that a precise estimation of the transfer function significantly improves the reconstruction of the flow in comparison with the application of the classical SQG solution.

关键词： Mesoscale processes Mediterranean Sea Geographic location/entity Atm/Ocean Structure/ Phenomena Sea surface temperature Streamfunction observational techniques and algorithms Currents Circulation/ Dynamics Remote sensing

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A Multidimensional Spectral Description of Ocean Variability

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JOURNAL OF PHYSICAL OCEANOGRAPHY 2014年第3期44卷 944-966页

作者： Wortham, Cimarron Wunsch, Carl Univ Washington Appl Phys Lab Seattle WA 98105 USA Harvard Univ Dept Earth & Planetary Sci Cambridge MA 02138 USA

An updated empirical, analytical model for the frequency and wavenumber distribution of balanced motion in the ocean is presented. The spectrum model spans periods longer than the inertial but shorter than a decade and wavelengths between 100 and 10 000 km. Assuming geostrophic dynamics, a spectrum model for the streamfunction is constructed to be consistent with a range of observations, including sea surface height from satellite altimetry, velocity from moored and shipboard instruments, and temperature from moorings. First-order characteristics of the observed spectra, including amplitude and spectral moments, vary slowly geographically. The spectrum model is horizontally anisotropic, accommodating observations that zonal wavenumber-frequency spectra are dominated by a nondispersive line. Qualitative and quantitative agreement is found with one-dimensional frequency and wavenumber spectra and observed vertical profiles of variance. Illustrative application is made of the model spectrum to observing-system design, data mapping, and uncertainty estimation for trends.

关键词： Circulation Dynamics Rossby waves Physical Meteorology and Climatology Kinetic energy observational techniques and algorithms Altimetry Mathematical and statistical techniques Spectral analysis models distribution Variability Oceanic variability

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The Sound of Tropical Cyclones

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JOURNAL OF PHYSICAL OCEANOGRAPHY 2014年第10期44卷 2763-2778页

作者： Zhao, Zhongxiang D'Asaro, Eric A. Nystuen, Jeffrey A. Univ Washington Appl Phys Lab Seattle WA 98105 USA

Underwater ambient sound levels beneath tropical cyclones were measured using hydrophones onboard Lagrangian floats, which were air deployed in the paths of Hurricane Gustav (2008) and Typhoons Megi (2010) and Fanapi (2010). The sound levels at 40 Hz-50 kHz from 1- to 50-m depth were measured at wind speeds up to 45 ms(-1). The measurements reveal a complex dependence of the sound level on wind speed due to the competing effects of sound generation by breaking wind waves and sound attenuation by quiescent bubbles. Sound level increases monotonically with increasing wind speed only for low frequencies (<200 Hz). At higher frequencies (>200 Hz), sound level first increases and then decreases with increasing wind speed. There is a wind speed that produces a maximum sound level for each frequency;the wind speed of the maximum sound level decreases with frequency. Sound level at >20 kHz mostly decreases with wind speed over the wind range 15-45 m(-1). The sound field is nearly uniform with depth in the upper 50 m with nearly all sound attenuation limited to the upper 2 m at all measured frequencies. A simple model of bubble trajectories based on the measured float trajectories finds that resonant bubbles at the high-frequency end of the observations (25 kHz) could easily be advected deeper than 2 m during tropical cyclones. Thus, bubble rise velocity alone cannot explain the lack of sound attenuation at these depths.

关键词： Circulation/ Dynamics Turbulence Wind waves Atm/Ocean Structure/ Phenomena Atmosphere-ocean interaction Tropical cyclones observational techniques and algorithms Acoustic measurements/effects Profilers oceanic

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Two Years of Observations of Warm-Core Anticyclones in the Labrador Sea and Their Seasonal Cycle in Heat and Salt Stratification

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JOURNAL OF PHYSICAL OCEANOGRAPHY 2014年第2期44卷 427-444页

作者： de Jong, M. F. Bower, A. S. Furey, H. H. Woods Hole Oceanog Inst Woods Hole MA 02543 USA

Between 25 September 2007 and 28 September 2009, a heavily instrumented mooring was deployed in the Labrador Sea, offshore of the location where warm-core, anticyclonic Irminger rings are formed. The 2-year time series offers insight into the vertical and horizontal structure of newly formed Irminger rings and their heat and salt transport into the interior basin. In 2 years, 12 Irminger rings passed by the mooring. Of these, 11 had distinct properties, while 1 anticyclone likely passed the mooring twice. Eddy radii (11-35 km) were estimated using the dynamic height signal of the anticyclones (8-18 cm) together with the observed velocities. The anticyclones show a seasonal cycle in core properties when observed (1.9 degrees C in temperature and 0.07 in salinity at middepth) that has not been described before. The temperature and salinity are highest in fall and lowest in spring. Cold, fresh caps, suggested to be an important source of freshwater, were seen in spring but were almost nonexistent in fall. The heat and freshwater contributions by the Irminger rings show a large spread (from 12 to 108 MJ m(-2) and from -0.5 to -4.7 cm, respectively) for two reasons. First, the large range of radii leads to large differences in transported volume. Second, the seasonal cycle leads to changes in heat and salt content per unit volume. This implies that estimates of heat and freshwater transport by eddies should take the distribution of eddy properties into account in order to accurately assess their contribution to the restratification.

关键词： Anticyclones Mesoscale processes North Atlantic Ocean In situ oceanic observations Circulation/ Dynamics Geographic location/entity Seasonal cycle Boundary currents Variability observational techniques and algorithms Atm/Ocean Structure/ Phenomena

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