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ChuanLi Jiang
,
Sarah T. Gille
,
Janet Sprintall
,
Kei Yoshimura
, and
Masao Kanamitsu

Abstract

High-resolution underway shipboard atmospheric and oceanic observations collected in Drake Passage from 2000 to 2009 are used to examine the spatial scales of turbulent heat fluxes and flux-related state variables. The magnitude of the seasonal cycle of sea surface temperature (SST) south of the Polar Front is found to be twice that north of the front, but the seasonal cycles of the turbulent heat fluxes show no differences on either side of the Polar Front. Frequency spectra of the turbulent heat fluxes and related variables are red, with no identifiable spectral peaks. SST and air temperature are coherent over a range of frequencies corresponding to periods between ~10 h and 2 days, with SST leading air temperature. The spatial decorrelation length scales of the sensible and latent heat fluxes calculated from two-day transects are 65 ± 6 km and 80 ± 6 km, respectively. The scale of the sensible heat flux is consistent with the decorrelation scale for air–sea temperature differences (70 ± 6 km) rather than either SST (153 ± 2 km) or air temperature (138 ± 4 km) alone. These scales are dominated by the Polar Front. When the Polar Front region is excluded, the decorrelation scales are 10–20 km, consistent with the first baroclinic Rossby radius.

These eddy scales are often unrepresented in the available gridded heat flux products. The Drake Passage ship measurements are compared with four recently available gridded turbulent heat flux products: the European Centre for Medium-Range Weather Forecasts high-resolution operational product in support of the Year of Coordinated Observing Modeling and Forcasting Tropical Convection (ECMWF-YOTC), ECMWF interim reanalysis (ERA-Interim), the Drake Passage reanalysis downscaling (DPRD10) regional product, and the objectively analyzed air–sea fluxes (OAFlux). The decorrelation length scales of the air–sea temperature difference, wind speed, and turbulent heat fluxes from these four products are significantly larger than those determined from shipboard measurements.

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Sarah T. Gille
,
Aaron Lombrozo
,
Janet Sprintall
,
Gordon Stephenson
, and
Richard Scarlet

Abstract

The high vertical resolution of temperature and salinity measurements from expendable conductivity–temperature–depth (XCTD) instruments can be useful for inferring small-scale mixing rates in the ocean. However, XCTD temperature profiles show distinct spectral spikes at frequencies of 5 and 10 Hz, corresponding to 1 and 2 cycles per five measurement points. Peaks at these same frequencies are often present in the conductivity spectra as well. The spectral spikes occur in XCTD profiles from both the subtropical and subpolar regions. They appear to originate as digital electronic noise within the probes. A finite impulse response filter design procedure was used to develop filters that could remove the spectral spikes while retaining as much high vertical resolution as possible. For most purposes, the application of an 11-point, least squares, low-pass filter proves sufficient for removing the spectral energy at 5 and 10 Hz, and results in an effective minimum vertical resolution of about 0.7 m.

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André Palóczy
,
Julie L. McClean
,
Sarah T. Gille
, and
He Wang

ABSTRACT

The depth-integrated vorticity budget of a global, eddy-permitting ocean/sea ice simulation over the Antarctic continental margin (ACM) is diagnosed to understand the physical mechanisms implicated in meridional transport. The leading-order balance is between the torques due to lateral friction, nonlinear effects, and bottom vortex stretching, although details vary regionally. Maps of the time-averaged depth-integrated vorticity budget terms and time series of the spatially averaged, depth-integrated vorticity budget terms reveal that the flow in the Amundsen, Bellingshausen, and Weddell Seas and, to a lesser extent, in the western portion of East Antarctica, is closer to an approximate topographic Sverdrup balance (TSB) compared to other segments of the ACM. Correlation and coherence analyses further support these findings, and also show that inclusion of the vorticity tendency term in the response (the planetary vorticity advection and the bottom vortex stretching term) increases the correlation with the forcing (the vertical net stress curl), and also increases the coherence between forcing and response at high frequencies across the ACM, except for the West Antarctic Peninsula. These findings suggest that the surface stress curl, imparted by the wind and the sea ice, has the potential to contribute to the meridional, approximately cross-slope, transport to a greater extent in the Amundsen, Bellingshausen, Weddell, and part of the East Antarctic continental margin than elsewhere in the ACM.

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Wilbert Weijer
,
Frédéric Vivier
,
Sarah T. Gille
, and
Henk A. Dijkstra

Abstract

Observations of the sea surface height in the Argentine Basin indicate that strong variability occurs on a time scale of 20−30 days. The aim of this study is to determine the physical processes responsible for this variability. First, results are presented from two statistical techniques applied to a decade of altimetric data. A complex empirical orthogonal function (CEOF) analysis identifies the recently discovered dipole mode as the dominant mode of variability. A principal oscillation pattern (POP) analysis confirms the existence of this mode, which has a period of 25 days. The second CEOF displays a propagating pattern in the northern Argentine Basin, plus a rotating dipole in the southwest corner. The POP analysis identifies both patterns as individual modes, with periods of 30 and 20 days, respectively. Second, the barotropic normal modes of the Argentine Basin are studied, using a shallow-water model capturing the full bathymetry of the basin. Coherences between the spatial patterns of these modes and altimeter data suggest that several of the basin modes are involved in the observed variability. This analysis implies that the 20-day mode detected by recent bottom-pressure measurements is a true barotropic mode. However, the 25-day variability, as found in altimeter data, cannot be directly attributed to the excitation of a free Rossby basin mode. This study indicates that the results of several apparently conflicting observations of the flow variability in the Argentine Basin can be reconciled by assuming that multiple basin modes are involved.

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Wilbert Weijer
,
Frédéric Vivier
,
Sarah T. Gille
, and
Henk A. Dijkstra

Abstract

In this paper the spectrum of barotropic basin modes of the Argentine Basin is shown to be connected to the classical Rossby basin modes of a flat-bottom (constant depth), rectangular basin. First, the spectrum of basin modes is calculated for the Argentine Basin, by performing a normal-mode analysis of the barotropic shallow-water equations. Then a homotopy transformation is performed that gradually morphs the full-bathymetry geometry through a flat-bottom configuration into a rectangular basin. Following the eigenmodes through this transition establishes a connection between most of the basin modes and the classical Rossby basin modes of a rectangular geometry. In particular, the 20-day mode of the Argentine Basin is identified with the lowest-order mode of classical theory. Sensitivity studies show that the decay rate of each mode is controlled by bottom friction, but that it is insensitive to lateral friction; lateral friction strongly impacts the oscillation frequency. In addition, the modes are found to be only slightly sensitive to the presence of a background flow.

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Jinbo Wang
,
Matthew R. Mazloff
, and
Sarah T. Gille

Abstract

The Kerguelen Plateau is a major topographic feature in the Southern Ocean. Located in the Indian sector and spanning nearly 2000 km in the meridional direction from the polar to the subantarctic region, it deflects the eastward-flowing Antarctic Circumpolar Current and influences the physical circulation and biogeochemistry of the Southern Ocean. The Kerguelen Plateau is known to govern the local dynamics, but its impact on the large-scale ocean circulation has not been explored. By comparing global ocean numerical simulations with and without the Kerguelen Plateau, this study identifies two major Kerguelen Plateau effects: 1) The plateau supports a local pressure field that pushes the Antarctic Circumpolar Current northward. This process reduces the warm-water transport from the Indian to the Atlantic Ocean. 2) The plateau-generated pressure field shields the Weddell Gyre from the influence of the warmer subantarctic and subtropical waters. The first effect influences the strength of the Antarctic Circumpolar Current and the Agulhas leakage, both of which are important elements in the global thermohaline circulation. The second effect results in a zonally asymmetric response of the subpolar gyres to Southern Hemisphere wind forcing.

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Kyla Drushka
,
Janet Sprintall
,
Sarah T. Gille
, and
Irsan Brodjonegoro

Abstract

The subsurface structure of intraseasonal Kelvin waves in two Indonesian Throughflow (ITF) exit passages is observed and characterized using velocity and temperature data from the 2004–06 International Nusantara Stratification and Transport (INSTANT) project. Scatterometer winds are used to characterize forcing, and altimetric sea level anomaly (SLA) data are used to trace the pathways of Kelvin waves east from their generation region in the equatorial Indian Ocean to Sumatra, south along the Indonesian coast, and into the ITF region.

During the 3-yr INSTANT period, 40 intraseasonal Kelvin waves forced by winds over the central equatorial Indian Ocean caused strong transport anomalies in the ITF outflow passages. Of these events, 21 are classed as “downwelling” Kelvin waves, forced by westerly winds and linked to depressions in the thermocline and warm temperature anomalies in the ITF outflow passages; 19 were “upwelling” Kelvin waves, generated by easterly wind events and linked to shoaling of the thermocline and cool temperature anomalies in the ITF. Both downwelling and upwelling Kelvin waves have similar vertical structures in the ITF outflow passages, with strong transport anomalies over all depths and a distinctive upward tilt to the phase that indicates downward energy propagation. A linear wind-forced model shows that the first two baroclinic modes account for most of the intraseasonal variance in the ITF outflow passages associated with Kelvin waves and highlights the importance of winds both in the eastern equatorial Indian Ocean and along the coast of Sumatra and Java for exciting Kelvin waves.

Using SLA as a proxy for Kelvin wave energy shows that 37% ± 9% of the incoming Kelvin wave energy from the Indian Ocean bypasses the gap in the coastal waveguide at Lombok Strait and continues eastward. Of the energy that continues eastward downstream of Lombok Strait, the Kelvin waves are split by Sumba Island, with roughly equal energy going north and south to enter the Savu Sea.

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Ru Chen
,
Julie L. McClean
,
Sarah T. Gille
, and
Alexa Griesel

Abstract

High spatial resolution isopycnal diffusivities are estimated in the Kuroshio Extension (KE) region (28°–40°N, 120°–190°E) from a global ° Parallel Ocean Program (POP) simulation. The numerical float tracks are binned using a clustering approach. The number of tracks in each bin is thus roughly the same leading to diffusivity estimates that converge better than those in bins defined by a regular geographic grid. Cross-stream diffusivities are elevated in the southern recirculation gyre region, near topographic obstacles and downstream in the KE jet, where the flow has weakened. Along-stream diffusivities, which are much larger than cross-stream diffusivities, correlate well with the magnitudes of eddy velocity. The KE jet suppresses cross-stream mixing only in some longitude ranges. This study estimates the critical layer depth both from linear local baroclinic instability analysis and from eddy phase speeds in the POP model using the Radon transform. The latter is a better predictor of large mixing length in the cross-stream direction. Critical layer theory is most applicable in the intense jet regions away from topography.

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Paul Chamberlain
,
Lynne D. Talley
,
Bruce Cornuelle
,
Matthew Mazloff
, and
Sarah T. Gille

Abstract

The core Argo array has operated with the design goal of uniform spatial distribution of 3° in latitude and longitude. Recent studies have acknowledged that spatial and temporal scales of variability in some parts of the ocean are not resolved by 3° sampling and have recommended increased core Argo density in the equatorial region, boundary currents, and marginal seas with an integrated vision of other Argo variants. Biogeochemical (BGC) Argo floats currently observe the ocean from a collection of pilot arrays, but recently funded proposals will transition these pilot arrays to a global array. The current BGC Argo implementation plan recommends uniform spatial distribution of BGC Argo floats. For the first time, we estimate the effectiveness of the existing BGC Argo array to resolve the anomaly from the mean using a subset of modeled, full-depth BGC fields. We also study the effectiveness of uniformly distributed BGC Argo arrays with varying float densities at observing the ocean. Then, using previous Argo trajectories, we estimate the Argo array’s future distribution and quantify how well it observes the ocean. Finally, using a novel technique for sequentially identifying the best deployment locations, we suggest the optimal array distribution for BGC Argo floats to minimize objective mapping uncertainty in a subset of BGC fields and to best constrain BGC temporal variability.

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Momme C. Hell
,
Bruce D. Cornelle
,
Sarah T. Gille
,
Arthur J. Miller
, and
Peter D. Bromirski

Abstract

Strong surface winds under extratropical cyclones exert intense surface stresses on the ocean that lead to upper-ocean mixing, intensified heat fluxes, and the generation of waves, that, over time, lead to swell waves (longer than 10-s period) that travel long distances. Because low-frequency swell propagates faster than high-frequency swell, the frequency dependence of swell arrival times at a measurement site can be used to infer the distance and time that the wave has traveled from its generation site. This study presents a methodology that employs spectrograms of ocean swell from point observations on the Ross Ice Shelf (RIS) to verify the position of high wind speed areas over the Southern Ocean, and therefore of extratropical cyclones. The focus here is on the implementation and robustness of the methodology in order to lay the groundwork for future broad application to verify Southern Ocean storm positions from atmospheric reanalysis data. The method developed here combines linear swell dispersion with a parametric wave model to construct a time- and frequency-dependent model of the dispersed swell arrivals in spectrograms of seismic observations on the RIS. A two-step optimization procedure (deep learning) of gradient descent and Monte Carlo sampling allows detailed estimates of the parameter distributions, with robust estimates of swell origins. Median uncertainties of swell source locations are 110 km in radial distance and 2 h in time. The uncertainties are derived from RIS observations and the model, rather than an assumed distribution. This method is an example of supervised machine learning informed by physical first principles in order to facilitate parameter interpretation in the physical domain.

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