Browse

You are looking at 1 - 10 of 7,904 items for :

  • Journal of Physical Oceanography x
  • User-accessible content x
Clear All
Chiung-Yin Chang and Malte F. Jansen

Abstract

Although the reconfiguration of the abyssal overturning circulation has been argued to be a salient feature of Earth’s past climate changes, our understanding of the physical mechanisms controlling its strength remains limited. In particular, existing scaling theories disagree on the relative importance of the dynamics in the Southern Ocean versus the dynamics in the basins to the north. In this study, we systematically investigate these theories and compare them with a set of numerical simulations generated from an ocean general circulation model with idealized geometry, designed to capture only the basic ingredients considered by the theories. It is shown that the disagreement between existing theories can be partially explained by the fact that the overturning strengths measured in the channel and in the basin scale distinctly with the external parameters, including surface buoyancy loss, diapycnal diffusivity, wind stress, and eddy diffusivity. The overturning in the reentrant channel, which represents the Southern Ocean, is found to be sensitive to all these parameters, in addition to a strong dependence on bottom topography. By contrast, the basin overturning varies with the integrated surface buoyancy loss rate and diapycnal diffusivity but is mostly unaffected by winds and channel topography. The simulated parameter dependence of the basin overturning can be described by a scaling theory that is based only on basin dynamics.

Open access
Ajitha Cyriac, Helen E. Phillips, Nathaniel L. Bindoff, Huabin Mao, and Ming Feng

Abstract

This study investigates the spatiotemporal variability of turbulent mixing in the eastern south Indian Ocean using a collection of data from electromagnetic autonomous profiling explorer (EM-APEX) profiling floats, shipboard CTD, and microstructure profilers. The floats collected 1566 profiles of temperature, salinity, and horizontal velocity data down to 1200 m over a period of about four months. A finescale parameterization is applied to the float and CTD data to estimate turbulent mixing. Elevated mixing is observed in the upper ocean, over bottom topography, and in mesoscale eddies. Mixing is enhanced in the anticyclonic eddies due to trapped near-inertial waves within the eddy. We found that cyclonic eddies contribute to turbulent mixing in the depth range of 500–1000 m, which is associated with downward-propagating internal waves. The mean diapycnal diffusivity over 250–500-m depth is O(10−6) m2 s−1, and it increases to O(10−5) m2 s−1 in 500–1000 m in cyclonic eddies. The turbulent mixing in this region has implications for water-mass transformation and large-scale circulation. Higher diffusivity [O(10−5) m2 s−1] is observed in the Antarctic Intermediate Water (AAIW) layer in cyclonic eddies, whereas weak diffusivity is observed in the Subantarctic Mode Water (SAMW) layer [O(10−6) m2 s−1]. Counterintuitively, then, the SAMW water-mass properties are strongly affected in cyclonic eddies, whereas the AAIW layer is less affected. Comparatively high diffusivity at the location of the South Indian Countercurrent (SICC) jets suggests there are wave–mean flow interactions in addition to the wave–eddy interactions that warrant further investigation.

Open access
Adrian Jenkins

Abstract

When the inclined base of an ice shelf melts into the ocean, it induces both a statically stable stratification and a buoyancy-forced, sheared flow along the interface. Understanding how those competing effects influence the dynamical stability of the boundary current is the key to quantifying the turbulent transfer of heat from far-field ocean to ice. The implications of the close coupling between shear, stability, and mixing are explored with the aid of a one-dimensional numerical model that simulates density and current profiles perpendicular to the ice. Diffusivity and viscosity are determined using a mixing length model within the turbulent boundary layer and empirical functions of the gradient Richardson number in the stratified layer below. Starting from rest, the boundary current is initially strongly stratified and dynamically stable, slowly thickening as meltwater diffuses away from the interface. Eventually, the current enters a second phase where dynamical instability generates a relatively well-mixed, turbulent layer adjacent to the ice, while beneath the current maximum, strong stratification suppresses mixing in the region of reverse shear. Under weak buoyancy forcing the time scale for development of the initial dynamical instability can be months or longer, but background flows, which are always present in reality, provide additional current shear that greatly accelerates the process. A third phase can be reached when the ice shelf base is sufficiently steep, with dynamical instability extending beyond the boundary layer into regions of geostrophic flow, generating a marginally stable pycnocline through which the heat flux is a simple function of ice–ocean interfacial slope.

Open access
Arjun Jagannathan, Kaushik Srinivasan, James C. McWilliams, M. Jeroen Molemaker, and Andrew L. Stewart

Abstract

Current–topography interactions in the ocean give rise to eddies spanning a wide range of spatial and temporal scales. The latest modeling efforts indicate that coastal and underwater topography are important generation sites for submesoscale coherent vortices (SCVs), characterized by horizontal scales of O(0.110)km. Using idealized, submesoscale and bottom boundary layer (BBL)-resolving simulations and adopting an integrated vorticity balance formulation, we quantify precisely the role of BBLs in the vorticity generation process. In particular, we show that vorticity generation on topographic slopes is attributable primarily to the torque exerted by the vertical divergence of stress at the bottom. We refer to this as the bottom stress divergence torque (BSDT). BSDT is a fundamentally nonconservative torque that appears as a source term in the integrated vorticity budget and is to be distinguished from the more familiar bottom stress curl (BSC). It is closely connected to the bottom pressure torque (BPT) via the horizontal momentum balance at the bottom and is in fact shown to be the dominant component of BPT in solutions with a well-resolved BBL. This suggests an interpretation of BPT as the sum of a viscous, vorticity-generating component (BSDT) and an inviscid, “flow-turning” component. Companion simulations without bottom drag illustrate that although vorticity generation can still occur through the inviscid mechanisms of vortex stretching and tilting, the wake eddies tend to have weaker circulation, be substantially less energetic, and have smaller spatial scales.

Open access
Bieito Fernández-Castro, Dafydd Gwyn Evans, Eleanor Frajka-Williams, Clément Vic, and Alberto C. Naveira-Garabato
Open access
Kaushik Srinivasan, James C. McWilliams, and Arjun Jagannathan

Abstract

Submesoscale coherent vortices (SCVs) are a ubiquitous feature of topographic wakes in the extratropical oceans. Recent studies demonstrate a mechanism wherein high-vorticity bottom boundary layers (BBLs) on the slopes of the topography separate (forming shear layers), undergo instabilities, and subsequently merge in the horizontal and align in the vertical to form vertically coherent, columnar SCVs (i.e., with low vertical shear). Background rotation is critical to the vertical alignment of unstable vortical filaments into coherent SCVs. In the tropics, however, the weakening of rotation prevents this alignment. Employing an idealized framework of steady barotropic flow past an isolated seamount in a background of constant stratification N and rotation rate f, we examine the wake structure for a range of f values spanning values from the poles to the tropics. We find a systematic increase in the interior vertical shear with decreasing f that manifests as a highly layered wake structure consisting of vertically thin, “pancake” SCVs possessing a high vertical shear. A monotonic increase in the wake energy dissipation rate is concomitantly observed with decreasing f. By examining the evolution equations for the vertical shear and vertical enstrophy, we find that the interior shear generation is an advective process, with the location of peak shear generation approximately collocated with maximum energy dissipation. This leads to the inference that high-wake dissipation in tropical topographic wakes is caused by parameterized shear instabilities induced by interior advective generation of vertical shear in the near wake region.

Open access
Lianxin Zhang, Xuefeng Zhang, William Perrie, Changlong Guan, Bo Dan, Chunjian Sun, Xinrong Wu, Kexiu Liu, and Dong Li

Abstract

A coupled ocean–wave–sea spray model system is used to investigate the impacts of sea spray and sea surface roughness on the response of the upper ocean to the passage of the Super Typhoon Haitang. Sea spray–mediated heat and momentum fluxes are derived from an improved version of Fairall’s heat fluxes formulation and Andreas’s sea spray–mediated momentum flux models. For winds ranging from low to extremely high speeds, a new parameterization scheme for the sea surface roughness is developed, in which the effects of wave state and sea spray are introduced. In this formulation, the drag coefficient has minimal values over the right quadrant of the typhoon track, along which the typhoon-generated waves are longer, smoother, and older, compared to other quadrants. Using traditional interfacial air–sea turbulent (sensible, latent, and momentum) fluxes, the sea surface cooling response to Typhoon Haitang is overestimated by 1°C, which can be compensated by the effects of sea spray and ocean waves on the right side of the storm. Inclusion of sea spray–mediated turbulent fluxes and sea surface roughness, modulated by ocean waves, gives enhanced cooling along the left edges of the cooling area by 0.2°C, consistent with the upper ocean temperature observations.

Open access
Edward J. Walsh, C. W. Fairall, and Ivan PopStefanija

Abstract

The airborne NOAA Wide Swath Radar Altimeter (WSRA) is a 16-GHz digital beamforming radar altimeter that produces a topographic map of the waves as the aircraft advances. The wave topography is transformed by a two-dimensional FFT into directional wave spectra. The WSRA operates unattended on the aircraft and provides continuous real-time reporting of several data products: 1) significant wave height; 2) directional ocean wave spectra; 3) the wave height, wavelength, and direction of propagation of the primary and secondary wave fields; 4) rainfall rate; and 5) sea surface mean square slope (mss). During hurricane flights the data products are transmitted in real-time from the NOAA WP-3D aircraft through a satellite data link to a ground station and on to the National Hurricane Center (NHC) for use by the forecasters for intensity projections and incorporation in hurricane wave models. The WSRA is the only instrument that can quickly provide high-density measurements of the complex wave topography over a large area surrounding the eye of the storm.

Open access
Hossein A. Kafiabad, Jacques Vanneste, and William R. Young

Abstract

Anticyclonic vortices focus and trap near-inertial waves so that near-inertial energy levels are elevated within the vortex core. Some aspects of this process, including the nonlinear modification of the vortex by the wave, are explained by the existence of trapped near-inertial eigenmodes. These vortex eigenmodes are easily excited by an initial wave with horizontal scale much larger than that of the vortex radius. We study this process using a wave-averaged model of near-inertial dynamics and compare its theoretical predictions with numerical solutions of the three-dimensional Boussinesq equations. In the linear approximation, the model predicts the eigenmode frequencies and spatial structures, and a near-inertial wave energy signature that is characterized by an approximately time-periodic, azimuthally invariant pattern. The wave-averaged model represents the nonlinear feedback of the waves on the vortex via a wave-induced contribution to the potential vorticity that is proportional to the Laplacian of the kinetic energy density of the waves. When this is taken into account, the modal frequency is predicted to increase linearly with the energy of the initial excitation. Both linear and nonlinear predictions agree convincingly with the Boussinesq results.

Open access
J. Thomas Farrar, Theodore Durland, Steven R. Jayne, and James F. Price

Abstract

Measurements from satellite altimetry are used to show that sea surface height (SSH) variability throughout much of the North Pacific Ocean is coherent with the SSH signal of the tropical instability waves (TIWs) that result from instabilities of the equatorial currents. This variability has regular phase patterns consistent with freely propagating barotropic Rossby waves radiating energy away from the unstable equatorial currents, and the waves clearly propagate from the equatorial region to at least 30°N. The pattern of SSH variance at TIW frequencies exhibits remarkable patchiness on scales of hundreds of kilometers, which we interpret as being due to the combined effects of wave reflection, refraction, and interference. North of 40°N, more than 6000 km from the unstable equatorial currents, the SSH field remains coherent with the near-equatorial SSH variability, but it is not as clear whether the variability at the higher latitudes is a simple result of barotropic wave radiation from the tropical instability waves. Even more distant regions, as far north as the Aleutian Islands off of Alaska and the Kamchatka Peninsula of eastern Russia, have SSH variability that is significantly coherent with the near-equatorial instabilities. The variability is not well represented in the widely used gridded SSH data product commonly referred to as the AVISO or DUACS product, and this appears to be a result of spatial variations in the filtering properties of the objective mapping scheme.

Open access