Amy F. Waterhouse

Global Patterns of Diapycnal Mixing from Measurements of the Turbulent Dissipation Rate

The authors present inferences of diapycnal diffusivity from a compilation of over 5200 microstructure profiles. As microstructure observations are sparse, these are supplemented with indirect measurements of mixingobtainedfrom(i)Thorpe-scaleoverturnsfrommooredprofilers,afinescaleparameterizationappliedto (ii) shipboard observations of upper-ocean shear, (iii) strain as measured by profiling floats, and (iv) shear and strainfromfull-depthloweredacousticDoppler currentprofilers (LADCP)andCTDprofiles. Verticalprofiles of the turbulent dissipation rate are bottom enhanced over rough topography and abrupt, isolated ridges. The geography of depth-integrated dissipation rate shows spatial variability related to internal wave generation, suggesting one direct energy pathway to turbulence. The global-averaged diapycnal diffusivity below 1000-m depth is O(10 24 )m 2 s 21 and above 1000-m depth is O(10 25 )m 2 s 21 . The compiled microstructure observations sample a wide range of internal wave power inputs and topographic roughness, providing a dataset with which to estimate a representative global-averaged dissipation rate and diffusivity. However, there is strong regional variabilityin theratiobetweenlocal internalwavegeneration and local dissipation.Insomeregions,the depthintegrateddissipationrateiscomparabletotheestimatedpowerinputintothelocalinternalwavefield.Inafew cases, more internal wave power is dissipated than locally generated, suggesting remote internal wave sources. However,atmostlocationsthetotalpowerlostthroughturbulentdissipationislessthantheinputintothelocal internal wave field. This suggests dissipation elsewhere, such as continental margins.

Estimating the Mean Diapycnal Mixing Using a Finescale Strain Parameterization

AbstractFinescale methods are currently being applied to estimate the mean turbulent dissipation rate and diffusivity on regional and global scales. This study evaluates finescale estimates derived from isopycnal strain by comparing them with average microstructure profiles from six diverse environments including the equator, above ridges, near seamounts, and in strong currents. The finescale strain estimates are derived from at least 10 nearby Argo profiles (generally <60 km distant) with no temporal restrictions, including measurements separated by seasons or decades. The absence of temporal limits is reasonable in these cases, since the authors find the dissipation rate is steady over seasonal time scales at the latitudes being considered (0°–30° and 40°–50°). In contrast, a seasonal cycle of a factor of 2–5 in the upper 1000 m is found under storm tracks (30°–40°) in both hemispheres. Agreement between the mean dissipation rate calculated using Argo profiles and mean from microstructure profiles is wi...

Tides in a System of Connected Estuaries

AbstractThe spatial structure of tidal amplitude and phase in a simplified system of connected estuaries, an idealized version of Florida’s Intracoastal Waterway, is analyzed with a linear analytical model. This model includes friction, the earth’s rotation, and variable bathymetry. It is driven at the connection with the ocean by a co-oscillating tide. Model results compare well with observations of pressure and currents in a section of the Intracoastal Waterway on the east coast of Florida. The comparison suggests that the waterway is highly frictional, causing the amplitude of the water elevation and tidal velocity to decrease away from the inlets to a minimum in the middle of the waterway. The local phase relationship between velocity and water elevation changed nonlinearly from 90° with no friction to 45° with maximum friction. In moderately to highly frictional basins, the phase lag was consistently less than 45°.

Climate Process Team on Internal Wave-Driven Ocean Mixing

Diapycnal mixing plays a primary role in the thermodynamic balance of the ocean and, consequently, in oceanic heat and carbon uptake and storage. Though observed mixing rates are on average consistent with values required by inverse models, recent attention has focused on the dramatic spatial variability, spanning several orders of magnitude, of mixing rates in both the upper and deep ocean. Away from ocean boundaries, the spatio-temporal patterns of mixing are largely driven by the geography of generation, propagation and dissipation of internal waves, which supply much of the power for turbulent mixing. Over the last five years and under the auspices of US CLIVAR, a NSF- and NOAA-supported Climate Process Team has been engaged in developing, implementing and testing dynamics-based parameterizations for internal-wave driven turbulent mixing in global ocean models. The work has primarily focused on turbulence 1) near sites of internal tide generation, 2) in the upper ocean related to wind-generated near inertial motions, 3) due to internal lee waves generated by low-frequency mesoscale flows over topography, and 4) at ocean margins. Here we review recent progress, describe the tools developed, and discuss future directions.

Internal tide convergence and mixing in a submarine canyon

AbstractObservations from Eel Canyon, located on the north coast of California, show that elevated turbulence in the full water column arises from the convergence of remotely generated internal wave energy. The incoming semidiurnal and bottom-trapped diurnal internal tides generate complex interference patterns. The semidiurnal internal tide sets up a partly standing wave within the canyon due to reflection at the canyon head, dissipating all of its energy within the canyon. Dissipation in the near bottom is associated with the diurnal trapped tide, while midwater isopycnal shear and strain is associated with the semidiurnal tide. Dissipation is elevated up to 600 m off the bottom, in contrast to observations over the flat continental shelf where dissipation occurs closer to the topography. Slope canyons are sinks for internal wave energy and may have important influences on the global distribution of tidally driven mixing.

ASIRI : an ocean–atmosphere initiative for Bay of Bengal

AbstractAir–Sea Interactions in the Northern Indian Ocean (ASIRI) is an international research effort (2013–17) aimed at understanding and quantifying coupled atmosphere–ocean dynamics of the Bay of Bengal (BoB) with relevance to Indian Ocean monsoons. Working collaboratively, more than 20 research institutions are acquiring field observations coupled with operational and high-resolution models to address scientific issues that have stymied the monsoon predictability. ASIRI combines new and mature observational technologies to resolve submesoscale to regional-scale currents and hydrophysical fields. These data reveal BoB’s sharp frontal features, submesoscale variability, low-salinity lenses and filaments, and shallow mixed layers, with relatively weak turbulent mixing. Observed physical features include energetic high-frequency internal waves in the southern BoB, energetic mesoscale and submesoscale features including an intrathermocline eddy in the central BoB, and a high-resolution view of the exchange...

Tidally Driven Processes Leading to Near-Field Turbulence in a Channel at the Crest of the Mendocino Escarpment

AbstractIn situ observations of tidally driven turbulence were obtained in a small channel that transects the crest of the Mendocino Ridge, a site of mixed (diurnal and semidiurnal) tides. Diurnal tides are subinertial at this latitude, and once per day a trapped tide leads to large flows through the channel giving rise to tidal excursion lengths comparable to the width of the ridge crest. During these times, energetic turbulence is observed in the channel, with overturns spanning almost half of the full water depth. A high-resolution, nonhydrostatic, 2.5-dimensional simulation is used to interpret the observations in terms of the advection of a breaking tidal lee wave that extends from the ridge crest to the surface and the subsequent development of a hydraulic jump on the flanks of the ridge. Modeled dissipation rates show that turbulence is strongest on the flanks of the ridge and that local dissipation accounts for 28% of the energy converted from the barotropic tide into baroclinic motion.

Submesoscale processes at shallow, salinity fronts in the Bay of Bengal: Observations during the winter monsoon

AbstractLateral submesoscale processes and their influence on vertical stratification at shallow salinity fronts in the central Bay of Bengal during the winter monsoon are explored using high-resol...

Measuring Ocean Turbulence

Ocean turbulence (and turbulence in general) tends to be tremendously intermittent, events often dominating average values. Or, put another way, the distribution of turbulence tends to be highly skewed, requiring significant systematic observations to capture the important dynamics that control time and space averages. It is thus imperative to link large-scale processes (macroscale) to turbulence energetics (microscale) to characterize the dynamics of a particular regime and to develop a quantitative understanding of the role of turbulence in ocean momentum and scalar budgets.

Observations of the Tasman Sea internal tide beam

AbstractLow-mode internal tides, a dominant part of the internal wave spectrum, carry energy over large distances, yet the ultimate fate of this energy is unknown. Internal tides in the Tasman Sea ...