Caitlin B. Whalen

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...

Impact of Parameterized Internal Wave Drag on the Semidiurnal Energy Balance in a Global Ocean Circulation Model

AbstractThe effects of a parameterized linear internal wave drag on the semidiurnal barotropic and baroclinic energetics of a realistically forced, three-dimensional global ocean model are analyzed. Although the main purpose of the parameterization is to improve the surface tides, it also influences the internal tides. The relatively coarse resolution of the model of ~8 km only permits the generation and propagation of the first three vertical modes. Hence, this wave drag parameterization represents the energy conversion to and the subsequent breaking of the unresolved high modes. The total tidal energy input and the spatial distribution of the barotropic energy loss agree with the Ocean Topography Experiment (TOPEX)/Poseidon (TPXO) tidal inversion model. The wave drag overestimates the high-mode conversion at ocean ridges as measured against regional high-resolution models. The wave drag also damps the low-mode internal tides as they propagate away from their generation sites. Hence, it can be considered...

A new characterization of the turbulent diapycnal diffusivities of mass and momentum in the ocean

The diapycnal diffusivity of mass supported by turbulent events in the ocean interior plays a fundamental role in controlling the global overturning circulation. The conventional representation of this diffusivity, due to Osborn (1980), assumes a constant mixing efficiency. We replace this methodology by a generalized-Osborn formula which involves a mixing efficiency that varies nonmonotonically with at least two nondimensional variables. Using these two variables, we propose dynamic parameterizations for mixing efficiency and turbulent Prandtl number (the latter quantifies the ratio of momentum to mass diapycnal diffusivities) based on the first synthesis of an extensive direct numerical simulation of inhomogeneously stratified shear-induced turbulence. Data from Argo floats are employed to demonstrate the extent of the spatial and statistical variability to be expected in both the diapycnal diffusivities of mass and momentum. We therefore suggest that previous estimates of these important characteristics of the global ocean require reconsideration.

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.

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...

Large-scale impacts of the mesoscale environment on mixing from wind-driven internal waves

Oceanic mesoscale structures such as eddies and fronts can alter the propagation, breaking and subsequent turbulent mixing of wind-generated internal waves. However, it has been difficult to ascertain whether these processes affect the global-scale patterns, timing and magnitude of turbulent mixing, thereby powering the global oceanic overturning circulation and driving the transport of heat and dissolved gases. Here we present global evidence demonstrating that mesoscale features can significantly enhance turbulent mixing due to wind-generated internal waves. Using internal wave-driven mixing estimates calculated from Argo profiling floats between 30° and 45° N, we find that both the amplitude of the seasonal cycle of turbulent mixing and the response to increases in the wind energy flux are larger to a depth of at least 2,000 m in the presence of a strong and temporally uniform field of mesoscale eddy kinetic energy. Mixing is especially strong within energetic anticyclonic mesoscale features compared to cyclonic features, indicating that local modification of wind-driven internal waves is probably one mechanism contributing to the elevated mixing observed in energetic mesoscale environments.Mesoscale ocean features can alter the magnitude of turbulent mixing caused by wind-driven internal waves, an analysis of Argo float and model data suggests.