A document by Ratnaksha Lele. Click on the document to view its contents.

Turbulent mixing in the ocean is often parameterized in terms of the downscale energy transfer by internal waves. Expressed in terms of the vertical wavenumber spectrum of oceanic velocity shear ($V^2_z $) and isopycnal strain ($ \zeta^2_z $), the “finescale parameterization” relies on several parameters, including key assumptions relating to the spectral properties. Here we use an unsupervised learning model to identify spatial correlations between embedded parameters of the finescale parameterization based upon data from 1875 full-depth hydrographic profiles from 15 sections traversing the global ocean. The clustered patterns along the sections have marked horizontal and vertical spatial dependence associated with distinct modes of spectral variation. Two clustered regions are identified where the underlying spectra deviate significantly from the canonical Garrett-Munk (GM) spectrum, suggesting potential departures from implicit assumptions about the downscale energy cascade. Spectral composites in these two regions show intensification of variance in the low and high wavenumber regimes respectively, as well as distinction in overall spectral levels and geographic prevalence. Furthermore, these clusters are found to be associated with regions where parameterized estimates of the turbulent dissipation rate $\epsilon$ differ significantly (exceeding a factor of 5) from co-located in-situ observations measured using $\chi$-pod temperature microstructure. Extending the methodology to other hydrographic datasets has the potential to reveal reasons for this parameterization bias and to identify the dynamical underpinnings leading to more robust parameterizations of oceanic turbulent mixing.ere

Oceanic transient tracers, such as chlorofluorocarbons (CFCs) and sulfur-hexafluoride (SF6), trace the propagation of intermediate-to-abyssal water masses in the ocean interior. Their temporal and spatial sparsity, however, has limited their utility in quantifying the global ocean circulation and its decadal variability. The Time-Correction Method presented here is a new approach to leverage the available CFCs and SF6 observations to solve for the Green’s functions describing the steady-state transport from the surface to the ocean interior. From the Green’s functions, we reconstruct global tracer concentrations (and associated uncertainties) in the ocean interior at annual resolution (1940 to 2021). The spatial resolution includes 50 neutral density levels that span the water column along WOCE/GO-SHIP lines. The reconstructed tracer concentrations return a global view of CFCs and SF6 spreading into new regions of the interior ocean, such as the deep north-western Pacific. For example, they capture the southward spreading and equatorial recirculation of distinct NADW components, and the spreading of CFC-rich AABW out of the Southern Ocean and into the North Pacific, East Indian, and West Atlantic. The reconstructed tracer concentrations fit the data in most locations (~75%), indicating that a steady-state circulation holds for the most part. Discrepancies between the reconstructed and observed concentrations offer insight into ventilation rate changes on decadal timescales. As an example, we infer decadal changes in Subantartic Mode Water (SAMW) and find an increase in SAMW ventilation from 1992 to 2014, highlighting the skill of the time-correction method in leveraging the sparse tracer observations.

Serendipitous measurements of deep internal wave signatures are evident in variations in the descent rates of certain Deep Argo floats (Deep SOLO models), which oscillate around a slow decrease with increasing pressure (and density). Averaged from 1000 dbar to the seafloor (using 10070 profiles that extend to at least 3000 dbar, and sometimes as deep as 6000 dbar) the mean of vertical velocity variances corresponds to a sinusoidal wave amplitude of about 0.007 dbar s-1. The distribution of variances is skewed towards larger values. They also exhibit notable regional variations among and within some deep ocean basins, with generally larger variances in regions of rougher topography or stronger deep currents. Dominant vertical wavelengths estimated from Morlet wavelet transform power spectra range from 393 to 1572 dbar, most frequently 786 and 935 dbar. Vertical wavelengths are weakly anticorrelated with bathymetric roughness, expectedly since shorter wavelengths should be found near generation regions.

As abyssal ocean properties are altered by climate change, density stratification may be expected to change in response. This shift can affect the buoyancy flux, internal wave generation, and turbulent dissipation, which may impact mixing and vertical transport. In this study, repeated surveys of three hydrographic sections in the Southwest Pacific Basin between the 1990s-2010s are used to estimate the change in buoyancy frequency N. We find that below Θ = 0.8◦C, N is reduced by a mean scaling factor of 0.88{plus minus}0.06 per decade. This reduction is intensified at depth, with the biggest change observed at Θ=0.63◦C by a scaling factor of 0.71{plus minus}0.07. Within the same time period, the magnitude of per unit area vertical diffusive heat flux is reduced by about 0.01 Wm, although this estimate is sensitive to the choice of estimated diffusivity. Finally, implications on heat budget and global ocean circulation are qualitatively discussed.