Dan E. Kelley

Identifying Overturns in CTD Profiles

Abstract The authors propose a scheme to test whether inversions in CTD density profiles are caused by overturning motions (from which mixing rates may be inferred) or by measurement noise. Following a common practice, possible overturning regions are found by comparing the observed profile ρ(z) and an imaginary profile ˆρ(z) constructed by reordering ρ(z) to make. it gravitationally stable. The resulting “reordering regions” are subjected to two tests. • The “Thorpe fluctuation” profile ρ′(z) = ρ(z) − ˆρ(z) is examined for “runs” of adjacent positive or negative values. The probability density function (PDF) of the run length is compared with the corresponding PDF of random noise. This yields a threshold value for rms run length within individual reordering regions that must be exceeded for adequate resolution of overturns, taking into account both CTD characteristics and local hydrographic properties. • Temperature and salinity covariations with respect to density are screened for systematic CTD errors ...

The diffusive regime of double-diffusive convection

The diffusive regime of double-diffusive convection is discussed, with a particular focus on unresolved issues that are holding up the development of large-scale parameterizations. Some of these issues, such as interfacial transports and layer-interface interactions, may be studied in isolation. Laboratory work should help with these. However, we must also face more difficult matters that relate to oceanic phenomena that are not represented easily in the laboratory. These lie beneath some fundamental questions about how double-diffusive structures are formed in the ocean, and how they evolve in the competitive ocean environment.

Effect of Baroclinicity on Double-Diffusive Interleaving

Although ocean fronts are often baroclinic, existing models of double-diffusive interleaving have ignored such baroclinic effects as velocity shear and horizontal density gradients. To determine the importance of these effects, the authors have formulated a linear instability analysis applicable to baroclinic fronts. Two limiting cases are considered: one for fronts with strong vertical and/or horizontal shear, the other for fronts with weak shear. In both limits, double-diffusive interleaving can be enhanced or suppressed by baroclinicity. Interleaving motion is enhanced if isopycnals rise toward the fresh side of the front. Conversely, interleaving is suppressed if isopycnals slope downward across the front. A significant result is that the salinity gradient along isopycnals is not a good indicator of interleaving strength. As an example, the model is applied to a Mediterranean salt lens. The effect of baroclinicity is significant: the predicted growth rates are increased by 35%‐90%. The large-scale velocity and hydrographic fields indicate that Meddy Sharon lies somewhere between the high- and low-shear limits. Nevertheless, the model predictions agree reasonably well with the observed interleaving characteristics.

Inferred influence of nutrient availability on the relationship between Sun-induced chlorophyll fluorescence and incident irradiance in the Bering Sea

[1] This study examines variability in the relationship between Sun-induced chlorophyll fluorescence and incident solar irradiance as a potential diagnostic of the nutritional status of phytoplankton. The study site is the Bering Sea, where two optical drifters were caught for more than 100 days in an anticyclonic eddy, while two others provided data from adjacent waters. Estimates of fluorescence emission normalized to the absorption of light by pigments were analyzed as a function of irradiance to describe variability of the quantum yield of fluorescence. Yields in bright sunlight and under lower light varied by a factor of 5 or more on the scale of days to weeks. For the one drifter that remained in the high-velocity region of the eddy, there was a lagged correlation between the eddy rotation period and fluorescence parameters, with higher fluorescence yields in both low and high irradiance associated with slower rotation. Since nutrient input to the photic zone may increase with increasing shear of the eddy flow, this is consistent with the established suggestion that Sun-induced fluorescence increases with nutrient stress in phytoplankton. Independent measurements of variable fluorescence (Fv/Fm, an indicator of photosynthetic efficiency) further support this interpretation. However, modeling shows that the established hypothesis of competition between photosynthesis and fluorescence for absorbed photons (i.e., photochemical quenching), with high fluorescence yields reflecting photosynthetic debility, does not apply near the sea surface, where photosynthesis is saturated, and dissipation of excess absorbed radiation by nonphotochemical quenching is the dominant influence on fluorescence yield.

Wave-induced boundary mixing in a partially mixed estuary

We present observations that reveal the existence of horizontally propagating, tidally-driven, high-frequency internal wave (IW) packets in a channel of the partially mixed St. Lawrence Estuary. The packets propagate transversely to the channel axis and collide with the shoaling lateral boundaries. The structure and energy of IWs are diagnosed with a two-dimensional, nonlinear nonhydrostatic model, and the results are compared with weakly nonlinear Korteweg-de-Vries (KdV) theory. The behavior of IWs running into the shoaling lateral boundary is examined in terms of published laboratory and numerical experiments. Our analysis indicates that IWs break on the slope, during which 6% of their energy is converted into potential energy through vertical mixing. The corresponding buoyancy flux, when averaged over the surf zone and the time of the mixing event, is more than an order of magnitude larger than values predicted by a published non-IW-resolving three-dimensional (3D) baroclinic circulation model of the region. Even averaging across the full domain and tidal period yields mixing rates that are a significant proportion of those in the 3D circulation model. These indirect inferences suggest that wave-induced boundary mixing may be of general significance in partially mixed estuaries.

On the Reflectance of Uniform Slopes for Normally Incident Interfacial Solitary Waves

Abstract The collision of interfacial solitary waves with sloping boundaries may provide an important energy source for mixing in coastal waters. Collision energetics have been studied in the laboratory for the idealized case of normal incidence upon uniform slopes. Before these results can be recast into an ocean parameterization, contradictory laboratory findings must be addressed, as must the possibility of a bias owing to laboratory sidewall effects. As a first step, the authors have revisited the laboratory results in the context of numerical simulations performed with a nonhydrostatic laterally averaged model. It is shown that the simulations and the laboratory measurements match closely, but only for simulations that incorporate sidewall friction. More laboratory measurements are called for, but in the meantime the numerical simulations done without sidewall friction suggest a tentative parameterization of the reflectance of interfacial solitary waves upon impact with uniform slopes.

Growth and steady state stages of thermohaline intrusions in the Arctic Ocean

Thermohaline intrusions are a widespread feature of the Arctic Ocean pycnocline and may be important for the lateral transport of heat, salt, and other tracers. In an attempt to understand the dynamics of the intrusions, we present an analysis of hydrographic observations of a front north of Svalbard. Following intrusions from profile to profile, we find that they slope upward toward the cold, fresh side of the front. However, their slope is less than that of the background isopycnals, so that the intrusions slope between horizontal and isopycnal surfaces in the “wedge” of baroclinic instability. Our analysis proceeds in two stages. First, to determine what might have caused the initial growth of the intrusions, we compare the observed intrusion properties with predictions of an instability theory. The results suggest that the intrusions developed as a form of double-diffusive interleaving, with fluxes dominated by salt fingering and additional forcing by baroclinicity. Second, to investigate the observed finite amplitude interleaving, we apply a steady state model to the observations. A key result is that diffusive convection, not salt fingering, must be the dominant form of double diffusion in order to reach steady state. This fundamental change in dynamics is discussed in the context of adjustment from the growth stage to an eventual steady state.

A Laterally Averaged Nonhydrostatic Ocean Model

A laterally averaged nonhydrostatic model for stratified flow in dynamically narrow domains is presented. Averaging laterally yields the computational efficiency of a two-dimensional model, while retaining some effects associated with variable domain width, such as flow acceleration through contracting channels. The model may be run in both hydrostatic and nonhydrostatic modes, and in the latter case it converges rapidly if the flow is approximately hydrostatic. The model’s strengths and weaknesses are illustrated with a series of test cases of increasing complexity. Side-by-side comparisons with laboratory observations show the ability of the model to simulate the structures of nonhydrostatic flows, including shear instabilities and overturning internal waves, with discrepancies becoming apparent mainly for transition to three-dimensional turbulence. Similar results are demonstrated in an application to the stratified sill flow in Knight Inlet, British Columbia. The model reproduces nonhydrostatic features thought to be dynamically important to this system, including the generation of largeamplitude lee waves and shear instabilities.

A Basinwide Estimate of Vertical Mixing in the Upper Pycnocline: Spreading of Bomb Tritium in the North Pacific Ocean

Abstract The vertical diffusivity KV in the upper half-kilometer of the North Pacific subtropical pycnocline is estimated from observations of the spreading rate of anthropogenic tritium. The calculation is based on approximately 300 ocean tritium profiles made since tritium was introduced to the atmosphere via bomb testing in the 1960s. The data coverage does not permit detailed mapping of tritium penetration, especially in the sparsely sampled western Pacific. For this reason, and to minimize advective effects, the spreading rate is averaged within the closed streamlines of the subtropical gyre spanning ∼5°N to ∼40°N. The result, KV = (1.5 ± 0.7) × 10−5 m2 s−1, is consistent with inferences from microstructure and purposefully released tracer measurements in the North Atlantic, confirming that the spatially averaged rate of mixing in the upper pycnocline is substantially lower than the canonical Munk estimate for the lower pycnocline.

Contrasting the interleaving in two baroclinic ocean fronts

To investigate the effects of baroclinicity on frontal interleaving, we contrast the interleaving characteristics of two fronts, one in the Arctic Ocean and the other surrounding a Mediterranean salt lens (Meddy). The Meddy is broken into two parts based on the vertical temperature and salinity structure, so our comparison involves three sets of interleaving observations. The cross-front slopes of intrusions relative to horizontal and isopycnal surfaces are taken to be key diagnostics of the interleaving dynamics. Assuming the observed slopes match those that were present during the initial growth of interleaving, we use an instability theory to infer the dominant form of double diffusion that was active during the growth stage. Then, to investigate the observed interleaving, we use a steady-state model to infer the dominant form of double diffusion at the time of observation. In the Arctic Ocean front, it appears that different forms of double diffusion dominated the two stages of interleaving (salt fingering during the growth stage and diffusive convection at steady state). In contrast, in the Meddy, the same form of double diffusion appears to have dominated both stages of interleaving (salt fingering in the lower part of the Meddy, diffusive convection in the upper part). In the Arctic Ocean front, the observations suggest that interleaving was driven by baroclinicity as well as double diffusion. In both parts of the Meddy, however, driving was by double diffusion only. Motivated by these differences, we suggest a new intrusion classification scheme, based on the slopes of intrusions relative to horizontal and isopycnal surfaces.