Dan E. Kelley

Bio: Dan E. Kelley is an academic researcher from Dalhousie University. The author has contributed to research in topics: Convection & Internal wave. The author has an hindex of 21, co-authored 48 publications receiving 1654 citations. Previous affiliations of Dan E. Kelley include Woods Hole Oceanographic Institution.

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 ...

Fluxes through diffusive staircases: A new formulation

Abstract: This paper deals with two aspects of the flux law for diffusive convection: (1) the dependence on density ratio, and (2) the dependence on ΔT. Empirical formulations of the dependence of temperature and salinity fluxes on density ratio are developed using published measurements. At high density ratios (Rρ > 4) our temperature fluxes agree with those given by Marmorino and Caldwell's (1976) formulation, but at low density ratios (Rρ < 2.5) our temperature fluxes are smaller by half. However, it is suggested that any formulation of the 4/3 flux law should be used with caution. A simple convection theory implies that fluxes are not proportional to ΔT4/3, as the 4/3 law states, but rather are proportional to ΔT raised to an power closer to 5/4. The theory accurately matches measurements of velocity and heat flux in thermal convection. There are insufficient data to test (or even fully formulate) the theory for diffusive convection, but if the thermal convection results carry over to the diffusive case, then the 4/3 flux law overestimates oceanic fluxes by up to about 30%.

Effective diffusivities within oceanic thermohaline staircases

Abstract: A scale for the thickness of layers in regular “diffusive”-type thermohaline staircases, derived from dimensional analysis, is found to collapse oceanic data. Combining this scale with laboratory-derived double-diffusive flux laws, we formulate effective diffusivities for salt, heat, and density. The diffusivities depend on the Turner number Rρ, but are independent of the Brunt-Vaisala frequency. For Rρ near 1 the diffusivities for salt and heat are approximately equal (≃10−4 m2 s−1). They decrease roughly as Rρ−4 and Rρ−2, respectively, over the oceanic range 1≤Rρ≤10.

Convection in ice-covered lakes: effects on algal suspension

Abstract: Convection occurs in ice-covered lakes if solar radiation warms near-surface water from the freezing point towards the temperature of maximal density. One effect of convective mixing may be to suspend non-motile phytoplankton in the upper water column, providing cells with enough light for growth during ice-covered periods. Observations of the diatom Aulacoseira baicalensis under the ice cover of Lake Baikal, Siberia, support the hypothesis that convective mixing causes net suspen- sion of cells. This paper presents a theoretical examination of the conditions under which convective flow fields can suspend algae in the photic zone of the upper water column. It is shown that the efficiency of algal suspension depends on the ratio of the still-water algal sinking rate, Wp, to con- vective updraft speed, Wu. The suspension efficiency is also shown to be affected by asymmetries in the flow field and night-time cessation of convection, but only if Wp and Wu are comparable in value. It is concluded that convection in Lake Baikal should be vigorous enough to increase the mixed-layer residence time of A.baicalensis from a few days to over a month, at least during years with thin snow

Oceanic vertical mixing: A review and a model with a nonlocal boundary layer parameterization

Abstract: If model parameterizations of unresolved physics, such as the variety of upper ocean mixing processes, are to hold over the large range of time and space scales of importance to climate, they must be strongly physically based. Observations, theories, and models of oceanic vertical mixing are surveyed. Two distinct regimes are identified: ocean mixing in the boundary layer near the surface under a variety of surface forcing conditions (stabilizing, destabilizing, and wind driven), and mixing in the ocean interior due to internal waves, shear instability, and double diffusion (arising from the different molecular diffusion rates of heat and salt). Mixing schemes commonly applied to the upper ocean are shown not to contain some potentially important boundary layer physics. Therefore a new parameterization of oceanic boundary layer mixing is developed to accommodate some of this physics. It includes a scheme for determining the boundary layer depth h, where the turbulent contribution to the vertical shear of a bulk Richardson number is parameterized. Expressions for diffusivity and nonlocal transport throughout the boundary layer are given. The diffusivity is formulated to agree with similarity theory of turbulence in the surface layer and is subject to the conditions that both it and its vertical gradient match the interior values at h. This nonlocal “K profile parameterization” (KPP) is then verified and compared to alternatives, including its atmospheric counterparts. Its most important feature is shown to be the capability of the boundary layer to penetrate well into a stable thermocline in both convective and wind-driven situations. The diffusivities of the aforementioned three interior mixing processes are modeled as constants, functions of a gradient Richardson number (a measure of the relative importance of stratification to destabilizing shear), and functions of the double-diffusion density ratio, Rρ. Oceanic simulations of convective penetration, wind deepening, and diurnal cycling are used to determine appropriate values for various model parameters as weak functions of vertical resolution. Annual cycle simulations at ocean weather station Papa for 1961 and 1969–1974 are used to test the complete suite of parameterizations. Model and observed temperatures at all depths are shown to agree very well into September, after which systematic advective cooling in the ocean produces expected differences. It is argued that this cooling and a steady salt advection into the model are needed to balance the net annual surface heating and freshwater input. With these advections, good multiyear simulations of temperature and salinity can be achieved. These results and KPP simulations of the diurnal cycle at the Long-Term Upper Ocean Study (LOTUS) site are compared with the results of other models. It is demonstrated that the KPP model exchanges properties between the mixed layer and thermocline in a manner consistent with observations, and at least as well or better than alternatives.

Open-ocean convection: Observations, theory, and models

Abstract: We review what is known about the convective process in the open ocean, in which the properties of large volumes of water are changed by intermittent, deep-reaching convection, triggered by winter storms. Observational, laboratory, and modeling studies reveal a fascinating and complex interplay of convective and geostrophic scales, the large-scale circulation of the ocean, and the prevailing meteorology. Two aspects make ocean convection interesting from a theoretical point of view. First, the timescales of the convective process in the ocean are sufficiently long that it may be modified by the Earth's rotation; second, the convective process is localized in space so that vertical buoyancy transfer by upright convection can give way to slantwise transfer by baroclinic instability. Moreover, the convective and geostrophic scales are not very disparate from one another. Detailed observations of the process in the Labrador, Greenland, and Mediterranean Seas are described, which were made possible by new observing technology. When interpreted in terms of underlying dynamics and theory and the context provided by laboratory and numerical experiments of rotating convection, great progress in our description and understanding of the processes at work is being made.

Iron cycling and nutrient-limitation patterns in surface waters of the World Ocean

Abstract: A global marine ecosystem mixed-layer model is used to study iron cycling and nutrient-limitation patterns in surface waters of the world ocean. The ecosystem model has a small phytoplankton size class whose growth can be limited by N, P, Fe, and/or light, a diatom class which can also be Si-limited, and a diazotroph phytoplankton class whose growth rates can be limited by P, Fe, and/or light levels. The model also includes a parameterization of calcification by phytoplankton and is described in detail by Moore et al. (Deep-Sea Res. II, 2002). The model reproduces the observed high nitrate, low chlorophyll (HNLC) conditions in the Southern Ocean, subarctic Northeast Pacific, and equatorial Pacific, and realistic global patterns of primary production, biogenic silica production, nitrogen fixation, particulate organic carbon export, calcium carbonate export, and surface chlorophyll concentrations. Phytoplankton cellular Fe/C ratios and surface layer dissolved iron concentrations are also in general agreement with the limited field data. Primary production, community structure, and the sinking carbon flux are quite sensitive to large variations in the atmospheric iron source, particularly in the HNLC regions, supporting the Iron Hypothesis of Martin (Paleoceanography 5 (1990) 1–13). Nitrogen fixation is also strongly influenced by atmospheric iron deposition. Nitrogen limits phytoplankton growth rates over less than half of the world ocean during summer months. Export of biogenic carbon is dominated by the sinking particulate flux, but detrainment and turbulent mixing account for 30% of global carbon export. Our results, in conjunction with other recent studies, suggest the familiar paradigm that nitrate inputs to the surface layer can be equated with particulate carbon export needs to be expanded to include multiple limiting nutrients and modes of export.

Climate change and the future for coral reef fishes

Abstract: Climate change will impact coral-reef fishes through effects on individual performance, trophic linkages, recruitment dynamics, population connectivity and other ecosystem processes. The most immediate impacts will be a loss of diversity and changes to fish community composition as a result of coral bleaching. Coral-dependent fishes suffer the most rapid population declines as coral is lost; however, many other species will exhibit long-term declines due to loss of settlement habitat and erosion of habitat structural complexity. Increased ocean temperature will affect the physiological performance and behaviour of coral reef fishes, especially during their early life history. Small temperature increases might favour larval development, but this could be counteracted by negative effects on adult reproduction. Already variable recruitment will become even more unpredictable. This will make optimal harvest strategies for coral reef fisheries more difficult to determine and populations more susceptible to overfishing. A substantial number of species could exhibit range shifts, with implications for extinction risk of small-range species near the margins of reef development. There are critical gaps in our knowledge of how climate change will affect tropical marine fishes. Predictions are often based on temperate examples, which may be inappropriate for tropical species. Improved projections of how ocean currents and primary productivity will change are needed to better predict how reef fish population dynamics and connectivity patterns will change. Finally, the potential for adaptation to climate change needs more attention. Many coral reef fishes have geographical ranges spanning a wide temperature gradient and some have short generation times. These characteristics are conducive to acclimation or local adaptation to climate change and provide hope that the more resilient species will persist if immediate action is taken to stabilize Earth’s climate.

Double diffusion in oceanography

Abstract: The modern study of double-diffusive convection began with Melvin Stern's article on "The Salt Fountain and Thermohaline Convection" in 1960. In that paper, he showed how opposing stratifications of two component species could drive convection if their diffusivities differed. Stommel ct al (1956) had earlier noted that there was significant potential energy available in the decrease of salinity with depth found in much of the tropical and subtropical ocean. While they suggested that a flow (the salt fountain) would be driven in a thermally-conducting pipe, it was Stern who realized that the two orders of magnitude difference in heat and salt diffusivities allowed the ocean to form its own pipes. These later came to be known as "salt fingers." Stern also identified the potential for the oscillatory instability when cold, fresh water overlies warm, salty water in the 1960 paper, though only in a footnote. Turner & Stommel (1964) demonstrated the "diffusive-convection" process a few years later. From these beginnings in oceanography over three decades ago, double diffusion has come to be recognized as an important convection process in a wide variety of fluid media, including magmas, metals, and stellar interiors (Schmitt 1983, Turner 1985). However, it is interesting to note that about one hundred years before Stern's paper, W. S. Jevons (1857) reported on the observation of long, narrow convection cells formed when warm, salty water was introduced over cold, fresh water. He correctly attributed the phenomenon to a difference in the diffusivities for heat and