Diapycnal Transport Research Articles

The Origin and Fate of Antarctic Intermediate Water in the Southern Ocean

Abstract Using observationally based hydrographic and eddy diffusivity datasets, a volume budget analysis is performed to identify the main mechanisms governing the spatial and seasonal variability of Antarctic Intermediate Water (AAIW) within the density range γn = (27.25–27.7) kg m−3 in the Southern Ocean. The subduction rates and water mass transformation rates by mesoscale and small-scale turbulent mixing are estimated. First, Ekman pumping upwells the dense variety of AAIW into the mixed layer south of the Polar Front, which can be advected northward by Ekman transport into the subduction regions of lighter-variety AAIW and Subantarctic Mode Water (SAMW). The subduction of light AAIW occurs mainly by lateral advection in the southeast Pacific and Drake Passage as well as eddy-induced flow between the Subantarctic and Polar Fronts. The circumpolar-integrated total subduction yields from −5 to 19 Sv (1 Sv ≡ 106 m3 s−1) of AAIW volume loss. Second, the diapycnal transport from subducted SAMW into the AAIW layer is predominantly by mesoscale mixing (2–13 Sv) near the Subantarctic Front and vertical mixing in the South Pacific, while AAIW is further replenished by transformation from Upper Circumpolar Deep Water by vertical mixing (1–10 Sv). Last, 3–14 Sv of AAIW are exported out of the Southern Ocean. Our results suggest that the distribution of AAIW is set by its formation due to subduction and mixing, and its circulation eastward along the ACC and northward into the subtropical gyres. The volume budget analysis reveals strong seasonal variability in the rate of subduction, vertical mixing, and volume transport driving volume change within the AAIW layer. The nonzero volume budget residual suggests that more observations are needed to better constrain the estimate of geostrophic flow and mesoscale and small-scale mixing diffusivities.

The Role of Eddies in the Zonal and Meridional Overturning Circulations of Buoyancy-Forced Basins

AbstractThe zonal and meridional overturning circulations of buoyancy-forced basins are studied in an eddy-resolving model. The zonal overturning circulation (ZOC) is driven by the meridional gradient of buoyancy at the surface and stratification at the southern boundary. The ZOC, in turn, produces zonal buoyancy gradients through upwelling and downwelling at the western and eastern boundaries, respectively. The meridional overturning circulation (MOC) is driven by these zonal gradients rather than being directly driven by meridional gradients. Eddies lead to a broadening of the upwelling and downwelling limbs of the ZOC, as well as a decoupling of the locations of vertical and diapycnal transport. This broadening is more prominent on the eastern boundary, where westward-moving eddies transport warm water away from a poleward-flowing eastern boundary current. Most of the diapycnal downwelling occurs in the “swash zone”—the region where the isopycnals intermittently come in contact with the surface and lose buoyancy to the atmosphere. A scaling for the overturning circulations, which depends on the background stratification and the surface buoyancy gradient, is derived and found to be an excellent fit to the numerical experiments.

Estimate of turbulent energy dissipation rate using free-fall and CTD-attached fast-response thermistors in weak ocean turbulence

The measurement of turbulence is necessary to quantify the vertical, diapycnal transport of heat, water and substances influencing climate, nutrient supply and marine ecosystems. As specialist instrumentation and ship-time are required to conduct microstructure measurements to quantify turbulence intensity, there is a need for more inexpensive and easy measurement methods. This study demonstrated that the turbulent energy dissipation rate, ε, estimated from fast-response thermistors Fastip Probe model 07 (FP07) with the depth-average of a > 10 m depth interval well agreed with those from current shear probes to a range of 10–11 W/kg (m2s−3) in the two casts of the most accurate and stable free-fall vertical microstructure profiler, VMP6000 in the Oyashio water. This range cannot be measured with velocity shear probes equipped in smaller profilers in which the lower limit of ε > O (10–10) W/kg. These results extend turbulence measurements using the FP07 to 10–11 W/kg. They may be especially useful for turbulence observations in deep oceans where ε is generally weak (< 10–10 W/kg). As FP07 are much less sensitive to instrument vibrations than current shear and may be attached to various observational platforms such as temperature-conductivity-depth (CTD) profilers and floats. The CTD-attached FP07 observations near the VMP6000 profiles demonstrated their capabilities in the ε range of 10–11–10–8 W/kg by data screening using a Wsd>0.1(W-0.3)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${W}_{\\mathrm{sd}}>0.1(W-0.3)$$\\end{document} criterion (1 s mean lowering rate W\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$W$$\\end{document} m/s and its standard deviation Wsd\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${W}_{\\mathrm{sd}}$$\\end{document}) under rough conditions where the cast-mean Wsd>\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${W}_{\\mathrm{sd}}>$$\\end{document} 0.07 m/s and the standard deviation of Wsd\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${W}_{\\mathrm{sd}}$$\\end{document} in each cast σ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sigma$$\\end{document} >0.05 m/s.

Diapycnal Transport near a Sloping Bottom Boundary

AbstractThe diapycnal motion in the stratified ocean near a sloping bottom boundary is studied using analytical solutions from one-dimensional boundary layer theory. Bottom-intensification of the diapycnal mixing intensity ensures that in the stratified mixing layer (SML), where isopycnals are relatively flat, the diapycnal motion is downward toward denser fluid. In contrast, convergence of the diffusive buoyancy flux near the seafloor drives diapycnal upwelling in what we define as the bottom boundary layer (BBL). Much of the one-dimensional BBL is characterized by a stratification only slightly reduced from that in the SML because the maximum in the buoyancy flux at the top of the BBL, where the diapycnal velocity changes sign, must occur in well-stratified fluid. The diapycnal upwelling in the BBL is determined by variations not only in the magnitude of the buoyancy gradient but also in the curvature of isopycnals. The net diapycnal upwelling is concentrated in the bottom half of the BBL where the magnitude of the buoyancy gradient changes most rapidly. The curvature effect drives upwelling near the seafloor that only makes a significant contribution to the net upwelling for steep slopes. The structure of the diapycnal velocity in this stratified BBL differs from the case of a turbulent well-mixed BBL that has been assumed in some recent theoretical studies on bottom-intensified mixing. This work therefore extends recent theories in a way that should be more applicable to abyssal ocean observations where well-mixed BBLs are not common.

Abyssal Pathways and the Double Silica Maximum in the Northeast Pacific Basin

AbstractThis study examines causes of the double silica maximum in the deep interior Northeast Pacific Basin using a stochastic Lagrangian tracer model based on steady‐state advective fields and diapycnal diffusion established by a hydrographic inverse method that conserves potential vorticity and salinity. Lateral diffusion, unresolved by the inverse model, is adjusted for overall agreement with radiocarbon distribution. The double silica maximum in vertical profiles arises from an eastern‐intensified single maximum in the North Pacific Deep Water along the northern domain boundary (originating in the western Pacific), and a strong subarctic bottom source supplying silica to Upper Circumpolar Deep Water density surfaces that successively intersect the seafloor over a broad area east of 150°W, associated geostrophically with southward flow. The existence of the double silica maximum requires weak diapycnal transport in the deep interior, with broader implications for the conceptual picture of meridional overturning circulation in the North Pacific.

Diapycnal mixing of passive tracers by Kelvin–Helmholtz instabilities

Abstract

Observations of the warm-tongue circulation in the northern East China Sea

A subsurface thermohaline front semi-permanently formed in association with near-bottom cyclonic circulation in the northern East China Sea was newly found from detailed hydrographic data collected during two cruises in February 2017 (winter) and April 2018 (spring) along with supplementary satellite remote sensing and historical hydrographic data. An alternate intruding frontal structure in water properties was observed across the cyclonic circulation in both seasons as formed by two contrasting water masses—low-temperature and low-salinity (i.e., low spiciness) water transported by the East China Sea Current and high-temperature and high-salinity (i.e., high spiciness) water transported by the Tsushima Warm Current. Consistent structures were confirmed from current observations during the two cruises, historical hydrographic observations, and satellite altimetry-derived sea surface height and surface frontal structure, indicative of retroflection of the Cheju Warm Current as deemed by the seasonal development of thermal stratification in spring. Our results reveal significant heat and material exchanges between the open Pacific and the broad shelf, particularly via diapycnal mixing and cross-front transports associated with across-front flow and cyclonic circulation, in the northern East China Sea.

The Atlantic Subtropical Cells Inferred from Observations

AbstractThe Atlantic Subtropical Cells (STCs) are shallow wind‐driven overturning circulations connecting the tropical upwelling areas to the subtropical subduction regions. In both hemispheres, they are characterized by equatorward transport at thermocline level, upwelling at the equator, and poleward Ekman transport in the surface layer. This study uses recent data from Argo floats complemented by ship sections at the western boundary as well as reanalysis products to estimate the meridional water mass transports and to investigate the vertical and horizontal structure of the STCs from an observational perspective. The seasonally varying depth of meridional velocity reversal is used as the interface between the surface poleward flow and the thermocline equatorward flow. The latter is bounded by the 26.0 kg m−3 isopycnal at depth. We find that the thermocline layer convergence is dominated by the southern hemisphere water mass transport (9.0 ± 1.1 Sv from the southern hemisphere compared to 2.9 ± 1.3 Sv from the northern hemisphere) and that this transport is mostly confined to the western boundary. Compared to the asymmetric convergence at thermocline level, the wind‐driven Ekman divergence in the surface layer is more symmetric, being 20.4 ± 3.1 Sv between 10°N and 10°S. The net poleward transports (Ekman minus geostrophy) in the surface layer concur with values derived from reanalysis data (5.5 ± 0.8 Sv at 10°S and 6.4 ± 1.4 Sv at 10°N). A diapycnal transport of about 3 Sv across the 26.0 kg m−3 isopycnal is required in order to maintain the mass balance of the STC circulation.

Estimating vertical mixing in the deep north Aegean Sea using argo data corrected for conductivity sensor drift

Successive CTD profiles of an Argo float trapped within a deep (~1250 m) sub-basin of the North Aegean Sea during 2014–2015 enabled identification of deep-water ventilation episodes and inference of a bulk (budget-based) vertical eddy diffusivity during the period that followed these events. Independent effective eddy diffusivity profiles were computed under the assumption that the turbulent exchange of heat, salt and buoyancy through each isobath below 400 m will balance the rate of change of the corresponding variable's content of the water volume within the basin below that depth. The initially estimated thermal, saline and density diffusivity profiles were significantly different from each other. Assuming turbulence was the dominant mixing process between the intermediate and deep water masses led to identification of conductivity sensor drift rate of 4.6 × 10−6 S m−1 day−1. After correcting the sensor's drift, bulk eddy diffusivities Kρ,KS and KT were found to span (2–30) × 10−3 m2 s−1, in close agreement below 400 m depth. These estimates are at least one order of magnitude higher than eddy diffusivities based on a finescale internal-wave strain parameterization. Enhanced boundary mixing and double diffusion are examined as candidates to explain the high effective diffusivity values. A hypsometric correction appears to provide sufficient diapycnal transport along the boundaries to reproduce bulk buoyancy transports.

Spatial and seasonal variability of global ocean diapycnal transport inferred from Argo profiles

The global diapycnal transport in the ocean interior is one of the significant branches to return the deep water back toward near-surface. However, the amount of the diapycnal transport and the seasonal variations are not determined yet. This paper estimates the dissipation rate and the associated diapycnal transports at 500 m, 750 m and 1 000 m depth throughout the global ocean from the wide-spread Argo profiles, using the finescale parameterizations and classic advection-diffusion balance. The net upwelling is ~5.2±0.81 Sv (Sverdrup) which is approximately one fifth in magnitude of the formation of the deep water. The Southern Ocean is the major region with the upward diapycnal transport, while the downwelling emerges mainly in the northern North Atlantic. The upwelling in the Southern Ocean accounts for over 50% of the amount of the global summation. The seasonal cycle is obvious at 500 m and vanishes with depth, indicating the energy source at surface. The enhancement of diapycnal transport occurs at 1 000 m in the Southern Ocean, which is pertinent with the internal wave generation due to the interaction between the robust deep-reaching flows and the rough topography. Our estimates of the diapycnal transport in the ocean interior have implications for the closure of the oceanic energy budget and the understanding of global Meridional Overturning Circulation.

The Internal-Wave-Driven Meridional Overturning Circulation

AbstractThe upwelling diapycnal limb of the ocean’s meridional overturning circulation is driven by divergence of diabatic turbulent buoyancy fluxes 〈w′b′〉 across density surfaces. A global assessment of zonally averaged internal-wave-driven turbulent diapycnal buoyancy fluxes from a strain-based finescale parameterization is used to infer mean diapycnal transports in the interior and near the bottom boundary. Bulk interior diabatic transports dominate above 2500-m depth (buoyancies |B| = gγn/ρ0 &lt; 0.267 m s−2, neutral densities γn &lt; 27.9 kg m−3), upwelling at 10–11 Sv (1 Sv = 106 m3 s−1); 2, 5, and 3–4 Sv in the Indian, Pacific, and Atlantic, respectively, but are weak in the abyss. Boundary water-mass transformations peak at 18–25 Sv (4–6, 10–14, and 4–5 Sv in the Indian, Pacific, and Atlantic) near buoyancy |B| ~ 0.268 m s−2 (γn ~ 28.1 kg m−3, 4500-m depth) between bottom and lower deep waters, consistent with published 20–30-Sv global Antarctic Bottom Water (AABW) transport estimates. Interior transports above 2500-m depth fall below inverse estimates, consistent with a more adiabatic ocean interior where diapycnal mixing occurs at Southern Hemisphere high-latitude surface density outcrops.

A reflecting, steepening, and breaking internal tide in a submarine canyon

AbstractSubmarine canyons are common features of the coastal ocean. Although they are known to be hotspots of turbulence that enhance diapycnal transport in their stratified waters, the dynamics of canyon mixing processes are poorly understood. Most studies of internal wave dynamics within canyons have focused on a handful of canyons with along‐axis slopes less steep than semidiurnal (D2) internal wave characteristics (subcritical). Here, we present the first tidally resolving observations within a canyon with a steeply sloping axis (supercritical). A process study consisting of two 24 h shipboard stations and a profiling mooring was conducted in the La Jolla Canyon off the coast of La Jolla, CA. Baroclinic energy flux is oriented up‐canyon and decreases from 182 ±18 W m−1 at the canyon mouth to W m−1 near the head. The ratio of horizontal kinetic energy to available potential energy and the observed group speed of each mode are lower than expected for freely propagating D2 internal waves at each station, indicating partial reflection. Harmonic analysis reveals that variance is dominated by the D2 tide. Moving up‐canyon, the relative importance of D2 decreases and its higher harmonics are needed to account for a majority of the observed variance, indicating steepening. Steep internal tides cause large isopycnal displacements (∼50 m in 100 m water depth) and high strain events. These events coincide with enhanced O( m2 s−3) dissipation of turbulent kinetic energy at mid‐depths.

Diapycnal Velocity in the Double-Diffusive Thermocline

A series of large-scale numerical simulations is presented, which incorporate parameterizations of vertical mixing of temperature and salinity by double-diffusion and by small-scale turbulence. These simulations reveal the tendency of double-diffusion to constrain diapycnal volume transport, both upward and downward. For comparable values of mixing coefficients, the average diapycnal velocity in the double-diffusive thermocline is much less than in the corresponding turbulent regime. The insulating effect of double-diffusion is rationalized using two theoretical models. The first argument is based on the assumed vertical advective-diffusive balance. The second theory uses the Rhines and Young technique to evaluate the net diapycnal transport across regions bounded by closed streamlines at a given density surface. The numerical simulations and associated analytical arguments in this study underscore fundamental differences between double-diffusive mixing and mechanically generated small-scale turbulence. When both double-diffusion and turbulence are taken into account, we find that the constraints on diapycnal velocity loosen (tighten) with the increase (decrease) of the fraction of the overall mixing attributed to turbulence. The range of diapycnal velocities that could be realized in doubly-diffusive fluids is determined by the variation in the heat/salt flux ratio. We hypothesize that the unique ability of double-diffusive mixing to actively control diapycnal volume transport may have significant ramifications for the structure and dynamics of thermohaline circulation in the ocean.

Local water mass modifications by a solitary meander in the Agulhas Current

AbstractWe present full‐depth hydrographic and velocity observations across a solitary meander within the Agulhas Current, and assess the degree to which the meander causes upwelling, cross‐frontal mixing, and diapycnal transport. These events can input nutrients onto the continental shelf as well as advect larvae offshore. We find that the meander drives inshore upwelling with vertical velocities of at least 13 m d−1. The meander also causes diapycnal transport resulting in 1°C cooler and 0.25 fresher central waters and 1°C warmer, 0.25 saltier intermediate waters below the thermocline. We introduce a new coordinate system that separates these changes into kinematic changes due to meandering and property changes along transport streamlines. This reveals that most of the observed diapycnal transport below the thermocline is due to property changes. We find a small amount of enhanced cross‐frontal mixing associated with solitary meanders of the Agulhas Current, but it is statistically insignificant. We believe this is due to the strongly barotropic nature of the meandering Agulhas Current, which maintains a deep steering level that inhibits mixing.

Overturning circulation that ventilates the intermediate layer of the Sea of Okhotsk and the North Pacific: The role of salinity advection

AbstractDense Shelf Water (DSW) formation in the northwestern continental shelf of the Sea of Okhotsk is the beginning of the lower limb of the overturning circulation that ventilates the intermediate layer of the North Pacific Ocean. The upper limb consisting of surface currents in the Okhotsk Sea and the subarctic gyre has not been clarified. Using a high‐resolution North Pacific Ocean model with a curvilinear grid as fine as 3 km × 3 km in the Sea of Okhotsk, we succeeded in representing the three‐dimensional structure of the overturning circulation including the narrow boundary currents and flows through straits that constitute the upper limb, as well as the lower limb consisting of DSW formation and ventilation. In particular, pathways and time scales from the Bering Sea to the intermediate layer via the ventilation in the Sea of Okhotsk were examined in detail using tracer experiments. Further, we found that the overturning circulation that connects the surface and intermediate layer is sensitive to wind stress. In the case of strong winds, the coastal current under polynyas where DSW forms is intensified, and consequently diapycnal transport from the surface layer to the intermediate layer increases. Strong winds also induce a positive sea surface salinity anomaly in the subarctic region, causing a significant decrease in the density stratification and increase in the DSW salinity (i.e., density). These processes act together to produce intense overturning circulation and deep ventilation, which may subduct even to the bottom of the Sea of Okhotsk if the wind is strong.

Diapycnal and Isopycnal Transports in the Southern Ocean Estimated by a Box Inverse Model

Abstract Quantitative descriptions of Circumpolar Deep Water upwelling and evolution into a lighter mode and heavier bottom waters in the Southern Ocean are still not well constrained. Here, data from two occupations of eight hydrographic sections are combined and used in a box inverse model to estimate isopycnal and diapycnal transports in the Southern Ocean. A mixed layer box allows diapycnal transports in the surface mixed layer to be estimated separately. Current velocity at 1000 dbar was constrained by the mean velocity field estimated from subsurface float drift data. The estimated isopycnal transports are largely consistent with past estimates and with outputs of three ocean general circulation models. The estimated subduction and upwelling at the base of the Southern Ocean mixed layer show that Upper Circumpolar Deep Water upwells [16 ± 15 and 17 ± 21 Sv (where 1 Sv ≡ 106 m3 s−1) by different inversion methods] and evolves into heavier Lower Circumpolar Deep Water (5 ± 13 and 6 ± 18 Sv) and Bottom Water (8 ± 9 and 8 ± 13 Sv) or lighter Mode and Intermediate Waters (9 ± 18 and 13 ± 24 Sv). Meridional transport in the surface mixed layer is due to northward Ekman transport and mostly southward eddy transport. In seasonal ice-covered areas near Antarctica, a significant (14 ± 14 Sv) southward transport was found. The southward eddy transport is largest north of the Antarctic Circumpolar Current and decreases poleward because of the poleward decrease in the eddy diffusivity. The interior diapycnal transports, which can be either upward (gaining buoyancy) or downward (gaining density), are comparable in magnitude to the horizontal diapycnal transports within the surface mixed layer.

Turbulent Mixing and Exchange with Interior Waters on Sloping Boundaries

Abstract Microstructure measurements along the axes of Monterey and Soquel Submarine Canyons reveal 200–300-m-thick well-stratified turbulent near-bottom layers with average turbulent kinetic energy dissipation rates 〈ɛ〉 = 4 × 10−8 W kg−1 and eddy diffusivities K = 16 × 10−4 m2 s−1 (assuming mixing efficiency γ = 0.2) to at least thalweg depths of 1200 m. Turbulent dissipation rates are an order of magnitude lower in overlying waters, whereas buoyancy frequencies are only 25% higher. Well-mixed bottom boundary layer thicknesses hN are an order of magnitude thinner than the stratified turbulent layer (hN ≪ hɛ). Because well-stratified turbulent layers are commonly observed above slopes, arguments that mixing efficiency should be reduced on sloping boundaries do not hold in cases of energetic internal-wave generation or interaction with topography. An advective–diffusive balance is used to infer velocities and transports, predicting horizontal upslope flows of 10–50 m day−1. Extrapolating this estimate globally suggests that canyon turbulence may contribute 2–3 times as much diapycnal transport to the World Ocean as interior mixing. The upcanyon turbulence-driven transports are not uniform, and the resulting upslope convergences will drive exchange between the turbulent layer and more quiescent interior. Predicted density surfaces of detrainment and entrainment are consistent with observed isopycnals of intermediate nepheloid and clear layers. These data demonstrate that turbulent mixing dynamics on sloping topography are fundamentally 2D or 3D in the ocean, so they cannot be accurately described by 1D models.

Diapycnal Transport and Mixing Efficiency in Stratified Boundary Layers near Sloping Topography

Abstract The interaction of shear, stratification, and turbulence in boundary layers on sloping topography is investigated with the help of an idealized theoretical model, assuming uniform bottom slope and homogeneity in the upslope direction. It is shown theoretically that the irreversible vertical buoyancy flux generated in the boundary layer is directly proportional to the molecular destruction rate of small-scale buoyancy variance, which can be inferred, for example, from microstructure observations. Dimensional analysis of the equations shows that, for harmonic boundary layer forcing and no rotation, the problem is governed by three nondimensional parameters (slope angle, roughness number, and ratio of forcing and buoyancy frequencies). Solution of the equations with a second-moment closure model for the turbulent fluxes reveals the periodic generation of gravitationally unstable boundary layers during upslope flow, consistent with available observations. Investigation of the nondimensional parameter space with the help of this model illustrates a systematic increase of the bulk mixing efficiencies for (i) steep slopes and (ii) low-frequency forcing. Except for very steep slopes, mixing efficiencies are substantially smaller than the classical value of Γ = 0.2.

Constraining the 2-component model of marine dissolved organic radiocarbon

Abstract Keeling plots of dissolved organic carbon (DOC) concentration and Δ 14 C depth profiles imply rapid, diapycnal transport of DOC to the meso- and bathypelagic zones, but do not constrain the mechanism of redistribution. We review the 2-component Keeling plot model, and present an alternative formulation explicitly based on homogenization of water parcels. Applying this new model to DOC and dissolved inorganic carbon (DIC) depth profiles suggests that the dominant controls on DOC redistribution differ throughout the water column. DOC concentration and Δ 14 C gradients were consistent with biogeochemical processing in the epipelagic and advection in the mesopelagic. Vertical gradients in DOC concentration and Δ 14 C were insufficient for further interpretation in the bathypelagic. Ultimately, additional concurrent measurements of DOC and DIC concentrations and Δ 14 C values throughout the water column at more locations are needed to constrain the applicability of two-component mixing models to marine DOC.

Effects of mass source/sink at the western boundary on the wind-driven gyres: Implications for the ventilation of the north pacific intermediate layer through convection in the Okhotsk Sea and tidal mixing at the Kuril Straits

A steady quasi-geostrophic 2.5-layer model, forced by both Ekman pumping and a mass source/sink situated at the western boundary has been constructed to investigate the effect of diapycnal transport due to convection in the Okhotsk Sea and tidal mixing at the Kuril Straits on the intermediate layer in the North Pacific. The model illustrates a combined effect of the wind-driven and mass-driven circulations. First, net mass input induces a “barotropic” mode inter-gyre flow along the western boundary through the dynamical influence of Kelvin waves. This flow creates characteristic curves (geostrophic contours) that facilitate inter-gyre communication through the western boundary layer from the location of the mass source to the subtropical gyre. Due to the effect of wind-driven circulation, the offshore part turns eastward into the interior, encircles the outer rim of the region (which would otherwise be the pool region in the absence of mass input), and then encounters the western boundary. Eventually, the water fed into the lower layer flows mostly along this path and later flows away to the equatorial region. Conversely, in the upper layer, water is fed from the equator to the subtropics, and to the subpolar interior region through the western boundary current. The water then circulates along the outer rim and is absorbed into the mass sink. The model is controlled mainly by three nondimensional parameters: (1) the ratio of net mass input rate to the maximum Sverdrup transport (Q/T Sv max ), which affects the inter-gyre communication by altering the paths of geostrophic contours, (2) the ratio of a mass input rate into the lower layer to that in total (Q 2/Q), which controls the vertical structure of the inter-gyre flow, and (3) the measure of the wind forcing effect relative to the β effect, which determines the horizontal extent of the area influenced by the mass input. The other parameter regimes with respect to Q/T Sv max and Q 2/Q are also presented.