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Ryan M. Holmes, Casimir de Lavergne, and Trevor J. McDougall

Abstract

In situ observations obtained over the last several decades have shown that the intensity of turbulent mixing in the abyssal ocean is enhanced toward the seafloor. Consequently, a new paradigm has emerged whereby dianeutral downwelling dominates in the ocean interior and dianeutral upwelling only occurs within thin bottom boundary layers. This study shows that when mixing is bottom intensified the net abyssal dianeutral transports and the stratification can depend on subtle features of the seafloor geometry. Under an assumption of depth-independent net dianeutral upwelling, small changes in the curvature of the seafloor can result in interior stratification that is bottom intensified, uniform, or surface intensified. Further, when the net dianeutral transport is allowed to vary in the vertical, changes in the seafloor slope and bathymetric contour length with height can drive lateral exchange between the boundary layer and interior, with particularly strong lateral outflows predicted at the crests of midocean ridges. Finally, using a realistic neutral density climatology the authors suggest that the increase in the perimeter of abyssal neutral density surfaces with height drives much of the dianeutral upwelling at depths greater than 4 km, while the increase in the slope of the seafloor at shallower depths acts to oppose upwelling. These results add to a growing body of literature highlighting the key control of seafloor geometry on the abyssal overturning circulation.

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Ryan M. Holmes, Jan D. Zika, and Matthew H. England

Abstract

, in a comment on , argue that under the incompressible Boussinesq approximation the “sum of the volume fluxes through any kind of control volume must integrate to zero at all times.” They hence argue that the expression in for the change in the volume of seawater warmer than a given temperature is inaccurate. Here we clarify what is meant by the term “volume flux” as used in and also more generally in the water-mass transformation literature. Specifically, a volume flux across a surface can occur either due to fluid moving through a fixed surface, or due to the surface moving through the fluid. Interpreted in this way, we show using several examples that the statement from quoted above does not apply to the control volume considered in . then derive a series of expressions for the water-mass transformation or volume flux across an isotherm G in the general, compressible case. In the incompressible Boussinesq limit these expressions reduce to a form (similar to that provided in ) that involves the temperature derivative of the diabatic heat fluxes. Due to this derivative, G can be difficult to robustly estimate from ocean model output. This emphasizes one of the advantages of the approach of , namely, G does not appear in the internal heat content budget and is not needed to describe the flow of internal heat content into and around the ocean.

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Christopher Bladwell, Ryan M. Holmes, and Jan D. Zika

Abstract

The global water cycle is dominated by an atmospheric branch that transfers freshwater away from subtropical regions and an oceanic branch that returns that freshwater from subpolar and tropical regions. Salt content is commonly used to understand the oceanic branch because surface freshwater fluxes leave an imprint on ocean salinity. However, freshwater fluxes do not actually change the amount of salt in the ocean and—in the mean—no salt is transported meridionally by ocean circulation. To study the processes that determine ocean salinity, we introduce a new variable “internal salt” along with its counterpart “internal fresh water.” Precise budgets for internal salt in salinity coordinates relate meridional and diahaline transport to surface freshwater forcing, ocean circulation, and mixing and reveal the pathway of freshwater in the ocean. We apply this framework to a 1° global ocean model. We find that for freshwater to be exported from the ocean’s tropical and subpolar regions to the subtropics, salt must be mixed across the salinity surfaces that bound those regions. In the tropics, this mixing is achieved by parameterized vertical mixing, along-isopycnal mixing, and numerical mixing associated with truncation errors in the model’s advection scheme, whereas along-isopycnal mixing dominates at high latitudes. We analyze the internal freshwater budgets of the Indo-Pacific and Atlantic Ocean basins and identify the transport pathways between them that redistribute freshwater added through precipitation, balancing asymmetries in freshwater forcing between the basins.

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Ryan M. Holmes, Leif N. Thomas, LuAnne Thompson, and David Darr

Abstract

Tropical instability vortices (TIVs) in the equatorial Pacific exhibit energetic horizontal and vertical circulation characterized by regions of high Rossby number and low Richardson number. Their strong anticyclonic vorticity and vertical shear can influence the broader-scale circulation by driving lateral mixing and vertical exchange between the ocean surface and interior. The authors use a set of nested high-resolution simulations of the equatorial Pacific, with a finest grid size of 3 km, to examine the vortex dynamics associated with TIV core water formation. TIV cores are characterized by low values of the Ertel potential vorticity (PV) as the relative vorticity is anticyclonic with magnitude comparable to the local Coriolis parameter. A study of the variation of PV and other scalars along Lagrangian fluid parcel tracks entering the TIVs shows that the low-PV water in their cores is a mix of Equatorial Undercurrent (EUC) water and North Equatorial Counter Current (NECC) water. The EUC water is characterized by strong horizontal vorticity, and thus, the baroclinic component of the PV is nonnegligible and acts as a source for the anticyclonic vorticity of TIVs. This horizontal vorticity is tilted by an ageostrophic secondary circulation associated with strain-induced frontogenesis that tends to form along the path of the EUC water that enters the vortex. Frontogenesis disrupts the cyclogeostrophic balance of the frontal flow and drives differential vertical motions across the front. These results emphasize the role of submesoscale physics in the equatorial region, which are active when both the Rossby and Richardson numbers are O(1).

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Ryan M. Holmes, Jan D. Zika, and Matthew H. England

Abstract

The rate at which the ocean moves heat from the tropics toward the poles, and from the surface into the interior, depends on diabatic surface forcing and diffusive mixing. These diabatic processes can be isolated by analyzing heat transport in a temperature coordinate (the diathermal heat transport). This framework is applied to a global ocean sea ice model at two horizontal resolutions (1/4° and 1/10°) to evaluate the partioning of the diathermal heat transport between different mixing processes and their spatial and seasonal structure. The diathermal heat transport peaks around 22°C at 1.6 PW, similar to the peak meridional heat transport. Diffusive mixing transfers this heat from waters above 22°C, where surface forcing warms the tropical ocean, to temperatures below 22°C where midlatitude waters are cooled. In the control 1/4° simulation, half of the parameterized vertical mixing is achieved by background diffusion, to which sensitivity is explored. The remainder is associated with parameterizations for surface boundary layer, shear instability, and tidal mixing. Nearly half of the seasonal cycle in the peak vertical mixing heat flux is associated with shear instability in the tropical Pacific cold tongue, highlighting this region’s global importance. The framework presented also allows for quantification of numerical mixing associated with the model’s advection scheme. Numerical mixing has a substantial seasonal cycle and increases to compensate for reduced explicit vertical mixing. Finally, applied to Argo observations the diathermal framework reveals a heat content seasonal cycle consistent with the simulations. These results highlight the utility of the diathermal framework for understanding the role of diabatic processes in ocean circulation and climate.

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Maurice F. Huguenin, Ryan M. Holmes, and Matthew H. England

Abstract

The equatorial Pacific warm water volume (WWV), defined as the volume of water warmer than 20°C near the equator, is a key predictor for El Niño–Southern Oscillation (ENSO), and yet much about the individual processes that influence it remains unknown. In this study, we conduct idealized ENSO simulations forced with symmetric El Niño– and La Niña–associated atmospheric anomalies as well as a historical 1979–2016 hindcast simulation. We use the water mass transformation framework to examine the individual contributions of diabatic and adiabatic processes to changes in WWV. We find that in both sets of simulations, El Niño’s discharge and La Niña’s recharge periods are initiated by diabatic fluxes of volume across the 20°C isotherm associated with changes in surface forcing and vertical mixing. Changes in adiabatic horizontal volume transport above 20°C between the equator and subtropical latitudes dominate at a later stage. While surface forcing and vertical mixing deplete WWV during El Niño, surface forcing during La Niña drives a large increase partially compensated for by a decrease driven by vertical mixing. On average, the ratio of diabatic to adiabatic contributions to changes in WWV during El Niño is about 40% to 60%; during La Niña this ratio changes to 75% to 25%. The increased importance of the diabatic processes during La Niña, especially the surface heat fluxes, is linked to the shoaling of the 20°C isotherm in the eastern equatorial Pacific and is a major source of asymmetry between the two ENSO phases, even in the idealized simulations where the wind forcing and adiabatic fluxes are symmetric.

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Sally J. Warner, Ryan M. Holmes, Elizabeth H. M. Hawkins, Martín S. Hoecker-Martínez, Anna C. Savage, and James N. Moum

Abstract

Two extremely sharp fronts with changes in sea surface temperature >0.4°C over lateral distances of ~1 m were observed in the equatorial Pacific at 0°, 140°W and at 0.75°N, 110°W. In both cases, layers of relatively warm and fresh water extending to ~30-m depth propagated to the southwest as gravity currents. Turbulent kinetic energy dissipation rates averaging 4.5 × 10−6 W kg−1 were measured with a microstructure profiler within the warm layer behind the leading edge of the fronts—1000 times greater than dissipation in the ambient water ahead of the fronts. From satellite images, these fronts were observed to propagate ahead of the trailing edge of a tropical instability wave (TIW) cold cusp. Results from an ocean model with 6-km grid resolution suggest that TIW fronts may release gravity currents through frontogenesis and loss of balance as the fronts approach the equator and the Coriolis parameter weakens. Sharp frontal features appear to be ubiquitous in the eastern tropical Pacific, have an influence on the distribution of biogeochemical tracers and organisms, and play a role in transferring energy out of the TIW field toward smaller scales and dissipation.

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Abhishek Savita, Jan D. Zika, Catia M. Domingues, Simon J. Marsland, Gwyn Dafydd Evans, Fabio Boeira Dias, Ryan M. Holmes, and Andrew McC. Hogg

Abstract

Ocean circulation and mixing regulate Earth’s climate by moving heat vertically within the ocean. We present a new formalism to diagnose the role of ocean circulation and diabatic processes in setting vertical heat transport in ocean models. In this formalism we use temperature tendencies, rather than explicit vertical velocities, to diagnose circulation. Using quasi-steady-state simulations from the Australian Community Climate and Earth-System Simulator Ocean Model (ACCESS-OM2), we diagnose a diathermal overturning circulation in temperature–depth space. Furthermore, projection of tendencies due to diabatic processes onto this coordinate permits us to represent these as apparent overturning circulations. Our framework permits us to extend the concept of “Super Residual Transport,” which combines mean and eddy advection terms with subgridscale isopycnal mixing due to mesoscale eddies but excludes small-scale three-dimensional turbulent mixing effect, to construct a new overturning circulation—the “Super Residual Circulation” (SRC). We find that in the coarse-resolution version of ACCESS-OM2 (nominally 1° horizontal resolution) the SRC is dominated by an ~11-Sv (1 Sv ≡ 106 m3 s−1) circulation that transports heat upward. The SRC’s upward heat transport is ~2 times as large in a finer-horizontal-resolution (0.1°) version of ACCESS, suggesting that a differing balance of super-residual and parameterized small-scale processes may emerge as eddies are resolved. Our analysis adds new insight into superresidual processes, because the SRC elucidates the pathways in temperature and depth space along which water mass transformation occurs.

Open access