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Bingrong Sun, Shengpeng Wang, Man Yuan, Hong Wang, Zhao Jing, Zhaohui Chen, and Lixin Wu

Abstract

Near-inertial internal waves (NIWs) are thought to play an important role in powering the turbulent diapycnal mixing in the ocean interior. Nevertheless, the energy flux into NIWs below the surface boundary layer (SBL) in the global ocean is still poorly understood. This key problem is addressed in this study based on a Community Earth System Model (CESM) simulation with a horizontal resolution of ~0.1° for its oceanic component and ~0.25° for its atmospheric component. The CESM shows good skill in simulating NIWs globally, reproducing the observed magnitude and spatial pattern of surface NIW currents and wind power on NIWs (W I). The simulated downward flux of NIW energy (F SBL) at the SBL base is positive everywhere. Its quasi-global integral (excluding the region within 5°S–5°N) is 0.13 TW, about one-third the value of W I. The ratio of local F SBL to W I varies substantially over the space. It exhibits an increasing trend with the enstrophy of balanced motions (BMs) and a decreasing trend with W I. The kinetic energy transfer from model-resolved BMs to NIWs is positive from the SBL base to 600 m but becomes negative farther downward. The quasi-global integral of energy transfer below the SBL base is two orders of magnitude smaller than that of F SBL, suggesting the resolved BMs in the CESM simulations making negligible contributions to power NIWs in the ocean interior.

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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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Ru Chen, James C. McWilliams, and Lionel Renault

Abstract

The California Undercurrent (CUC) transport, with significant variability ranging from weeks to decades, has consequences for both the climate and biogeochemistry of the California Current system. This study evaluates the governors of the CUC transport and its temporal variability from a momentum perspective, using a mesoscale-resolving regional model. From a 16-year mean perspective, the along-isobath pressure gradient acts to accelerate the CUC, whereas eddy advection retards it. The topographic form stress, which is part of the volume integrated along-isobath pressure gradient, not only acts in the direction of the time-mean CUC, but also greatly modulates the temporal variability of the CUC transport. This temporal variability is also correlated with the eddy momentum advection. The eddy stress plays a role in transferring both the equatorward wind stress and poleward CUC momentum downward. A theory is formulated to show that, in addition to the conventional vertical redistribution of momentum, the eddy stress can also redistribute momentum horizontally in the area where the correlation between the pressure anomaly and isopycnal fluctuations has large spatial variability.

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Ryuichiro Inoue and Satoshi Osafune

Abstract

Part of near-inertial wind energy dissipates locally below the surface mixed layer. Here, their role in the climate system is studied by adopting near-inertial, near-field wind-mixing parameterization to a coarse-forward ocean general circulation model. After confirming a problem with the parameterization in the equatorial region, we investigate effects of near-field wind mixing due to storm track activities in the North Pacific. We found that, in the center of the Pacific decadal oscillation (PDO) around 170°W in the midlatitude, near-field wind mixing transfers the PDO signal into deeper layers. Since the results suggest that near-field wind mixing is important in the climate system, we also compared the parameterization with velocity observations by a float in the North Pacific. The float observed abrupt and local propagation of near-inertial internal waves and shear instabilities in the main thermocline along the Kuroshio Extension for 460 km. Vertical diffusivities inferred from the parameterization do not reproduce the enhanced diffusivities in the deeper layer inferred from the float. Wave-ray tracing indicates that wave trapping near the Kuroshio front is responsible for the elevated diffusivities. Therefore, enhanced mixing due to trapping should be included in the parameterization.

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Ajitha Cyriac, Helen E. Phillips, Nathaniel L. Bindoff, Huabin Mao, and Ming Feng

Abstract

This study investigates the spatiotemporal variability of turbulent mixing in the eastern south Indian Ocean using a collection of data from electromagnetic autonomous profiling explorer (EM-APEX) profiling floats, shipboard CTD, and microstructure profilers. The floats collected 1566 profiles of temperature, salinity, and horizontal velocity data down to 1200 m over a period of about four months. A finescale parameterization is applied to the float and CTD data to estimate turbulent mixing. Elevated mixing is observed in the upper ocean, over bottom topography, and in mesoscale eddies. Mixing is enhanced in the anticyclonic eddies due to trapped near-inertial waves within the eddy. We found that cyclonic eddies contribute to turbulent mixing in the depth range of 500–1000 m, which is associated with downward-propagating internal waves. The mean diapycnal diffusivity over 250–500-m depth is O(10−6) m2 s−1, and it increases to O(10−5) m2 s−1 in 500–1000 m in cyclonic eddies. The turbulent mixing in this region has implications for water-mass transformation and large-scale circulation. Higher diffusivity [O(10−5) m2 s−1] is observed in the Antarctic Intermediate Water (AAIW) layer in cyclonic eddies, whereas weak diffusivity is observed in the Subantarctic Mode Water (SAMW) layer [O(10−6) m2 s−1]. Counterintuitively, then, the SAMW water-mass properties are strongly affected in cyclonic eddies, whereas the AAIW layer is less affected. Comparatively high diffusivity at the location of the South Indian Countercurrent (SICC) jets suggests there are wave–mean flow interactions in addition to the wave–eddy interactions that warrant further investigation.

Open access
C. A. Luecke, H. W. Wijesekera, E. Jarosz, D. W. Wang, J. C. Wesson, S. U. P. Jinadasa, H. J. S. Fernando, and W. J. Teague

Abstract

Long-term measurements of turbulent kinetic energy dissipation rate (ε), and turbulent temperature variance dissipation rate (χ T) in the thermocline, along with currents, temperature, and salinity were made at two subsurface moorings in the southern Bay of Bengal (BoB). This is a part of a major international program, conducted between July 2018 and June 2019, for investigating the role of the BoB on the monsoon intraseasonal oscillations. One mooring was located on the typical path of the Southwest Monsoon Current (SMC), and the other was in a region where the Sri Lanka dome is typically found during the summer monsoon. Microstructure and finescale estimates of vertical diffusivity revealed the long-term subthermocline mixing patterns in the southern BoB. Enhanced turbulence and large eddy diffusivities were observed within the SMC during the passage of a subsurface-intensified anticyclonic eddy. During this time, background shear and strain appeared to influence high-frequency motions such as near-inertial waves and internal tides, leading to increased mixing. Near the Sri Lanka dome, enhanced dissipation occurred at the margins of the cyclonic feature. Turbulent mixing was enhanced with the passage of Rossby waves and eddies. During these events, values of χ T exceeding 10−4 °C2 s−1 were recorded concurrently with ε values exceeding 10−5 W kg−1. Inferred diffusivity peaked well above background values of 10−6 m2 s−1, leading to an annually averaged diffusivity near 10−4 m2 s−1. Turbulence appeared low throughout much of the deployment period. Most of the mixing occurred in spurts during isolated events.

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Constantin W. Arnscheidt, John Marshall, Pierre Dutrieux, Craig D. Rye, and Ali Ramadhan

Abstract

Antarctic glacial meltwater is thought to play an important role in determining large-scale Southern Ocean climate trends, yet recent modeling efforts have proceeded without a good understanding of how its vertical distribution in the water column is set. To rectify this, here we conduct new large-eddy simulations of the ascent of a buoyant meltwater plume after its escape from beneath an Antarctic ice shelf. We find that the meltwater’s settling depth is primarily a function of the buoyancy forcing per unit width of the source and the ambient stratification, consistent with the classical theory of turbulent buoyant plumes and in contrast to previous work that suggested an important role for centrifugal instability. Our results further highlight the significant role played by localized variability in stratification; this helps explain observed interannual variability in the vertical meltwater distribution near Pine Island Glacier. Because of the vast heterogeneity in mass loss rates and ambient conditions at different Antarctic ice shelves, a dynamic parameterization of meltwater settling depth may be crucial for accurately simulating high-latitude climate in a warming world; we discuss how this may be developed following this work, and where the remaining challenges lie.

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Ali Tamizi, Jose-Henrique Alves, and Ian R. Young

Abstract

A series of numerical experiments with the WAVEWATCH III spectral wave model are used to investigate the physics of wave evolution in tropical cyclones. Buoy observations show that tropical cyclone wave spectra are directionally skewed with a continuum of energy between locally generated wind-sea and remotely generated waves. These systems are often separated by more than 90°. The model spectra are consistent with the observed buoy data and are shown to be governed by nonlinear wave–wave interactions that result in a cascade of energy from the wind-sea to the remotely generated spectral peak. The peak waves act in a “parasitic” manner taking energy from the wind-sea to maintain their growth. The critical role of nonlinear processes explains why one-dimensional tropical cyclone spectra have characteristics very similar to fetch-limited waves, even though the generation system is far more complex. The results also provide strong validation of the critical role nonlinear interactions play in wind-wave evolution.

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Adrian Jenkins

Abstract

When the inclined base of an ice shelf melts into the ocean, it induces both a statically stable stratification and a buoyancy-forced, sheared flow along the interface. Understanding how those competing effects influence the dynamical stability of the boundary current is the key to quantifying the turbulent transfer of heat from far-field ocean to ice. The implications of the close coupling between shear, stability, and mixing are explored with the aid of a one-dimensional numerical model that simulates density and current profiles perpendicular to the ice. Diffusivity and viscosity are determined using a mixing length model within the turbulent boundary layer and empirical functions of the gradient Richardson number in the stratified layer below. Starting from rest, the boundary current is initially strongly stratified and dynamically stable, slowly thickening as meltwater diffuses away from the interface. Eventually, the current enters a second phase where dynamical instability generates a relatively well-mixed, turbulent layer adjacent to the ice, while beneath the current maximum, strong stratification suppresses mixing in the region of reverse shear. Under weak buoyancy forcing the time scale for development of the initial dynamical instability can be months or longer, but background flows, which are always present in reality, provide additional current shear that greatly accelerates the process. A third phase can be reached when the ice shelf base is sufficiently steep, with dynamical instability extending beyond the boundary layer into regions of geostrophic flow, generating a marginally stable pycnocline through which the heat flux is a simple function of ice–ocean interfacial slope.

Open access
Elizabeth Brasseale and Parker MacCready

Abstract

The inflow to an estuary originates on the shelf. It flushes the estuary and can bring in nutrients, heat, salt, and hypoxic water, having consequences for estuarine ecosystems and fjordic glacial melt. However, the source of estuarine inflow has only been explored in simple models that do not resolve interactions between inflow and outflow outside of the estuarine channel. This study addressed the estuary inflow problem using variations on a three-dimensional primitive equation model of an idealized estuarine channel next to a sloping, unstratified shelf with mixing provided by a single-frequency, 12-h tide. Inflow was identified using particle tracking, momentum budgets, and total exchange flow. Inflow sources were found in shelf water downstream of the estuary, river plume water, and shelf water upstream of the estuary. Downstream is defined here with respect to the direction of coastal trapped wave propagation, which is to the right for an observer looking seaward from the estuary mouth in the Northern Hemisphere. Downstream of the estuary and offshore of the plume, the dynamics were quasigeostrophic, consistent with previous simple models. The effect of this inflowing current on the geometry of the river plume front was found to be small. Novel sources of inflow were identified which originated from within the plume and upstream of the estuary.

Open access