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Ying Zhang, Yan Du, Tangdong Qu, Yu Hong, Catia M. Domingues, and Ming Feng

Abstract

The Subantarctic Mode Water (SAMW) plays an essential role in the global heat, freshwater, carbon, and nutrient budgets. In this study, decadal changes in the SAMW properties in the southern Indian Ocean (SIO) and associated thermodynamic and dynamic processes are investigated during the Argo era. Both temperature and salinity of the SAMW in the SIO show increasing trends during 2004–18. A two-layer structure of the SAMW trend, with more warm and salty light SAMW but less cool and fresh dense SAMW, is identified. The heaving and spiciness processes are important but have opposite contributions to the temperature and salinity trends of the SAMW. A significant deepening of isopycnals (heaving), peaking at σθ = 26.7–26.8 kg m−3 in the middle layer of the SAMW, expands the warm and salty light SAMW and compresses the cool and fresh dense SAMW corresponding to the change in subduction rate during 2004–18. The change in the SAMW subduction rate is dominated by the change in the mixed layer depth, controlled by the changes in wind stress curl and surface buoyancy fluxes. An increase in the mixed layer temperature due to weakening northward Ekman transport of cool water leads to a lighter surface density in the SAMW formation region. Consequently, density outcropping lines in the SAMW formation region shift southward and favor the intrusion and entrainment of the cooler and fresher Antarctic surface water from the south, contributing to the cooling/freshening trend of isopycnals (spiciness). Subsequently, the cooler and fresher SAMW spiciness anomalies spread in the SIO via the subtropical gyre.

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Astrid Pacini, Robert S. Pickart, Isabela A. Le Bras, Fiammetta Straneo, N. Penny Holliday, and Michael A. Spall

Abstract

The boundary current system in the Labrador Sea plays an integral role in modulating convection in the interior basin. Four years of mooring data from the eastern Labrador Sea reveal persistent mesoscale variability in the West Greenland boundary current. Between 2014 and 2018, 197 middepth intensified cyclones were identified that passed the array near the 2000-m isobath. In this study, we quantify these features and show that they are the downstream manifestation of Denmark Strait Overflow Water (DSOW) cyclones. A composite cyclone is constructed revealing an average radius of 9 km, maximum azimuthal speed of 24 cm s−1, and a core propagation velocity of 27 cm s−1. The core propagation velocity is significantly smaller than upstream near Denmark Strait, allowing them to trap more water. The cyclones transport a 200-m-thick lens of dense water at the bottom of the water column and increase the transport of DSOW in the West Greenland boundary current by 17% relative to the background flow. Only a portion of the features generated at Denmark Strait make it to the Labrador Sea, implying that the remainder are shed into the interior Irminger Sea, are retroflected at Cape Farewell, or dissipate. A synoptic shipboard survey east of Cape Farewell, conducted in summer 2020, captured two of these features that shed further light on their structure and timing. This is the first time DSOW cyclones have been observed in the Labrador Sea—a discovery that could have important implications for interior stratification.

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Chiung-Yin Chang and Malte F. Jansen

Abstract

Although the reconfiguration of the abyssal overturning circulation has been argued to be a salient feature of Earth’s past climate changes, our understanding of the physical mechanisms controlling its strength remains limited. In particular, existing scaling theories disagree on the relative importance of the dynamics in the Southern Ocean versus the dynamics in the basins to the north. In this study, we systematically investigate these theories and compare them with a set of numerical simulations generated from an ocean general circulation model with idealized geometry, designed to capture only the basic ingredients considered by the theories. It is shown that the disagreement between existing theories can be partially explained by the fact that the overturning strengths measured in the channel and in the basin scale distinctly with the external parameters, including surface buoyancy loss, diapycnal diffusivity, wind stress, and eddy diffusivity. The overturning in the reentrant channel, which represents the Southern Ocean, is found to be sensitive to all these parameters, in addition to a strong dependence on bottom topography. By contrast, the basin overturning varies with the integrated surface buoyancy loss rate and diapycnal diffusivity but is mostly unaffected by winds and channel topography. The simulated parameter dependence of the basin overturning can be described by a scaling theory that is based only on basin dynamics.

Open access
Charles W. McMahon, Joseph J. Kuehl, and Vitalii A. Sheremet

Abstract

The dynamics of gap-leaping western boundary currents (e.g., the Kuroshio intrusion, the Loop Current) are explored through rotating table experiments and a numerical model designed to replicate the experimental apparatus. Simplified experimental and numerical models of gap-leaping systems are known to exhibit two dominant states (leaping or penetrating into the gap) as the inertia of the current competes with vorticity constraints (in this case the β effect). These systems are also known to admit multiple states with hysteresis. To advance toward more realistic oceanographic scenarios, recent studies have explored the effects of islands, mesoscale eddies, and variable baroclinic deformation radii on the dynamical system. Here, the effect of throughflow forcing is considered, with particle tracking velocimetry (PTV) used in the laboratory experiments. Mean transport in or out of the gap is found to significantly shift the hysteresis range as well as change its width. Because of these transformations, changes in throughflow can induce transitions in the gap-leaping system when near a critical state (leaping-to-penetrating/penetrating-to-leaping). Results from the study are interpreted within a nonlinear dynamical framework and various properties of the system are explored.

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Preston Spicer, Kelly L. Cole, Kimberly Huguenard, Daniel G. MacDonald, and Michael M. Whitney

Abstract

The mixing of river plumes into the coastal ocean influences the fate of riverborne tracers over the inner shelf, though the relative importance of mixing mechanisms under different environmental conditions is not fully understood. In particular, the contribution to plume mixing from bottom-generated shear stresses, referred to as tidal mixing, is rarely considered important relative to frontal and stratified shear (interfacial) mixing in surface advected plumes. The effect of different mixing mechanisms is investigated numerically on an idealized, tidally pulsed river plume with varying river discharge and tidal amplitudes. Frontal, interfacial, and tidal mixing are quantified via a mixing energy budget to compare the relative importance of each to the overall buoyancy flux over one tide. Results indicate that tidal mixing can dominate the energy budget when the tidal mixing power exceeds that of the input buoyancy flux. This occurs when the nondimensional number, RiER01 (the estuarine Richardson number divided by the mouth Rossby number), is generally less than 1. Tidal mixing accounts for between 60% and 90% of the net mixing when RiER01<1, with the largest contributions during large tides and low discharge. Interfacial mixing varies from 10% to 90% of total mixing and dominates the budget for high discharge events with relatively weaker tides (RiER01>1). Frontal mixing is always less than 10% of total mixing and never dominates the budget. This work is the first to show tidal mixing as an important mixing mechanism in surface advected river plumes.

Open access
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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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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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