Browse

You are looking at 1 - 10 of 8,442 items for :

  • Journal of Physical Oceanography x
  • Refine by Access: Content accessible to me x
Clear All
Anda Vladoiu
,
Ren-Chieh Lien
, and
Eric Kunze

Abstract

Shipboard ADCP velocity and towed CTD chain density measurements from the eastern North Pacific pycnocline are used to segregate energy between linear internal waves (IW) and linear vortical motion [quasigeostrophy (QG)] in 2D wavenumber space spanning submesoscale horizontal wavelengths λx ∼ 1–50 km and finescale vertical wavelengths λz ∼ 7–100 m. Helmholtz decomposition and a new Burger number (Bu) decomposition yield similar results despite different methodologies. While these wavelengths are conventionally attributed to internal waves, both QG and IW contribute significantly at all measured scales. Partition between IW and QG total energies depends on Bu. For Bu < 0.01, available potential energy EP exceeds horizontal kinetic energy EK and is contributed mostly by QG. In contrast, energy is nearly equipartitioned between QG and IW for Bu ≫ 1. For Bu < 2, EK is contributed mainly by IW, and EP by QG, while, for Bu > 2, contributions are reversed. Finescale near-inertial IW dominate vertical shear variance, implying negligible QG contribution to vertical shear instability. In contrast, both QG and IW at the smallest λx ∼ 1 km contribute large horizontal shear variance, so that both may lead to horizontal shear instability, while QG, with its longer time scales, likely dominates isopycnal stirring. Both QG and IW contribute to vortex stretching at small vertical scales. For QG, the relative vorticity contribution to linear potential vorticity anomaly increases with decreasing horizontal and increasing vertical scales.

Open access
Margarita Y. Markina
,
Helen L. Johnson
, and
David P. Marshall

Abstract

A large part of the variability in the Atlantic Meridional Overturning Circulation (AMOC) and thus uncertainty in its estimates on interannual timescales comes from atmospheric synoptic eddies and mesoscale processes. In this study, a suite of experiments with a 1/12° regional configuration of the MITgcm is performed where low pass filtering is applied to surface wind forcing to investigate the impact of subsynoptic (< 2 days) and synoptic (2-10 days) atmospheric processes on the ocean circulation. Changes in the wind magnitude and hence the wind energy input in the region have a significant effect on the strength of the overturning; once this is accounted for, the magnitude of the overturning in all sensitivity experiments is very similar to that of the control run. Synoptic and subsynoptic variability in atmospheric winds reduce the surface heat loss in the Labrador Sea, resulting in anomalous advection of warm and salty waters into the Irminger Sea and lower upper ocean densities in the eastern subpolar North Atlantic. Other effects of high-frequency variability in surface winds on the AMOC are associated with changes in Ekman convergence in the midlatitudes. Synoptic and subsynoptic winds also impact the strength of the boundary currents and density structure in the subpolar North Atlantic. In the Labrador Sea, the overturning strength is more sensitive to the changes in density structure, whereas in the eastern subpolar North Atlantic, the role of density is comparable to that of the strength of the East Greenland Current.

Open access
T. Sohail
and
J. D. Zika

Abstract

The ocean surrounding Antarctica, also known as the Antarctic margins, is characterized by complex and heterogeneous process interactions, which have major impacts on the global climate. A common way to understand changes in the Antarctic margins is to categorize regions into similar “regimes,” thereby guiding process-based studies and observational analyses. However, this categorization is traditionally largely subjective and based on temperature, density, and bathymetric criteria that are bespoke to the dataset being analyzed. In this work, we introduce a method to classify Antarctic shelf regimes using unsupervised learning. We apply Gaussian mixture modeling to the across-shelf temperature and salinity properties along the Antarctic margins from a high-resolution ocean model, ACCESS-OM2-01. Three clusters are found to be optimum based on the Bayesian information criterion and an assessment of regime properties. The three clusters correspond to the fresh, dense, and warm regimes identified canonically via subjective approaches. Our analysis allows us to track changes to these regimes in a future projection of the ACCESS-OM2-01 model. We identify the future collapse of dense water formation, and the merging of dense and fresh shelf regions into a single fresh regime that covers the entirety of the Antarctic shelf except for the West Antarctic. Our assessment of these clusters indicates that the Antarctic margins may shift into a two-regime system in the future, consisting only of a strengthening warm shelf in the West Antarctic and a fresh shelf regime everywhere else.

Significance Statement

The Antarctic margins are characterized by complex interactions of surface and ocean processes, producing distinct regions or “regimes.” Understanding where these regimes are and their future state is critical to understanding climate change. Based on a subjective assessment of ocean conditions, past research has identified fresh, dense, and warm regimes in the Antarctic margins. In this work, we use an unsupervised classification tool, Gaussian mixture modeling, to objectively identify the location of regimes around the Antarctic margins. Our method detects three regimes in an ocean model, which match the location of subjectively identified fresh, dense, and warm regimes, and indicates a future shrinking of the dense regime. Our method is adaptable to multiple datasets, enabling us to identify trends and processes in the Antarctic margins.

Open access
Qiang Wang

Abstract

The Arctic Beaufort Gyre plays a critical role in climate and marine ecosystems. This study investigates the response of the liquid freshwater in the Beaufort Gyre to various wind perturbations using numerical simulations. A new diagnostic called “freshwater renewal” is introduced, which quantifies the amount of freshwater that has entered the Beaufort Gyre since a specific point in time. The findings reveal that the process of freshwater renewal is persistently efficient in the Beaufort Gyre region, occurring irrespective of the gyre’s status. The spatial distribution of freshwater renewal varies, influenced by factors such as wind forcing and gyre circulation patterns. Cyclonic wind perturbation associated with a negative Beaufort high sea level pressure anomaly triggers freshwater release from the Beaufort Gyre, with freshwater export and renewal dependent on wind-perturbation locations and time scales. While some released Beaufort Gyre freshwater exits the Arctic Ocean through the Davis and Fram Straits, a considerable portion could remain within the Arctic Ocean for many years under specific conditions. Wind perturbation associated with the positive Arctic Oscillation enhances the Arctic export of Beaufort Gyre freshwater, mainly through the Fram Strait. The Arctic export of total freshwater and the Arctic export of the portion originating from the Beaufort Gyre have different time scales and magnitudes. Hence, it is essential to collectively examine different freshwater components in order to assess the role of Arctic export in the climate system.

Open access
Walter I. Torres
and
James L. Hench

Abstract

This study adopts a curvature dynamics approach to understand and predict the trajectory of an idealized depth-averaged barotropic outflow onto a slope in shallow water. A novel equation for streamwise curvature dynamics was derived from the barotropic vorticity equation and applied to a momentum jet subject to bottom friction, topographic slope, and planetary rotation. The terms in the curvature dynamics equation have a natural geometric interpretation whereby each physical process can influence the flow direction. It is shown that a weakly spreading jet onto a steep slope admits the formulation of a 1D ordinary differential equation system in a streamline coordinate system, yielding an integrable ordinary differential equation system that predicts the kinematical behavior of the jet. The 1D model was compared with a set of high-resolution idealized depth-averaged circulation model simulations where bottom friction, planetary rotation, and bottom slope were varied. Favorable performance of the 1D reduced physics model was found, especially in the nearfield of the outflow. The effect of nonlinear processes such as topographic stretching and bottom torque on the fate of the jet outflow are explained using curvature dynamics. Planetary rotation has a surprisingly strong influence on the nearfield deflection of an intermediate-scale jet given a sufficient topographic slope. The deflection of the jet across steep slopes affects cross- and alongshelf transport patterns in the tropics.

Open access
Free access
D. A. Cherian
,
Y. Guo
, and
F. O. Bryan

Abstract

We assess the representation of mesoscale stirring in a suite of models against an estimate derived from microstructure data collected during the North Atlantic Tracer Release Experiment (NATRE). We draw heavily from the approximate temperature variance budget framework of Ferrari and Polzin. This framework assumes two sources of temperature variance away from boundaries: first, the vertical stirring of the large-scale mean vertical gradient by small-scale turbulence; and second, the lateral stirring of large-scale mean along-isopycnal gradients by mesoscale eddies. Temperature variance so produced is transformed and on average transferred down scales for ultimate dissipation at the microscale at a rate χ estimated using microstructure observations. Ocean models represent these pathways by a vertical mixing parameterization, and an along-isopycnal lateral mixing parameterization (if needed). We assess the rate of variance production by the latter as a residual from the NATRE dataset and compare against the parameterized representations in a suite of model simulations. We find that variance production due to lateral stirring in a Parallel Ocean Program version 2 (POP2) 1/10° simulation agrees well, to within the estimated error bars, with that inferred from the NATRE estimate. A POP2 1° simulation and the Estimating the Circulation and Climate of the Ocean Version 4 release 4 (ECCOV4r4) simulation appear to dissipate an order of magnitude too much variance by applying a lateral diffusivity, when compared to the NATRE estimate, particularly below 1250 m. The ECCOV4r4-adjusted lateral diffusivities are elevated where the microstructure suggests elevated χ sourced from mesoscale stirring. Such elevated values are absent in other diffusivity estimates suggesting the possibility of compensating errors and caution in interpreting ECCOV4r4’s adjusted lateral diffusivities.

Significance Statement

We look at whether microstructure turbulence observations can provide a useful metric for judging the fidelity of representation of mesoscale stirring in a suite of models. We focus on the region of the North Atlantic Tracer Release Experiment (NATRE), the site of a major ocean turbulence observation campaign, and use an approximate variance budget framework for the region with observational estimates from Ferrari and Polzin (2005). The approach provides a novel framework to evaluate the approximate representation of mesoscale stirring in a variety of models.

Open access
Peiran Yang
,
Zhao Jing
,
Haiyuan Yang
, and
Lixin Wu

Abstract

The vertical buoyancy flux Bf under the turbulent thermal wind (TTW) balance B f TTW plays an important role in restratifying the surface mixed layer in winter. So far, most of the global ocean models are too coarse to resolve this process. In this paper, a scale-aware parameterization is proposed for B f TTW and implemented in a hierarchy of regional ocean simulations over the winter Kuroshio Extension with horizontal resolutions ranging from 27 to 1 km. The parameterization depends on the Coriolis parameter, model-simulated turbulent vertical viscosity, horizontal density gradient, and a scaling relationship to adjust for the effects of model horizontal resolution on the simulated horizontal density gradient. It shows good skills in reconciling the difference between B f TTW in the coarse-resolution simulations (27, 9, and 3 km) and in the 1-km simulation where B f TTW is well resolved. Furthermore, implementation of the parameterization improves the simulated stratification in the surface mixed layer in coarse-resolution simulations.

Open access
J. Anselin
,
P. R. Holland
,
A. Jenkins
, and
J. R. Taylor

Abstract

Efforts to parameterize ice shelf basal melting within climate models are limited by an incomplete understanding of the influence of ice base slope on the turbulent ice shelf-ocean boundary current (ISOBC). Here we examine the relationship between ice base slope, boundary current dynamics, and melt rate using 3-D, turbulence-permitting large-eddy simulations (LES) of an idealized ice shelf-ocean boundary current forced solely by melt-induced buoyancy. The range of simulated slopes (3-10%) is appropriate to the grounding zone of small Antarctic ice shelves and to the flanks of relatively wide ice base channels, and the initial conditions are representative of warm-cavity ocean conditions. In line with previous studies, the simulations feature the development of an Ekman boundary layer adjacent to the ice, overlaying a broad pycnocline. The time-averaged flow within the pycnocline is in thermal wind balance, with a mean shear that is only weakly dependent on the ice base slope angle α, resulting in a mean gradient Richardson number 〈Rig〉 that decreases approximately linearly with sinα. Combining this inverse relationship with a linear approximation to the density profile, we derive formulations for the friction velocity, thermal forcing, and melt rate in terms of slope angle and total buoyancy input. This theory predicts that melt rate varies like the square root of slope, which is consistent with the LES results and differs from a previously proposed linear trend. The derived scalings provide a potential framework for incorporating slope-dependence into parameterizations of mixing and melting at the base of ice shelves.

Open access
Free access