scholarly journals The Effects of Mesoscale Ocean–Atmosphere Coupling on the Large-Scale Ocean Circulation

2009 ◽  
Vol 22 (15) ◽  
pp. 4066-4082 ◽  
Author(s):  
Andrew Mc C. Hogg ◽  
William K. Dewar ◽  
Pavel Berloff ◽  
Sergey Kravtsov ◽  
David K. Hutchinson
Keyword(s):  
Spatial Scale ◽  
Ocean Circulation ◽  
Large Scale ◽  
Ocean Model ◽  
Small Scale ◽  
Western Boundary ◽  
Ekman Pumping ◽  
Boundary Current ◽  
Ocean Atmosphere ◽  
Gyre Circulation

Abstract Small-scale variation in wind stress due to ocean–atmosphere interaction within the atmospheric boundary layer alters the temporal and spatial scale of Ekman pumping driving the double-gyre circulation of the ocean. A high-resolution quasigeostrophic (QG) ocean model, coupled to a dynamic atmospheric mixed layer, is used to demonstrate that, despite the small spatial scale of the Ekman-pumping anomalies, this phenomenon significantly modifies the large-scale ocean circulation. The primary effect is to decrease the strength of the nonlinear component of the gyre circulation by approximately 30%–40%. This result is due to the highest transient Ekman-pumping anomalies destabilizing the flow in a dynamically sensitive region close to the western boundary current separation. The instability of the jet produces a flux of potential vorticity between the two gyres that acts to weaken both gyres.

scholarly journals Thermodynamic Analysis of Ocean Circulation

2007 ◽  
Vol 37 (8) ◽  
pp. 2038-2052 ◽  
Author(s):  
J. Nycander ◽  
J. Nilsson ◽  
K. Döös ◽  
G. Broström
Keyword(s):  
Ocean Circulation ◽  
Large Scale ◽  
Three Dimensional ◽  
Boussinesq Equations ◽  
Ocean Model ◽  
Global Ocean ◽  
Small Scale ◽  
Ekman Pumping ◽  
Overturning Circulation ◽  
Thermodynamic Work

Abstract Calculating a streamfunction as function of depth and density is proposed as a new way of analyzing the thermodynamic character of the overturning circulation in the global ocean. The sign of an overturning cell in this streamfunction directly shows whether it is driven mechanically by large-scale wind stress or thermally by heat conduction and small-scale mixing. It is also shown that the integral of this streamfunction gives the thermodynamic work performed by the fluid. The analysis is also valid for the Boussinesq equations, although formally there is no thermodynamic work in an incompressible fluid. The proposed method is applied both to an idealized coarse-resolution three-dimensional numerical ocean model, and to the realistic high-resolution Ocean Circulation and Climate Advanced Model (OCCAM). It is shown that the overturning circulation in OCCAM between the 200- and 1000-m depth is dominated by a thermally indirect cell of 24 Sverdrups (1 Sv ≡ 106 m3 s−1), forced by Ekman pumping. In the densest and deepest waters there is a thermally direct cell of 18 Sv, which requires a forcing by around 100 GW of parameterized small-scale mixing.

scholarly journals Ocean Eddy Dynamics in a Coupled Ocean–Atmosphere Model*

2007 ◽  
Vol 37 (5) ◽  
pp. 1103-1121 ◽  
Author(s):  
P. Berloff ◽  
W. Dewar ◽  
S. Kravtsov ◽  
J. McWilliams
Keyword(s):  
Random Process ◽  
Large Scale ◽  
Decadal Variability ◽  
Ocean Model ◽  
Large Reynolds Number ◽  
Western Boundary ◽  
Boundary Current ◽  
Nonstationary Random Process ◽  
Ocean Atmosphere ◽  
Atmosphere Model

Abstract The role of mesoscale oceanic eddies is analyzed in a quasigeostrophic coupled ocean–atmosphere model operating at a large Reynolds number. The model dynamics are characterized by decadal variability that involves nonlinear adjustment of the ocean to coherent north–south shifts of the atmosphere. The oceanic eddy effects are diagnosed by the dynamical decomposition method adapted for nonstationary external forcing. The main effects of the eddies are an enhancement of the oceanic eastward jet separating the subpolar and subtropical gyres and a weakening of the gyres. The flow-enhancing effect is due to nonlinear rectification driven by fluctuations of the eddy forcing. This is a nonlocal process involving generation of the eddies by the flow instabilities in the western boundary current and the upstream part of the eastward jet. The eddies are advected by the mean current to the east, where they backscatter into the rectified enhancement of the eastward jet. The gyre-weakening effect, which is due to the time-mean buoyancy component of the eddy forcing, is a result of the baroclinic instability of the westward return currents. The diagnosed eddy forcing is parameterized in a non-eddy-resolving ocean model, as a nonstationary random process, in which the corresponding parameters are derived from the control coupled simulation. The key parameter of the random process—its variance—is related to the large-scale flow baroclinicity index. It is shown that the coupled model with the non-eddy-resolving ocean component and the parameterized eddies correctly simulates climatology and low-frequency variability of the control eddy-resolving coupled solution.

scholarly journals Estimating Eddy Stresses by Fitting Dynamics to Observations Using a Residual-Mean Ocean Circulation Model and Its Adjoint

2005 ◽  
Vol 35 (10) ◽  
pp. 1891-1910 ◽  
Author(s):  
David Ferreira ◽  
John Marshall ◽  
Patrick Heimbach
Keyword(s):  
Ocean Circulation ◽  
Large Scale ◽  
Three Dimensional ◽  
Circulation Model ◽  
Eddy Diffusivity ◽  
Ocean Model ◽  
Global Ocean ◽  
Western Boundary ◽  
Mean Flow ◽  
Ocean Circulation Model

Abstract A global ocean circulation model is formulated in terms of the “residual mean” and used to study eddy–mean flow interaction. Adjoint techniques are used to compute the three-dimensional eddy stress field that minimizes the departure of the coarse-resolution model from climatological observations of temperature. The resulting 3D maps of eddy stress and residual-mean circulation yield a wealth of information about the role of eddies in large-scale ocean circulation. In eddy-rich regions such as the Southern Ocean, the Kuroshio, and the Gulf Stream, eddy stresses have an amplitude comparable to the wind stress, of order 0.2 N m−2, and carry momentum from the surface down to the bottom, where they are balanced by mountain form drag. From the optimized eddy stress, 3D maps of horizontal eddy diffusivity κ are inferred. The diffusivities have a well-defined large-scale structure whose prominent features are 1) large values of κ (up to 4000 m2 s−1) in the western boundary currents and on the equatorial flank of the Antarctic Circumpolar Current and 2) a surface intensification of κ, suggestive of a dependence on the stratification N 2. It is shown that implementation of an eddy parameterization scheme in which the eddy diffusivity has an N 2 dependence significantly improves the climatology of the ocean model state relative to that obtained using a spatially uniform diffusivity.

scholarly journals On the Total, Mean, and Eddy Heat and Freshwater Transports in the Southern Hemisphere of a ⅛° × ⅛° Global Ocean Model

2007 ◽  
Vol 37 (2) ◽  
pp. 277-295 ◽  
Author(s):  
A. J. Meijers ◽  
N. L. Bindoff ◽  
J. L. Roberts
Keyword(s):  
Southern Hemisphere ◽  
Ocean Circulation ◽  
General Circulation ◽  
Large Scale ◽  
Circulation Model ◽  
Drake Passage ◽  
Ocean Model ◽  
Global Ocean ◽  
Western Boundary ◽  
Eddy Transport

Abstract The large-scale volume, heat, and freshwater ocean transports in the Southern Hemisphere are investigated using time-averaged output from a seasonless, high-resolution general circulation model. The ocean circulation is realistic, and property transports are comparable to observations. The Antarctic Circumpolar Current (ACC) carries 144 Sv (Sv ≡ 106 m3 s−1) of water eastward across Drake Passage, increasing to 155 Sv south of Australia because of the Indonesian Throughflow (ITF). There is a clear Indo-Pacific gyre around Australia exchanging −10 Sv, 0.9 PW of heat, and 0.2 Sv of freshwater through the ITF, and there is a 9-Sv leakage from the Tasman Sea to the Indian Ocean. The transport of heat and freshwater by eddies is localized to the upper 1000 m of the water column and specific regions, such as western boundary currents, confluences, and the subantarctic front (SAF). Eddy transport of heat and freshwater is negligible in gyre interiors and south of the SAF but is vital across the northern edge of the ACC, in particular at the Agulhas Retroflection where eddies accomplish almost 100% of the net ocean heat and 60% of the southward freshwater transport. The eddy transport is almost zero across the latitude of Drake Passage while in a quasi-Lagrangian frame eddy transports are significant across the ACC but surprisingly are still smaller than the mean transport of heat. Mean and eddy property transport divergences are found to be strongly compensating in areas of high eddy activity. This is caused by increased baroclinic instability in strong mean flows, which induces an opposing eddy transport. This relationship is observed to be stronger in the case of horizontal heat transport than in corresponding horizontal freshwater transports.

scholarly journals Frequency-Domain Analysis of the Energy Budget in an Idealized Coupled Ocean–Atmosphere Model

2020 ◽  
Vol 33 (2) ◽  
pp. 707-726 ◽  
Author(s):  
Paige E. Martin ◽  
Brian K. Arbic ◽  
Andrew McC. Hogg ◽  
Andrew E. Kiss ◽  
James R. Munroe ◽  
...  
Keyword(s):  
Frequency Domain ◽  
Time Scales ◽  
Energy Budget ◽  
Western Boundary Current ◽  
Spectral Energy ◽  
Western Boundary ◽  
Intrinsic Variability ◽  
Boundary Current ◽  
Ocean Atmosphere ◽  
Atmosphere Model

AbstractClimate variability is investigated by identifying the energy sources and sinks in an idealized, coupled, ocean–atmosphere model, tuned to mimic the North Atlantic region. The spectral energy budget is calculated in the frequency domain to determine the processes that either deposit energy into or extract energy from each fluid, over time scales from one day up to 100 years. Nonlinear advection of kinetic energy is found to be the dominant source of low-frequency variability in both the ocean and the atmosphere, albeit in differing layers in each fluid. To understand the spatial patterns of the spectral energy budget, spatial maps of certain terms in the spectral energy budget are plotted, averaged over various frequency bands. These maps reveal three dynamically distinct regions: along the western boundary, the western boundary current separation, and the remainder of the domain. The western boundary current separation is found to be a preferred region to energize oceanic variability across a broad range of time scales (from monthly to decadal), while the western boundary itself acts as the dominant sink of energy in the domain at time scales longer than 50 days. This study paves the way for future work, using the same spectral methods, to address the question of forced versus intrinsic variability in a coupled climate system.

Internal Wave Generation by a Combination of Tidal and Steady Currents

2021 ◽  
Author(s):  
Xiaolin Bai ◽  
Kevin Lamb ◽  
José da Silva
Keyword(s):  
Internal Wave ◽  
Wave Generation ◽  
Satellite Observations ◽  
Internal Tides ◽  
Small Scale ◽  
Western Boundary ◽  
Boundary Current ◽  
Reanalysis Dataset ◽  
Amazon Shelf ◽  
Internal Wave Generation

<p>In the presence of topography, two main contributors for internal wave energy are tide-topography interaction transferring energy from the barotropic tide to internal tides, and lee wave generation when geostrophic currents or eddying abyssal flows interact with topography. In the past few decades, many studies considered the respective contribution of the oscillating flows or steady background flows, but few investigations have considered both.  </p><p>In this talk, we consider the joint effects of tidal and steady currents to investigate internal wave generation and propagation on the Amazon shelf, a hotspot for internal solitary wave (ISW) generation. The Amazon Shelf is off the mouth of the Amazon River in the southwest tropical Atlantic Ocean, affected by strong tidal constituents over complex bottom bathymetry and a strong western boundary current, the North Brazilian Current (NBC). Both satellite observations and numerical modelling are used in this study. Satellite observations provide a clear visualization of the wave characteristics, such as temporal and spatial distributions, propagating direction and its relation to background currents. Based on parameters from satellite observations and reanalysis dataset, we set up a model to numerically investigate the dynamics of the ISW generation. We demonstrate that the small-scale topography contributes to a rich generation of along-shelf propagating ISW, which significantly contribute to the ocean mixing and potentially cause sediment resuspension. Moreover, the ISW-induced currents also contribute to the sea surface wave breaking as observed by satellite measurements. In addition, statistics based on a decade of satellite images and numerical investigations on seasonal variations of the ISWs and the NBC improve our understanding of the generation and evolution of these nonlinear internal waves in the presence of background currents.</p>

Overlooked current estimation biases arising from the Lagrangian Argo trajectory derivation method

2021 ◽  
Author(s):  
Tianyu Wang ◽  
Yan Du ◽  
Minyang Wang
Keyword(s):  
Ocean Model ◽  
Ocean Dynamics ◽  
Western Boundary ◽  
Boundary Current ◽  
Current Estimation ◽  
Quantitative Metrics ◽  
Circumpolar Current ◽  
Derivation Method ◽  
Jet Interactions ◽  
Reference Layer

AbstractAn Argo simulation system is used to provide synthetic Lagrangian trajectories based on the Estimating the Circulation and Climate of the Ocean model, Phase II (ECCO2). In combination with ambient Eulerian velocity at the reference layer (1000 m) from the model, quantitative metrics of the Lagrangian trajectory-derived velocities are computed. The result indicates that the biases induced by the derivation algorithm are strongly linked with ocean dynamics. In low latitudes, Ekman currents and vertically sheared geostrophic currents influence both the magnitude and the direction of the derivation velocity vectors. The maximal shear-induced biases exist near the equator with the amplitudes reaching up to about 1.2 cm s-1. The angles of the shear biases are pronounced in the low latitude oceans, ranging from -8° to 8°. Specifically, the study shows an overlooked bias from the float drifting motions that mainly occurs in the western boundary current and Antarctic circumpolar current (ACC) regions. In these regions, a recently reported horizontal acceleration measured via Lagrangian floats is significantly associated with the strong eddy-jet interactions. The acceleration could induce an overestimation of Eulerian current velocity magnitudes. For the common Argo floats with a 9-day float parking period, the derivation speed biases induced by velocity acceleration would be as large as 3 cm s-1, approximately 12% of the ambient velocity. It might have implications to map the mean mid-depth ocean currents from Argo trajectories, as well as understand the dynamics of eddy-jet interactions in the ocean.

Topographic Rossby Waves in the Abyssal South China Sea

2021 ◽  
Author(s):  
QI QUAN ◽  
ZHONGYA CAI ◽  
GUANGZHEN JIN ◽  
ZHIQIANG LIU
Keyword(s):  
South China Sea ◽  
Group Velocity ◽  
South China ◽  
Large Scale ◽  
Rossby Waves ◽  
Western Boundary ◽  
Horizontal Shear ◽  
Boundary Current ◽  
China Sea ◽  
Topographic Rossby Waves

AbstractTopographic Rossby waves (TRWs) in the abyssal South China Sea (SCS) are investigated using observations and high-resolution numerical simulations. These energetic waves can account for over 40% of the kinetic energy (KE) variability in the deep western boundary current and seamount region in the central SCS. This proportion can even reach 70% over slopes in the northern and southern SCS. The TRW-induced currents exhibit columnar (i.e., in-phase) structure in which the speed increases downward. Wave properties such as the period (5–60 days), wavelength (100–500 km), and vertical trapping scale (102–103 m) vary significantly depending on environmental parameters of the SCS. The TRW energy propagates along steep topography with phase propagation offshore. TRWs with high frequencies exhibit a stronger climbing effect than low-frequency ones and hence can move further upslope. For TRWs with a certain frequency, the wavelength and trapping scale are dominated by the topographic beta, whereas the group velocity is more sensitive to the internal Rossby deformation radius. Background circulation with horizontal shear can change the wavelength and direction of TRWs if the flow velocity is comparable to the group velocity, particularly in the central, southern, and eastern SCS. A case study suggests two possible energy sources for TRWs: mesoscale perturbation in the upper layer and large-scale background circulation in the deep layer. The former provides KE by pressure work, whereas the latter transfers the available potential energy (APE) through baroclinic instability.

scholarly journals Variational data assimilation with superparameterization

2015 ◽  
Vol 2 (2) ◽  
pp. 513-536 ◽  
Author(s):  
I. Grooms ◽  
Y. Lee
Keyword(s):  
Data Assimilation ◽  
Large Scale ◽  
Climate Models ◽  
Low Cost ◽  
Ocean Model ◽  
Variational Data Assimilation ◽  
Scale Model ◽  
Small Scale ◽  
New Applications ◽  
Lorenz 96

Abstract. Superparameterization (SP) is a multiscale computational approach wherein a large scale atmosphere or ocean model is coupled to an array of simulations of small scale dynamics on periodic domains embedded into the computational grid of the large scale model. SP has been successfully developed in global atmosphere and climate models, and is a promising approach for new applications. The authors develop a 3D-Var variational data assimilation framework for use with SP; the relatively low cost and simplicity of 3D-Var in comparison with ensemble approaches makes it a natural fit for relatively expensive multiscale SP models. To demonstrate the assimilation framework in a simple model, the authors develop a new system of ordinary differential equations similar to the two-scale Lorenz-'96 model. The system has one set of variables denoted {Yi}, with large and small scale parts, and the SP approximation to the system is straightforward. With the new assimilation framework the SP model approximates the large scale dynamics of the true system accurately.

scholarly journals Controlling factors of the OMZ in the Arabian Sea

2012 ◽  
Vol 9 (5) ◽  
pp. 5509-5550
Author(s):  
L. Resplandy ◽  
M. Lévy ◽  
L. Bopp ◽  
V. Echevin ◽  
S. Pous ◽  
...  
Keyword(s):  
Spatial Distribution ◽  
Oxygen Concentration ◽  
Seasonal Cycle ◽  
Arabian Sea ◽  
Coastal Upwelling ◽  
Western Boundary ◽  
Ekman Pumping ◽  
Low Oxygen ◽  
Boundary Current ◽  
Ocean Dynamic

Abstract. In-situ observations indicate that the Arabian Sea oxygen minimum zone (OMZ) is only weakly influenced by the strong seasonal cycle of ocean dynamic and biogeochemistry forced by the asian monsoon system and it is spatially decorrelated from the coastal upwelling systems where the biological production is the strongest. In this study we examine the factors controlling the seasonality and the spatial distribution of the OMZ in the Arabian Sea using a coupled bio-physical model. We find that the oxygen concentration in the OMZ displays a seasonal cycle with an amplitude of 5–15 % of the annual mean oxygen concentration. The OMZ is ventilated by lateral ventilation along the western boundary current and in the coastal undercurrent along India during the summer monsoon and by coastal downwelling and negative Ekman pumping during the fall intermonsoon and winter monsoon. This ventilation is counterbalanced by strong coastal upwelling and positive Ekman pumping of low oxygen waters at the base of the OMZ during the spring intermonsoon. Although the factors controlling the OMZ seasonality are associated with the men circulation, we find that mesoscale dynamics modulates them by limiting the vertical ventilation during winter and enhancing it through lateral advection during the rest of the year. Processes explaining the establishment and spatial distribution of the OMZ were quantified using a perturbation experiment initialised with no OMZ. As expected, the oxygen depletion is triggered by strong biological activity in central Arabian Sea during winter and in western and eastern boundary coastal upwelling systems during summer. We find that the 3-D ocean dynamic largely controls the spatial distribution of the OMZ. The eastward shift ensues from the northward lateral transport of ventilated waters along the western and eastern coasts and the advection offshore of low oxygen waters formed in the upwelling system.

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