scholarly journals The Scale of Submesoscale Baroclinic Instability Globally

2020 ◽  
Vol 50 (9) ◽  
pp. 2649-2667 ◽  
Author(s):  
Jihai Dong ◽  
Baylor Fox-Kemper ◽  
Hong Zhang ◽  
Changming Dong
Keyword(s):  
Mixed Layer ◽  
Spatial Scales ◽  
Baroclinic Instability ◽  
Ocean Model ◽  
Median Surface ◽  
Surface Mixed Layer ◽  
Vertical Resolution ◽  
High Vertical Resolution ◽  
Mixed Layers ◽  
Strong Seasonality

AbstractThe spatial scale of submesoscales is an important parameter for studies of submesoscale dynamics and multiscale interactions. The horizontal spatial scales of baroclinic, geostrophic-branch mixed layer instabilities (MLI) are investigated globally (without the equatorial or Arctic oceans) based on observations and simulations in the surface and bottom mixed layers away from significant topography. Three high-vertical-resolution boundary layer schemes driven with profiles from a MITgcm global submesoscale-permitting model improve robustness. The fastest-growing MLI wavelength decreases toward the poles. The zonal median surface MLI wavelength is 51–2.9 km when estimated from the observations and from 32, 25, and 27 km to 2.5, 1.2, and 1.1 km under the K-profile parameterization (KPP), Mellor–Yamada (MY), and κ–ε schemes, respectively. The surface MLI wavelength has a strong seasonality with a median value 1.6 times smaller in summer (10 km) than winter (16 km) globally from the observations. The median bottom MLI wavelengths estimated from simulations are 2.1, 1.4, and 0.41 km globally under the KPP, MY, and κ–ε schemes, respectively, with little seasonality. The estimated required ocean model grid spacings to resolve wintertime surface mixed layer eddies are 1.9 km (50% of regions resolved) and 0.92 km (90%) globally. To resolve summertime eddies or MLI seasonality requires grids finer than 1.3 km (50%) and 0.55 km (90%). To resolve bottom mixed layer eddies, grids finer than 257, 178, and 51 m (50%) and 107, 87, and 17 m (90%) are estimated under the KPP, MY, and κ–ε schemes.

Parameterization of Mixed Layer Eddies. Part I: Theory and Diagnosis

2008 ◽  
Vol 38 (6) ◽  
pp. 1145-1165 ◽  
Author(s):  
Baylor Fox-Kemper ◽  
Raffaele Ferrari ◽  
Robert Hallberg
Keyword(s):  
Mixed Layer ◽  
Mixed Layer Depth ◽  
Baroclinic Instability ◽  
Finite Amplitude ◽  
Climate Impacts ◽  
Surface Mixed Layer ◽  
Rotation Rates ◽  
Wide Range ◽  
Horizontal Density Gradient ◽  
Mixed Layers

Abstract Ageostrophic baroclinic instabilities develop within the surface mixed layer of the ocean at horizontal fronts and efficiently restratify the upper ocean. In this paper a parameterization for the restratification driven by finite-amplitude baroclinic instabilities of the mixed layer is proposed in terms of an overturning streamfunction that tilts isopycnals from the vertical to the horizontal. The streamfunction is proportional to the product of the horizontal density gradient, the mixed layer depth squared, and the inertial period. Hence restratification proceeds faster at strong fronts in deep mixed layers with a weak latitude dependence. In this paper the parameterization is theoretically motivated, confirmed to perform well for a wide range of mixed layer depths, rotation rates, and vertical and horizontal stratifications. It is shown to be superior to alternative extant parameterizations of baroclinic instability for the problem of mixed layer restratification. Two companion papers discuss the numerical implementation and the climate impacts of this parameterization.

scholarly journals Baroclinic Instability with a Simple Model for Vertical Mixing

2019 ◽  
Vol 49 (12) ◽  
pp. 3273-3300 ◽  
Author(s):  
Matthew N. Crowe ◽  
John R. Taylor
Keyword(s):  
Growth Rate ◽  
Stability Analysis ◽  
Simple Model ◽  
Numerical Simulations ◽  
Mixed Layer ◽  
Numerical Models ◽  
Baroclinic Instability ◽  
Vertical Mixing ◽  
Small Scale ◽  
Surface Mixed Layer

AbstractHere, we examine baroclinic instability in the presence of vertical mixing in an idealized setting. Specifically, we use a simple model for vertical mixing of momentum and buoyancy and expand the buoyancy and vorticity in a series for small Rossby numbers. A flow in subinertial mixed layer (SML) balance (see the study by Young in 1994) exhibits a normal mode linear instability, which is studied here using linear stability analysis and numerical simulations. The most unstable modes grow by converting potential energy associated with the basic state into kinetic energy of the growing perturbations. However, unlike the inviscid Eady problem, the dominant energy balance is between the buoyancy flux and the energy dissipated by vertical mixing. Vertical mixing reduces the growth rate and changes the orientation of the most unstable modes with respect to the front. By comparing with numerical simulations, we find that the predicted scale of the most unstable mode matches the simulations for small Rossby numbers while the growth rate and orientation agree for a broader range of parameters. A stability analysis of a basic state in SML balance using the inviscid QG equations shows that the angle of the unstable modes is controlled by the orientation of the SML flow, while stratification associated with an advection/diffusion balance controls the size of growing perturbations for small Ekman numbers and/or large Rossby numbers. These results imply that baroclinic instability can be inhibited by small-scale turbulence when the Ekman number is sufficiently large and might explain the lack of submesoscale eddies in observations and numerical models of the ocean surface mixed layer during summer.

Submesoscale Currents in the Subtropical Upper Ocean Observed by Long-Term High-Resolution Mooring Arrays

2021 ◽  
Vol 51 (1) ◽  
pp. 187-206
Author(s):  
Zhiwei Zhang ◽  
Xincheng Zhang ◽  
Bo Qiu ◽  
Wei Zhao ◽  
Chun Zhou ◽  
...  
Keyword(s):  
Mixed Layer ◽  
Source Term ◽  
Dominant Role ◽  
Baroclinic Instability ◽  
Northwestern Pacific ◽  
Subtropical Countercurrent ◽  
Vertical Heat Flux ◽  
Mixed Layers ◽  
Submesoscale Currents

AbstractAlthough observational efforts have been made to detect submesoscale currents (submesoscales) in regions with deep mixed layers and/or strong mesoscale kinetic energy (KE), there have been no long-term submesoscale observations in subtropical gyres, which are characterized by moderate values of both mixed layer depths and mesoscale KE. To explore submesoscale dynamics in this oceanic regime, two nested mesoscale- and submesoscale-resolving mooring arrays were deployed in the northwestern Pacific subtropical countercurrent region during 2017–19. Based on the 2 years of data, submesoscales featuring order one Rossby numbers, large vertical velocities (with magnitude of 10–50 m day−1) and vertical heat flux, and strong ageostrophic KE are revealed in the upper 150 m. Although most of the submesoscales are surface intensified, they are found to penetrate far beneath the mixed layer. They are most energetic during strong mesoscale strain periods in the winter–spring season but are generally weak in the summer–autumn season. Energetics analysis suggests that the submesoscales receive KE from potential energy release but lose a portion of it through inverse cascade. Because this KE sink is smaller than the source term, a forward cascade must occur to balance the submesoscale KE budget, for which symmetric instability may be a candidate mechanism. By synthesizing observations and theories, we argue that the submesoscales are generated through a combination of baroclinic instability in the upper mixed and transitional layers and mesoscale strain-induced frontogenesis, among which the former should play a more dominant role in their final generation stage.

scholarly journals Submesoscale Baroclinic Instability in the Bottom Boundary Layer

2018 ◽  
Vol 48 (11) ◽  
pp. 2571-2592 ◽  
Author(s):  
Jacob O. Wenegrat ◽  
Jörn Callies ◽  
Leif N. Thomas
Keyword(s):  
Boundary Layer ◽  
Mixed Layer ◽  
Baroclinic Instability ◽  
Slope Angle ◽  
Deep Ocean ◽  
Bottom Boundary Layer ◽  
Surface Mixed Layer ◽  
Nonlinear Simulation ◽  
Bottom Boundary ◽  
Topographic Slope

AbstractWeakly stratified layers over sloping topography can support a submesoscale baroclinic instability mode, a bottom boundary layer counterpart to surface mixed layer instabilities. The instability results from the release of available potential energy, which can be generated because of the observed bottom intensification of turbulent mixing in the deep ocean, or the Ekman adjustment of a current on a slope. Linear stability analysis suggests that the growth rates of bottom boundary layer baroclinic instabilities can be comparable to those of the surface mixed layer mode and are relatively insensitive to topographic slope angle, implying the instability is robust and potentially active in many areas of the global oceans. The solutions of two separate one-dimensional theories of the bottom boundary layer are both demonstrated to be linearly unstable to baroclinic instability, and results from an example nonlinear simulation are shown. Implications of these findings for understanding bottom boundary layer dynamics and processes are discussed.

A Modeling Study of Circulation and Eddies in the Persian Gulf

2010 ◽  
Vol 40 (9) ◽  
pp. 2122-2134 ◽  
Author(s):  
Prasad G. Thoppil ◽  
Patrick J. Hogan
Keyword(s):  
Persian Gulf ◽  
Spatial Scales ◽  
Baroclinic Instability ◽  
Mesoscale Eddies ◽  
Ocean Model ◽  
Coastal Current ◽  
Basin Scale ◽  
Strait Of Hormuz ◽  
Moderate Resolution Imaging Spectroradiometer ◽  
The Persian Gulf

Abstract The circulation and mesoscale eddies in the Persian Gulf are investigated using results from a high-resolution (∼1 km) Hybrid Coordinate Ocean Model (HYCOM). The circulation in the Persian Gulf is composed of two spatial scales: basin scale and mesoscale. The progression of a cyclonic circulation cell dominates the basin-scale circulation in the eastern half of the gulf (52°–55°E) during March–July. This is primarily the consequence of density-driven outflow–inflow through the Strait of Hormuz and strong stratification. A northwestward-flowing Iranian Coastal Current (ICC; 30–40 cm s−1) between the Strait of Hormuz and north of Qatar (∼52°E) forms the northern flank of the cell. Between July and August the ICC becomes unstable because of the baroclinic instability mechanism by releasing the potential energy stored in the cross-shelf density gradient. As a result, the meanders in the ICC evolve into a series of mesoscale eddies, which is denoted as the Iranian coastal eddies (ICE). The ICE have a diameter of about 115–130 km and extend vertically over most of the water column. Three cyclonic eddies produced by the model during August–September 2005 compared quite well with the Moderate Resolution Imaging Spectroradiometer (MODIS) SST and chlorophyll-a observations. The remnants of ICE are seen until November, after which they dissipate as the winter cooling causes the thermocline to collapse.

scholarly journals Embedding a One-column Ocean Model (SIT 1.06) in the Community Atmosphere Model 5.3 (CAM5.3; CAM5–SIT v1.0) to Improve Madden–Julian Oscillation Simulation in Boreal Winter

2021 ◽  
Author(s):  
Yung-Yao Lan ◽  
Huang-Hsiung Hsu ◽  
Wan-Ling Tseng ◽  
Li-Chiang Jiang
Keyword(s):  
Mixed Layer ◽  
Diurnal Cycle ◽  
Ocean Model ◽  
Boreal Winter ◽  
Ocean Temperature ◽  
Vertical Resolution ◽  
Madden Julian Oscillation ◽  
Oceanic Mixed Layer ◽  
Sit Model ◽  
Community Atmosphere Model

Abstract. The effect of the air–sea interaction on the Madden–Julian Oscillation (MJO) was investigated using the one-column ocean model Snow–Ice–Thermocline (SIT 1.06) embedded in the Community Atmosphere Model 5.3 (CAM5.3; hereafter CAM5–SIT v1.0). The SIT model with 41 vertical layers was developed to simulate sea surface temperature (SST) and upper-ocean temperature variations with a high vertical resolution that resolves the cool skin and diurnal warm layer and the upper oceanic mixed layer. A series of 30-year sensitivity experiments were conducted in which various model configurations (e.g., coupled versus uncoupled, vertical resolution and depth of the SIT model, coupling domains, and absence of the diurnal cycle) were considered to evaluate the effect of air–sea coupling on MJO simulation. Most of the CAM5–SIT experiments exhibited higher fidelity than the CAM5-alone experiment in characterizing the basic features of the MJO such as spatiotemporal variability and the eastward propagation in boreal winter. The overall MJO simulation performance of CAM5–SIT benefited from (1) better resolving the fine structure of upper-ocean temperature and therefore the air–sea interaction that resulted in more realistic intraseasonal variability in both SST and atmospheric circulation and (2) the adequate thickness and vertical resolution of the oceanic mixed layer. The sensitivity experiments demonstrated the necessity of coupling the tropical eastern Pacific in addition to the tropical Indian Ocean and the tropical western Pacific. Enhanced MJO could be obtained without considering the diurnal cycle in coupling.

scholarly journals The Vertical Structure of Open-Ocean Submesoscale Variability during a Full Seasonal Cycle

2020 ◽  
Vol 50 (1) ◽  
pp. 145-160 ◽  
Author(s):  
Zachary K. Erickson ◽  
Andrew F. Thompson ◽  
Jörn Callies ◽  
Xiaolong Yu ◽  
Alberto Naveira Garabato ◽  
...  
Keyword(s):  
Structure Function ◽  
Mixed Layer ◽  
Seasonal Cycle ◽  
Spatial Scales ◽  
Horizontal Resolution ◽  
Ocean Model ◽  
Open Ocean ◽  
Order Structure ◽  
Numerical Ocean Model ◽  
Ocean Region

AbstractSubmesoscale dynamics are typically intensified at boundaries and assumed to weaken below the mixed layer in the open ocean. Here, we assess both the seasonality and the vertical distribution of submesoscale motions in an open-ocean region of the northeast Atlantic. Second-order structure functions, or variance in properties separated by distance, are calculated from submesoscale-resolving ocean glider and mooring observations, as well as a 1/48° numerical ocean model. This dataset combines a temporal coverage that extends through a full seasonal cycle, a horizontal resolution that captures spatial scales as small as 1 km, and vertical sampling that provides near-continuous coverage over the upper 1000 m. While kinetic and potential energies undergo a seasonal cycle, being largest during the winter, structure function slopes, influenced by dynamical characteristics, do not exhibit a strong seasonality. Furthermore, structure function slopes show weak vertical variations; there is not a strong change in properties across the base of the mixed layer. Additionally, we compare the observations to output from a high-resolution numerical model. The model does not represent variability associated with superinertial motions and does not capture an observed reduction in submesoscale kinetic energy that occurs throughout the water column in spring. Overall, these results suggest that the transfer of mixed layer submesoscale variability down to depths below the traditionally defined mixed layer is important throughout the weakly stratified subpolar mode waters.

scholarly journals Baroclinic Instability in the Presence of Convection

2018 ◽  
Vol 48 (1) ◽  
pp. 45-60 ◽  
Author(s):  
Jörn Callies ◽  
Raffaele Ferrari
Keyword(s):  
Numerical Simulations ◽  
Mixed Layer ◽  
Baroclinic Instability ◽  
Relative Strength ◽  
Small Scale ◽  
Upper Ocean ◽  
Bulk Properties ◽  
Scale Turbulence ◽  
Balanced Dynamics ◽  
Mixed Layers

AbstractBaroclinic mixed-layer instabilities have recently been recognized as an important source of submesoscale energy in deep winter mixed layers. While the focus has so far been on the balanced dynamics of these instabilities, they occur in and depend on an environment shaped by atmospherically forced small-scale turbulence. In this study, idealized numerical simulations are presented that allow the development of both baroclinic instability and convective small-scale turbulence, with simple control over the relative strength. If the convection is only weakly forced, baroclinic instability restratifies the layer and shuts off convection, as expected. With increased forcing, however, it is found that baroclinic instabilities are remarkably resilient to the presence of convection. Even if the instability is too weak to restratify the layer and shut off convection, the instability still grows in the convecting environment and generates baroclinic eddies and fronts. This suggests that despite the vigorous atmospherically forced small-scale turbulence in winter mixed layers, baroclinic instabilities can persistently grow, generate balanced submesoscale turbulence, and modify the bulk properties of the upper ocean.

Comparison of the Observed Mixed Layer Depth in the Lee of the Hawaiian Island to the Modeled Mixed Layer Depth of the Regional Navy Coastal Ocean Model

2013 ◽  
Vol 47 (1) ◽  
pp. 55-66 ◽  
Author(s):  
Jeffery Todd Rayburn ◽  
Vladimir M. Kamenkovich
Keyword(s):  
Mixed Layer ◽  
Mixed Layer Depth ◽  
Coastal Ocean ◽  
Ocean Model ◽  
Sea Surface ◽  
Height Anomaly ◽  
Surface Mixed Layer ◽  
Layer Depth ◽  
Coastal Ocean Model

AbstractThis study evaluates the ability of the Hawaii Regional Navy Coastal Ocean Model to accurately predict the depth of the surface mixed layer in the lee of the Hawaiian Islands. Accurately modeling the depth of the surface mixed layer in this complex wake island environment is important to naval operations because the area hosts numerous training exercises. The simulated data were compared to CTD data collected from sea gliders, and tests for correlation were conducted. For mixed layer depths that did show correlation, match-paired t tests were used to determine the significance of the correlations. It was determined that the Hawaii Regional Navy Coastal Ocean Model has difficulty accurately predicting the depth of the surface mixed layer. It was also determined that the model has difficulty with unusual oceanographic features such as mode water eddies. These features are too uncommon and short-lived to be depicted in the climatology data. The climatology data are a major component of the synthetic profiles that the model generates, and these profiles tend to smooth out the unusual subsurface isothermal layer.List of AbbreviationsBT ‐ bathythermographsCCE ‐ cold core eddyCOAMPS ‐ Coupled Ocean/Atmosphere Mesoscale Prediction SystemCTD ‐ conductivity, temperature, and depthGDEM ‐ Generalized Digital Environmental ModelIR ‐ infraredMLD ‐ mixed layer depthMODAS ‐ Modular Ocean Data Assimilation SystemMOODS ‐ Master Oceanographic Observation DatasetNCODA ‐ Navy Coupled Ocean Data AssimilationNCOM1 ‐ Hawaii Regional Navy Coastal Ocean Model with in situ assimilationNCOM2 ‐ Hawaii Regional Navy Coastal Ocean Model without in situ assimilationPAVE ‐ Profile Analysis and Visualization EnvironmentSSHa ‐ sea surface height anomaly derived from altimetrySST ‐ sea surface temperatureWCE ‐ warm core eddy

scholarly journals The scale and activity of symmetric instability estimated from a global submesoscale-permitting ocean model

2021 ◽  
Author(s):  
Jihai Dong ◽  
Baylor Fox-Kemper ◽  
Hong Zhang ◽  
Changming Dong
Keyword(s):  
Kinetic Energy ◽  
Upper Bound ◽  
Linear Prediction ◽  
Ocean Model ◽  
Surface Mixed Layer ◽  
Western Atlantic ◽  
Structure And Dynamics ◽  
The Western Pacific ◽  
Strong Seasonality ◽  
Symmetric Instability

AbstractSymmetric instability (SI) extracts kinetic energy from fronts in the surface mixed layer (SML), potentially affecting the SML structure and dynamics. Here, a global submesoscale-permitting ocean model named MITgcm LLC4320 simulation is used to examine the Stone (1966) linear prediction of the maximum SI scale to estimate grid spacings needed to begin resolving SI. Furthermore, potential effects of SI on the usable wind-work are estimated roughly: this estimate of SI “activity” is useful for assessing if these modes should be resolved or parameterized. The maximum SI scale varies by latitude with median values of 568 m to 23 m. Strong seasonality is observed in the SI scale and activity. The median scale in winter is 188 m globally, 2.5 times of that of summer (75 m). SI is more active in winter: 15% of the time compared with 6% in summer. The strongest SI activity is found in the western Pacific, western Atlantic, and Southern Oceans. The required grid spacings for a global model to begin resolving SI eddies in the SML are 24 m (50% of regions resolved) and 7.9 m (90%) in winter, decreasing to 9.4 m (50%) and 3.6 m (90%) in summer. It is also estimated that SI may reduce usable wind-work by an upper bound of 0.83 mW m−2 globally, or 5% of the global magnitude. The sensitivity of these estimates to empirical thresholds is provided in the text.

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