scholarly journals Properties and evolution of a submesoscale cyclonic spiral

2021 ◽  
Author(s):  
Reiner Onken ◽  
Burkard Baschek
Keyword(s):  
Extreme Values ◽  
Euphotic Zone ◽  
Horizontal Resolution ◽  
Surface Flow ◽  
Ocean Modeling ◽  
Vortex Center ◽  
Regional Ocean Modeling System ◽  
Near Surface ◽  
Surface Drifters ◽  
Radial Outflow

Abstract. The evolution of a submesoscale cyclonic spiral of 1 km in diameter is simulated with ROMS (Regional Ocean Modeling System) using 33.3 m horizontal resolution in a triple-nested configuration. The generation of the spiral starts from a dense filament that is rolled into a vortex and detaches from the filament. During spin-up, extreme values are attained by various quantities, that are organized in single-arm and multi-arm spirals. The spin-down starts when the cyclone separates from the filament. At the same time, the horizontal speed develops a dipole-like pattern and isotachs form closed contours around the vortex center. The amplitudes of most quantities decrease significantly, but the instantaneous vertical velocity w exhibits high-frequency oscillations and more pronounced extremes than during spin-up. The oscillations are due to vortex Rossby waves (VRWs), that circle the eddy counterclockwise and generate multi-arm spirals with alternating signs by means of azimuthal vorticity advection. Experiments with virtual surface drifters and isopycnal floats indicate downwelling everywhere near the surface. The downwelling is most intense in the center of the spiral at all depth levels, leading to a radial outflow in the thermocline and weak upwelling at the periphery. This overturning circulation is driven by convergent near-surface flow and associated subduction of isopycnals. While the downwelling in the center may support the export of particulate organic carbon from the mixed layer into the main thermocline, the upwelling at the periphery effectuates an upward isopycnal transport of nutrients, enhancing the growth of phytoplankton in the euphotic zone.

scholarly journals Permanent Meanders in the California Current System

2008 ◽  
Vol 38 (8) ◽  
pp. 1690-1710 ◽  
Author(s):  
L. R. Centurioni ◽  
J. C. Ohlmann ◽  
P. P. Niiler
Keyword(s):  
Sea Level ◽  
Current System ◽  
California Current ◽  
Ocean Model ◽  
Ocean Modeling ◽  
Surface Velocity ◽  
Regional Ocean Modeling System ◽  
Near Surface ◽  
Geostrophic Velocity ◽  
California Current System

Abstract Surface Velocity Program (SVP) drifter data from 1987 through 2005; Archiving, Validation, and Interpretation of Satellite Oceanographic data (AVISO) sea level anomalies; and NCEP reanalysis winds are used to assemble a time-averaged map of the 15-m-deep geostrophic velocity field in the California Current System seaward of about 50 km from the coast. The wind data are used to compute the Ekman currents, which are then subtracted from the drifter velocity measurements. The resulting proxy for geostrophic velocity anomalies computed from drifters and from satellite sea level measurements are combined to form an unbiased mean geostrophic circulation map. The result shows a California Current System that flows southward with four permanent meanders that can extend seaward for more than 800 km. Bands of alternating eastward and westward zonal currents are connected to the meanders and extend several thousand kilometers into the Pacific Ocean. This observed time-mean circulation and its associated eddy energy are compared to those produced by various high-resolution OGCM solutions: Regional Ocean Modeling System (ROMS; 5 km), Parallel Ocean Program model (POP; 1/10°), Hybrid Coordinate Ocean Model (HYCOM; 1/12°), and Naval Research Laboratory (NRL) Layered Ocean Model (NLOM; 1/32°). Simulations in closest agreement with observations come from ROMS, which also produces four meanders, geostrophic time-mean currents, and geostrophic eddy energy consistent with the observed values. The time-mean ageostrophic velocity in ROMS is strongest within the cyclonic part of the meanders and is similar to the ageostrophic velocity produced by nonlinear interaction of Ekman currents with the near-surface vorticity field.

Linear Wind-Forced Beta Plumes with Application to the Hawaiian Lee Countercurrent*

2013 ◽  
Vol 43 (10) ◽  
pp. 2071-2094 ◽  
Author(s):  
Ali Belmadani ◽  
Nikolai A. Maximenko ◽  
Julian P. Mccreary ◽  
Ryo Furue ◽  
Oleg V. Melnichenko ◽  
...  
Keyword(s):  
General Circulation Model ◽  
General Circulation ◽  
Circulation Model ◽  
Ocean Modeling ◽  
North Equatorial Current ◽  
Regional Ocean Modeling System ◽  
The West ◽  
Near Surface ◽  
Zonal Jets ◽  
Hawaiian Lee Countercurrent

Abstract Two numerical ocean models are used to study the baroclinic response to forcing by localized wind stress curl (i.e., a wind-forced β plume, which is a circulation cell developing to the west of the source region and composed of a set of zonal jets) with implications for the Hawaiian Lee Countercurrent (HLCC): an idealized primitive equation model [Regional Ocean Modeling System (ROMS)], and a global, eddy-resolving, general circulation model [Ocean General Circulation Model for the Earth Simulator (OFES)]. In addition, theoretical ideas inferred from a linear continuously stratified model are used to interpret results. In ROMS, vertical mixing preferentially damps higher-order vertical modes. The damping thickens the plume to the west of the forcing region, weakening the near-surface zonal jets and generating deeper zonal currents. The zonal damping scale increases monotonically with the meridional forcing scale, indicating a dominant role of vertical viscosity over diffusion, a consequence of the small forcing scale. In the OFES run forced by NCEP reanalysis winds, the HLCC has a vertical structure consistent with that of idealized β plumes simulated by ROMS, once the contribution of the North Equatorial Current (NEC) has been removed. Without this filtering, a deep HLCC branch appears artificially separated from the surface branch by the large-scale intermediate-depth NEC. The surface HLCC in two different OFES runs exhibits sensitivity to the meridional wind curl scale that agrees with the dynamics of a β plume in the presence of vertical viscosity. The existence of a deep HLCC extension is also suggested by velocities of Argo floats.

scholarly journals Observing and modeling currents on the continental slope: assimilation of high frequency radar currents and hydrography profiles

2014 ◽  
Vol 11 (3) ◽  
pp. 1357-1390
Author(s):  
A. K. Sperrevik ◽  
K. H. Christensen ◽  
J. Röhrs
Keyword(s):  
Data Assimilation ◽  
High Frequency ◽  
Ocean Modeling ◽  
Hf Radar ◽  
Regional Ocean Modeling System ◽  
Background Error ◽  
Realistic Case ◽  
Analysis Scheme ◽  
Surface Drifters ◽  
The Impact

Abstract. Assimilation of High Frequency (HF) radar current observations and CTD hydrography is performed with the 4D-Var analysis scheme implemented in the Regional Ocean Modeling System (ROMS). We consider both an idealized case, with a baroclinic slope current in a periodic channel, and a realistic case for the coast of Vesterålen in Northern Norway. In the realistic case the results of the data assimilation are compared with independent data from acoustic profilers and surface drifters. Best results are obtained when background error correlation scales are small (10 km or less) and when the data assimilation window is short, i.e. about one day. Furthermore, we find that the impact of assimilating HF radar currents is generally larger than the impact of CTD hydrography, which implies that the amount of hydrographic data is insufficient to constrain the solution. Combining the HF radar currents with a few hydrographic profiles gives significantly better results, which demonstrates the importance of complementing surface observations with observations of the vertical structure of the ocean.

scholarly journals Very high-resolution modelling of submesoscale turbulent patterns and processes in the Baltic Sea

2019 ◽  
Author(s):  
Reiner Onken ◽  
Burkard Baschek ◽  
Ingrid M. Angel-Benavides
Keyword(s):  
Baltic Sea ◽  
Horizontal Resolution ◽  
Ocean Modeling ◽  
Regional Ocean Modeling System ◽  
The Baltic Sea ◽  
Operational Model ◽  
The Baltic ◽  
Patterns And Processes ◽  
Comparison Of The Results ◽  
Good Agreement

Abstract. In order to simulate submesoscale turbulent patterns and processes (STPPs) and to analyse their properties and dynamics, the Regional Ocean Modeling System (ROMS) is applied to a subregion of the Baltic Sea around the island of Bornholm. The modeled STPPs provide an aid for the interpretation of observations that were taken during the Expedition Clockwork Ocean in the same region in June 2016. To create a realistic mesoscale and submesoscale environment, ROMS with 500-m horizontal resolution is one-way nested into an existing operational model. The comparison of the results with satellite images shows good agreement. STPPs with horizontal scales

scholarly journals Mechanisms of Lateral Spreading in a Near-Field Buoyant River Plume Entering a Fjord

2021 ◽  
Vol 8 ◽  
Author(s):  
Rebecca A. McPherson ◽  
Craig L. Stevens ◽  
Joanne M. O'Callaghan ◽  
Andrew J. Lucas ◽  
Jonathan D. Nash
Keyword(s):  
Shear Flow ◽  
Pressure Gradient ◽  
Near Field ◽  
Lateral Spreading ◽  
Surface Flow ◽  
River Plume ◽  
Horizontal Shear ◽  
Near Surface ◽  
Surface Drifters ◽  
Spreading Rates

Observations collected from a fast-flowing buoyant river plume entering the head of Doubtful Sound, New Zealand, were analysed to examine the drivers of plume lateral spreading. The near-field plume is characterised by flow speeds of over 2 ms−1, and strong stratification (N2 > 0.1 s−2), resulting in enhanced shear which supports the elevated turbulence dissipation rates (ϵ > 10−3 W kg−1). Estimates of plume lateral spreading rates were derived from the trajectories of Lagrangian GPS surface drifters and from cross-plume hydrographic transects. Lateral spreading rates derived from the latter compared favourably with estimates derived from a control volume technique in a previous study. The lateral spreading of the plume was driven by a baroclinic pressure gradient toward the base of the plume. However, spreading rates were underestimated by the surface drifters. A convergence of near-surface flow from the barotropic pressure gradient concentrated the drifters within the plume core. The combination of enhanced internal turbulence stress and mixing at the base of the surface layer, and the presence of steep fjord sidewalls likely reduced the rate of lateral spreading relative to the theoretical spreading rate. The estimates of plume width from the observations provided evidence of scale-dependent dispersion which followed a 4/3 power law. Two theoretical models of dispersion, turbulence and shear flow dispersion, were examined to assess which was capable of representing the observed spreading. An analytical horizontal shear-flow dispersion model generated estimates of lateral dispersion that were consistent with the observed 4/3 law of dispersion. Therefore, horizontal shear dispersion appeared to be the dominant mechanism of dispersion, thus spreading, in the surface plume layer.

Spatial and Temporal Variability of the M2 Internal Tide Generation and Propagation on the Oregon Shelf

2011 ◽  
Vol 41 (11) ◽  
pp. 2037-2062 ◽  
Author(s):  
J. J. Osborne ◽  
A. L. Kurapov ◽  
G. D. Egbert ◽  
P. M. Kosro
Keyword(s):  
Spatial Variability ◽  
Hot Spot ◽  
Time Windows ◽  
Internal Tide ◽  
Tidal Energy ◽  
Horizontal Resolution ◽  
Ocean Modeling ◽  
Spatial And Temporal Variability ◽  
Affect Intensity ◽  
Regional Ocean Modeling System

Abstract A 1-km-horizontal-resolution model based on the Regional Ocean Modeling System is implemented along the Oregon coast to study average characteristics and intermittency of the M2 internal tide during summer upwelling. Wind-driven and tidally driven flows are simulated in combination, using realistic bathymetry, atmospheric forcing, and boundary conditions. The study period is April through August 2002, when mooring velocities are available for comparison. Modeled subtidal and tidal variability on the shelf are in good quantitative agreement with moored velocity time series observations. Depth-integrated baroclinic tidal energy flux (EF), its divergence, and topographic energy conversion (TEC) from the barotropic to baroclinic tide are computed from high-pass-filtered, harmonically analyzed model results in a series of 16-day time windows. Model results reveal several “hot spots” of intensive TEC on the slope. At these locations, TEC is well balanced by EF divergence. Changes in background stratification and currents associated with wind-driven upwelling and downwelling do not appreciably affect TEC hot spot locations but may affect intensity of internal tide generation at those locations. Relatively little internal tide is generated on the shelf. Areas of supercritical slope near the shelf break partially reflect baroclinic tidal energy to deeper water, contributing to spatial variability in seasonally averaged on-shelf EF. Despite significant temporal and spatial variability in the internal tide, the alongshore-integrated flux of internal tide energy onto the Oregon shelf, where it is dissipated, does not vary much with time. Approximately 65% of the M2 baroclinic tidal energy generated on the slope is dissipated there, and the rest is radiated toward the shelf and interior ocean in roughly equal proportions. An experiment with smoother bathymetry reveals that slope-integrated TEC is more sensitive to bathymetric roughness than on-shelf EF.

scholarly journals Constraining energetic slope currents through assimilation of high-frequency radar observations

Ocean Science ◽  
2015 ◽  
Vol 11 (2) ◽  
pp. 237-249 ◽  
Author(s):  
A. K. Sperrevik ◽  
K. H. Christensen ◽  
J. Röhrs
Keyword(s):  
Data Assimilation ◽  
High Frequency ◽  
Ocean Modeling ◽  
Hf Radar ◽  
Regional Ocean Modeling System ◽  
Background Error ◽  
Realistic Case ◽  
Analysis Scheme ◽  
Surface Drifters ◽  
The Impact

Abstract. Assimilation of high-frequency (HF) radar current observations and CTD hydrography is performed with the 4D-Var analysis scheme implemented in the Regional Ocean Modeling System (ROMS). We consider both an idealized case, with a baroclinic slope current in a periodic channel, and a realistic case for the coast of Vesterålen in northern Norway. In the realistic case, the results of the data assimilation are compared with independent data from acoustic profilers and surface drifters. Best results are obtained when background error correlation scales are small (10 km or less) and when the data assimilation window is short, i.e. about 1 day. Furthermore, we find that the impact of assimilating HF radar currents is generally larger than the impact of CTD hydrography. However, combining the HF radar currents with a few hydrographic profiles gives significantly better results, which demonstrates the importance of complementing surface observations with observations of the vertical structure of the ocean.

scholarly journals The Seasonality of Convective Events in the Labrador Sea

2014 ◽  
Vol 27 (17) ◽  
pp. 6456-6471 ◽  
Author(s):  
Hao Luo ◽  
Annalisa Bracco ◽  
Fan Zhang
Keyword(s):  
Large Scale ◽  
Current System ◽  
Horizontal Resolution ◽  
Surface Heat ◽  
Heat Fluxes ◽  
Ocean Modeling ◽  
Labrador Sea ◽  
Boundary Current ◽  
Regional Ocean Modeling System ◽  
Surface Heat Fluxes

Abstract Modeling deep convection is a key challenge for climate science. Here two simulations of the Labrador Sea circulation obtained with the Regional Ocean Modeling System (ROMS) run at a horizontal resolution of 7.5 km are used to characterize the response of convection to atmospheric forcing and its seasonal variability over the period 1980–2009. The integrations compare well with the sparse observations available. The modeled convection varies in three key aspects over the 30 years considered. First, its magnitude changes greatly at decadal scales. This aspect is supported by the in situ observations. Second, the initiation and peak of convection (i.e., initiation and maximum) shift by 2–3 weeks between strong and weak convective years. Third, the duration of convection varies by approximately one month between strong and weak years. The last two changes are associated with the variability of the time-integrated surface heat fluxes over the Labrador Sea during winter and spring, while the first results from changes in both atmospheric heat fluxes and oceanic conditions through the lateral inflow of warm Irminger Water from the boundary current system to the basin interior. Changes in surface heat fluxes over the convective region are linked to large-scale modes of variability, the North Atlantic Oscillation and Arctic Oscillation. Implications for modeling the climate variability of the Labrador basin are discussed.

Improvements to an intermediate complexity atmospheric model for high-resolution downscaling in very complex terrain.

2021 ◽  
Author(s):  
Dylan Reynolds ◽  
Bert Kruyt ◽  
Ethan Gutmann ◽  
Tobias Jonas ◽  
Michael Lehning ◽  
...  
Keyword(s):  
High Resolution ◽  
Complex Terrain ◽  
Large Scale ◽  
Atmospheric Model ◽  
Horizontal Resolution ◽  
Surface Flow ◽  
Near Surface ◽  
Precipitation Patterns ◽  
Intermediate Complexity ◽  
Nwp Model

<p>            Snow deposition patterns in complex terrain are heavily dependent on the underlying topography. This topography affects precipitating clouds at the kilometer-scale and causes changes to the wind field at the sub-kilometer scale, resulting in altered advection of falling hydrometeors. Snow particles are particularly sensitive to changes in the near-surface flow field due to their low density. Atmospheric models which run at the kilometer scale cannot resolve the actual heterogeneity of the underlying terrain, resulting in precipitation maps which do not capture terrain-affected precipitation patterns. Thus, snow-atmosphere interactions such as preferential deposition are often not resolved in precipitation data used as input to snow models. To bridge this spatial gap and resolve snow-atmosphere interactions at the sub-kilometer scale, we couple an intermediate complexity atmospheric model (ICAR) to the COSMO NWP model. Applying this model to sub-kilometer terrain (horizontal resolution of 50 and 250 m) required changes to ICAR’s computational grid, atmospheric dynamics, and boundary layer flow. As a result, the near-surface flow now accounts for surface roughness and topographically induced speed up. This has been achieved by using terrain descriptors calculated once at initialization which consider a point’s exposure or sheltering relative to surrounding terrain. In particular, the use of a 3-dimensional Sx parameter allows us to simulate areas of stagnation and recirculation on the lee of terrain features. Our approach maintains the accurate large-scale precipitation patterns from COSMO but resolves the dynamics induced by terrain at the sub-kilometer scale without adding additional computational burden. We find that solid precipitation patterns at the ridge scale, such as preferential deposition of snow, are better resolved in the high-resolution version of ICAR than the current ICAR or COSMO models. This updated version of ICAR presents a new tool to dynamically downscale NWP output for snow models and enables future studies of snow-atmosphere interactions at domain scales of 100’s of kilometers.</p>

scholarly journals CAPS v1.0: An improved regional coupled modeling system for Arctic sea ice and climate simulation and prediction

2021 ◽  
Author(s):  
Chao-Yuan Yang ◽  
Jiping Liu ◽  
Dake Chen
Keyword(s):  
Sea Ice ◽  
Regional Scale ◽  
Arctic Sea Ice ◽  
Ocean Modeling ◽  
The Arctic ◽  
Ice Thickness ◽  
Local Error ◽  
Regional Ocean Modeling System ◽  
Near Surface ◽  
Arctic Sea

Abstract. The updated Coupled Arctic Prediction System (CAPS) is evaluated, which is built on new versions of Weather Research and Forecasting model (WRF), the Regional Ocean Modeling System (ROMS), the Community Ice CodE (CICE), and a data assimilation based on the Local Error Subspace Transform Kalman Filter. A set of Pan-Arctic prediction experiments with improved/changed physical parameterizations in WRF, ROMS and CICE as well as different configurations are performed for the year 2018 to assess their impacts on the predictive skill of Arctic sea ice at seasonal timescale. The key improvements of WRF, including cumulus, boundary layer, and land surface schemes, result in improved simulation in near surface air temperature and downward radiation. The major changes of ROMS, including tracer advection and vertical mixing schemes, lead to improved evolution of the predicted total ice extent (particularly correcting the late ice recovery issue in the previous CAPS), and reduced biases in sea surface temperature. The changes of CICE, that include improved ice thermodynamics and assimilation of new sea ice thickness product, have noticeable influences on the predicted ice thickness and the timing of ice recovery. Results from the prediction experiments suggest that the updated CAPS can better predict the evolution of total ice extent compared with its predecessor, though the predictions still have certain biases at the regional scale. We further show that the CAPS can remain skillful beyond the melting season, which may have potential values for stakeholders making decisions for socioeconomical activities in the Arctic.

Sign in / Sign up

Export Citation Format

Share Document