scholarly journals Seasonality in Transition Scale from Balanced to Unbalanced Motions in the World Ocean

2018 ◽  
Vol 48 (3) ◽  
pp. 591-605 ◽  
Author(s):  
Bo Qiu ◽  
Shuiming Chen ◽  
Patrice Klein ◽  
Jinbo Wang ◽  
Hector Torres ◽  
...  
Keyword(s):  
Mixed Layer ◽  
Seasonal Changes ◽  
General Circulation ◽  
World Ocean ◽  
Circulation Model ◽  
Internal Tides ◽  
Western Boundary ◽  
Massachusetts Institute Of Technology ◽  
Boundary Current ◽  
Tropical Oceans

AbstractThe transition scale Lt from balanced geostrophic motions to unbalanced wave motions, including near-inertial flows, internal tides, and inertia–gravity wave continuum, is explored using the output from a global 1/48° horizontal resolution Massachusetts Institute of Technology general circulation model (MITgcm) simulation. Defined as the wavelength with equal balanced and unbalanced motion kinetic energy (KE) spectral density, Lt is detected to be geographically highly inhomogeneous: it falls below 40 km in the western boundary current and Antarctic Circumpolar Current regions, increases to 40–100 km in the interior subtropical and subpolar gyres, and exceeds, in general, 200 km in the tropical oceans. With the exception of the Pacific and Indian sectors of the Southern Ocean, the seasonal KE fluctuations of the surface balanced and unbalanced motions are out of phase because of the occurrence of mixed layer instability in winter and trapping of unbalanced motion KE in shallow mixed layer in summer. The combined effect of these seasonal changes renders Lt to be 20 km during winter in 80% of the Northern Hemisphere oceans between 25° and 45°N and all of the Southern Hemisphere oceans south of 25°S. The transition scale’s geographical and seasonal changes are highly relevant to the forthcoming Surface Water and Ocean Topography (SWOT) mission. To improve the detection of balanced submesoscale signals from SWOT, especially in the tropical oceans, efforts to remove stationary internal tidal signals are called for.

scholarly journals Local Sensitivities of the Gulf Stream Separation

2017 ◽  
Vol 47 (2) ◽  
pp. 353-373 ◽  
Author(s):  
Joseph Schoonover ◽  
William K. Dewar ◽  
Nicolas Wienders ◽  
Bruno Deremble
Keyword(s):  
Gulf Stream ◽  
General Circulation ◽  
Climate Modeling ◽  
Circulation Model ◽  
Ocean Modeling ◽  
Western Boundary ◽  
Massachusetts Institute Of Technology ◽  
Boundary Current ◽  
Institute Of Technology ◽  
The Impact

AbstractRobust and accurate Gulf Stream separation remains an unsolved problem in general circulation modeling whose resolution will positively impact the ocean and climate modeling communities. Oceanographic literature does not face a shortage of plausible hypotheses that attempt to explain the dynamics of the Gulf Stream separation, yet a single theory that the community agrees on is missing. In this paper, the authors investigate the impact of the deep western boundary current (DWBC), coastline curvature, and continental shelf steepening on the Gulf Stream separation within regional configurations of the Massachusetts Institute of Technology General Circulation Model. Artificial modifications to the regional bathymetry are introduced to investigate the sensitivity of the separation to each of these factors. Metrics for subsurface separation detection confirm the direct link between flow separation and the surface expression of the Gulf Stream in the Mid-Atlantic Bight. It is shown that the Gulf Stream separation and mean surface position are most sensitive to the continental slope steepening, consistent with a theory proposed by Melvin Stern in 1998. In contrast, the Gulf Stream separation exhibits minimal sensitivity to the presence of the DWBC and coastline curvature. The implications of these results to the development of a “separation recipe” for ocean modeling are discussed. This study concludes adequate topographic resolution is a necessary, but not sufficient, condition for proper Gulf Stream separation.

scholarly journals Numerical investigation of the Arctic ice–ocean boundary layer and implications for air–sea gas fluxes

Ocean Science ◽  
2017 ◽  
Vol 13 (1) ◽  
pp. 61-75 ◽  
Author(s):  
Arash Bigdeli ◽  
Brice Loose ◽  
An T. Nguyen ◽  
Sylvia T. Cole
Keyword(s):  
Gas Exchange ◽  
Sea Ice ◽  
Mixed Layer ◽  
General Circulation ◽  
Mixed Layer Depth ◽  
Circulation Model ◽  
Horizontal Resolution ◽  
The Arctic ◽  
Massachusetts Institute Of Technology ◽  
Institute Of Technology

Abstract. In ice-covered regions it is challenging to determine constituent budgets – for heat and momentum, but also for biologically and climatically active gases like carbon dioxide and methane. The harsh environment and relative data scarcity make it difficult to characterize even the physical properties of the ocean surface. Here, we sought to evaluate if numerical model output helps us to better estimate the physical forcing that drives the air–sea gas exchange rate (k) in sea ice zones. We used the budget of radioactive 222Rn in the mixed layer to illustrate the effect that sea ice forcing has on gas budgets and air–sea gas exchange. Appropriate constraint of the 222Rn budget requires estimates of sea ice velocity, concentration, mixed-layer depth, and water velocities, as well as their evolution in time and space along the Lagrangian drift track of a mixed-layer water parcel. We used 36, 9 and 2 km horizontal resolution of regional Massachusetts Institute of Technology general circulation model (MITgcm) configuration with fine vertical spacing to evaluate the capability of the model to reproduce these parameters. We then compared the model results to existing field data including satellite, moorings and ice-tethered profilers. We found that mode sea ice coverage agrees with satellite-derived observation 88 to 98 % of the time when averaged over the Beaufort Gyre, and model sea ice speeds have 82 % correlation with observations. The model demonstrated the capacity to capture the broad trends in the mixed layer, although with a significant bias. Model water velocities showed only 29 % correlation with point-wise in situ data. This correlation remained low in all three model resolution simulations and we argued that is largely due to the quality of the input atmospheric forcing. Overall, we found that even the coarse-resolution model can make a modest contribution to gas exchange parameterization, by resolving the time variation of parameters that drive the 222Rn budget, including rate of mixed-layer change and sea ice forcings.

Evaluation of Interior Circulation in a High-Resolution Global Ocean Model. Part I: Deep and Bottom Waters

2004 ◽  
Vol 34 (12) ◽  
pp. 2592-2614 ◽  
Author(s):  
Alexander Sen Gupta ◽  
Matthew H. England
Keyword(s):  
Time Scales ◽  
Mixed Layer ◽  
Deep Water ◽  
General Circulation ◽  
Bottom Water ◽  
Circulation Model ◽  
Global Ocean ◽  
Western Boundary ◽  
Model Skill ◽  
The Antarctic

Abstract Global watermass ventilation pathways and time scales are investigated using an “eddy permitting” (¼°) offline tracer model. Unlike previous Lagrangian trajectory studies, here an offline model based on a complete tracer equation that includes three-dimensional advection and mixing is employed. In doing so, the authors are able to meaningfully simulate chlorofluorocarbon (CFC) uptake and assess model skill against observation. This is the first time an eddy-permitting model has been subjected to such an assessment of interior ocean ventilation. The offline model is forced by seasonally varying prescribed velocity, temperature, and salinity fields of a state-of-the-art ocean general circulation model. A seasonally varying mixed layer parameterization is incorporated to account for the degradation of surface convection processes resulting from the temporal averaging. A series of CFC simulations are assessed against observations to investigate interdecadal-time-scale ventilation using a variety of mixed layer criteria. Simulated tracer inventories and penetration depths are in good agreement with observations, especially for thermocline, mode, and surface waters. Deep water from the Labrador Sea is well represented, forming a distinct deep western boundary current that branches at the equator, although concentrations are lower than observed. The formation of bottom water, which occurs around the Antarctic margin, is also generally too weak, although there is excellent qualitative agreement with observations in the region of the Ross and Weddell Seas. Multicentury ventilation of the outflow of North Atlantic Deep Water and bottom water from the Antarctic Margin are investigated using 1000-yr passive tracer experiments with specified interior source regions. The model captures many of the detailed pathways evident from observations, with much of the discrepancy accounted for by differences between actual and modeled topography. A comparison between model-derived “tracer age” and Δ14C “advection age” provides a semiquantitative assessment of model skill at these longer time scales.

scholarly journals Sea Level Expression of Intrinsic and Forced Ocean Variabilities at Interannual Time Scales

2011 ◽  
Vol 24 (21) ◽  
pp. 5652-5670 ◽  
Author(s):  
Thierry Penduff ◽  
Mélanie Juza ◽  
Bernard Barnier ◽  
Jan Zika ◽  
William K. Dewar ◽  
...  
Keyword(s):  
Sea Level ◽  
Time Scales ◽  
General Circulation ◽  
Large Scale ◽  
Full Range ◽  
Circulation Model ◽  
Global Ocean ◽  
Western Boundary ◽  
Intrinsic Variability ◽  
Boundary Current

Abstract This paper evaluates in a realistic context the local contributions of direct atmospheric forcing and intrinsic oceanic processes on interannual sea level anomalies (SLAs). A ¼° global ocean–sea ice general circulation model, driven over 47 yr by the full range of atmospheric time scales, is quantitatively assessed against altimetry and shown to reproduce most observed features of the interannual SLA variability from 1993 to 2004. Comparing this simulation with a second driven only by the climatological annual cycle reveals that the intrinsic part of the total interannual SLA variance exceeds 40% over half of the open-ocean area and exceeds 80% over one-fifth of it. This intrinsic contribution is particularly strong in eddy-active regions (more than 70%–80% in the Southern Ocean and western boundary current extensions) as predicted by idealized studies, as well as within the 20°–35° latitude bands. The atmosphere directly forces most of the interannual SLA variance at low latitudes and in most midlatitude eastern basins, in particular north of about 40°N in the Pacific. The interannual SLA variance is almost entirely due to intrinsic processes south of the Antarctic Circumpolar Current in the Indian Ocean sector, while half of this variance is forced by the atmosphere north of it. The same simulations were performed and analyzed at 2° resolution as well: switching to this laminar regime yields a comparable forced variability (large-scale distribution and magnitude) but almost suppresses the intrinsic variability. This likely explains why laminar ocean models largely underestimate the interannual SLA variance.

scholarly journals Variability and Coherence of the Agulhas Undercurrent in a High-Resolution Ocean General Circulation Model

2009 ◽  
Vol 39 (10) ◽  
pp. 2417-2435 ◽  
Author(s):  
A. Biastoch ◽  
L. M. Beal ◽  
J. R. E. Lutjeharms ◽  
T. G. D. Casal
Keyword(s):  
High Resolution ◽  
General Circulation ◽  
Current System ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Ocean Model ◽  
Western Boundary ◽  
Boundary Current ◽  
Sensitivity Experiment ◽  
Agulhas Current

Abstract The Agulhas Current system has been analyzed in a nested high-resolution ocean model and compared to observations. The model shows good performance in the western boundary current structure and the transports off the South African coast. This includes the simulation of the northward-flowing Agulhas Undercurrent. It is demonstrated that fluctuations of the Agulhas Current and Undercurrent around 50–70 days are due to Natal pulses and Mozambique eddies propagating downstream. A sensitivity experiment that excludes those upstream perturbations significantly reduces the variability as well as the mean transport of the undercurrent. Although the model simulates undercurrents in the Mozambique Channel and east of Madagascar, there is no direct connection between those and the Agulhas Undercurrent. Virtual float releases demonstrate that topography is effectively blocking the flow toward the north.

Anatomy and Decadal Evolution of the Pacific Subtropical–Tropical Cells (STCs)*

2005 ◽  
Vol 18 (18) ◽  
pp. 3739-3758 ◽  
Author(s):  
Antonietta Capotondi ◽  
Michael A. Alexander ◽  
Clara Deser ◽  
Michael J. McPhaden
Keyword(s):  
Time Scales ◽  
General Circulation ◽  
Phase Relationship ◽  
Circulation Model ◽  
Adjustment Process ◽  
Western Boundary ◽  
Boundary Current ◽  
Meridional Transport ◽  
The Pacific ◽  
Equatorial Upwelling

Abstract The output from an ocean general circulation model driven by observed surface forcing (1958–97) is used to examine the evolution and relative timing of the different branches of the Pacific Subtropical–Tropical Cells (STCs) at both interannual and decadal time scales, with emphasis on the 1976–77 climate shift. The STCs consist of equatorward pycnocline transports in the ocean interior and in the western boundary current, equatorial upwelling, and poleward flow in the surface Ekman layer. The interior pycnocline transports exhibit a decreasing trend after the mid-1970s, in agreement with observational transport estimates, and are largely anticorrelated with both the Ekman transports and the boundary current transports at the same latitudes. The boundary current changes tend to compensate for the interior changes at both interannual and decadal time scales. The meridional transport convergence across 9°S and 9°N as well as the equatorial upwelling are strongly correlated with the changes in sea surface temperature (SST) in the central and eastern equatorial Pacific. However, meridional transport variations do not occur simultaneously at each longitude, so that to understand the phase relationship between transport and SST variations it is important to consider the baroclinic ocean adjustment through westward-propagating Rossby waves. The anticorrelation between boundary current changes and interior transport changes can also be understood in terms of the baroclinic adjustment process. In this simulation, the pycnocline transport variations appear to be primarily confined within the Tropics, with maxima around 10°S and 13°N, and related to the local wind forcing; a somewhat different perspective from previous studies that have emphasized the role of wind variations in the subtropics.

scholarly journals Labrador Slope Water Connects the Subarctic with the Gulf Stream

2021 ◽  
Author(s):  
Adrian New ◽  
David Smeed ◽  
Arnaud Czaja ◽  
Adam Blaker ◽  
Jenny Mecking ◽  
...  
Keyword(s):  
Gulf Stream ◽  
General Circulation ◽  
Circulation Model ◽  
Western Boundary Current ◽  
Water Masses ◽  
Meridional Overturning Circulation ◽  
Western Slope ◽  
Western Boundary ◽  
Boundary Current ◽  
Shelf Slope

<p>Labrador Slope Water (LSLW) is found in the Slope Sea on the US-Canadian eastern shelf-slope as a relatively fresh and cool water mass, lying between the upper layer water masses and those carried by the Deep Western Boundary Current. It originates from the Labrador Current and has previously only been reported in the Eastern Slope Sea (east of 66°W). We here use the EN4 gridded database and the Line W hydrographic observations to show for the first time that the LSLW also penetrates into the Western Slope Sea, bringing it into close contact with the Gulf Stream. We also show that the LSLW spreads across the entire Slope Sea north of the Gulf Stream, and is both fresher and thicker when the Atlantic Meridional Overturning Circulation (AMOC) is high at the RAPID array at 26°N. The fresher, thicker LSLW is likely to contribute an additional 1.5 Sv of Gulf Stream transport. The spreading of the LSLW is also investigated in a high-resolution ocean general circulation model (NEMO), and is found to occur both as a western boundary current and through the extrusion of filaments following interaction with Gulf Stream meanders and eddies. The mechanism results in downward vertical motion as the filaments are entrained into the Gulf Stream. We conclude that the LSLW (rather than the deeper Labrador Sea Water) provides the intermediate depth water masses which maintain the density contrast here which partly drives the Gulf Stream, and that the transport of the LSLW from the Labrador shelf-slope offers a potential new mechanism for decadal variability in the Atlantic climate system, through connecting high latitude changes in the Subarctic with subsequent variability in the Gulf Stream and AMOC.</p>

scholarly journals Dynamics of the Northern Tsuchiya Jet*

2009 ◽  
Vol 39 (9) ◽  
pp. 2024-2051 ◽  
Author(s):  
Ryo Furue ◽  
Julian P. McCreary ◽  
Zuojun Yu
Keyword(s):  
General Circulation ◽  
Circulation Model ◽  
North Equatorial Current ◽  
Western Boundary ◽  
Ekman Pumping ◽  
Boundary Current ◽  
Wind Fields ◽  
Equatorial Undercurrent ◽  
Southern Boundary ◽  
The North

Abstract The Tsuchiya jets (TJs) are narrow eastward currents located along thermal fronts at the poleward edges of thermostad water in the Pacific Ocean. In this study, an oceanic general circulation model (OGCM) is used to explore the dynamics of the northern TJ. Solutions are found in a rectangular basin, extending 100° zonally and from 40°S to 40°N. They are forced by three idealized forcings: several patches of idealized wind fields, including one that simulates the strong Ekman pumping region in the vicinity of the Costa Rica Dome (CRD); surface heating that warms the ocean in the tropics; and a prescribed interocean circulation (IOC) that enters the basin through the southern boundary and exits through the western boundary from 2° to 6°N (the model’s Indonesian passages). Solutions forced by all the aforementioned processes and with minimal diffusion resemble the observed flow field in the tropical North Pacific. A narrow eastward current, the model’s northern TJ, flows across the basin along the northern edge of a thick equatorial thermostad. Part of the TJ water upwells at the CRD upwelling region and the rest returns westward in the lower part of the North Equatorial Current. The deeper part of the TJ is supplied by water that leaves the western boundary current somewhat north of the equator. Its shallower part originates from water that diverges from the deep portion of the Equatorial Undercurrent (EUC); as a result, the TJ transport increases to the east and the TJ warms as it flows across the basin. These and other properties suggest that the dynamics of the model’s TJ are those of an arrested front, which in a 2½-layer model are generated when characteristics of the flow converge strongly or intersect. Eddy form stress, due to instability waves generated at the CRD region, extends the TJ circulation to deeper levels. When diffusivity is increased to commonly used values, the thermostad is less well defined and the TJ is weak. In a solution without the IOC, the TJ is shifted to higher temperatures with its water supplied by the subtropical cell. When horizontal viscosity is reduced, the TJ becomes narrower and is flanked by a westward current on its equatorward side.

Surface flux errors in asynchronous coupling of GCM

2020 ◽  
Author(s):  
Xavier Perrot ◽  
Jean-Philippe Duvel ◽  
Lionel Guez
Keyword(s):  
Mixed Layer ◽  
General Circulation ◽  
Circulation Model ◽  
Coupled System ◽  
Surface Flux ◽  
Surface Fluxes ◽  
Solar Flux ◽  
Western Boundary ◽  
Time Step ◽  
The Impact

<div> <div> <div> <p>In coupled general circulation model, the accuracy of momentum and energy exchange at the air-sea interface is still a potential source of significant bias. In the framework of the COCOA project we investigate new methods (both mathematical and numerical) to have a more correct flux representation. One important source of error is the asynchronous coupling between oceanic and atmospheric model. Indeed, the time step of the coupling is generally longer than time steps used by either the atmospheric or the oceanic model. This introduces inconsistencies between the free evolution of the two models due to the exchange parameters that are held constant since the last coupling time step. In particular, non-synchronous exchange coefficients may lead to error in the diurnal evolution of the coupled system, or to bias in the ocean mixed layer temperature for period where surface fluxes increases or decrease linearly.</p> <p>In order to evaluate the potential amplitude of this error, and its regional and sea- sonal distribution, we use the hourly fluxes that are available in the new ECMWF ERA5 re-analyses. The error due to asynchronous coupling is first evaluated by inspecting the flux difference between two successive time-steps. Results show more important differences over the western boundary currents and the circumpolar current for all the fluxes except for the solar flux. We also observe larger differences in summer compared to winter in the respective hemisphere. By taking in account the geometrical variation of the solar flux we show how we can reduce the error for the solar flux.</p> <p>In a second time we are calculating the statistics for the linear increase and decrease of the flux for a fixed period (ig one day, two days...) over all the ocean for all the fluxes except the solar one. The results are showing coefficients that are decreasing as the period increase. We also use those coefficient in a simple mixed layer model to calculate the error made over the period of calcul. On the contrary we see the appearance of a plateau at two-three days on the impact of this linear bias. Finally, using the De Boyer de Montaigu climatology for the mixed layer height we show that the linear bias could lead to temperature change up to 0.1K.</p> </div> </div> </div>

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