scholarly journals Topographic Influence on Baroclinic Instability and the Mesoscale Eddy Field in the Northern North Atlantic Ocean and the Nordic Seas

2018 ◽  
Vol 48 (11) ◽  
pp. 2593-2607 ◽  
Author(s):  
Marta Trodahl ◽  
Pål Erik Isachsen
Keyword(s):  
North Atlantic ◽  
Bottom Topography ◽  
Baroclinic Instability ◽  
Mesoscale Eddy ◽  
Ocean Model ◽  
Topographic Effects ◽  
Nordic Seas ◽  
Boundary Currents ◽  
Eddy Characteristics ◽  
Eddy Field

AbstractA weak planetary vorticity gradient and weak density stratification in the northern North Atlantic and Nordic seas lead to time-mean currents that are strongly guided by bottom topography. The topographic steering sets up distinct boundary currents with strong property fronts that are prone to both baroclinic and barotropic instability. These instability processes generate a macroturbulent eddy field that spreads buoyancy and other tracers out from the boundary currents and into the deep basins. In this paper we investigate the particular role played by baroclinic instability in generating the observed eddy field, comparing predictions from linear stability calculations with diagnostics from a nonlinear eddy-permitting ocean model hindcast. We also look into how the bottom topography impacts instability itself. The calculations suggest that baroclinic instability is a consistent source of the eddy field but that topographic potential vorticity gradients impact unstable growth significantly. We also observe systematic topographic effects on finite-amplitude eddy characteristics, including a general suppression of length scales over the continental slopes. Investigation of the vertical structure of unstable modes reveal that Eady theory, even when modified to account for a bottom slope, is unfit as a lowest-order model for the dynamics taking place in these ocean regions.

Interannual Variability of the North Pacific Subtropical Countercurrent and Its Associated Mesoscale Eddy Field

2010 ◽  
Vol 40 (1) ◽  
pp. 213-225 ◽  
Author(s):  
Bo Qiu ◽  
Shuiming Chen
Keyword(s):  
North Pacific ◽  
Southern Oscillation ◽  
North Pacific Ocean ◽  
Baroclinic Instability ◽  
Mesoscale Eddy ◽  
Vertical Shear ◽  
North Equatorial Current ◽  
Altimeter Data ◽  
Subtropical Countercurrent ◽  
Eddy Field

Abstract Interannual changes in the mesoscale eddy field along the Subtropical Countercurrent (STCC) band of 18°–25°N in the western North Pacific Ocean are investigated with 16 yr of satellite altimeter data. Enhanced eddy activities were observed in 1996–98 and 2003–08, whereas the eddy activities were below average in 1993–95 and 1999–2002. Analysis of repeat hydrographic data along 137°E reveals that the vertical shear between the surface eastward-flowing STCC and the subsurface westward-flowing North Equatorial Current (NEC) was larger in the eddy-rich years than in the eddy-weak years. By adopting a 2½-layer reduced-gravity model, it is shown that the increased eddy kinetic energy level in 1996–98 and 2003–08 is because of enhanced baroclinic instability resulting from the larger vertical shear in the STCC–NEC’s background flow. The cause for the STCC–NEC’s interannually varying vertical shear can be sought in the forcing by surface Ekman temperature gradient convergence within the STCC band. Rather than El Niño–Southern Oscillation signals as previously hypothesized, interannual changes in this Ekman forcing field, and hence the STCC–NEC’s vertical shear, are more related to the negative western Pacific index signals.

Nonlinear Dynamics of Two Western Boundary Currents Colliding at a Gap

2012 ◽  
Vol 42 (11) ◽  
pp. 2030-2040 ◽  
Author(s):  
Zheng Wang ◽  
Dongliang Yuan
Keyword(s):  
Hopf Bifurcation ◽  
Mesoscale Eddy ◽  
Ocean Model ◽  
Viscous Force ◽  
Hopf Bifurcations ◽  
Western Boundary ◽  
Western Basin ◽  
Western Boundary Currents ◽  
Boundary Currents ◽  
Eddy Shedding

Abstract The nonlinear collision of two western boundary currents (WBCs) of equal transport at a gap of the western boundary is studied using a 1.5-layer reduced-gravity quasigeostrophic ocean model. It is found that, when the gap (of width 2a) is narrow, a ≤ 7.3LM (LM the Munk thickness), neither of the WBCs can penetrate into the western basin because of the restriction of the viscous force. When 7.3LM < a < 9.0LM, both WBCs penetrate into the western basin for small transport and choke for large transport. When 9.0LM ≤ a ≤ 9.6LM, the two WBCs penetrate for small transport, choke for intermediate transport, and shed eddies periodically for large transport. When a > 9.6LM, no steady choking state is found. Instead, the WBCs have only two equilibrium states: the penetrating and the periodic eddy shedding states. A Hopf bifurcation is found for a > 9.0LM. The Reynolds number (Re) of the Hopf bifurcation is sensitive to the magnitude of γ(a/LM) and the baroclinic deformation radius, being small for larger γ or deformation radius. In addition, a reverse Hopf bifurcations is identified in the decreased Re experiments, occurring at a smaller Re than that of the Hopf bifurcation. The Re of the reverse Hopf bifurcation is not sensitive to the magnitude of the baroclinic deformation radius. Hysteresis behavior of the WBCs is found for a > 9.0LM, because of the existence of the Hopf and reverse Hopf bifurcations. In between them, steady penetrating or choking states coexist with eddy-shedding states. The steady states are found to be sensitive to perturbations of relative vorticity and can transit to periodic eddy-shedding states at the forcing of a mesoscale eddy.

scholarly journals Observations of Water Mass Transformation and Eddies in the Lofoten Basin of the Nordic Seas

2015 ◽  
Vol 45 (6) ◽  
pp. 1735-1756 ◽  
Author(s):  
Clark G. Richards ◽  
Fiammetta Straneo
Keyword(s):  
Water Mass ◽  
Heat Budget ◽  
Mesoscale Eddy ◽  
Surface Fluxes ◽  
Meridional Overturning Circulation ◽  
Boundary Current ◽  
Mode Water ◽  
Nordic Seas ◽  
Eddy Field ◽  
Water Mass Transformation

AbstractThe Lofoten basin of the Nordic Seas is recognized as a crucial component of the meridional overturning circulation in the North Atlantic because of the large horizontal extent of Atlantic Water and winter surface buoyancy loss. In this study, hydrographic and current measurements collected from a mooring deployed in the Lofoten basin from July 2010 to September 2012 are used to describe water mass transformation and the mesoscale eddy field. Winter mixed layer depths (MLDs) are observed to reach approximately 400 m, with larger MLDs and denser properties resulting from the colder 2010 winter. A heat budget of the upper water column requires lateral input, which balances the net annual heat loss of ~80 W m−2. The lateral flux is a result of mesoscale eddies, which dominate the velocity variability. Eddy velocities are enhanced in the upper 1000 m, with a barotropic component that reaches the bottom. Detailed examination of two eddies, from April and August 2012, highlights the variability of the eddy field and eddy properties. Temperature and salinity properties of the April eddy suggest that it originated from the slope current but was ventilated by surface fluxes. The properties within the eddy were similar to those of the mode water, indicating that convection within the eddies may make an important contribution to water mass transformation. A rough estimate of eddy flux per unit boundary current length suggests that fluxes in the Lofoten basin are larger than in the Labrador Sea because of the enhanced boundary current–interior density difference.

Stationary Sea Surface Height Anomalies in Cyclonic Boundary Currents: Conservation of Potential Vorticity and Deviations from Strict Topographic Steering

2016 ◽  
Vol 46 (8) ◽  
pp. 2437-2456 ◽  
Author(s):  
Sara Broomé ◽  
Johan Nilsson
Keyword(s):  
Potential Vorticity ◽  
Bottom Topography ◽  
Relative Vorticity ◽  
Boundary Current ◽  
Nordic Seas ◽  
Boundary Currents ◽  
First Order ◽  
Topographic Slope ◽  
Depth Contour ◽  
Steady State Model

AbstractIn high-latitude subpolar seas, such as the Nordic seas and the Labrador Sea, time-mean geostrophic currents mediate the bulk of the meridional oceanic heat transport. These currents are primarily encountered along the continental slopes as intense cyclonic boundary currents, which, because of the relatively weak stratification, should be strongly steered by the bottom topography. However, analyses of hydrographic and satellite altimetric data along depth contours in Nordic seas boundary currents reveal some remarkable, stationary, along-stream variations in the depth-integrated buoyancy and bottom pressure. A closer examination shows that these variations are linked to changes in steepness and curvature of the continental slope. To examine the underlying dynamics, a steady-state model of a cyclonic stratified boundary current over a topographic slope is developed in the limit of small Rossby numbers. Based on potential vorticity conservation, equations for the zeroth- and first-order pressure and buoyancy fields are derived. To the lowest order, the flow is completely aligned with the bottom topography. However, the first-order results show that where the lowest-order flow increases (decreases) its relative vorticity along a depth contour, the first-order pressure and depth-integrated buoyancy increase (decrease). This response is associated with cross-isobath flows, which induce stretching/compression of fluid elements that compensates for the changes in relative vorticity. The model-predicted along-isobath variations in pressure and depth-integrated buoyancy are comparable in magnitude to the ones found in the observational data from the Nordics Seas.

scholarly journals Mesoscale Eddy Buoyancy Flux and Eddy-Induced Circulation in Eastern Boundary Currents

2013 ◽  
Vol 43 (6) ◽  
pp. 1073-1095 ◽  
Author(s):  
François Colas ◽  
Xavier Capet ◽  
James C. McWilliams ◽  
Zhijin Li
Keyword(s):  
Coastal Upwelling ◽  
Baroclinic Instability ◽  
Mesoscale Eddy ◽  
Generation Process ◽  
Boundary Currents ◽  
Eastern Boundary ◽  
Instability Theory ◽  
Eastern Boundary Currents ◽  
Eddy Fluxes ◽  
Induced Velocity

Abstract A dynamical interpretation is made of the mesoscale eddy buoyancy fluxes in the Eastern Boundary Currents off California and Peru–Chile, based on regional equilibrium simulations. The eddy fluxes are primarily shoreward and upward across a swath several hundred kilometers wide in the upper ocean; as such they serve to balance mean offshore air–sea heating and coastal upwelling. In the stratified interior the eddy fluxes are consistent with the adiabatic hypothesis associated with a mean eddy-induced velocity advecting mean buoyancy and tracers. Furthermore, with a suitable gauge choice, the horizontal fluxes are almost entirely aligned with the mean horizontal buoyancy gradient, consistent with the advective parameterization scheme of Gent and McWilliams. The associated diffusivity κ is surface intensified, matching the vertical stratification profile. The fluxes span the across-shore band of high eddy energy, but their alongshore structure is unresolved because of sampling limitations. In the surface layer the eddy flux is significantly diabatic with a shallow eddy-induced circulation cell and downgradient lateral diapycnal flux. The dominant eddy generation process is baroclinic instability, but there are significant regional differences between the upwelling systems in the flux and κ that are not consistent with simple instability theory.

scholarly journals The Influence of Topography on the Stability of the Norwegian Atlantic Current off Northern Norway

2018 ◽  
Vol 48 (11) ◽  
pp. 2761-2777 ◽  
Author(s):  
Peygham Ghaffari ◽  
Pål Erik Isachsen ◽  
Ole Anders Nøst ◽  
Jan Erik Weber
Keyword(s):  
Shallow Water ◽  
Continental Slope ◽  
Bottom Topography ◽  
Baroclinic Instability ◽  
Mesoscale Eddy ◽  
Water Model ◽  
Barotropic Instability ◽  
Northern Norway ◽  
The Stability ◽  
Atlantic Current

AbstractThe steep continental slope off the Lofoten–Vesterålen islands of northern Norway appears to be the source of the most intense mesoscale eddy field in all of the Nordic Seas. Here we use linearized two-layer shallow-water equations to study the stability of the Norwegian Atlantic Current in this region. The study extends previous works that used linearized quasigeostrophic vertical mode equations to examine the effects of bottom topography on baroclinic instability here. We find evidence of baroclinic instability in the stacked shallow-water model but also of barotropic instability that is associated with the upper-layer lateral shear. The calculations give an indication that growth rates of barotropic instability may be comparable to or larger than those of baroclinic instability over the steepest parts of the continental slope.

scholarly journals The Behavior of Jet Currents over a Continental Slope Topography with a Possible Application to the Northern Current

2005 ◽  
Vol 35 (5) ◽  
pp. 790-810 ◽  
Author(s):  
M. M. Flexas ◽  
G. J. F. van Heijst ◽  
R. R. Trieling
Keyword(s):  
High Speed ◽  
Bottom Topography ◽  
Laboratory Model ◽  
Topographic Effects ◽  
Topographic Slope ◽  
Eddy Formation ◽  
Slow Flows ◽  
Bottom Depth ◽  
Source Sink ◽  
Northern Current

Abstract The Northern Current is a slope current in the northwest Mediterranean that shows high mesoscale variability, generally associated with meander and eddy formation. A barotropic laboratory model of this current is used here to study the role of the bottom topography on the current variability. For this purpose, a source–sink setup in a cylindrical tank placed on a rotating table is used to generate an axisymmetric barotropic current. To study inviscid topographic effects, experiments are performed over a topographic slope and also over a constant-depth setup, the latter being used as a reference for the former. With the aim of obtaining a fully comprehensive view of the vorticity balance at play, the flow may be forced in either azimuthal direction, leading to a “westward” prograde current (similar to the Northern Current) or an “eastward” retrograde current. For slow flows, eastward and westward currents showed similar patterns, dominated by Kelvin–Helmholtz-type instabilities. For high-speed flows, eastward and westward currents showed very different behavior. In eastward currents, the variability is observed to concentrate toward the center of the jet and shows strong meandering formation. Westward currents, instead, showed major variability toward the edges of the jet, together with a strong variability over the uppermost slope, which has been associated here with a topographic Rossby wave trapped over the shelf break. The differences between eastward and westward jets are explained through the balance between shear-induced and topographically induced vorticity at play in each case. Moreover, a model of jets over a beta plane is successfully applied here, allowing its extension to any ambient potential vorticity gradient caused either by latitudinal or bottom depth changes.

Ocean model formulation influences climate sensitivity

2021 ◽  
Author(s):  
Tido Semmler ◽  
Johann Jungclaus ◽  
Christopher Danek ◽  
Helge F Goessling ◽  
Nikolay Koldunov ◽  
...  
Keyword(s):  
Sea Ice ◽  
Climate Sensitivity ◽  
North Atlantic ◽  
Heat Content ◽  
Ocean Model ◽  
Max Planck ◽  
Coupled Models ◽  
Ocean Heat Uptake ◽  
Marine Research ◽  
Transient Climate Response

<p>The climate sensitivity is known to be mainly determined by the atmosphere model but here we discover that the ocean model can change a given transient climate response (TCR) by as much as 20% while the equilibrium climate sensitivity (ECS) change is limited to 10%. In our study, two different coupled CMIP6 models (MPI-ESM and AWI-CM) in two different resolutions each are compared. The coupled models share the same atmosphere-land component ECHAM6.3, which has been developed at the Max-Planck-Institute for Meteorology (MPI-M). However, as part of MPI-ESM and AWI-CM, ECHAM6.3 is coupled to two different ocean models, namely the MPIOM sea ice-ocean model developed at MPI-M and the FESOM sea ice-ocean model developed at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI). A reason for the different TCR is different ocean heat uptake through greenhouse gas forcing in AWI simulations compared to MPI-M simulations. Specifically, AWI-CM simulations show stronger surface heating than MPI-ESM simulations while the MPI-M model accumulates more heat in the deeper ocean. The vertically integrated ocean heat content is increasing stronger in MPI-M model configurations compared to AWI model configurations in the high latitudes. Strong vertical mixing in MPI-M model configurations compared to AWI model configurations seems to be key for these differences. The strongest difference in vertical ocean mixing occurs inside the Weddell Gyre, but there are also important differences in another key region, the northern North Atlantic. Over the North Atlantic, these differences materialize in a lack of a warming hole in AWI model configurations and the presence of a warming hole in MPI-M model configurations. All these differences occur largely independent of the considered model resolutions.</p>

Sign in / Sign up

Export Citation Format

Share Document