scholarly journals Baroclinic instability and the mesoscale eddy field around the Lofoten Basin

2015 ◽  
Vol 120 (4) ◽  
pp. 2884-2903 ◽  
Author(s):  
Pål Erik Isachsen
Keyword(s):  
Baroclinic Instability ◽  
Mesoscale Eddy ◽  
Eddy Field

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.

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.

scholarly journals Emergence of Wind-Driven Near-Inertial Waves in the Deep Ocean Triggered by Small-Scale Eddy Vorticity Structures

2011 ◽  
Vol 41 (7) ◽  
pp. 1297-1307 ◽  
Author(s):  
Eric Danioux ◽  
Patrice Klein ◽  
Matthew W. Hecht ◽  
Nobumasa Komori ◽  
Guillaume Roullet ◽  
...  
Keyword(s):  
Mesoscale Eddy ◽  
Deep Ocean ◽  
Mean Amplitude ◽  
Relative Vorticity ◽  
Wind Forcing ◽  
Small Scale ◽  
Inertial Waves ◽  
Deep Interior ◽  
Eddy Field ◽  
Baroclinic Modes

Abstract Using numerical simulations forced by a uniform realistic wind time series, the authors show that the presence of a mesoscale eddy field at midlatitudes accelerates the vertical propagation of the wind-forced near-inertial waves (NIW) and produces the emergence of a maximum of vertical velocity into the deep ocean (around 2500 m) characterized by a mean amplitude of 25 m day−1, a dominant 2f frequency, and scales as small as O(30 km). These results differ from previous studies that reported a smaller depth and larger scales. The authors show that the larger depth observed in the present study (2500 m instead of 1700 m) is due to the wind forcing duration that allows the first five baroclinic modes to disperse and to impact the deep NIW maximum (instead of the first two modes as reported before). The smaller scales (30 km instead of 90 km) are explained by a resonance mechanism (described in previous studies) that affects the high NIW baroclinic modes, but only when small-scale relative vorticity structures (related to the mesoscale eddy field) have an amplitude that is large enough. These results, which point out the importance of the wind forcing duration and the resolution, indicate that the emergence of a deep NIW maximum with a 2f frequency reported before is a robust feature that is enhanced with more realistic conditions. Such 2f frequency in the deep interior raises the question of the mechanisms, still unresolved, that may ultimately transfer this superinertial energy into mixing at these depths.

scholarly journals Seasonal Modulation of Eddy Kinetic Energy and Its Formation Mechanism in the Southeast Indian Ocean

2011 ◽  
Vol 41 (4) ◽  
pp. 657-665 ◽  
Author(s):  
Fan Jia ◽  
Lixin Wu ◽  
Bo Qiu
Keyword(s):  
Indian Ocean ◽  
Kinetic Energy ◽  
Vertical Velocity ◽  
Baroclinic Instability ◽  
Mesoscale Eddy ◽  
Eddy Kinetic Energy ◽  
Velocity Shear ◽  
Austral Spring ◽  
Eddy Activity ◽  
South Indian

Abstract Mesoscale eddy activity in the southeast Indian Ocean (15°–30°S, 60°–110°E) is investigated based on available satellite altimetry observations. The observed sea level anomaly data show that this region is the only eastern basin among the global oceans where strong eddy activity exists. Furthermore, the eddy kinetic energy (EKE) level in this region displays a distinct seasonal cycle with the maximum in austral summer and minimum in austral winter. It is found that this seasonal modulation of EKE is mediated by baroclinic instability associated with the surface-intensified South Indian Countercurrent (SICC) and the underlying South Equatorial Current (SEC) system. In austral spring and summer the enhanced flux forcing of combined meridional Ekman and geostrophic convergence strengthens the upper-ocean meridional temperature gradient, intensifying the SICC front and its vertical velocity shear. Modulation of the vertical velocity shear results in the seasonal changes in the strength of baroclinic instability, leading to the seasonal EKE variations in the southeast Indian Ocean.

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.

scholarly journals Continental Shelf Baroclinic Instability. Part II: Oscillating Wind Forcing

2016 ◽  
Vol 46 (2) ◽  
pp. 569-582 ◽  
Author(s):  
K. H. Brink ◽  
H. Seo
Keyword(s):  
Flow Field ◽  
Continental Shelf ◽  
Large Scale ◽  
Baroclinic Instability ◽  
Wind Forcing ◽  
Develop Model ◽  
Bottom Slope ◽  
Coriolis Parameter ◽  
Wide Range ◽  
Eddy Field

AbstractContinental shelf baroclinic instability energized by fluctuating alongshore winds is treated using idealized primitive equation numerical model experiments. A spatially uniform alongshore wind, sinusoidal in time, alternately drives upwelling and downwelling and so creates highly variable, but slowly increasing, available potential energy. For all of the 30 model runs, conducted with a wide range of parameters (varying Coriolis parameter, initial stratification, bottom friction, forcing period, wind strength, and bottom slope), a baroclinic instability and subsequent eddy field develop. Model results and scalings show that the eddy kinetic energy increases with wind amplitude, forcing period, stratification, and bottom slope. The dominant alongshore length scale of the eddy field is essentially an internal Rossby radius of deformation. The resulting depth-averaged alongshore flow field is dominated by the large-scale, periodic wind forcing, while the cross-shelf flow field is dominated by the eddy variability. The result is that correlation length scales for alongshore flow are far greater than those for cross-shelf velocity. This scale discrepancy is qualitatively consistent with midshelf observations by Kundu and Allen, among others.

The mesoscale eddy field of the middle Adriatic Sea during fall 1988

1993 ◽  
Vol 40 (7) ◽  
pp. 1365-1377 ◽  
Author(s):  
Elio Paschini ◽  
Antonio Artegiani ◽  
Nadia Pinardi
Keyword(s):  
Adriatic Sea ◽  
Mesoscale Eddy ◽  
Eddy Field

scholarly journals A Resonance Mechanism Leading to Wind-Forced Motions with a 2f Frequency

2008 ◽  
Vol 38 (10) ◽  
pp. 2322-2329 ◽  
Author(s):  
Eric Danioux ◽  
Patrice Klein
Keyword(s):  
Shallow Water Equations ◽  
Mesoscale Eddy ◽  
Relative Vorticity ◽  
General Situation ◽  
Small Scale ◽  
Wave Interactions ◽  
Resonance Mechanism ◽  
Eddy Field ◽  
Baroclinic Modes ◽  
Parametric Subharmonic Instability

Abstract This study revisits the mechanisms that spatially reorganize wind-forced inertial motions embedded in an oceanic mesoscale eddy field. Inertial motions are known to be affected by the eddy relative vorticity, being expelled from cyclonic structures and trapped within anticyclonic ones. Using shallow water equations (involving a single baroclinic mode), the authors show that nonlinear wave–wave interactions excite, through a resonance mechanism, motions with a 2f frequency (with f being the inertial frequency) and a specific length scale. In a more general situation involving several baroclinic modes, this resonance is found to be reinforced and principally efficient for the lowest baroclinic modes. Such characteristics make the energy of the wind-forced motions potentially available to small-scale mixing through parametric subharmonic instability (not taken into account in the present study).

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