Michael Spall | Woods Hole Oceanographic Institution (original) (raw)

Papers by Michael Spall

Research paper thumbnail of Downfront Winds over Buoyant Coastal Plumes

Journal of Physical Oceanography, Oct 1, 2016

Research paper thumbnail of The influence of the large scale circulation on an eastern boundary current

AGUFM, Dec 1, 2010

ABSTRACT The wind-driven gyre circulation in the ocean interior varies across large temporal and ... more ABSTRACT The wind-driven gyre circulation in the ocean interior varies across large temporal and spatial scales, while the current along the eastern boundary is concentrated in a narrow jet with smaller temporal and spatial scales. These boundary currents are often hydrodynamically unstable and generate mesoscale and sub-mesoscale variability. In this study, we investigate the influence of the large scale circulation on an unstable eastern boundary current. One example is the influence of the Pacific subtropical gyre on the California current system. We study the problem using both a linear stability analysis and a nonlinear numerical model in a barotropic and quasi-geostrophic framework. The large scale circulation and the boundary current are specified in the linear analysis and are generated by an Ekman forcing in the numerical model. The linear stability analysis shows that to the lowest order the eastward (westward) flow of the large scale circulation stabilizes (destabilizes) the boundary current. Additionally, the meridional flow contributed by the large scale circulation accelerates or decelerates the originally parallel boundary current and modifies the stability of the current through the Doppler effect. Unstable perturbations which can be represented by normal modes for a parallel current then develop streamwise spatial structures. In the nonlinear numerical simulations, the streamwise nonuniformity of the boundary current influenced by the large scale circulation is clearly shown in the eddy kinetic energy. The location of the maximum eddy kinetic energy depends on the relative strength of the large scale circulation and the boundary current. The meridionally nonuniform eddy activities are important in offshore tracer transport. The nonlinear numerical simulation is forced by a wind curl field which generates a southward eastern boundary current and a large scale circulation with double gyres (white contours). The mean eddy kinetic energy of the boundary current (in color) exhibits a meridionally varying structure showing the influence of the large scale circulation on the boundary current stability.

Research paper thumbnail of Response to a Steady Poleward Outflow. Part II: Oscillations and Eddies

Journal of Physical Oceanography, Jul 1, 2009

Research paper thumbnail of Circulation Induced by Isolated Dense Water Formation over Closed Topographic Contours

Journal of Physical Oceanography, Sep 1, 2017

Research paper thumbnail of Influences of Time-Dependent Precipitation on Water Mass Transformation, Heat Fluxes, and Deep Convection in Marginal Seas

Journal of Physical Oceanography, Jul 1, 2015

Research paper thumbnail of Interaction of Ekman Layers and Islands

Journal of Physical Oceanography, May 1, 2013

Research paper thumbnail of Circulation and Exchange in Choked Marginal Seas

Journal of Physical Oceanography, Dec 1, 2008

Research paper thumbnail of Eddy Memory Mode of Multidecadal Variability in Residual-Mean Ocean Circulations with Application to the Beaufort Gyre

Journal of Physical Oceanography, Apr 1, 2017

Research paper thumbnail of Large-Scale Circulations Forced by Localized Mixing over a Sloping Bottom*

Journal of Physical Oceanography, Aug 1, 2001

Research paper thumbnail of Effect of Sea Surface Temperature–Wind Stress Coupling on Baroclinic Instability in the Ocean

Journal of Physical Oceanography, Apr 1, 2007

Research paper thumbnail of Wind‐driven freshwater buildup and release in the Beaufort Gyre constrained by mesoscale eddies

Geophysical Research Letters, Jan 6, 2016

Research paper thumbnail of A numerical investigation on the energetics of a current along an ice-covered continental slope

Research paper thumbnail of A Three-Dimensional Inertial Model for Coastal Upwelling along Western Boundaries

Journal of Physical Oceanography, Oct 1, 2022

A three-dimensional inertial model that conserves quasigeostrophic potential vorticity is propose... more A three-dimensional inertial model that conserves quasigeostrophic potential vorticity is proposed for wind-driven coastal upwelling along western boundaries. The dominant response to upwelling favorable winds is a surface-intensified baroclinic meridional boundary current with a subsurface countercurrent. The width of the current is not the baroclinic deformation radius but instead scales with the inertial boundary layer thickness while the depth scales as the ratio of the inertial boundary layer thickness to the baroclinic deformation radius. Thus, the boundary current scales depend on the stratification, wind stress, Coriolis parameter, and its meridional variation. In contrast to two-dimensional wind-driven coastal upwelling, the source waters that feed the Ekman upwelling are provided over the depth scale of this baroclinic current through a combination of onshore barotropic flow and from alongshore in the narrow boundary current. Topography forces an additional current whose characteristics depend on the topographic slope and width. For topography wider than the inertial boundary layer thickness the current is bottom intensified, while for narrow topography the current is wave-like in the vertical and trapped over the topography within the inertial boundary layer. An idealized primitive equation numerical model produces a similar baroclinic boundary current whose vertical length scale agrees with the theoretical scaling for both upwelling and downwelling favorable winds.

Research paper thumbnail of Radiating Instability

Cambridge University Press eBooks, Feb 28, 2019

Research paper thumbnail of Wind-Driven Freshwater Buildup in the Beaufort Gyre is Inevitably Constrained by Mesoscale Eddies

2015 AGU Fall Meeting, Dec 16, 2015

Research paper thumbnail of Forward Problem in Numerical Models

Research paper thumbnail of Wind-driven variations in an overturning circulation

EGU General Assembly Conference Abstracts, Apr 1, 2017

Research paper thumbnail of On the sea-ice cover of the Nordic Seas in an idealized MITgcm-setup

EGU General Assembly Conference Abstracts, Apr 1, 2016

Research paper thumbnail of Buoyancy-Forced Downwelling in Boundary Currents

Journal of Physical Oceanography, Dec 1, 2008

Research paper thumbnail of An overlooked component of the meridional overturning circulation

Journal of physical oceanography, Jun 13, 2024

Research paper thumbnail of Downfront Winds over Buoyant Coastal Plumes

Journal of Physical Oceanography, Oct 1, 2016

Research paper thumbnail of The influence of the large scale circulation on an eastern boundary current

AGUFM, Dec 1, 2010

ABSTRACT The wind-driven gyre circulation in the ocean interior varies across large temporal and ... more ABSTRACT The wind-driven gyre circulation in the ocean interior varies across large temporal and spatial scales, while the current along the eastern boundary is concentrated in a narrow jet with smaller temporal and spatial scales. These boundary currents are often hydrodynamically unstable and generate mesoscale and sub-mesoscale variability. In this study, we investigate the influence of the large scale circulation on an unstable eastern boundary current. One example is the influence of the Pacific subtropical gyre on the California current system. We study the problem using both a linear stability analysis and a nonlinear numerical model in a barotropic and quasi-geostrophic framework. The large scale circulation and the boundary current are specified in the linear analysis and are generated by an Ekman forcing in the numerical model. The linear stability analysis shows that to the lowest order the eastward (westward) flow of the large scale circulation stabilizes (destabilizes) the boundary current. Additionally, the meridional flow contributed by the large scale circulation accelerates or decelerates the originally parallel boundary current and modifies the stability of the current through the Doppler effect. Unstable perturbations which can be represented by normal modes for a parallel current then develop streamwise spatial structures. In the nonlinear numerical simulations, the streamwise nonuniformity of the boundary current influenced by the large scale circulation is clearly shown in the eddy kinetic energy. The location of the maximum eddy kinetic energy depends on the relative strength of the large scale circulation and the boundary current. The meridionally nonuniform eddy activities are important in offshore tracer transport. The nonlinear numerical simulation is forced by a wind curl field which generates a southward eastern boundary current and a large scale circulation with double gyres (white contours). The mean eddy kinetic energy of the boundary current (in color) exhibits a meridionally varying structure showing the influence of the large scale circulation on the boundary current stability.

Research paper thumbnail of Response to a Steady Poleward Outflow. Part II: Oscillations and Eddies

Journal of Physical Oceanography, Jul 1, 2009

Research paper thumbnail of Circulation Induced by Isolated Dense Water Formation over Closed Topographic Contours

Journal of Physical Oceanography, Sep 1, 2017

Research paper thumbnail of Influences of Time-Dependent Precipitation on Water Mass Transformation, Heat Fluxes, and Deep Convection in Marginal Seas

Journal of Physical Oceanography, Jul 1, 2015

Research paper thumbnail of Interaction of Ekman Layers and Islands

Journal of Physical Oceanography, May 1, 2013

Research paper thumbnail of Circulation and Exchange in Choked Marginal Seas

Journal of Physical Oceanography, Dec 1, 2008

Research paper thumbnail of Eddy Memory Mode of Multidecadal Variability in Residual-Mean Ocean Circulations with Application to the Beaufort Gyre

Journal of Physical Oceanography, Apr 1, 2017

Research paper thumbnail of Large-Scale Circulations Forced by Localized Mixing over a Sloping Bottom*

Journal of Physical Oceanography, Aug 1, 2001

Research paper thumbnail of Effect of Sea Surface Temperature–Wind Stress Coupling on Baroclinic Instability in the Ocean

Journal of Physical Oceanography, Apr 1, 2007

Research paper thumbnail of Wind‐driven freshwater buildup and release in the Beaufort Gyre constrained by mesoscale eddies

Geophysical Research Letters, Jan 6, 2016

Research paper thumbnail of A numerical investigation on the energetics of a current along an ice-covered continental slope

Research paper thumbnail of A Three-Dimensional Inertial Model for Coastal Upwelling along Western Boundaries

Journal of Physical Oceanography, Oct 1, 2022

A three-dimensional inertial model that conserves quasigeostrophic potential vorticity is propose... more A three-dimensional inertial model that conserves quasigeostrophic potential vorticity is proposed for wind-driven coastal upwelling along western boundaries. The dominant response to upwelling favorable winds is a surface-intensified baroclinic meridional boundary current with a subsurface countercurrent. The width of the current is not the baroclinic deformation radius but instead scales with the inertial boundary layer thickness while the depth scales as the ratio of the inertial boundary layer thickness to the baroclinic deformation radius. Thus, the boundary current scales depend on the stratification, wind stress, Coriolis parameter, and its meridional variation. In contrast to two-dimensional wind-driven coastal upwelling, the source waters that feed the Ekman upwelling are provided over the depth scale of this baroclinic current through a combination of onshore barotropic flow and from alongshore in the narrow boundary current. Topography forces an additional current whose characteristics depend on the topographic slope and width. For topography wider than the inertial boundary layer thickness the current is bottom intensified, while for narrow topography the current is wave-like in the vertical and trapped over the topography within the inertial boundary layer. An idealized primitive equation numerical model produces a similar baroclinic boundary current whose vertical length scale agrees with the theoretical scaling for both upwelling and downwelling favorable winds.

Research paper thumbnail of Radiating Instability

Cambridge University Press eBooks, Feb 28, 2019

Research paper thumbnail of Wind-Driven Freshwater Buildup in the Beaufort Gyre is Inevitably Constrained by Mesoscale Eddies

2015 AGU Fall Meeting, Dec 16, 2015

Research paper thumbnail of Forward Problem in Numerical Models

Research paper thumbnail of Wind-driven variations in an overturning circulation

EGU General Assembly Conference Abstracts, Apr 1, 2017

Research paper thumbnail of On the sea-ice cover of the Nordic Seas in an idealized MITgcm-setup

EGU General Assembly Conference Abstracts, Apr 1, 2016

Research paper thumbnail of Buoyancy-Forced Downwelling in Boundary Currents

Journal of Physical Oceanography, Dec 1, 2008

Research paper thumbnail of An overlooked component of the meridional overturning circulation

Journal of physical oceanography, Jun 13, 2024