-Plumes Created by Overflows - JAMSTEC-Plumes Created by Overflows Shinichiro Kida, James F. Price,...
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Ogzokmen, T.M. et al. 2001, On the dynamics of b-plumes. J Phys. Oceanogr 31/12, 3569-3680 Stommel, H. 1982, Is the South Pacific Helium dynamically active? Earth Planet. Sci. lett. 61, 63-67 Swaters, G.E. 1991, On the baroclinic instability of cold core coupled den- sity fronts on a sloping continental shelf. J. Fluid Mech. 224, 361-382 is research was supported by NSF Grant # ***** References: .. .. 5. Conclusions and Future Work e Overflow can force a time mean circulation in the upper oceanic layer that has a magnitude as large as the over- flow itself. Baroclinic Instability creates a double β-plume in the oce- anic layer through form drag. is effect can enhance the diabatic β-plume. e β-plumes formed in this study depend on topograph- ic β and are thus locally confined. How these β−plumes can lead to a planetary scale feature, such as Azores Current, needs further investigation. 4. Diabatic β-plume Diabatic β-plume Eddy driven β-plume Figure 8: How eddy driven β - plume can enhance the diabatic β -plume. Figure 7 Time averaged flow 3Sv e effect of entrainment on the oceanic layer was studied by imposing a local diapycnal mass flux in the downstream basin. is created a dense layer going down the slope and a diabatic β-plume in the upper layer (Figure 7ab.) (e mass source/sink in the up/downstream basin was turned off.) e transport of the β-plume was 3Sv which was larger than a linear solution of 2Sv. is was due to the eddy β− plume as shown schematically in Figure 8. Diapycnal cooling source (1.5 Sv Mass flux) (a)Overflow layer thickness (b) Sea surface height 3. Eddy Driven β-plume Figure 5: The basic mechanism for the formation of eddy driven β-plume. 400km 0 200 400 600 800 km 300m thick 400km 0 Bathymetry lines 5cm 40cm/s 40km Figure 3: Snap shot view(birds eye view) of the two layers Eddies formed. Figure 4: Time average the oceanic layer shows the double β -plume. (a)Overflow layer thickness (b) Sea surface height (a)Overflow layer thickness (b) Sea surface height 400km 0 400km 0 400km 0 400km 0 No induced flow! Figure 6: No eddy induced flow when there is no instability. (a)Overflow layer thickness (b) Sea surface height 2 Sv 5cm/s Lower Basin Oceanic layer: Double β -plume Overflow Form Drag >ν/f =ν/f e adiabatic interaction between the overflow and upper layer was studied by shutting off mixing be- tween the layers. e overflow (Figure 3a) was baroclinically unstable (Swaters 1991) and the upper oceanic layer flow (Figure 3b, shown by the sea surface height) had strong eddies [40cm/s, 40km wide.] ese eddies in- duced an eddy driven mean flow in the upper layer as a double β-plume (Figure 4b.) Its transport was 2Sv, a size comparable to the overflow transport. Figure 5 shows the mechanism of how the double β-plume was driven. As the overflow exited the strait, it became baroclincally unstable and created eddies. en part of its momentum is transfered across layers as a form drag. is form drag created the eddy PV flux divergence term in the vorticity equation (Section 1) and thus drove the eddy driven β-plume. e momentum transfer was a drag to the overflow layer (Figure 4a) and caused it to go down the slope with a steeper angle than the Ekman number. Figure 6 shows a case when the upper layer was thicker (+1000m.) Baroclinic instability did not happen and no eddies formed, so there was no eddy induced upper layer flow 2. Model setup Figure 2: Model Setup Upstream Basin (Marginal sea) Downstream Basin (Open Ocean) Input Output Strait 40km wide & 1000m deep Slope 0.01 Dense Overflow Layer We used a two layer (overflow and upper oceanic layer) isopycnal model (Hallberg Isopycnal Model) on a f-plane. β is introduced by the bottom topography shown below. 1.5Sv of dense water is pumped from the upstream basin to the downstream basin. is creates the overflow layer going through the narrow strait. Mixing between layers is either shut off or specified. A linear bottom drag acts on the layer that is in contact with the bottom topography. Sea surface Overflows vigorously entrain oceanic water. How does the upper ocean respond to this local entrainment? A small local forcing can induce a strong horizontal circulation(Figure 1, Stommel 1984). is concept is the β− plume and will be used in this study. e equation that ex- presses this concept is the vorticity equation written below. is study focuses on the overflow ocean interaction in the presence of a sloping bottom topography using a two layer model. 1. Introduction U,q, and w* are Transport, PV, and cross isopycnal velocity. Diabatic β-plume: Cross PV gradient flow by a diapycnal mass flux Eddy driven β-plume: Cross PV gradient flow by an eddy PV flux divergence Local entrainment w*>0 causing a Cross PV gradient flow Figure 1: Schematic of a diabatic β-plume in a rectangular basin. ) ( N Uq = -q’U’+qw*+Friction -Plumes Created by Overflows Shinichiro Kida, James F. Price, and Jiayan Yang 1. MIT/WHOI Joint Program in Oceanography 2. Woods Hole Oceanographic Institution 2 2 1 Contact: [email protected] β
Transcript of -Plumes Created by Overflows - JAMSTEC-Plumes Created by Overflows Shinichiro Kida, James F. Price,...
Ogzokmen, T.M. et al. 2001, On the dynamics of b-plumes. J Phys. Oceanogr 31/12, 3569-3680Stommel, H. 1982, Is the South Pacific Helium dynamically active? Earth Planet. Sci. lett. 61, 63-67Swaters, G.E. 1991, On the baroclinic instability of cold core coupled den- sity fronts on a sloping continental shelf. J. Fluid Mech. 224, 361-382This research was supported by NSF Grant # *****
References:....
5. Conclusions and Future Work The Overflow can force a time mean circulation in the upper oceanic layer that has a magnitude as large as the over-flow itself. Baroclinic Instability creates a double β-plume in the oce-anic layer through form drag. This effect can enhance the diabatic β-plume. The β-plumes formed in this study depend on topograph-ic β and are thus locally confined. How these β−plumes can lead to a planetary scale feature, such as Azores Current, needs further investigation.
4. Diabatic β-plume
Diabatic β-plume Eddy driven β-plume
Figure 8: How eddy driven β-plume can enhance the diabatic β-plume.Figure 7 Time averaged flow
3Sv
The effect of entrainment on the oceanic layer was studied by imposing a local diapycnal mass flux in the downstream basin. This created a dense layer going down the slope and a diabatic β-plume in the upper layer (Figure 7ab.) (The mass source/sink in the up/downstream basin was turned off.) The transport of the β-plume was 3Sv which was larger than a linear solution of 2Sv. This was due to the eddy β−plume as shown schematically in Figure 8.
Diapycnal cooling source(1.5 Sv Mass flux)
(a)Overflow layer thickness
(b) Sea surface height3. Eddy Driven β-plume
Figure 5: The basic mechanism for theformation of eddy driven β-plume.
400km0
200
400
600
800km
300mthick
400km0
Bathymetry lines
5cm40cm/s40km
Figure 3: Snap shot view(birds eye view) of the two layers Eddies formed.
Figure 4: Time average the oceanic layer shows the double β-plume.
(a)Overflow layer thickness
(b) Sea surface height
(a)Overflow layer thickness
(b) Sea surface height
400km0
400km0
400km0
400km0
No inducedflow!
Figure 6: No eddy induced flowwhen there is no instability.
(a)Overflow layer thickness
(b) Sea surface height
2 Sv5cm/s
Lower Basin
Oceanic layer:Double β-plume
Overflow
Form Drag
>ν/f
=ν/f
The adiabatic interaction between the overflow and upper layer was studied by shutting off mixing be-tween the layers. The overflow (Figure 3a) was baroclinically unstable (Swaters 1991) and the upper oceanic layer flow (Figure 3b, shown by the sea surface height) had strong eddies [40cm/s, 40km wide.] These eddies in-duced an eddy driven mean flow in the upper layer as a double β-plume (Figure 4b.) Its transport was 2Sv, a size comparable to the overflow transport. Figure 5 shows the mechanism of how the double β-plume was driven. As the overflow exited the strait, it became baroclincally unstable and created eddies. Then part of its momentum is transfered across layers as a form drag. This form drag created the eddy PV flux divergence term in the vorticity equation (Section 1) and thus drove the eddy driven β-plume.
The momentum transfer was a drag to the overflow layer (Figure 4a) and caused it to go down the slope with a steeper angle than the Ekman number. Figure 6 shows a case when the upper layer was thicker (+1000m.) Baroclinic instability did not happen and no eddies formed, so there was no eddy induced upper layer flow
2. Model setup
Figure 2: Model Setup
Upstream Basin(Marginal sea)
Downstream Basin(Open Ocean)
Input
Output
Strait40km wide & 1000m deep
Slope 0.01
DenseOverflowLayer
We used a two layer (overflow and upper oceanic layer) isopycnal model (Hallberg Isopycnal Model) on a f-plane. β is introduced by the bottom topography shown below. 1.5Sv of dense water is pumped from the upstream basin to the downstream basin. This creates the overflow layer going through the narrow strait. Mixing between layers is either shut off or specified. A linear bottom drag acts on the layer that is in contact with the bottom topography.
Sea surface
Overflows vigorously entrain oceanic water. How does the upper ocean respond to this local entrainment? A small local forcing can induce a strong horizontal circulation(Figure 1, Stommel 1984). This concept is the β−plume and will be used in this study. The equation that ex-presses this concept is the vorticity equation written below. This study focuses on the overflow ocean interaction in the presence of a sloping bottom topography using a two layer model.
1. Introduction
U,q, and w* are Transport, PV, and cross isopycnal velocity.
Diabatic β-plume: Cross PV gradient flow by a diapycnal mass flux
Eddy driven β-plume: Cross PV gradient flow by an eddy PV flux divergence
Local entrainment w*>0 causing a Cross PV gradient flow
Figure 1: Schematic of a diabaticβ-plume in a rectangular basin.
)(
NUq = -q’U’+qw*+Friction
-Plumes Created by OverflowsShinichiro Kida, James F. Price, and Jiayan Yang1. MIT/WHOI Joint Program in Oceanography2. Woods Hole Oceanographic Institution
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Contact: [email protected]
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