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Some Idealized Thought Experiments of Wind-driven
Circulation in the Arctic Ocean Jiayan Yang Woods Hole Oceanographic Institution Co-PI: Andrey Proshutinsky Nov. 2, 2011, 15th AOMIP Workshop, Woods Hole, MA
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28-Year (1978-2006) daily Ekman transport and
upwelling on 25x25 km resolution over the whole Arctic (Yang, 2006, J. Climate) Data Sources: Ice motion: NSIDC (Fowler, 2003); Ice concentration: NASA GSFC, Greenbelt, Maryland; Surface wind: derived from geostrophic wind from IABP
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Today I am not going to talk about the real Arctic Ocean circulation.
I will discuss some simple but fundamental (I believe) issues that have puzzled me in the last a few years. Something as simple as: Question 1: how the Ekman cell is maintained? Question 2: what is the 0th order balance in dynamics? Question 3: can there be a quasi-steady circulation? Question 4: where the vorticity and energy are dissipated (related to 3)?
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The 0th order dynamics of a wind-driven gyre:
Ekman Transport Sverdrup Transport Western Boundary Current Ekman pumping z The 0th order dynamics of a wind-driven gyre: Wind-stress curl -> Ekman pumping -> Sverdrup Transport; Western Boundary Current closes the transport and dissipates the vorticity and energy
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What drives this return flow?
North Pole Canada Basin What drives this return flow?
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Two-layer, nonlinear model that allows
interface outcropping or grounding. Model resolution is 5 km and uses C-grid.
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(the reason of keeping same f0
b-plane at 85N f-plane at 90N b-plane: f0 set at 90N b at 45N (the reason of keeping same f0 is to have a same deformation radius as other exps)
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Eddies transport mass meridionally and
North Pole b-plane at 85N Canada Basin Eddies transport mass meridionally and flux vorticity to boundary layer Eddy fluxes b-plane: f0 set at 90N b at 45N bV transport mass meridionally (Sverdrup relation) but eddies flux vorticity to boundary layer
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b-plane at 85N Inserting a western boundary
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An Active role of eddies in the Arctic Ocean:
Mean vorticity convergence of wind-stress curl frictional torque Advection eddy vorticity flux A A A A4 In interior: Along boundary:
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The spin-up of a wind-driven circulation:
A classic view (Veronis and Stommel, 1956; Anderson and Gill, 1975): Wind-stress curl -> barotropic and baroclinic Rossby waves -> western boundary current -> boundary adjustment by Kelvin waves -> Rossby wave radiation from the eastern boundary -> the barotropic and baroclinic modes are adjusted so that their velocities cancel each in the abyssal layer -> wind-driven gyre is confined in the upper layer above thermocline The spin-up in the Arctic Ocean ?
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Time-mean (averaged in the 11th year) flow and pressure
b-plane at 85N Is this barotropic flow due to stagnation of planetary Rossby waves?
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Time-mean (averaged in the
11th year) flow and pressure b-plane: f0 set at 90N and b at 45N. The lower layer circulation is still not shut down
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Time-mean (averaged in the 11th year) flow and pressure
The classic Anderson-Gill spin-up is recovered by inserted a western boundary
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Summary: Because the Arctic Ocean has a unique geographic shape (no western boundary) and different physical parameter (b is small), the dynamical balance is different from that in a typical mid-latitude gyre; (2) Eddies play a leading role in vorticity and mass fluxes; (3) Eddies are not necessarily generated by baroclinically unstable boundary currents. They can be originated in the interior and are required by some fundamental dynamical balances; (4) A non-trivial b is necessary but insufficient in achieving a Anderson-Gill spin up that shut down the abyssal flow. A western boundary is needed.
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