atoc 5051 introduction to physical oceanography …whan/atoc5051/lecture...wbc (x’) and transport...
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ATOC 5051 INTRODUCTION TO PHYSICAL OCEANOGRAPHY
Lecture 24:Wind-driven ocean circulationLearning objectives: Qualitative & quantitative: physical causes of westward
intensification;Qualitative: coastal and equatorial ocean circulation
features & processes that causes them. Specifics:1. Westward intensification (continue);2. Wind-driven coastal ocean circulation;3. Wind-driven equatorial ocean circulation.
Previous class: scale analysis the vorticity equation in WB region:
(i) Stommel 1948 (linear bottom drag)
1. Westward intensification (continue)
Interior solution: 45N
15N
30N
Constant f varying f
β ≠ 0
Vmax @ 1.2
(ii) Munk’s solution
Munk, W. H., On the wind-driven ocean circulation, J. Meteorol., Vol. 7,1950
Boundary conditions of Munk:
Munk 1950(horizonal harmonic mixing;more realistic)
Vorticity equation in WB region:
Physically, WBC is a response tointerior circulation
Munk (1950)
WBC (X’) and transport X: more realistic.Number “n” in zonal axis shows “n x Lx”, which is n x 100km in our case. Vmax ~120km
Note the “return current”or recirculation of Munk
layer!
Munklayer
Interior
Recirculation
Degree
Green to purple: small to large
+V -V
Stream function contours are the lines that currents (or winds in the atmosphere) flow along
Interior & WB regions: Major physical differences(steady circulation vorticity equation:
(5)
(6)or
Ocean interior away from western boundary:Western boundary region:Fundamental difference interior/western boundary!Interior: wind forcing;western boundary: remote response to interior forcing &mixing is important.
Summary• What is the scientific insight and limitation of
Stommel’s WBC?
• What progress has Munk made comparing to Stommel’s solution? What modification did he do that led to this improvement?
Pedlosky1987
15N
30N40N
Physics behind: Why do we integrate the equation from eastern boundary westward rather than from west to east in solving interior solution?Observational evidence:
hmix
thermocline
Easternboundary
[3] continuous wind forcing
[1][2]
Because long Rossby waves radiate westward!
This will also help to explain why westward intensification disappears when beta=0.
effect; Rossby waves[1][2] Vorticity balance (entire basin)
Westward intensification:
f2
f1
-vorticity inputby wind
45N
15N
30N
f2
f1
+vorticity inputby friction!
Validity of Sverdrup theory? Topography renders the integrated circulation deviates from Sverdrup balanceIs the North Atlantic in Sverdrup Balance? JPO, https://doi.org/10.1175/1520-0485(1985)015<1876:ITNAIS>2.0.CO;2A note on the validity of the Sverdrup balance in the Atlantic North Equatorial Countercurrent. DSR, https://doi.org/10.1016/j.dsr.2004.05.014
Eastern boundary coastal upwellingImportant for fishery industry
Indian Ocean:Somali&Omancoasts: west
2. Coastal ocean circulation
Observation
Satellite observed surface winds
Mean surface winds & SST from 1950-2009
CA
June, 121993,finerscale:
CentralCACoast
Intense&Mesoscalefilaments
On a specific day
PhytoplanktonBloom!
Satellite observed SST Ocean color (Chlorophyll)
Fishery: marine food chain
Coastal ocean circulation dynamics: 2-layer model
Critical Thinking (1)How do the eastern boundary surface
currents compare with the local winds? (2) Can you think of a cause?
Equatorward surface winds
Poleward coastal undercurrent (CUC)
Boad demo
Dynamics: 2-layer model
Coastal jet
thermocline
Low sea level (idealized)Ekman transport
2-layer model (f-plane):
Coastal jet
Denote
Assume alongshore geostrophy, which isalongshore current vobeys geostrophy.
Then we obtain solutions as:
H1 is layer 1 initial thickness;h is its perturbation;
is Rossby radius for the baroclinic mode, which is the e-folding decay scale for coastal jet & CUC.
Solutions: Coastal jet & CUC:Spatial scale: Rossby radius;Alongshore geostrophy;Add mixing – solution more realistic
3. Equatorial ocean circulation
Obs:ZonalCurrentU in EQAtlantic
Obs: EQ Pacific
Equatorial ocean circulation: Dynamics
Linear model Solution forced by Easterly trade wind
SEC:EUC – Pressure Gradient force + mixing
Problem: heat unbalanced.This linear model does not conserve heat. The EUCis cold. Why?
This point to the subtropical-tropical exchange idea,the SubTropical Cells (STCs; Luyten, Pedlosky,
Stommel, 1983; McCreary and Lu 1994).
The colder, subtropical water subducts into the thermocline; then flows southward and westward alongthe Rossby waves characteristics. After it arrives at
the western boundary of the North Pacific, it flows eastward to feed the EUC. In the south Pacific, there are interior pathways.