ATOC 5051 INTRODUCTION TO PHYSICAL OCEANOGRAPHY Lecture 24:Wind-driven ocean circulation Learning 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.
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ATOC 5051 INTRODUCTION TO PHYSICAL OCEANOGRAPHY …whan/ATOC5051/Lecture...WBC (X’) and transport X: more realistic. Number “n” in zonal axis shows “n x L x ”, which is n
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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.
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
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.