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.