ATOC 5051 INTRODUCTION TO PHYSICAL OCEANOGRAPHYwhan/ATOC5051/Lecture_Notes/ATOC5… · ATOC 5051...

ATOC 5051 INTRODUCTION TO PHYSICAL OCEANOGRAPHY

Lecture 22: Wind driven ocean circulation Learning objectives: should be able to identify the observed features in subtropical ocean circulation, and understand their causes. Specifics: 1. Observations: upper ocean circulation (today)2. Sverdrup relation, Sverdrup balance (today);3. Westward intensification

1. Observed upper-ocean circulation

Schematic diagram

Pedlosky1987

Westward intensification!

2: Ocean Interior SolutionScale analysis, Sverdrup relation, Sverdrup balance

Development of Sverdrup theory:

Step 1: Derive a simplified vorticity balance for theinterior ocean;

Step 2: Vertically integrate of the vorticity equation from step 1 under appropriate conditions, which yields a remarkable relationship between the vertically integrated meridional flow and the local value of the wind stress.

Scale analysis: What are the scales for large-scale motion in ocean interior? Ro and Ekman numbers?

(earlier classes: scale analysis for interior ocean,with Lx=Ly=1000km)

Horizontal momentum equations:

Nonlinear terms (advection), mixing terms,and time varying term are small & neglected.

Ro<<1 & Ex<<1, Ey <<1 & steady circulation:

Oceanic layers: surface mixed layer;deeper ocean below the mixed layer.

D

(a) Surface Mixed layer:

(b) Below the mixed layer:

Linear Vorticityequation

For a northward motion (NH),

y (N)z

Potential vorticity conservation:

Here, H representsvortex’s height

Questions

Below the oceanic surface mixed layer, a water column moves southward in NH; how will the column change, stretching or compressed?

Step 2: vertically integrate the vorticity equation

NOTE: we used Boussinesque approximation; so it is the approximate form.

Schematic:Surface winds

Observations

Pedlosky 1987

EQ

+

-

15N-45N, STG region:

<0

maximumL

45N

15N

y

30N

- +

45N

15N

y

30N

26.1b

L

0

45N

15N

30N

y=L

y=0X=0 x=

40N

70N

10N

y

westerly

easterly

Schematic diagram: wind-driven circulation in the ocean interior

Question• Why is the upper ocean circulation in the

northern hemisphere subtropical ocean anticyclonic?

• What relationship have you derived and used to explain this circulation?