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