Blank Black Slide

1
Blank Black Slide
2
Southern Ocean - the region of lowest SST increase
CMIP2 results at time of CO2 doubling
IPCC 2001
3
Ekman Transport in theSouthern Ocean

• David Webb
• National Oceanographic Centre
• Southampton

4
Part 1Where the World is inSteady State
5
Wind Stress (dynes/cm2)
6
J. Richman (OSU)
7
Ekman Transport
8
Ekman Transport Divergence
9
Overturning Stream Function
On Levels
10
FRAM Surface Fields
11
FRAM North-South Section
12
WOCE Section I6
Feb - Mar 1996
13
WOCE Section P18
Feb - Apr 1994
14
OCCAM - March 1996 WOCE Section I6
15
OCCAM - March 1994 - WOCE Section P18
16
Part 2Where in the Real World Things Change
with Time
17
Wind Stress (dynes/cm2)
18
Average Wind Stress between 55S and 65S (1993-94)
19
Ekman Pumping in the SE Pacific
20
RMS Ekman Pumping
21
Trapped Modes
22
South-East Pacific ModeResponse Function
23
Part 2aWhen the Wind Stress changeswhat
happens to the Large Scale ACC Transportand
the Momentum Balance?
24
Drake Passage Transport- during the Fram Spin-Up
25
Correlation of Tide Gauges(C. Hughes et al)
26
Hughes and Meredith (Phil Trans, in press)
27
OCCAM 1º Global Model (66 levels)
28
Drake Passage Transport - Control Run
29
Wind Stress Increment (dynes/cm2)
30
Drake Passage Transport
31
Drake Passage Transport
32
SSH Difference (1 dyne - Control) over10
days
33
SSH Difference(Control 0.1 dyne) - (Control)
34
Overturning Stream Function - Control Run
35
Overturning Stream Function - Increased Wind in
Band
36
Overturning Stream Function - Change with
Increased Wind
30 days
37
North-South Velocity Difference(Control 0.1
dyne) - (Control)
After 2 days
38
North-South Velocity Difference(Control 0.1
dyne) - (Control)
After 30 days
39
Barotropic Model of the Overturning Circulation
40
Overturning Stream Function - Theoretical Change
30 days
41
So How Barotropic is the Change?
Variance of Difference Field 1.37x1027
Variance of Difference Field - Theoretical
Estimate 7.66x1024
Variance Unexplained 0.56
(Unexplained after 2 days 6.3)
30 days
42
Balance of Terms after 2 Days
Surface Layer
Wind Stress
Mid Layer
SSH-Bottom Pressure
Abyssal Layer
43
Balance of Terms after 30 Days
Surface Layer
Wind Stress
Mid Layer
Abyssal Layer
SSH-Bottom Pressure
44
Conclusions

• Wind Stress creates inertial oscillations which
  die out within a day or two leaving an Ekman
  Layer
• The Ekman Layer appears to generate strong fronts
  where it meets saline western boundary currents.
  Mixing is caused by the Ekman layer driving dense
  water over lighter water.
• In the eastern side of the oceans the structures
  are much weaker. Why is intermediate water
  formed in these regions?

45
Conclusions cont

• There is a large variability in the Wind Stress,
  and the resulting Ekman Transport and Ekman
  Pumping.
• This can excite barotropic modes of the Southern
  Ocean
• There is experimental evidence that changes in
  ACC transport are also associated with barotropic
  waves that propagate around Antarctica

46
Conclusions cont

• Ocean models show that the ocean responds to
  changes in the wind stress within a few days.
• The initial ocean response is primarily
  barotropic
• The associated current does not follow f/h
  contours
• The barotropic current explains the change in the
  Deacon cell
• The change in topographic form drag due to the
  change in SSH balances the change in wind stress.
• After a month baroclinic/density terms reach the
  10 level.

47
Blank Black Slide
48
North-South Velocity Difference(Control 0.1
dyne) - (Control)
49
(No Transcript)
50
Drake Passage Transport
51
Analysis - FRAM