1. No category

Why is the Indonesian
Throughflow strengthening
the Agulhas leakage?
Dewi Le Bars
Will De Ruijter
Henk Dijkstra
IMAU, Utrecht University
AGU Chapman Conference,
Stellenbosch, South Africa, October 2012
Satellite altimetry
Contours of Mean Dynamic Topography and standard deviation
(colors, cm). Averaged over the last 20 years.
•
•
The Indian Ocean is opened at low latitude: unique feature
Impact on the Agulhas system? Let’s close the Indonesian
Throughflow.
Ocean model: Parallel Ocean Program (POP)
•
Primitive equations
model
•
Simulations used:
Global, 0.1º resolution
•
•
•
z-coordinates, 42 levels
Climatological forcing
Comparison of an
open vs closed ITF
simulations, each of 40
years.
Satellite altimetry
POP
Contours of SSH and standard deviation (colors, cm)
Drawbacks: Retroflection too early leads to a regular path of the
Agulhas eddies.
But it is the first time a global high resolution model is used to
look at the ITF impact on the leakage.
Opening the Indonesian Throughflow
NBC
MC
AL
BC
SEMC
AC
EAC
TL
Section
ITF
EAC
TL
SEMC
MC
AC
AL
NBC
BC
# Agulhas eddies per year
POP CTRL
POP NoITF
Difference
15
0
+15
39
52
-13
9
12
-3
19
17
+2
17
2
+15
92
82
+10
21
17
+4
24
23
+1
37
34
+3
3.6
2.9
+0.7
Velocity difference (open minus closed ITF) averaged over the top
1500m (arrows) and difference of the magnitude (colors) in cm/s
POP Open ITF
POP Closed ITF
Contours of SSH and standard deviation (colors, cm)
The ITF increases the variability in the “eddy corridor” through an
increased number of eddies.
POP conclusions
•
The ITF increases the frequency of Agulhas eddy shedding
and the total volume of Agulhas leakage.
•
The additional input of water from the ITF doesn’t change
the retroflection location.
What about the retroflection index?
(Dijkstra and De Ruijter 2001)
ΦL
R=1−
ΦAC
ΦL : Leakage
ΦAC : AC transport
Doesn’t change:
Open ITF: R = 0.78
Closed ITF: R = 0.79
Mechanism
Opening of the Indonesian Passages
Increase transport of the Mozambique
Channel and Agulhas Current
Same proportion of retroflection
Increase leakage
How general is
this result?
Hallberg Isopycnal Model (HIM)
Solve the primitive equations on a sphere in isopycnal coordinates
Flat bottom, 2 layers, 0.1º horizontal resolution
-τ0
Wind
stress
τ0
Indonesian
Throughflow
Mozambique
Channel
Agulhas
Current
Leakage
Periodic
Boundary
Conditions
HIM Results
a
b
Open ITF (black) and closed ITF (red)
Le Bars et al. 2012
The linear case
We write the momentum equations as F = 0
with :
1 ∂p
τx
F1 = −f v +
−
− AH ∇2 u,
ρ ∂x ρH
1 ∂p
− AH ∇ 2 v
F2 = f u +
ρ ∂y
Integrating F on
different
contours leads
to three vorticity
balances:
=0
F .ds
Ci
C2
Φin
C3
ΦAC
ΦL
ΦD
Φ2
C1 Φ1
ΦACC
C
Combined with
the mass balances
we can solve for
the transport
through each
section as a
function of the
wind stress.
The linear case: Results
Open ITF (black) and closed ITF (red)
Dynamics
•
2 processes could cancel each other:
(i) The (large scale) requirement that the Indonesian
Throughflow water has to flow back south of Australia leads
to reduce R.
(ii) The additional input of water from the ITF leads to
increase (locally) the inertia/instability of the AC and to
increase R.
Conclusions
Mozambique
Channel
ITF
Agulhas
Current
Constant R
From 3 different
models opening the
ITF leads to increase
the Agulhas leakage.
The fact that the proportion of
retroflection is insensitive to the ITF
seems to be very robust when nonlinearities are taken into account.
a
Satellite
altimetry
Agulhas
Plateau
Comparison of
the
retroflection
position
b
POP
with ITF
c
POP
no ITF
Retroflection
too far east
No change due
to the ITF