Diapositive%201

1
Bifurcation Dynamics L. Gourdeau(1) , B.
Kessler(2) 1), LEGOS/IRD Nouméa, New Caledonia,
2) NOAA/PMEL, Seattle, USA
Why is it important to study the
bifurcation ?? What do we know about the
bifurcation dynamics ?? Which are the
questions that we want to solve ?? What
means do we need ??
2
Why is it important to study the bifurcation ??
The bifurcation controls the partition of the
westward zonal currents which cross the oceanic
basins between the equatorial and the poleward
western boundary currents existing at the eastern
coast of the continents.
The role of the western boundary currents to
connect the tropics and the subtropics via the
STCs
Importance on the decadal modulation of ENSO
3
What do we know about the bifurcation dynamics?
- Steady Sverdrup theory Latitude of the
bifurcation at the zero zonally integrated wind
stress curl line
- A more complex reality depth dependence of the
subtropical gyre circulation Time and space
variability of the wind forcing Complex
bathymetry of the coastal region
- Few works, mostly in the north Pacific Toole et
al., 1990, Observations Qiu and Lukas, 1996
Seasonal and interannual variability (model) Qu,
and Lukas, 2003 Seasonal variability
(climatologic data) Kim et al., 2004 Seasonal
and interannual variability (model)
4
What about the bifurcation in the south
Pacific??
Climatological interpretation Qu and Lindstrom,
2002 Ridgway and Dunn, 2003
Role of the complex regional topography on the
circulation and the bifurcation
Queensland plateau Solomon New Caledonia
Vanuatu
5
A model study of the south Pacific
circulation/bifurcation
ORCA05 model An interannual global simulation
1992-2002
Evidence for zonal jets
ORCA model
SCJ
Cars data
6
A subsurface circulation
Orca model
Validation with XBT data
7
Bifurcation
Depth dependent
15S
100 m
500 m
19S
8
Which definition for the western boundary
currents??
1,5 extension to the east of the topography
500 m
Mean latitude of the bifurcation function of
depth
From 15S to 20S
0 m
EAC
NQC
1400 m
30S
12S
20S
9
Qu and Lindstrom Figure 8
Orca model
EAC
23S
EAC
GBRUC
17S
NQC
13S
10
Ridgway and Dunn Figure 10
Orca model
EAC
EAC
GBRUC
NQC
11
Seasonal cycle
The 3 Bifurcation variation explains the 10 Sv
WBCs variation
SEC 19S-13S 162E EAC 23S NQC 13S Bif
22-26 Sv 10-20 Sv 6-16 Sv 18S-21S
4 Sv 10 Sv 10 Sv 3
Relation to the wind stress curl seasonal
variability?
Seasonal anomalies in the 20S-23S latitudinal
band
12
Seasonal variability of the bifurcation fonction
of depth
June
January
0 m
1400 m
Jan Juil Dec
0-300m de 12S à 17S 600-1000m de 19S à 24S
The bifurcation shifts southward during the
austral winter northward during the austral
summer
13
Interannual Variability of the bifurcation
A /- 4 latitude variation Low correlation
between the bifurcation in the surface layers and
the deep layers C 0.4 (100/1000 m) 0.72
(100/300 m) 0.95 (700/1000 m)
14
How are the WBCs related to the bifurcation and
the zonal inflow ??
SCJ
15
How are the WBCs related to the bifurcation and
the zonal inflow ??
WBCs / SEC
Correlation JNV/JNC 0.26 Difference of Rossby
waves propagation function of latitude Correlati
on JNC/WBCs 0.85 The JNC as a primary source of
variability for the WBCs
16
WBCs / Bifurcation
EAC and NQC poorly correlated 0.26
NQC
Bif. at 150 m
JNV
Bif. at 700 m
EAC
JNC
NQC related to the bifurcation in the surface
layers EAC related to the bifurcation in the deep
layers
17
A SSH signature of the WBCs
SSH Anomalies (cm)
WBCs transport Anomalies (1E6 Sv)
18
Conclusion
The climatological solution seems realistic
Seasonnal variability The bifurcation varies
from 12S to 24S depending of the season and of
depth The annual cycle of the bifurcation
explains the EAC/NQC variability

• Interannual variability
• Strong relation between the JNC and the WBCs
• Low correlation between the bifurcation in the
  surface layers and the deep layers
• The EAC is related to the bifurcation at depth
• The NQC is related to the bifurcation in the
  surface layers
• Signature on SSH of the LLWBCs

19
What are the questions that we want to solve ??
Which are the sources of variability, at
seasonnal/interannual time scale, for the
bifurcation ? Rossby waves, local wind forcing,
circulation around Australia???
How are the non linearities important?
How is the bifurcation related to ENSO events ?
Why is the bifurcation uncorelated between the
surface and the deeper layers?
Is the North Caledonian Jet the principal source
of the western Australian boundary currents?
What are the exact role of the north Queensland
current to feed the equatorial basin ?
20
Relation between the North Queensland Current
and the North Guinea Coastal Current
21
What means do we need ??
in terms of observations - Need for a specific
observational program (oceanographic cruise,
moorings, gliders, Argo buoys, others??)
in terms of numerical simulation - Are the
global simulations sufficient? - Do we need
regional simulations with high resolution?