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 ??
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
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 forcingComplex bathymetry of the coastal region
- Few works, mostly in the north PacificToole et al., 1990, ObservationsQiu 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)
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
A model study of the south Pacific circulation/bifurcation
ORCA05 model
An interannual global simulation 1992-2002
Evidence for zonal jets
EAC
NQC
NCJ
NVJ
NVJ_nNQC
NGCC
SCJ
ORCA model
Cars data
A subsurface circulation
NCJ
SCJ
NVJ
NVJ_n
Validation with XBT data
NCJ
SCJ
NVJ
NVJ_n
NCJ
SCJ
NVJ
NVJ_n
SCJ
NCJ
NVJ
Orca model
Bifurcation:
100 m
500 m
15°S
19°S
Depth dependent
Which definition for the western boundary currents??
1400 m
0 m
30S 12S20S
1,5° extension to the east of the topography
Mean latitude of the bifurcation function of depth:
From 15°S to 20°S
500 m
NQCEAC
Qu and Lindstrom: Figure 8
Orca model
23°S
17°S
13°S
EAC
EAC
GBRUC
NQC
Ridgway and Dunn: Figure 10
Orca model
EAC
EAC
GBRUC
NQC
Seasonal cycle
EAC
NQC
SEC/2
Bif20
12
4
The 3° Bifurcation variation explains the 10 Sv
WBCs variation
SEC19°S-13°S;
162°E
EAC23°S
NQC13°S
Bif
22-26 Sv
10-20 Sv
6-16 Sv
18°S-21°S
4 Sv 10 Sv 10 Sv 3°
Relation to the wind stress curl seasonal variability?
Seasonal anomalies in the 20°S-23°S latitudinal band
Seasonal variability of the bifurcation fonction of depth
0-300m: de 12°S à 17°S
600-1000m: de 19°S à 24°S
Jan Juil Dec
0 m
1400 m
The bifurcation shifts
southward during the austral winter
northward during the austral summer
JuneJanuary
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)
Interannual Variabilityof the bifurcation
EAC
NQC
NCJ
NVJ
NVJ_nNQC
NGCC
SCJ
How are the WBCs related to the bifurcation and the zonal inflow ??
How are the WBCs related to the bifurcation and the zonal inflow ??
Correlation JNV/JNC : 0.26Difference of Rossby waves propagation function
of latitude
Correlation JNC/WBCs: 0.85The JNC as a primary source of variability for the
WBCs
WBCs / SEC
EAC-NQC
JNC
JNV
WBCs / Bifurcation
EAC and NQC poorly correlated: 0.26
NQC related to the bifurcation in the surface layers
EAC related to the bifurcation in the deep layers
NQC
JNV
JNC
EAC
Bif. at 150 m
Bif. at 700 m
A SSH signature of the WBCs
NQC
SSH
SSH
EAC
C:-0.74
C:-0.76
1992 1998 2002
8
8
-8
-8
SSH Anomalies
(cm)
WBCs transport
Anomalies (1E6 Sv)
Conclusion
The climatological solution seems realistic
Seasonnal variability:
The bifurcation varies from 12°S to 24°S 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
What are the questions that we want to solve ??
Is the North Caledonian Jet the principal source of the western Australian boundary currents?
Which are the sources of variability, at seasonnal/interannual time scale, for the bifurcation ?
Rossby waves, local wind forcing, circulation around Australia???
How is the bifurcation related to ENSO events ?
What are the exact role of the north Queensland current to feed the equatorial basin ?
How are the non linearities important?
Why is the bifurcation uncorelated between the surface and the deeper layers?
NEGATIVE = STRONGER FLOW OF EACH CURRENT
150°E
14°S
Interannual Variability
Seasonal Cycle
Relation between the
North Queensland Current and the
North Guinea Coastal Current
NEGATIVE = STRONGER FLOW OF EACH CURRENT
NVJ_nNQC
What means do we need ??
in terms of numerical simulation
- Are the global simulations sufficient?
- Do we need regional simulations with high resolution?
in terms of observations
- Need for a specific observational program(oceanographic cruise, moorings, gliders, Argo buoys, others…??)