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…??)