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 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)

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 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 30S12S20S 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 NQC EAC

Qu and Lindstrom: Figure 8 Orca model 23°S 17°S 13°S EAC GBRUC NQC

Ridgway and Dunn: Figure 10 Orca model EAC GBRUC NQC

Seasonal cycle EAC NQC SEC/2 Bif The 3° Bifurcation variation explains the 10 Sv WBCs variation SEC 19°S-13°S; 162°E EAC 23°S NQC 13°S Bif Sv Sv 6-16 Sv 18°S- 21°S 4 Sv10 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 m: 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 Variability of the bifurcation

EAC NQC NCJ NVJ NVJ_nNQC NGCC SCJ How are the WBCs related to the bifurcation and the zonal inflow ??

Correlation JNV/JNC : 0.26 Difference of Rossby waves propagation function of latitude Correlation JNC/WBCs: 0.85 The 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 EAC C:-0.74 C: 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_n NQC

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…??)