Seasonal Variation of the South Equatorial Current
Bifurcation off the Australian Coast
Zhaohui Chen and Lixin Wu
Physical Oceanography Laboratory, Ocean University of China, Qingdao, China
Abstract
Role of ITF and islands in the east of Coral Sea in modulating SEC Ab
Seasonal variation of the South Equatorial Current (SEC) bifurcation off the Australian
coast in the upper Southwest Pacific is investigated based on observations and a 1.5-layer
non-linear reduced gravity model. The mean SEC bifurcation latitude (SBL) integrated
over the upper thermocline is around 17.5°S, almost 2° south of the position predicted by
the Sverdrup theory. In terms of its seasonal variation, the SBL reaches the southernmost
position in June/July and the northernmost position in November/December, with southnorth migration (Ab) of 2.5°, which is over twice larger than its counterpart in the North
Pacific. It is found that, in addition to linear Rossby wave dynamics, the large SEC Ab
attributes to the existence of Indonesian Throughflow (ITF) and islands in the east of
Coral Sea. A series of numerical experiments indicate that the seasonally varying ITF
could enhance the Ab by 0.5°, while the islands could reduce it by 1°, comparing with the
model runs without the islands. This suggests the islands obstacle and the seasonal pulse
from ITF play explicit roles in modulating SEC Ab, which are all absent in the NEC
bifurcation issue in the North Pacific.
A simple expression of the seasonal bifurcation involving the wind stress forcing in
conjunction with baroclinic adjustment is proposed under the framework of linear Rossby
wave dynamics. It is found that the seasonal variation of the bifurcation latitude is
predominantly controlled by the spatial pattern of the seasonally varying wind forcing,
while the baroclinic adjustment only determines Ab. This expression works well in
reproducing the seasonal cycles of the SEC/NEC bifurcation under different spatial wind
forcing in both hemispheres, and it provides a simple way to understand the critical role
of local wind forcing in amplifying the south-north migration of the SBL off the
Australian coast in comparison with the NEC bifurcation.
FIG 3 (a) Seasonal cycle of the SBL in the control run (red),
with ITF closed (blue), and tropical South Pacific wind forcing
with ITF closed (green). (b) The seasonal evolution of the
bifurcation latitude shift (Ctrl Run minus Close ITF, blue) and
ITF transport in the Ctrl Run (red).
FIG 4 (a) Seasonal cycle of the SBL in the control
run (red) and sensitivity run without the islands
east of Coral Sea (blue). (b) Same as (a) but for
the cases with the ITF closed.
To clarify why SEC Ab exhibits larger than NEC Ab, we performed sensitivity experiments using
a 1.5-layer, non-linear, reduced gravity model to investigate the role of ITF and islands in
modulating the SEC Ab. If the ITF is closed, the SBL would shift equatorward by over 2° (Fig. 3a)
and the Ab would be reduced by about 0.5°, which could be attributed to the seasonally varying
ITF (Fig. 3b). Further experiments show that the islands will be obstacles to the SEC inflow by
‘island rule’, leading to the northward shift of the bifurcation latitude (Fig. 4) and the reduction
in Ab (about 1°) compared with the case where no islands exist (Fig. 4a). If ITF is closed, as
shown in Fig. 4b, Ab has no significant change, implying a farther south bifurcation will
inevitably be influenced by the northern tip of New Caledonia.
A simple bifurcation expression
Seasonal variation of the SBL off the Australian coast
FIG 1 Seasonal variation of the SBL
derived from the satellite altimetry SSH
data (black), the geostrophic flow
averaged in the upper 400 m (red) derived
from the WOD T/S data, and the
meridional flow averaged in the upper 410
m from the ECCO2 product (blue). The
pluses denote individual bifurcation
latitude estimated from the SSH data and
the shaded bars denote the standard
deviation range. The dashed lines
represent mean values.
The mean SBL integrated over the upper 400 m is located between 17.5 ºS and 17.8 ºS based on
the calculations from WOD 09 and ECCO2, but 15.5 ºS from the 20-year altimetry SSH data
analysis (Fig. 1). This 2ºdifference is largely due to the poleward tilting of the SEC bifurcation
with an increasing depth. In terms of the seasonal variation, it moves to the southernmost
position in June/July and the northernmost position in November/December, with south-north
migration of 2.5º, which is over twice larger than its counterpart, i.e., NEC bifurcation off the
Philippines in the North Pacific (Fig. 2).
c)
FIG 5 (a) Seasonal cycle of the NEC/SEC bifurcation latitude derived from the model runs, in which only tropical
wind stress forcing is considered. (b) Same as (a) but for the linear Rossby model forced by wind stress curl. (c)
(c) Annual migration of the zero line as a function of longitude.
If we eliminate the impacts of the islands and ITF, it is shown in Fig. 5a, b that SEC Ab is still
larger than NEC Ab. To explain their difference, we propose a simple bifurcation expression
under the framework of linear Rossby wave dynamics: the seasonal bifurcation is regarded as
the overall response to the south-north migration of the zero curl line integrated over the basin.
Suppose the wind forcing is expressed by a sinusoidal function, and it
F (t )   A sin(t )
indicates the zero line reaches its southernmost position in March:
As in Fig. 5c, the forcing exhibits a spatial-dependent form
(L indicates the basin width; k describes zonal level of the forcing)

cos(t  )
2
1
2 a
baroclinic adjustment coefficient, where a is transit years of Rossby wave.




R(t ) 
A
cos(

t
)

e
cos(

t

2

a
)

sin(

t
)

e
sin(

t

2

a
)


2
( / k )  1 
k
k


1

k
Response to
westernmost
wind forcing (x=0)
with 3-month lag
1

k
Response to
easternmost wind
forcing (x=L)
with 3-month lag

A 
Y (t ) 
cos(t )  e cos(t  2 a)   MeanLat (curl )
2
( / k )  1 

NP A=13/2，k=1.5
SP A=22/2，k=0.5
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om

The overall response at the WB is the integration over the entire basin (from 0 to L), we have:
1

k
RESEARCH POSTER PRESENTATION DESIGN © 2011
x
kL
The response of the western boundary to wind forcing at any
x


x
kL
point can be regarded as the time-lag response due to baroclinic r ( x, t )  Ae cos(t    )
2
c
Rossby wave propagation (c is the phase speed of Rossby waves)
Damping
Coefficient
FIG 2 Seasonal cycle of the NEC/SEC bifurcation latitude derived from WOD geostrophic flow, satellite
altimetry SSH data, and ECCO2 product.
F ( x, t )  Ae

NP Peak Month: 9
SP Peak Month: 8
Suppose the mean
curl is at 15°N/15°S
Cr=0.16m/s
LNP 120°
LSP 130°
Response to non-zonal wind
forcing due to the dependence of
A on longitude (x), but it is very
small (high-order terms).