1.
State of the art
The inflow to the Coral sea is supplied by the westward South Equatorial Current (SEC). The
Sec bifurcates at the Australian coast to feed Low Latitude Western Boundary currents
(LLWBCs), namely the poleward Eastern Australian Current (EAC), and the Northwards
North Queensland Current (NQC) and Great Barrier Reef Undercurrent (GBRC). The
equatorward-flowing LLWBCs recirculates in the gulf of Papua New Guinea through the
New Guinea Coastal Undercurrent (NGCUC) which is a primary source for the Equatorial
Undercurrent (EUC).
Variations in the strength or water mass properties of the LLWBCs will influence the
structures of the equatorial thermocline and cold tongue, and thereby feedback to affect the
atmospheric circulation that link the tropics and subtropics. This scenario have been
hypothesized to be important in determining Pacific decadal climate variability (Gu and
Philander 1997; Keeman et al., 1999]).
The amount of subtropical water imported to the tropics via the LLWBCs depends critically
on the latitude where the SEC bifurcate at the western boundary.According to Sverdrup
theory, the bifurcation of the SEC should occurs at the zero zonally integrated wind stress curl
line situated at about 15°S. This simple steady-state-depth-averaged theory is not sufficient to
describe the actual flow field.
The recent publications (Qu and Lindstom 2002; Ridgway and Dunn 2003; Sokolov and
Rintoul 2000) describe with great details the mean circulation of the South West Pacific.
Particularly, they point out the influence of the complex regional topography on the
circulation.
The SEC is not a broad westward flow extending in depth, as the normally accepted picture,
because there is significant blocking by the large number of island arcs and reef systems in
the region. When the size of the topographic features is large compared to the Rossby radius,
one would normally expect to find boundary currents at the eastern side of the topography and
zonal jets extending westward from the northern and southern extremities.
There are some evidence of such zonal jets associated to the presence of Vanuatu and New
Caledonia islands in the papers mentionned above. The modeling study of Webb (2000)
confirms the split of the SEC into zonal jets even if theSEC is shift to the south, and the
circulation at the surface is too intense in their model.
Therefore, the SEC is described as spanning a 13°S-19°S latitudinal range with a northern
branch centered at 14°S and a southern branch centered at 17°S. Therefore, the SEC provides
a strong inflow to the Coral sea both north of Vanuatu and between Vanuatu and New
Caledonia. The core of the SEC moves southward at increasing depth. It has a maximum
intensity along 17°S and at a depth of 200 m.
The nature of the SEC bifurcation appears to be control by the he topography, and in
particular the complex reef structure on the Queensland plateau. The vertically integrated
circulation shows a bifurcation latitude around 18°S. There is also a southward shift of the
bifurcation with increasing depth. The bifurcation moves from about 15°S near the surface to
about 22°S at depths around 800m.
II. ORCA05 model analysis
At this time, only the mean state is described. The ORCAO05 model is used to investigate the
variability of the SEC and of its bifurcation. It was running for the 1992-2002 period and
outputs are sampled every 5 days.
First, a description of the mean circulation in the Coral sea is necessary to validate the mean
model state.
II.1 The mean model state
The streamfunction over the Coral sea provides a picture of the depth-integrated circulation.
The presence of New Caledonia and Vanuatu blocks the westward extension of the SEC and
as a result, cells of the zonal flow are formed extending westward from the northern and
southern extremities of the islands. There are evidence for a north Vanuatu zonal jet at 14°S
(NVJ), a north New Caledonia jet at 18°S (NCJ), and a south New Caledonia jet (SCJ). The
mean bifurcation point seems situated at 17°S. Therefore, most of the NVJ and NCJ will feed
the GBRUC and the EAC respectively, and the SCJ will feed the EAC.
The 0-1500m depth–integrated zonal velocity shows that the zonal jets are centered at 13°5
and 18°S, and they have a 2° latitudinal extension. The order of the transport is of 14 sv by
jet. It is twice the transport calculated with the 0-400m depth-integrated zonal velocity,
therefore a major part of the jets flow in depth. Superimposed on the plots is the bathymetry at
1000m, showing how the jets, particularly the NCJ, are influenced by the Queensland plateau
situated 5° east of the Australian coast. The strong eastward current south of New Guinea is
the NGCUC.
A latitudinal section at 161°E of the depth-integrated zonal velocity, shows how well the jets
are defined, The NVJ and the NCJ are separated by a region of null transport at 16°S. The
NCJ extends more in depth than the NVJ. If the transport from the surface to 400m are similar
for the two jets, the transport from the surface to 1500 of the NCJ is higher than the one of the
NVJ. Also if the limits of the NCJ are well defined, it is not the case at the southern boundary
of the NVJ. It means that west of Vanuatu, the NVJ is feeded by the southwestward equatorial
SEC, and in the surface layer by recirculation cells east of New Guinea.
On the vertical section at 161°E of zonal velocity, the NCJ extends as deep as 1400m. Its core
is situated at 200 m with a magnitude of 8 cm/s, deeper than the core of the NVJ (100m). The
NVJ shifts northward from 14°S at depth to 12°S in the surface layer, suggesting that a
cyclonic cell (part of the New Guinea Coastal Surface current and the NGCUC??) feed the
NVJ and bounded it at 11°S.
The two branches of the SEC have different water properties with warmer and saltier water
found in the NVJ compared to the NCJ
The two branches of the SEC bifurcates at the Australian coast. The bifurcation latitude varies
greatly with depth from 15.5°S at the surface to 20°S at 800 m. At 300 m, the bifurcation
latitude is at 17°S.
Due to the Queensland plateau, from 154°E the NCJ is deviated northward and by example at
700 m a part of it flows at 15°S. Because of the latitude range of the bifurcation, between
15°S-19°S, the NCJ appears to be of prime importance to feed both the EAC and the GBRUC
depending on depth. The NVJ feeds principally the GBRUC and the NQC even if a part of it
may turn south in the first 100 m deep. There is a local recirculation of the EAC at 21°S
which intensifies the current along the boundary. At 23°S, deeper than 300m, there is a
westward current, which could be the SCJ, feeding the EAC. South of 20°S, near the
boundary, the EAC flows at speed of order 15-20 cm/s.
The GBRUC is first detectable at 20°S at about 1000 m. Further to the north, the GBRUC
intensifies and merges with the NQC.
The description above is very close to the state of the art presented above. It gives confidence
in the model simulation even if the .5° resolution must smooth the influence of the
topography.
The SEC defined between 19°S and 13°S carries 23 Sv into the Coral sea (30 sv between 20S10S). Most of this transport is carried by the two jets, the northern being the weakest (9 sv
against 12 sv for the NCJ). The NGCUC returns 14 sv, the remainder turns south to feed the
EAC. The SCJ (26°s-21°S, 200m-1500m, 161°E) carries 9 sv which must fee the EAC. (A
revoir les chiffres qui paraissent faibles)!
II.2. Model Variability
XBT sections passing just west of New Caledonia are used to study the variability of the SEC
between NC and the Solomon islands, and to validate model outputs. The transport calculated
from the XBT is very similar to the model one. There is a 20 sv bias between the XBT
transport limited to the first 400 m depth and the total transport from the model. The
variability is not modified by the choice of the reference level, therefore the XBT may
provide useful information on the variability of the SEC. This comparison gives confidence in
the model variability. There are strong interannual variations of the zonal transport by the
SEC going from 50 sv in 1998 to 15 sv in 2000.
II.2.a seasonal variability
The location of the jets is defined by the topography, therefore there is no spatial variability of
their location. The NVJ is influenced by a strong seasonal signal at its northern boundary.
The net transport between 19s and 13s, very close to the sum of the the NCJ and NVJ, does
not have a well marked seasonal cycle (The half transport is ploted). It is not the case for the
individual jets which are out of phase. The transport of the NCJ (NVJ) in black (in red) is
maximum (minimum) during the austral winter with a 14 sv magnitude (7sv). Also is ploted
on the figure, the strong seasonal cycle at the northern boundary of the NVJ (13°S-11°S),
minimum in april and maximum in october which may interact with the NVJ.
If the total transport of the SECis relatively constant in time, the latitude bifurcation point,
averaged with depth, varies in time at seasonal time scale. It varies from 17°S to 19°S, being
at its souhern position during the austral winter. There is a phase relationship between the
amplitude of the jets and the latitude where the bifurcation takes place.
The latitude bifurcation point varies with depth in a 5° range from 13°S to 18°S between
september and february, and from 18°S to 23°s between march and august. Therefore the
latitudinal variation of the bifurcation has a well marked seasonal cycle.
The LLBWCs are fed by the SEC (mostly by the NCJ and the NVJ). The northward
component (the GBRUC, in red) is fed mostly during the winter Austral when the latitude
bifurcation is at its southern location and where the transport of the NCJ is stronger. At the
opposite the southward component (the EAC, in black) is fed mostly during the Austral
summer