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 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 zonal jets are centered at 13°5 and 18°S, and they have a 2° latitudinal extension.
They are clearly generated by the New Caledonia and Vanuatu islands. 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.
At 161°E, the NVJ and the NCJ are separated by a region of null transport at 16°S.
the 0-1500 m depth-integrated zonal velovity gives a transport of 14 sv for each jet
(considering a 2° extension in latitude). It is twice the transport calculated with the 0-400m
depth-integrated zonal velocity. Therefore a major part of the jets flow in depth. It is
particularly true for the NCJ which extends deeper than the NVJ. If the transport from the
surface to 400 m are similar for the two jets, the NCJ gives a transport from the surface to
1500 m of the NCJ higher than the NVJ. Also if the boundaries of the NCJ are well defined, it
is not the case at the northern boundary of the NVJ where the NVJ may be fed by the
southwestward equatorial SEC, and by recirculation cells east of New Guinea.
At 161°E, the NCJ extends as deep as 1400m. Its core is situated at 200 m with a
magnitude of 8 cm/s. It is 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 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 20S-10S). 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
II.2.a Validation
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 0-400 m
depth transport calculated from the XBT is very similar to the model one. But there is a 20 sv
bias between the 0-400m depth transport and the total transport. 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.
At 165°E, altimetry shows high interannual variability around 10°S with positive anomalies in
1996, 2000 and 2001, and negative anomaly in 1998. These anomalies are the signature of Rossby
waves which propagates westward with a speed depending of latitude. It explains the southward
propagation of the anomalies. SSH anomalies from the model are very close to the SLA from
altimetry.
The location of the jets is defined by the topography, therefore there is no spatial variability in
time of their location. A strong variability is observed at the northern boundary of the NVJ.
The SSH, at the location of the jets, exhibits strong annual and interannual variability.
These both variability are described separately.
II.2.a seasonal variability
As seen above, a clear annual cycle on SSH exits at the location of the jets, but the 01500 transport between 19s and 13s (in green is the half transport), very close to the sum of
the NCJ and NVJ transports, does not have a well marked seasonal cycle. It means that the
annual cycle on SSH mostly illustrates a surface signature associated to an Ekman circulation.
The annual cycles on SSH at 18°S and 14°s are in phase, but the NCJ and NVJ transports 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), and minimum (maximum) during the austral
summer with a 10 sv magnitude (10sv). Also is plotted 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 transport by the SEC is 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 southern position during the austral winter. The latitudinal range of the bifurcation
corresponds with the latitudinal location of the NCJ, therefore it could be principally
influenced by the NCJ. The bifurcation point is at its southern position at the time when the
NCJ is maximum. Also, at the surface, the bifurcation latitude seems related to the variability
of the northern branch of the SEC.
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 cycle, especially above 250 m
depth, and below 400 m depth. Between 250 m and 400 m depth the latitude bifurcation is
stationary at around 18°S
The bifurcation, function of depth, for each month is shown:
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. It means that water mass properties of the equatorward GBRUC can be influenced
by subtropical water, and inversely water mass properties of the southward EAC can be
influenced by tropical water.
Why the latitude bifurcation point is at its southern position when the NCJ is maximum???
See at the seasonal cycle of the wind stress forcing.II.2.b Interannual Variability
Is there a clear relation at interannual time scale between the variability of the jets, the
latitude bifurcation point, and the LLWBCs?
The 1992-2002 period is marked by the strong 1997-1998 ENSO event, therefore most
of the interannual variability could be conditioned by this event.
The zonal velocity, characteristic of the jets, exhibits a strong interannual variability
from the surface to 700 m depth. Westward velocity increases in 1992, 1993, and 1998. The
current reverses in 1996, and 2001. The NVJ has higher interannual variability than the NCJ.
Sometimes, as in 1999-2000, the NVJ flows eastward when the NCJ flows westward.
The annual cycle of the latitude bifurcation is greatly modulated at interannual time
scale. At 100 m depth, the latitude of the bifurcation is around 12°S in October 1997 against
16°S in October 1999. At depth (800 m), the latitude of the bifurcation is around 18°S in July
1998 against 24°S in July 1999.
Interannual anomalies are particularly significant deeper than 500m. Strong positive
anomalies (the bifurcation latitude is shifted to the north) exists on July 1992, 1997, 1998;
strong negative anomalies (the bifurcation latitude is shifted to the south) exists on October
1994, 1996, 1999. The upper layer (50-400m) seems out of phase with the deeper layers
except in 1999-2000 where the bifurcation latitude point has shifted south at every depth.
Therefore, there are strong interannual variability of the LLWBCs. At 14°S, anomalies
are concentrated in the surface layer. Most of the time, the flow (NQC) goes northward with
an annual cycle. It is particularly intense in 1999-2000. It reverses in December 1994, 1997,
and 2001. At 23°S, Interannual anomalies are located in depth at around 700 m where the
GBRUC is detectable. In 1998, the GBRUC vanishes, at the opposite it increases in 1996 and
2000. Following the intensification of the GBRUC at 23°S in 2000, there is an intensification
of the NQC at 14°S.
Part of the variability of the south west Pacific may be explain by Rossby waves as
shown by Billy. Interannual SSH anomalies are plotted to be compared with the Rossby
model from Billy. The Rossby waves are generated in the central basin. Two upwelling
rossby waves are generated in 1992 and 1998, and two downwelling Rossby waves are
generated in 1996 and 2001. Their signature on SSH is particularly visible at the latitude of
the NVJ with amplitude up to 10 cm.
Downwelling (upwelling) Rossby waves generates eastward (westward) zonal current
anomalies (???? À verifier) which are visible on the zonal velocity plot above. The zonal
velocity anomalies created by the Rossby waves may impact the transport of mass through the
jets. Transport anomalies propagates westward but the relationship with SSH is not clear.
As the propagation speed of the Rossby waves is function of latitude (The speed
decreases with latitude), a phase lag is suspected between the anomalies of the JNC (blue) and
JNV (red) in the case where the Rossby waves are generated at the same date. In term of SSH,
The JNC lags the JNV by 10-11 months with a 0.53 correlation. This computation is strongly
influenced by the 1997-1998 enso event. Noted also that the topography of the South West
Pacific may influence the Rossby waves, and it can explain why the correlation between the
Rossby model and TOPEX/Poseidon falls down south of 10°S.
At the opposite from SSH, transport anomalies from the JNC and JNV are in phase during
the 1997-1998 ENSO, and out of phase the rest of the time. The relation with the SOI index is
evident for the 1997-1998 period, and the jets lags the SOI by 6-7 months.
The SEC is the addition of the NCJ with the NVJ. At interannual time scale, the variability of
the SEC (black line, half transport) has a magnitude, similar to its mean value, reaching 20 sv
from peak to peak. The transport of the SEC doubles in relation with El Nino; it reduces
dramatically prior to El Nino (1996), and following the 1997-1998 ENSO event.
What is the reason why SSH and transport are so different. Does i tmean that there are
changes in the water mass characteristics? Which variables are the best for this study?
The interannual variability of the latitude bifurcation point is strongly correlated to the SOI..
It lags the SOI by 5-6 months with a -0.7 correlation (depth averaged latitude black). The
bifurcation latitude point shifts equatorward during ENSO, and southward in 1999. In red, is
the latitude variability at 300m, and in blue at 800 m. The bifurcation varies with depth, the
deeper layer seems to lag behind the upper layer.
Is it possible that the strength of the jets could control the bifurcation latitude point?
In blue is the SEC transport anomalies, in red the inverse of the bifurcation laltitude
anomalies. When the SEC intensifies, the bifurcation lattitude point shifts equatorward.
Is there a difference between the JNC and the JNV. We look at the relation between the
bifurcation in the deeper layer and the JNC (top) and between the bifurcation in th eupper
layer and the JNV (bottom). There is no clear relation between the JNV and the bifurcation, at
the opposite the relation between the JNC and the bifurcation is evident. Does it mean that the
JNC control the bifurcation?
What is the influence of the jets and bifurcation variability on the LLWBCs?
The llWBCs are characterized here by the meridional transport of the coastal curren. On the
left plot the zero transport shows the location of the bifurcation. The maximum of the EAC
transport is south of 24°S, and the the maximum of the NQC is north of 14°S. On the right are
the anomalies. Both LLWBCs decreases during the onset and the maturation phase of the
1997 El Nino before to increase since 1998.
There is a clear signature on SSH of the transport anomalies along the Australian coast.
Positive transport anomalies are related to negative SSH anomalies When the bifurcation
point shifts southward (like in 2000), the NQC increases and negative SSH anomalies are
observed north of the bifurcation. When the bifurcation shifts northward (like in Dec 2001),
the EAC decreases and negative SSH anomalies are observed south of the bifurcation.
Anomalies of transport function of latitude are in phase in the EAC and the NQC, but there is
phase lag between the EAC (at 26°S) and the NQC (at 14°S). The EAC is correlated at 0.71 to
the NQC with a 15 months lag.
There is a good relationship between anomalies of transport in the EAC and the anomalies of
latitude bifurcation at 800m. Also, there is a relationship between the NQC and the latitude
bifurcation at 250m.
STOP HERE
The latitude of the bifurcation defined how the LLWBCs are fed.
The GBRUC is defined as the equatorward coastal current at 14°S, and the EAC is defined as
the southward coastal current at 30°S.
Like the bifurcation latitude point, both currents exhibit a strong seasonal signal. When the
bifurcation latitude point is at its northernmost position (in December), transport through the
EAC is maximum, minimum for the GBRUC. In July, the bifurcation latitude point is at its
southernmost position, the transport through the GBRUC is maximum, and minimum for the
EAC. The amplitude of the EAC seasonal cycle seems to increase with time. If the cycle of
the EAC is relatively regular, the GBRUC exhibits a strong interannual signal.
The low frequency variability of the EAC shows that there is tendancy for the EAC to
decrease with time. It is not the case for the GBRUC which exhibits strong interannual
variability particularly in December 1999 and 2001. The GBRUC is up to 6sv strongest than
normal in December 1999, and down to 6 sv weakest than normal in December 2001.
The relation with the SOI indexl is relatively unclear