Cotella, 1990, 14(4): 113-118
The SeasAround Us - Number 2
THE LEEUWINCURRENT
GEORGE CRESSWELL
CSIRO,Divisionof Oceanography,
Box 1538G.P.O.,Hobart,Tas.7001
Recetved.
6 JuIy, 1990
The LeeuwinCurrentis an unexpected
currentbecauseit carrieswaterfromthetropicsto southpossiblyas far as Tasmania.
westernAustraliaandtheneastward,
In someplacesandat sometimes
it hasspeedsup to 3.5 knots,therebyrivalling
the EaslAustralian
Current.Thispaperoutlinessome
of its features.
INTRODUCTION
The earliest evidencefor a southward-flowins
warm current off western Australia came fro;
observationsof warm waters and troDicalmarine
flora and fauna around the Abrolhos Islands
(-29'S) by naturalist Saville-Kent in 1897. In
7922 Halligan, a hydrographer, inferred such a
current from temperature maps derived from
merchant ship measurements. Further, he
reported "a warm southerly and easterlysurface
current from the vicinity of Cape Leeuwin" with
speedsof 0.3-{.4 knots.
Sincethat time studiesof currents,water properties and Dlankton have contributed to the
description of a warm, low-salinity current that
starts off NW Australia and runs down to Cape
Leeuwin where it Divots to flow eastwardacross
the Great Austraiian Bight. The response of
CSIRO satellite-trackeddrifters to this current
was quite dramaticsomefifteen yearsago and led
to it beins called the Leeuwin Current. The
Leeuwin (Lioness in English) was a Dutch ship
that expforedeastwardtowardsthe Bi ght in 7622.
Features of the Leeuwin Current and the
marine environment of Western Australia are
outlined in the following sections.
THE LEEUWIN CT]RRENT SYSTEM:
DIMENSIONS AND SPEEDS
Off NW Australia the Leeuwin Current is
broad and shallow (200 km by 50 m) and has
speedsof up to 1 knot. It acceleratesalong its
southward path and by the time Cape Naturaliste
is reached(33"S)it can have speedsup to 2 knots.
There its warm waters are some tens of km wide
and can be as deep as200m. The Current further
acceleratesin the Cape Leeuwin vicinity and
speedsof up to 3.5 knots have been measured
just beyond the shelf edgebetweenthe Cape and
Esperance. Eastward from Cape Leeuwin its
warm watersspreadquite closeto shore,although
the maximumspeedsat midshelfare only 1 knot.
Changesin the strengthof the Leeuwin Current
are reflectedin mean sealevel changes
- at coastal
stations. These typically show an annual
amplitude of 1.5-20cm, with highest values in
March (NW shelf) to May or June (SW Australia): this seasonalmovement of the sea-level
maximum reflects the southward passageof the
Leeuwin Current pulse. It is worth mentioning
that passingweather patternscommonlyproduce
sealevel changesof ,10cm; thesehave periodsof
severaldays.
All along its path southward from NW Australia, the Leeuwin Current both meandersand
"spins up" eddies. It provides these eddieswith
warm "fresh" waters while, at the same time,
The Leeuwin Current flows principally, but not
exclusively, in autumn and winter. It probably
influencesthe coastalclimate; it actsas a conduit
to bring tropical marine fauna and flora to southern Australia; and it appearsto play an important
role in the migration of severalspeciesof fish and
birds. It is unusualin that it flows southwardand
into the wind. Other current systems on the
easternsidesof oceans- the Benguela,Canary,
Peru, and Califomia current systems of the
Atlantic and PacificOceans-flow eouatorward.
113
114
G. C/ess!a/e//.
The LeeuwinCurrent
takes cold "salty" watersfrom them. This mixing
contributesto a progressivecooling and increase
in salinity along the path of the Cunent. The
eddiesout from the west coastare both clockwise
and anticlockwise and are roughly 150 km in
diameter. Speedsof up to 2 knots have been
measuredin them and their lifetimes can be over
two months(Fig. 1).
Corella14(4)
As it runs eastwardthe Leeuwin Current loses
vast amounts of water into offshoots that can
extend up to severalhundred kilometres southward. The offshoots are 50 km wide and 150 m
deep and have trough-shapedcrosssections.On
their western sides are southgoingcurrents and
on their easternsidesare northgoingcurrents;the
speedsare up to 2 knots and turbulencebetween
the two streams gives rise to overturning and
mixing. The offshoots drive clockwise and anticlockwise eddy-pairs 100 km diameter on their
easternand westernsidesrespectively.
Below the Leeuwin Current, and extending
over a depth range of severalhundred metres, is
a reverseundercurrentof up to 0.5 knots.
WATER TYPES OF THE WESTERN
AUSTRALIAN REGION
The following waters are encounteredin the
upper 500 m in the subtropicswest of Australia:
The Leeuwin Current: Strface, low-salinity
( < 3 5 . 0 0p a r t sp e r t h o u s a n d )h. i g h - t e m p e r a t u r e
(>24"C), low-nutrient,tropical watersthat spread
south from 20'S in autumn and winter. Off
RottnestIsland (32'S)the winter salinity is 35.2035.40 and the Leeuwin Current holds the sea
surfacetemperaturethere above 22'C ]until afler
JUne.
Figure 1. The surface temperature distribution off v)esten
Austrulia as detemined from an infrared satellite image
from 15 lune 19U. The Leewein Cufient starts as a
brood fan of wam watet (>24'C) off the no h-westand
prcgressessouthward to Cape Leeut/",ihand then eastward
b the Great Australian Bight. Therc arc eddies, meande\
and offshoots associated h,ith the Leeuwin CwrenL
Mixing, radiation and eyoporation cause its watersto cool
and becomesaltier os they progress.
South Ind tn Central waterc are hieh-salinitv
(>35.80)with temperatures
of t7'-19'e and ari
found near the surfaceoff Australia's SW corner.
They sink to several hundred metres as they
spreadnorthwardasan undercurrent,with mixing
progressivelylowering their salinity. In summer
they extend to 12'S and in winter to 16'5. The
northwardsoreadis arrestedbv the westwardflow
of the relitively low-saliniiy (<34.5) South
Equatorial Current. The South Indian Central
waters are also preventedfrom reachingthe continental shelf between 10" and 20'S by the lowsalinity Leeuwin Current sourcewaters.
The West Australian Current: Sufiace, highsalinity (>35.90), low-nutrient, subtropical
waters with temperaturesof 20"-22"Cat 30"S.In
summer they are carried northward to 25'S and
down to 100m depth.
September,1990
G,C.esswe/fTheLeeuwin
Current
Tropical oxygen minimum: Subsurface(100-150
m), low salinity(<35.00),low-orygen(< 150pM-),
tropical water spreadingsouthwardto about 26'5
in late summerand autumn.
0
200
gq)
r
-E
4oo
6
E ooo
'115
10
f
fr
C)
-9.
o-
E
o
F
The Leeuwin Current spreadssouthwardover,
and mixes with, South Indian Central water,
thereby becomingprogressivelymore saline and
cooler with increasinglatitude. Seasonalvariations in solar heating, evaporation-precipitation
effects, dynamical uplifting of South Indian
Central water, mixing, and the supply of waters
of the Leeuwin and West Australian Currentsare
controllingfactorsfor the temperatureand salinity
of the surfacelayer off westernAustralia.
bs
When the Leeuwin Current turns to the eastat
CapeLeeuwin it leavesa region where its salinity
is less than the ambient (-35.8 South Indian
Central water) and entersone where its salinity is
higher than the ambient 35.5. In the second
(fresher) regime the Leeuwin Current has been
observedto carry with it a sheathof saltySouth
Indian Centralwater. The sheathis then slowlv
lost downstreamthrough energeticmixing witir
the fresher offshorewaters (Fig. 2).
8.
800
1000
0
200
-E 400
-c
-oo
o 600
Subtropical oxygen mqximum.. Subsurface
(400-500m), low-salinity (<35.00) waters of the
subtropicaloxygenmaximum (>175pM) drifting
northward to about 14'S in winter.
:=
.E
E
U)
800
1000
0
v,
c
200
.Y
c
c)
-E 400
c
5
(J
3 ooo
=
800
F
o
0)
art
1000
50 km
EFFECTS ON SHELF WATERS
The effectsof the variouscurrentson the waters
of the continentalshelf were first monitored at a
hydrology station on Rottnest Island at a water
depth of 50 m. There is a southwardflow of lowsalinity water in autumn/winterand a northward
flow of high-salinitywater in summer.The northward flow is opposite to the southward open
ocean summerflow and is probably the effect of
a predominantly southerly wind stress on the
shallow waters of the shelf. The arrival each
'Micro-molar, a rough division by 50 givesmg/|.
Figure 2 Sectigls of temperuture, salinity and current speed.(inferred from density) along a tine out from the coast westof Albany
in June 1987. The current is driven by the slopes of the temperature and salinitj siuctures. Note the warm surface Leeuwin
Current "river" with itssaltysheathsittingover the outerpart of the continentalshelf (C.5.); also the nearsutace'currentspeed
of I 8 knots eastwards(+) and the undercuftent of 0.4 knots reestwalds(-). Off tie trestern continental sheli rhisuntlercurrent
flows northward awl caries salty South Indian Central water.
G.Cresswe//--The
Leeuwin
Current
116
?\o
;;;
"; ;
;;;
18"
16'
Corella14(4)
autumn of the low-salinity waters of the
Leeuwin Current at the Rotinesl lsland 50 m
hydrology station producesan annual salinity
variationof roughly0.5. Recentwork aspart of the
Leeuwin Current InterdisciplinaryExperiment.
LUCIE. quite clearly showedthe major effect
of the Leeuwin Curent to occur at the outer
shelf.
Current measurementsat the southern Dart
of the NW Shelf showedthe Leeuwin Curient
to run parallel to the bottom topography and
to be strongest at the shelf break in February
and June, reaching a maximum speed of 0.5
knots (Fig. 3).
E
- 100
a
o
150
THE SOURCE REGION
200
20 km
Fi8lne 3. A section(bottompanel) showingthe currentstnrcfrrre
measuredby RV Ftankl\n several days after the tefiperaturc and salinity sections of Figurc 2. Current speeds
greater than 1.0, 2.0 and 3.0 knots are shadcd progressively darker. The isotheht measured by expendable
kmperature probes are shorsn. Note the surface temperature and salinity front (top panel) on the offshore edgeof
the Leeuwin CurrenL This is where the speeddramoticallt
decreasesand where considerable overturn and miring
take place.
105"E
110"
The watersNW of Australia appearto contain
the source region for the Leeuwin Current.
Historical temperature-depth profiles were
interpreted by Gentilli in 1972 to show that
the autumn and winter throughflow pulse from
the Pacificto the Indian Oceanbecomesisolated
off NW Australia by a reversal of the flow
in spring. Over the summer this water becomes
a warm homogeneous 'raft', which then
spreads southward (as the Leeuwin Current)
during the following autumn and winter (Figs 4
and 5).
115"
120"
125"E
Figure 4. A chart of the triangle befieeenNW Australia and Indonesia showihg the station positions
occupied.by RV Ffinklin in October 1987 entuute to and betonil Christmas Island.
September,1990
G.C.esswe/fTheLeeuwin
Current
117
TEMPERATURE
(C1
1
IO
a
a
a
15
a
a
a
a
a
a
20
a a
a
24
a a
0
^ 200
€
s
o
400
600
0
200
400
600
800
SALINITY
a
a
a
a
1400
a
E
0
200
400
600
800
OXYGEN
a
a
a
1400
a
E
o
600
NW
Slrell
200
400
600
800
Distance {km)
1000
1200
1400
Christmas
F\gtrre 5. The station positions . corresponding to thoseshown in Figure 4 from the North WestShelf to
Christmas Isl&nd are shoyrn at the top of eachpanel. The wam, low salinitt suiace layer se es as
the source of the Leeut/in Current. The slope of the subsurfaceisotherms bettreenstations 16 and 24
driyes the South Equatorial Current from east to v)estpast Christmas Island. The salinity section
shows a maximum that is due to the northward flohting South Indian Central water. The oxygen
section shows both the Tropical Oxygen Minimum water spreading southward at 150250 m and
the Subtropical Orygen Maximum water spreading nofthward. at 300-500 m.
Current
TheLeeuwin
G.Cresswe//.
118
INTERANNUAL
VARIABILITY
The extent of interannual variations in the
strensth of the Leeuwin Current is not known. It
has b?en found that anti-ENSO (ENSO: el Niflo
- Southern Oscillation) eventsare accompanied
bv hieher than normal sealevel at Darwin and
that the enhanced
Pbrt iledland, and suggested
sea-level alonq Westein Australia is associated
with a stronedr than normal Leeuwin Current.
This suggesti-onfinds support in the Rottnest
lslandrdiords.which,torihe 197F76anti-ENSO
event, showed low salinities suggestiveof an
enhanced flow of low-salinity water from the
troprcs.
WHY IS THERE A LEEIJWIN CURRENT?
According to Stuart Godfrey of CSIRO
the Leeuwin Current is dilferent from other
eastern boundary currents in a number o[
wavs. which appear to be linked in a feedbaik looo. Th; first unusual feature of the
Leeuwin eurrent is that the current flows directly
into the wind, thereby carryingwarm water polewards.
This is probably the reason for a second
unusualfeaiure: thi:re is a strongheat flux out of
the ocean from the waters of the Leeuwin
Current, rather than into the ocean as in all
other eastem boundary currents. This heat
loss favours convectionand hence the formation
which are a third unusual
of deep mixed lavers,
-Leeuwin
Current. Thompson
feature of the
argued in 1984 that these deep-mixed layers
ol the observed
are essentialto the maintenance
larae alongshorepressuregradient (a fourth
u n - u s u a lf i a r u r e b I t h e L e e u w i n C u r r e n t ) .
Drawing thesepointstogether.the warm waters
flow doinhill to the south' in responseto the
alonsshore Dressuregradient, thereby overcoming the ;fforts of-the wind to push them
northiard. The wind is much less effective
in moving a deep surface mixed layer than a
shallow one.
So far, there is no explanation why a!- these
linked features occur off Western Australia and
not in other eastern boundary currents, but it
mieht be due to the Pacific to Indian Ocean
thr-oughflow,as Gentilli suggested.The question
is still verv much open.
BIOIOGICAL
Corella14(4)
IMPLICATIONS
The Leeuwin Current obviously can carry
oassiveorqanismshundredsof kilometresand
llons the louth coast this was Rrst noted by
a currentto be responsible
who suggested
Woo,-d.
dinoflagellatesthat he
of
disrriSdtion
tor the
observed from SW Australia to Tasmania. It
is also possible that other creatures may
actively Ake advantageof the Leeuwin Current
to tra;sDort their eggs, larvae and juveniles;
example; of this are the Southern Bluefin
Tuna that spawn off l'IW Australia and the
Australian salmon and herring that migrate
from South Australian waters to spawn off SW
Australia.
Also, while the exactmechanismmay be debatable. Pearceand Phillips havefound recentlythat
the ;ettlement of wesiern rock lobster puerulus
larvaeto the nearshorereefsof WesternAustralia
is higher in anti-ENSO years with a strong
Leeuwin Current.
On a smallerscale,during LUCIE the seaward
edee of the Leeuwin Current on the continental
sh;lf at Rottnest Island was observed to be a
for dolphins'yellow-nosed
resionof congregation
and pilchards.Depth soundersrevealed
alSatross
considerablemarine li{e in the watersdown to
the bottom.
SELECTED FURTHER READING
R and Godfrey, J. S (1989)'
church. J. A., Cresswell,G.'Poleward
Flows along Eastern
The Leeuwin Curent ln
Ocean Boundaries'. (Eds S. Neshyba, C. N K. Mooers
and R. L. Smith.) Pp, 230_252.(Springer-Verlag:New
York. )
in the EasternIndian
Centilli.J. ( 1972).Thermalanomalies
238:.9195.
Ocean.Nature, Phvsi.al Scicnces
Codfrev. J. S. and Ridgway. K R (1985) The large'scale
environment of the poleward-ffowingLeeuwin Current'
Western Australia: longshoresteric height gradients,wind
sttesses and geostrophic flow J. P,,yr. Oceangr' 15:
481-495.
Pearce,A. F. and Phillips, B F. (1988).ENSO events,the
Leeuwin Cu ent and lawal recruitmentof the westemrock
lobster. J. Cons. Int. Explor. Mer 45| 13-21.
Rochford, D. J. (1969). Seasonalinterchangeof high and
low salinity su ace waters off South_westAustralia'
Division oa Fishe es and Oceanography Technical Paper
No.29.
Wood. E. J. F. (1954) Dinoflagellatesin the Australian
resion.,Ausl.J. Mar. Frcshw Res 5: 171-351.