boundary currents, like other frontal features,
spawn meanders, rings and eddies that transfer
momentum, vorticity, heat and other properties
between the current and the surrounding ocean.
These transfers create and sustain recirculation
gyres that may extend over the full ocean depth,
creating an expression of the boundary current
within the deep and bottom waters beneath the
main current. The recirculation gyres increase the
transports of the boundary currents by factors of
three to four.
Within WOCE, the measurement of boundary
currents was an essential element of the determination of oceanic transports of heat and fresh water
in each ocean gyre. The measurements were also
sustained over periods of at least 20 months in
order to deal with ocean variability associated with
eddies and boundary current instabilities (Rossby
and Gottlieb, 1998). Since boundary currents are
not described explicitly in any single chapter, we
provide some detail here.
The most comprehensive western boundary
current measurement programme during the
WOCE period was that in the Kuroshio, in which
a combination of in-situ current measurements,
geostrophy and satellite altimetry provided (due to
the close linear relationship between sea surface
slope and transport) a multiyear transport record
(Imawaki et al., 1997). The transport was estimated at 63<1310
6 m
3 s
91 over a 3-year period
and revealed no clear annual cycle (Fig. 1.2.6).
This estimate is larger than the likely transport of
the Kuroshio alone because of the presence of a
warm anticyclonic eddy on the offshore side of the
Kuroshio (Dickson et al., Chapter 7.3).
Mata et al. (2000) analysed data from a 2-year
array of current meters in the East Australian Current (Fig. 1.2.6). The strongest variability was centred on periods between 144 and 45 days but
again no annual cycle was evident. The high variability results in periodic reversals of the current
direction. The estimated mean southward transport was 22<510
6 m
3 s
91 . The measurements
also reveal a persistent equatorward deep undercurrent. The Agulhas measurements by Beal and
Bryden (1999) also gave direct confirmation of the
presence of an undercurrent previously postulated
on the basis of water mass properties. Beal and
Bryden’s Agulhas transport estimate gives a value
of 7310
6 m
3 s
91 (Fig. 1.2.6). The clearest evidence of the wind-driven response of a boundary
current is the seasonal reversal of the Somali current under the influence of the monsoonal wind
regime. Logistical problems precluded sustained
measurements of the Somali current during WOCE
but our present understanding of this current
and its relationship to the Indian Ocean equatorial
current system are described in Chapter 4.3.
In the Atlantic, monitoring of the Gulf Stream
system in the Florida Strait (Larsen, 1992) and
between Bermuda and the US mainland (Rossby
and Gottlieb, 1998) continued during WOCE. A
current meter and inverted echo sounder array
plus hydrographic surveys measured the Gulf
Stream/North Atlantic Current transport off Newfoundland (Fig. 1.2.6, Clarke et al., 1998) over
640 days during 1993–95. The median poleward
transport of 14010
6 m
3 s
91 included a substantial contribution from a strong recirculation gyre
within the Newfoundland Basin. Gulf Stream
transports and their relationship to the wider
Atlantic circulation are well summarized in
Schmitz (1996a) but this does not include most
estimates made during WOCE.
In the South Atlantic, Maamaatuaiahutapu et al.
(1998) made measurements in the confluence
region of the Brazil and Falklands/Malvinas currents. This is an area of high spatial and temporal
variability and while consistent values of the Falklands/Malvinas current transport (40<7 m
3 s
91 )
were obtained from inversions of hydrographic
data constrained by current meter measurements,
values for the Brazil current ranged between
30<7 and 56<8 10
6 m
3 s
91 .
Coarse-resolution ocean and coupled atmosphere–ocean models (Wood, Chapter 2.3) are able
to capture the direct response of subtropical gyres
and their western boundary currents to the largescale forcing of the wind. However, such models
are unable to reproduce either the recirculation
gyres and underestimate the transports of the
boundary currents. Finer-resolution ocean models
(Böning and Semtner, Chapter 2.2) are better able
to reproduce the eddy dynamics and hence provide
more realistic boundary current mass, heat and
property transports.
The eastern boundaries of ocean basins exhibit
equatorward flow linked to the subtropical gyres
(e.g. California Current, Canary Current, Peru Current) and are generally regions of intense upwelling.
A notable exception in the South Indian Ocean is
the Leeuwin Current, which flows poleward. There
1.2 Ocean Processes and Climate Phenomena
19
Clarke, Church and Gould
spawn meanders, rings and eddies that transfer
momentum, vorticity, heat and other properties
between the current and the surrounding ocean.
These transfers create and sustain recirculation
gyres that may extend over the full ocean depth,
creating an expression of the boundary current
within the deep and bottom waters beneath the
main current. The recirculation gyres increase the
transports of the boundary currents by factors of
three to four.
Within WOCE, the measurement of boundary
currents was an essential element of the determination of oceanic transports of heat and fresh water
in each ocean gyre. The measurements were also
sustained over periods of at least 20 months in
order to deal with ocean variability associated with
eddies and boundary current instabilities (Rossby
and Gottlieb, 1998). Since boundary currents are
not described explicitly in any single chapter, we
provide some detail here.
The most comprehensive western boundary
current measurement programme during the
WOCE period was that in the Kuroshio, in which
a combination of in-situ current measurements,
geostrophy and satellite altimetry provided (due to
the close linear relationship between sea surface
slope and transport) a multiyear transport record
(Imawaki et al., 1997). The transport was estimated at 63<1310
6 m
3 s
91 over a 3-year period
and revealed no clear annual cycle (Fig. 1.2.6).
This estimate is larger than the likely transport of
the Kuroshio alone because of the presence of a
warm anticyclonic eddy on the offshore side of the
Kuroshio (Dickson et al., Chapter 7.3).
Mata et al. (2000) analysed data from a 2-year
array of current meters in the East Australian Current (Fig. 1.2.6). The strongest variability was centred on periods between 144 and 45 days but
again no annual cycle was evident. The high variability results in periodic reversals of the current
direction. The estimated mean southward transport was 22<510
6 m
3 s
91 . The measurements
also reveal a persistent equatorward deep undercurrent. The Agulhas measurements by Beal and
Bryden (1999) also gave direct confirmation of the
presence of an undercurrent previously postulated
on the basis of water mass properties. Beal and
Bryden’s Agulhas transport estimate gives a value
of 7310
6 m
3 s
91 (Fig. 1.2.6). The clearest evidence of the wind-driven response of a boundary
current is the seasonal reversal of the Somali current under the influence of the monsoonal wind
regime. Logistical problems precluded sustained
measurements of the Somali current during WOCE
but our present understanding of this current
and its relationship to the Indian Ocean equatorial
current system are described in Chapter 4.3.
In the Atlantic, monitoring of the Gulf Stream
system in the Florida Strait (Larsen, 1992) and
between Bermuda and the US mainland (Rossby
and Gottlieb, 1998) continued during WOCE. A
current meter and inverted echo sounder array
plus hydrographic surveys measured the Gulf
Stream/North Atlantic Current transport off Newfoundland (Fig. 1.2.6, Clarke et al., 1998) over
640 days during 1993–95. The median poleward
transport of 14010
6 m
3 s
91 included a substantial contribution from a strong recirculation gyre
within the Newfoundland Basin. Gulf Stream
transports and their relationship to the wider
Atlantic circulation are well summarized in
Schmitz (1996a) but this does not include most
estimates made during WOCE.
In the South Atlantic, Maamaatuaiahutapu et al.
(1998) made measurements in the confluence
region of the Brazil and Falklands/Malvinas currents. This is an area of high spatial and temporal
variability and while consistent values of the Falklands/Malvinas current transport (40<7 m
3 s
91 )
were obtained from inversions of hydrographic
data constrained by current meter measurements,
values for the Brazil current ranged between
30<7 and 56<8 10
6 m
3 s
91 .
Coarse-resolution ocean and coupled atmosphere–ocean models (Wood, Chapter 2.3) are able
to capture the direct response of subtropical gyres
and their western boundary currents to the largescale forcing of the wind. However, such models
are unable to reproduce either the recirculation
gyres and underestimate the transports of the
boundary currents. Finer-resolution ocean models
(Böning and Semtner, Chapter 2.2) are better able
to reproduce the eddy dynamics and hence provide
more realistic boundary current mass, heat and
property transports.
The eastern boundaries of ocean basins exhibit
equatorward flow linked to the subtropical gyres
(e.g. California Current, Canary Current, Peru Current) and are generally regions of intense upwelling.
A notable exception in the South Indian Ocean is
the Leeuwin Current, which flows poleward. There
1.2 Ocean Processes and Climate Phenomena
19
Clarke, Church and Gould