5–15 Sv (see above), must disperse northwards and
southwards into the interior. Zonal surface
currents estimated from buoys and ship drifts
(Molinari et al., 1990a) show a semiannual signal on
both sides of the equator; however, annual mean
surface flow is predominantly to the southeast,
along the Indonesian coast. The predominantly
southward movement of the equatorial flow is
expected from the Sverdrup relation, as noted by
McPhaden (1982a). These annual mean results are
consistent with the observations of Schott et al.
(1994), who find that the annual mean westward
flow between Sri Lanka and 4°11ЈN, along
80°30ЈE (just north of the section described by
Reppin et al., 1999) was only 2–3 Sv. From mass
conservation, one might expect it to equal the fraction of equatorial eastward flow that turns north,
east of 80°30ЈE. The Sri Lankan westward flow
was largely confined to the top 100 m, occurs
mainly in winter, and is fresh (see, for example,
the section near this line in March 1995, Talley
and Baringer, 1997). By contrast the eastward
equatorial zonal jet is about 200 m deep, and is
salty. These results are consistent with summer
freshening of the inflow in the Bay of Bengal (e.g.
Shetye et al., 1991). However, even when flow on
the equator and south of Sri Lanka is westward,
saline Arabian Sea water flowed east (at least in
1995) in a continuous North Equatorial CounterCurrent (NECC). This flowed from 3°N at 80°E to
the north end of Sumatra (Hacker et al., 1998).
Semiannual flows along the Indonesian coast
are evident from coastal tide gauge data (Wyrtki,
1961a), and from XBT data (Meyers et al., 1995).
Red Sea Water is apparent near Bali (Fieux et al.,
1994), indicating that flow must occur on annual
mean along the south coast of Indonesia. This is in
agreement with current meter data from March
1997 to March 1998 at 115 m and 175 m south
of Java, despite unusually strong southeasterly
winds (Sprintall et al., 1999). Since the Indonesian
Throughflow is southward on annual mean (e.g.
Meyers et al., 1995), the flow diverging southward
from the equator must all join the ECC and SEC,
at least in the top 400 m.
Clarke and Liu (1994) showed that on interannual time scales, sea level varies coherently by a
few centimetres from Java to Bombay in India.
They find that any sea level discontinuity in proceeding around the south tip of Sri Lanka does
not vary greatly interannually; thus the zonal jet
emanating from this point should not vary much
on interannual time scales. They ascribe this broadscale sea level variation to interannual variations
of the equatorial Kelvin wave along the equator,
i.e. to interannual variations in zonal equatorial
winds.
4.3.4.3 Deeper circulation and equatorial jets
A major issue for the Indian Ocean is: Is there a
moderate to large conversion of Antarctic Bottom
Water into Deep Water in this basin, as suggested
by Toole and Warren (1993), Toole and Raymer
(1985) and Robbins and Toole (1997)? If so,
where does the mixing take place that feeds heat
to these depths, and what physical processes are
responsible for it? The equatorial region and the
connecting western boundary regions are likely
places for strong mixing, because strong currents
are observed there (at least near the surface). Traditional analysis of top-to-bottom hydrographic
sections may assist in solving such questions,
though care needs to be taken with possible aliasing of high-frequency effects such as those seen in
Fig. 4.3.11. However, relative to other oceans,
equatorial data on deep currents (or even on the
occurrence of low Richardson numbers in the thermocline, which may control downward diffusion
there) is quite sparse in the Indian Ocean. In
the following, we briefly consider: seasonality of
deep zonal currents; deep cross-equatorial flows;
abyssal flows; and deep reversing jets, and their
associated shear mixing zones.
Luyten and Roemmich (1982) analysed yearlong current meter records at 200 m, 500 m and
750 m, in 47–59°E; they have been discussed
earlier. Reppin et al. (1999) give amplitudes and
phases of annual and semiannual zonal currents,
at or near the equator at 80°30ЈE. Semiannual
amplitudes at 600 m are about 0.1–0.15 m s
91
,
close to those found by Luyten and Roemmich,
with smaller values near 1000 m and 2000 m. An
annual cycle of order 0.05 m s
91 is apparent at
600, 1000 and 2000 m.
As for deep cross-equatorial flows, these are
liable to be confined to western boundary currents. Schott et al. (1989) reported observations of
the deep Somali Current at the equator, from two
moorings in October 1984 to September 1985,
and two more from September 1985 to September
1986. Flows at 1000, 1500, 2000 and 3000 m
show substantial variability on time scales of weeks
4.3 The Tropical Ocean Circulation
243
Godfrey, Johnson, McPhaden, Reverdin and Wijffels
southwards into the interior. Zonal surface
currents estimated from buoys and ship drifts
(Molinari et al., 1990a) show a semiannual signal on
both sides of the equator; however, annual mean
surface flow is predominantly to the southeast,
along the Indonesian coast. The predominantly
southward movement of the equatorial flow is
expected from the Sverdrup relation, as noted by
McPhaden (1982a). These annual mean results are
consistent with the observations of Schott et al.
(1994), who find that the annual mean westward
flow between Sri Lanka and 4°11ЈN, along
80°30ЈE (just north of the section described by
Reppin et al., 1999) was only 2–3 Sv. From mass
conservation, one might expect it to equal the fraction of equatorial eastward flow that turns north,
east of 80°30ЈE. The Sri Lankan westward flow
was largely confined to the top 100 m, occurs
mainly in winter, and is fresh (see, for example,
the section near this line in March 1995, Talley
and Baringer, 1997). By contrast the eastward
equatorial zonal jet is about 200 m deep, and is
salty. These results are consistent with summer
freshening of the inflow in the Bay of Bengal (e.g.
Shetye et al., 1991). However, even when flow on
the equator and south of Sri Lanka is westward,
saline Arabian Sea water flowed east (at least in
1995) in a continuous North Equatorial CounterCurrent (NECC). This flowed from 3°N at 80°E to
the north end of Sumatra (Hacker et al., 1998).
Semiannual flows along the Indonesian coast
are evident from coastal tide gauge data (Wyrtki,
1961a), and from XBT data (Meyers et al., 1995).
Red Sea Water is apparent near Bali (Fieux et al.,
1994), indicating that flow must occur on annual
mean along the south coast of Indonesia. This is in
agreement with current meter data from March
1997 to March 1998 at 115 m and 175 m south
of Java, despite unusually strong southeasterly
winds (Sprintall et al., 1999). Since the Indonesian
Throughflow is southward on annual mean (e.g.
Meyers et al., 1995), the flow diverging southward
from the equator must all join the ECC and SEC,
at least in the top 400 m.
Clarke and Liu (1994) showed that on interannual time scales, sea level varies coherently by a
few centimetres from Java to Bombay in India.
They find that any sea level discontinuity in proceeding around the south tip of Sri Lanka does
not vary greatly interannually; thus the zonal jet
emanating from this point should not vary much
on interannual time scales. They ascribe this broadscale sea level variation to interannual variations
of the equatorial Kelvin wave along the equator,
i.e. to interannual variations in zonal equatorial
winds.
4.3.4.3 Deeper circulation and equatorial jets
A major issue for the Indian Ocean is: Is there a
moderate to large conversion of Antarctic Bottom
Water into Deep Water in this basin, as suggested
by Toole and Warren (1993), Toole and Raymer
(1985) and Robbins and Toole (1997)? If so,
where does the mixing take place that feeds heat
to these depths, and what physical processes are
responsible for it? The equatorial region and the
connecting western boundary regions are likely
places for strong mixing, because strong currents
are observed there (at least near the surface). Traditional analysis of top-to-bottom hydrographic
sections may assist in solving such questions,
though care needs to be taken with possible aliasing of high-frequency effects such as those seen in
Fig. 4.3.11. However, relative to other oceans,
equatorial data on deep currents (or even on the
occurrence of low Richardson numbers in the thermocline, which may control downward diffusion
there) is quite sparse in the Indian Ocean. In
the following, we briefly consider: seasonality of
deep zonal currents; deep cross-equatorial flows;
abyssal flows; and deep reversing jets, and their
associated shear mixing zones.
Luyten and Roemmich (1982) analysed yearlong current meter records at 200 m, 500 m and
750 m, in 47–59°E; they have been discussed
earlier. Reppin et al. (1999) give amplitudes and
phases of annual and semiannual zonal currents,
at or near the equator at 80°30ЈE. Semiannual
amplitudes at 600 m are about 0.1–0.15 m s
91
,
close to those found by Luyten and Roemmich,
with smaller values near 1000 m and 2000 m. An
annual cycle of order 0.05 m s
91 is apparent at
600, 1000 and 2000 m.
As for deep cross-equatorial flows, these are
liable to be confined to western boundary currents. Schott et al. (1989) reported observations of
the deep Somali Current at the equator, from two
moorings in October 1984 to September 1985,
and two more from September 1985 to September
1986. Flows at 1000, 1500, 2000 and 3000 m
show substantial variability on time scales of weeks
4.3 The Tropical Ocean Circulation
243
Godfrey, Johnson, McPhaden, Reverdin and Wijffels