will be apparent that near the depth of about
100 m (Fig. 4.3.12), seasonally varying currents do
seem to advect water masses northward nearly the
full length from the equator to 10°N, and to mix
them up in the Great Whirl. Winter advection
similarly returns salty water to most of the region.
Observed current magnitudes near 100 m also support this conclusion. In other words, annual flushing of the SCS is rather complete, at least in the
top 100 m. It is therefore physically plausible that
an ‘eddy flux’ heat transport mechanism like the
one described above may contribute to heat
removal from the northern Indian Ocean.
The equator-spanning Southern Gyre appears to
mediate the transfer from the SCS to the equatorial flows. Schott et al. (1998) undertook hydrographic surveys in the offshore regions of the
Somali Current in summer 1995. They showed
that the Somali Current becomes disconnected
from the interior Arabian Sea between 4°N and
12°N, both in currents and water masses. The
Southern Gyre is clearly evident in their maps of
near-surface currents. In the later map, salinity is
actually higher south of the equator than north of
it along 50°E, suggesting that the Southern Gyre is
transporting water from the Somali Current across
the equator, and then eastwards.
The qualitative picture just presented suggests
that the most of the water formed in the top 100 m
in summer in the SCS may escape via the southflowing cross-equatorial western boundary current, and thence eastward via the Wyrtki Jet (in
October–November) or by the EUC (in December–February). However, the quantitative details
of this picture are important for assessing the contribution of SCS flushing to the net meridional
heat flux out of the Indian Ocean – and to its
interannual variability.
Models with only one active layer but high horizontal resolution are quite successful in simulating
several features of Fig. 4.3.13; for example, Luther
and O’Brien (1985, 1989) and Luther et al. (1985)
obtained quite realistic simulations of the ‘Great
Whirl’ and ‘Southern Gyre’ in their runs. A run
with a fixed seasonal cycle of winds shows much
less interannual variability than one with realistic,
interanually varying winds. However, such models
cannot resolve subtleties of the vertical circulation
such as whether the SCS outflow joins the EUC.
Visbeck and Schott (1992) addressed the issue of
equatorial divergence, at least in the 150–800 m
depth range, by comparing available observations
with the output of a Geophysical Fluid Dynamics
Laboratory (GFDL) model run, driven by Hellerman
and Rosenstein (1983) mean seasonal winds. This
limited-area model had a zonal resolution of 0.5°
in the western Indian Ocean, and a meridional resolution of 0.33° in the region of interest. Figure
4.3.10b shows the mean seasonal cycle of surface
zonal currents along the equator from the model,
in fair agreement with Reverdin’s (1987) climatology. A similar level of agreement is found between
model and observation for vertical sections through
the EUC at 55°E, due to Leetmaa and Stommel
(1980). The model’s annual-frequency zonal currents along the equator have amplitudes and phases
that are in reasonable accord with observations in
the western boundary current and in the EUC. The
model is somewhat less successful in simulating the
Somali Current at the equator, but model longshore currents showed a large degree of asymmetry
about the equator; longshore semiannual transport
amplitude reached 5 Sv near 2°N, S.
Before leaving the topic of the western boundary current, we consider the seasonal relationship
of the wind-driven overturning cell to water mass
formation and SST change. Hsiung et al. (1989)
used observed surface fluxes and heat content
changes to estimate seasonal net northward crossequatorial heat flux into the Indian. In winter, it is
northward; but at this time, it may not contribute
to water mass conversion. Instead the heat transport is probably balanced by adiabatic deepening
of the thermocline (WP). Water mass conversion is
probably primarily confined to summer – especially late summer, when the thermocline is shallowest. It is interesting also that in a typical
summer intraseasonal event, SST falls by about
2°C off Somalia (Webster et al., 2000), possibly
due to upwelling induced by the strengthened
Findlater Jet at this time. Thus the overturning cell
may be linked to water mass conversion and SST
change on sub-seasonal time scales; such SST
changes may be of importance to our understanding of the Madden–Julian Oscillation (e.g. Madden
and Julian, 1994) as a coupled ocean–atmosphere
phenomenon.
Relation of equatorial flows to higher-latitude flows,
outside the western boundary current
The eastward flow along the equator, mostly in
the two WJs but with a substantial annual mean of
SECTION 4 THE GLOBAL FLOW FIELD
242
100 m (Fig. 4.3.12), seasonally varying currents do
seem to advect water masses northward nearly the
full length from the equator to 10°N, and to mix
them up in the Great Whirl. Winter advection
similarly returns salty water to most of the region.
Observed current magnitudes near 100 m also support this conclusion. In other words, annual flushing of the SCS is rather complete, at least in the
top 100 m. It is therefore physically plausible that
an ‘eddy flux’ heat transport mechanism like the
one described above may contribute to heat
removal from the northern Indian Ocean.
The equator-spanning Southern Gyre appears to
mediate the transfer from the SCS to the equatorial flows. Schott et al. (1998) undertook hydrographic surveys in the offshore regions of the
Somali Current in summer 1995. They showed
that the Somali Current becomes disconnected
from the interior Arabian Sea between 4°N and
12°N, both in currents and water masses. The
Southern Gyre is clearly evident in their maps of
near-surface currents. In the later map, salinity is
actually higher south of the equator than north of
it along 50°E, suggesting that the Southern Gyre is
transporting water from the Somali Current across
the equator, and then eastwards.
The qualitative picture just presented suggests
that the most of the water formed in the top 100 m
in summer in the SCS may escape via the southflowing cross-equatorial western boundary current, and thence eastward via the Wyrtki Jet (in
October–November) or by the EUC (in December–February). However, the quantitative details
of this picture are important for assessing the contribution of SCS flushing to the net meridional
heat flux out of the Indian Ocean – and to its
interannual variability.
Models with only one active layer but high horizontal resolution are quite successful in simulating
several features of Fig. 4.3.13; for example, Luther
and O’Brien (1985, 1989) and Luther et al. (1985)
obtained quite realistic simulations of the ‘Great
Whirl’ and ‘Southern Gyre’ in their runs. A run
with a fixed seasonal cycle of winds shows much
less interannual variability than one with realistic,
interanually varying winds. However, such models
cannot resolve subtleties of the vertical circulation
such as whether the SCS outflow joins the EUC.
Visbeck and Schott (1992) addressed the issue of
equatorial divergence, at least in the 150–800 m
depth range, by comparing available observations
with the output of a Geophysical Fluid Dynamics
Laboratory (GFDL) model run, driven by Hellerman
and Rosenstein (1983) mean seasonal winds. This
limited-area model had a zonal resolution of 0.5°
in the western Indian Ocean, and a meridional resolution of 0.33° in the region of interest. Figure
4.3.10b shows the mean seasonal cycle of surface
zonal currents along the equator from the model,
in fair agreement with Reverdin’s (1987) climatology. A similar level of agreement is found between
model and observation for vertical sections through
the EUC at 55°E, due to Leetmaa and Stommel
(1980). The model’s annual-frequency zonal currents along the equator have amplitudes and phases
that are in reasonable accord with observations in
the western boundary current and in the EUC. The
model is somewhat less successful in simulating the
Somali Current at the equator, but model longshore currents showed a large degree of asymmetry
about the equator; longshore semiannual transport
amplitude reached 5 Sv near 2°N, S.
Before leaving the topic of the western boundary current, we consider the seasonal relationship
of the wind-driven overturning cell to water mass
formation and SST change. Hsiung et al. (1989)
used observed surface fluxes and heat content
changes to estimate seasonal net northward crossequatorial heat flux into the Indian. In winter, it is
northward; but at this time, it may not contribute
to water mass conversion. Instead the heat transport is probably balanced by adiabatic deepening
of the thermocline (WP). Water mass conversion is
probably primarily confined to summer – especially late summer, when the thermocline is shallowest. It is interesting also that in a typical
summer intraseasonal event, SST falls by about
2°C off Somalia (Webster et al., 2000), possibly
due to upwelling induced by the strengthened
Findlater Jet at this time. Thus the overturning cell
may be linked to water mass conversion and SST
change on sub-seasonal time scales; such SST
changes may be of importance to our understanding of the Madden–Julian Oscillation (e.g. Madden
and Julian, 1994) as a coupled ocean–atmosphere
phenomenon.
Relation of equatorial flows to higher-latitude flows,
outside the western boundary current
The eastward flow along the equator, mostly in
the two WJs but with a substantial annual mean of
SECTION 4 THE GLOBAL FLOW FIELD
242