Hall et al., 1997; Thierry, 2000). The mooring
array at 44°W suggests that near the western
boundary flow direction varies but remains parallel to the topography and that transports of the
deep waters change by more than a factor of 2,
whereas flow changes are mostly zonal further east
along the equator. Along the equator, the phase is
observed to increase from the bottom upward
(Hall et al., 1997; Thierry, 2000), which suggests
the energy propagates downward. The large vertical-scale variability seems to be captured in highresolution models of the tropical Atlantic (Schott
and Böning, 1991; Fischer and Schott, 1997) and
is reminiscent of the other tropical oceans. (The
smaller scales associated with the equatorial jets
are however not present in these models, even in
more recent higher-resolution models; C. Böning
and A.-M. Treguier, personal communications.)
Evidence for a seasonal cycle at the equator near
the western boundary is less pronounced in the
upper layers of the ocean than at depth. This also
occurs in the western equatorial Indian Ocean,
where current meters (Schott et al., 1989) show
strong northward flow at depth in boreal summer.
In the Atlantic, Fischer and Schott (1997) note
that the observed annual cycle of mean deep
transport is quite close to the Sverdrup estimate,
in both amplitude and phase (though this does
not hold for the observed large semiannual cycle).
The upward phase propagation probably reflects
Rossby wave activity.
Mooring arrays near the western boundary
have also identified energetic higher-frequency
variability in the upper ocean layers (Johns et al.,
1990; Colin et al., 1994; Schott et al., 1993a, 1998;
Johns et al., 1998). There is evidence of variability
close to the surface at 20- to 30-day periods in the
early boreal summer, similar in period to the equatorial instability waves found north of the equator
that are initiated at the beginning of the upwelling
season, when the meridional shear of the nearequatorial currents increases (Legeckis and
Reverdin, 1987; Weisberg, 1984). During the second part of the year, large oscillations at periods
of 40 to 60 days are present near the boundary
and extend below the thermocline. North of 4°N,
these oscillations are associated with the shedding
of rings from the NBC retroflection. These rings
drift to the northwest towards the Lesser Antilles,
and can be identified and monitored using satellite
altimetry (Didden and Schott, 1993) or ocean
colour (Johns et al., 1990). They are responsible
for a significant transport (up to 3 Sv) of southern
water into the north Atlantic subtropical gyre
(Fratantoni et al., 1995). Only a few numerical
models generate these eddies, for reasons that need
to be examined in detail. Important variability
associated often with meanders is also observed in
the North Equatorial Countercurrent (Arnault
et al., 1999).
4.3.3.4 Meridional heat and freshwater
transports
Meridional heat and freshwater transports have
been investigated based on single hydrographic
sections across the basin, usually several degrees
away from the equator. This helps to minimize the
error associated with the large variability near the
equator, which is not properly sampled during
the sections. The tropical Atlantic gains heat from
the exchange with the atmosphere, and gains fresh
water because of the presence of the ITCZ (IntraTropical Convergence Zone) and the outflow of
rivers. The Amazon River in particular contributes
0.2 Sv. Peterson and Stramma (1991) and Speer
et al. (1996) provide a review of earlier estimates
of the meridional heat flux. MacDonald and
Wunsch (1996) provide estimates at 11°S and 11°N
(0.9 pW and 1.4 pW, respectively) based on single
sections. Lux et al. (2001) also use inverse modelling to provide estimates based on a one-shot
hydrographic survey 7.5°N–4.5°S (January–March
1993). Interestingly, their mass budget requires
considerable conversion of intermediate and deep
waters, in particular the transformation of AAIW
into uNADW in the equatorial region. Analysis of
high-resolution model simulations (A.-M. Treguier,
personal communication) suggests that this does
not result from seasonal variability of mass convergence not properly taken into account in the
inverse model, although these issues remain to be
thoroughly examined. The hydrographic sections
also suggest that significant meridional transports
occur in the eastern equatorial Atlantic, something
that has been hitherto ignored in experimental
analyses based mostly on measurements in the
western boundary currents. At the time of the
hydrographic survey, the Ekman transport contributes a significant share of the meridional heat
transport at 7.5°N, illustrating how sensitive the
heat and freshwater transport estimates will be to
uncertainties in the wind stress and to its seasonal
SECTION 4 THE GLOBAL FLOW FIELD
232
array at 44°W suggests that near the western
boundary flow direction varies but remains parallel to the topography and that transports of the
deep waters change by more than a factor of 2,
whereas flow changes are mostly zonal further east
along the equator. Along the equator, the phase is
observed to increase from the bottom upward
(Hall et al., 1997; Thierry, 2000), which suggests
the energy propagates downward. The large vertical-scale variability seems to be captured in highresolution models of the tropical Atlantic (Schott
and Böning, 1991; Fischer and Schott, 1997) and
is reminiscent of the other tropical oceans. (The
smaller scales associated with the equatorial jets
are however not present in these models, even in
more recent higher-resolution models; C. Böning
and A.-M. Treguier, personal communications.)
Evidence for a seasonal cycle at the equator near
the western boundary is less pronounced in the
upper layers of the ocean than at depth. This also
occurs in the western equatorial Indian Ocean,
where current meters (Schott et al., 1989) show
strong northward flow at depth in boreal summer.
In the Atlantic, Fischer and Schott (1997) note
that the observed annual cycle of mean deep
transport is quite close to the Sverdrup estimate,
in both amplitude and phase (though this does
not hold for the observed large semiannual cycle).
The upward phase propagation probably reflects
Rossby wave activity.
Mooring arrays near the western boundary
have also identified energetic higher-frequency
variability in the upper ocean layers (Johns et al.,
1990; Colin et al., 1994; Schott et al., 1993a, 1998;
Johns et al., 1998). There is evidence of variability
close to the surface at 20- to 30-day periods in the
early boreal summer, similar in period to the equatorial instability waves found north of the equator
that are initiated at the beginning of the upwelling
season, when the meridional shear of the nearequatorial currents increases (Legeckis and
Reverdin, 1987; Weisberg, 1984). During the second part of the year, large oscillations at periods
of 40 to 60 days are present near the boundary
and extend below the thermocline. North of 4°N,
these oscillations are associated with the shedding
of rings from the NBC retroflection. These rings
drift to the northwest towards the Lesser Antilles,
and can be identified and monitored using satellite
altimetry (Didden and Schott, 1993) or ocean
colour (Johns et al., 1990). They are responsible
for a significant transport (up to 3 Sv) of southern
water into the north Atlantic subtropical gyre
(Fratantoni et al., 1995). Only a few numerical
models generate these eddies, for reasons that need
to be examined in detail. Important variability
associated often with meanders is also observed in
the North Equatorial Countercurrent (Arnault
et al., 1999).
4.3.3.4 Meridional heat and freshwater
transports
Meridional heat and freshwater transports have
been investigated based on single hydrographic
sections across the basin, usually several degrees
away from the equator. This helps to minimize the
error associated with the large variability near the
equator, which is not properly sampled during
the sections. The tropical Atlantic gains heat from
the exchange with the atmosphere, and gains fresh
water because of the presence of the ITCZ (IntraTropical Convergence Zone) and the outflow of
rivers. The Amazon River in particular contributes
0.2 Sv. Peterson and Stramma (1991) and Speer
et al. (1996) provide a review of earlier estimates
of the meridional heat flux. MacDonald and
Wunsch (1996) provide estimates at 11°S and 11°N
(0.9 pW and 1.4 pW, respectively) based on single
sections. Lux et al. (2001) also use inverse modelling to provide estimates based on a one-shot
hydrographic survey 7.5°N–4.5°S (January–March
1993). Interestingly, their mass budget requires
considerable conversion of intermediate and deep
waters, in particular the transformation of AAIW
into uNADW in the equatorial region. Analysis of
high-resolution model simulations (A.-M. Treguier,
personal communication) suggests that this does
not result from seasonal variability of mass convergence not properly taken into account in the
inverse model, although these issues remain to be
thoroughly examined. The hydrographic sections
also suggest that significant meridional transports
occur in the eastern equatorial Atlantic, something
that has been hitherto ignored in experimental
analyses based mostly on measurements in the
western boundary currents. At the time of the
hydrographic survey, the Ekman transport contributes a significant share of the meridional heat
transport at 7.5°N, illustrating how sensitive the
heat and freshwater transport estimates will be to
uncertainties in the wind stress and to its seasonal
SECTION 4 THE GLOBAL FLOW FIELD
232