patterns of intraseasonal and seasonal variability,
and it included also significant extratropical aspects
of ENSO.
A focal point for a host of modelling studies has
been the seasonal cycle in the wind-driven gyre
circulation, manifested primarily in a variability
of meridional volume transports at the western
boundary. It had been noted by Gill and Niiler
(1973) that, in mid-latitudes, the seasonal and
mean responses to the large-scale wind stress cannot be the same since the time scale for establishing the mean fields is much longer than a year.
The various processes affecting the annual variation of boundary current transport were elucidated
by Anderson and Corry (1985a) within the context of an idealized model. Unless forcing adjacent
to the western boundary is considered, baroclinic
Rossby waves had only little effect on transport
variations in mid-latitudes. The seasonal response
to large-scale forcing is essentially barotropic, and
thus strongly influenced by topography; the phase
lag is roughly zero. The particular situation in the
western boundary regime of the subtropical North
Atlantic was first investigated by Anderson and
Corry (1985b) with a linear, two-layer model,
forced with monthly wind stress anomalies of
Hellerman and Rosenstein (1983). The simple
model demonstrated the important role of the
Bahamian Archipelago which, due to an effective
blocking of the primarily barotropic response to
large-scale forcing variations, leads to a seasonal
cycle in the Florida Current different in origin and
phase from large-scale Sverdrup dynamics, with a
transport maximum in July–August and minimum
in October–November. Since it is well measured in
the Straits of Florida, the seasonal cycle has been
used repeatedly for model–data comparisons, usually indicating a good reproduction of the phase,
but discrepancies in the amplitude. However, due
to the local generation it can neither serve as an
indicator of variability in the ocean’s interior nor
as an ideal test for large-scale models; in fact, the
most critical model factor for simulations of the
seasonal variation in the Straits is the (local) wind
forcing: the difference between solutions of different models, forced by the same wind stress, are
usually much smaller than the changes obtained by
using alternative wind stress climatologies (Böning
et al., 1991, 2001).
A conspicuous, common feature of early model
studies (e.g. Anderson and Corry, 1985b; Sarmiento,
1986; Greatbach and Goulding, 1989; Smith et al.,
1990) was a strong annual variation of <10–15 Sv
east of the Bahamas which, for a long time, stood in
sharp contrast to the observational evidence. In
fact, a first analysis of direct current meter observations by Lee et al. (1990), obtained at 26.5°N over
a 14-month period, gave no indication of an annual
signal; instead, they showed a surprisingly large (up
to about 90 Sv) variation of the volume transport
on a time scale of 70–100 days. An explanation for
the apparent discrepancy was offered by solutions
of the high-resolution CME model experiments,
which also showed the transports at that site to be
governed by strong, weakly depth-dependent fluctuations arising from local instabilities of the upperlayer flow field. However, a seasonal signal very
similar to that of the earlier model solutions was
present also, but could be revealed only after a
multiyear averaging (Böning et al., 1991). The
model results were confirmed when a similar averaging became possible after a multiyear continuation of the current meter measurements (Lee et al.,
1996). Figure 2.2.4 shows the remarkable agreement in both phase and amplitude between model
results and measurements averaged over a 5.8-year
span. A recent evaluation of the suite of different
models used in the DYNAMO programme showed
the annual cycle in the boundary regime east of the
Bahamas to be a very robust feature of the solutions, suggesting that model issues such as vertical
coordinate schemes and corresponding representation of local topography representation, or parameterization of sub-grid-scale mixing effects, are of
minor importance for the simulation of wind-driven
variations (Böning et al., 2000). Obviously, the single most important factor for a realistic simulation
of transport variations at seasonal time scales is the
wind stress; indeed, while similar to each other, the
DYNAMO simulations gave an annual range of less
than half of that obtained by Lee et al. (1996),
implying that the annual variation of the curl of the
ECMWF-based wind stresses at this latitude was
too weak.
The capability of high-resolution ocean models
to aid in the interpretation of the sparse observations of current fluctuations on interannual and
shorter time scales has also been utilized to explain
the fragmented information on wind-driven changes
in the North Pacific. For example, time series of
absolute volume transport across a single section
of the Kuroshio estimated from T/P altimeter data
2.2 Modelling of Thermohaline and Wind-Driven Circulation
67
Böning and Semtner
and it included also significant extratropical aspects
of ENSO.
A focal point for a host of modelling studies has
been the seasonal cycle in the wind-driven gyre
circulation, manifested primarily in a variability
of meridional volume transports at the western
boundary. It had been noted by Gill and Niiler
(1973) that, in mid-latitudes, the seasonal and
mean responses to the large-scale wind stress cannot be the same since the time scale for establishing the mean fields is much longer than a year.
The various processes affecting the annual variation of boundary current transport were elucidated
by Anderson and Corry (1985a) within the context of an idealized model. Unless forcing adjacent
to the western boundary is considered, baroclinic
Rossby waves had only little effect on transport
variations in mid-latitudes. The seasonal response
to large-scale forcing is essentially barotropic, and
thus strongly influenced by topography; the phase
lag is roughly zero. The particular situation in the
western boundary regime of the subtropical North
Atlantic was first investigated by Anderson and
Corry (1985b) with a linear, two-layer model,
forced with monthly wind stress anomalies of
Hellerman and Rosenstein (1983). The simple
model demonstrated the important role of the
Bahamian Archipelago which, due to an effective
blocking of the primarily barotropic response to
large-scale forcing variations, leads to a seasonal
cycle in the Florida Current different in origin and
phase from large-scale Sverdrup dynamics, with a
transport maximum in July–August and minimum
in October–November. Since it is well measured in
the Straits of Florida, the seasonal cycle has been
used repeatedly for model–data comparisons, usually indicating a good reproduction of the phase,
but discrepancies in the amplitude. However, due
to the local generation it can neither serve as an
indicator of variability in the ocean’s interior nor
as an ideal test for large-scale models; in fact, the
most critical model factor for simulations of the
seasonal variation in the Straits is the (local) wind
forcing: the difference between solutions of different models, forced by the same wind stress, are
usually much smaller than the changes obtained by
using alternative wind stress climatologies (Böning
et al., 1991, 2001).
A conspicuous, common feature of early model
studies (e.g. Anderson and Corry, 1985b; Sarmiento,
1986; Greatbach and Goulding, 1989; Smith et al.,
1990) was a strong annual variation of <10–15 Sv
east of the Bahamas which, for a long time, stood in
sharp contrast to the observational evidence. In
fact, a first analysis of direct current meter observations by Lee et al. (1990), obtained at 26.5°N over
a 14-month period, gave no indication of an annual
signal; instead, they showed a surprisingly large (up
to about 90 Sv) variation of the volume transport
on a time scale of 70–100 days. An explanation for
the apparent discrepancy was offered by solutions
of the high-resolution CME model experiments,
which also showed the transports at that site to be
governed by strong, weakly depth-dependent fluctuations arising from local instabilities of the upperlayer flow field. However, a seasonal signal very
similar to that of the earlier model solutions was
present also, but could be revealed only after a
multiyear averaging (Böning et al., 1991). The
model results were confirmed when a similar averaging became possible after a multiyear continuation of the current meter measurements (Lee et al.,
1996). Figure 2.2.4 shows the remarkable agreement in both phase and amplitude between model
results and measurements averaged over a 5.8-year
span. A recent evaluation of the suite of different
models used in the DYNAMO programme showed
the annual cycle in the boundary regime east of the
Bahamas to be a very robust feature of the solutions, suggesting that model issues such as vertical
coordinate schemes and corresponding representation of local topography representation, or parameterization of sub-grid-scale mixing effects, are of
minor importance for the simulation of wind-driven
variations (Böning et al., 2000). Obviously, the single most important factor for a realistic simulation
of transport variations at seasonal time scales is the
wind stress; indeed, while similar to each other, the
DYNAMO simulations gave an annual range of less
than half of that obtained by Lee et al. (1996),
implying that the annual variation of the curl of the
ECMWF-based wind stresses at this latitude was
too weak.
The capability of high-resolution ocean models
to aid in the interpretation of the sparse observations of current fluctuations on interannual and
shorter time scales has also been utilized to explain
the fragmented information on wind-driven changes
in the North Pacific. For example, time series of
absolute volume transport across a single section
of the Kuroshio estimated from T/P altimeter data
2.2 Modelling of Thermohaline and Wind-Driven Circulation
67
Böning and Semtner