How to calculate the effect of lower sea level during the Last Glacial Maximum on Gulf Stream flow t

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How to calculate the effect of lower sea level
during the Last Glacial Maximum on Gulf Stream
flow through the Florida StraitsDana Ionita,
Emanuele Di Lorenzo, and Jean Lynch-StieglitzScho
ol of Earth and Atmospheric Sciences, Georgia
Institute of Technology
A geostrophic transport estimate for the Gulf
Stream flow through the Florida Straits during
the Last Glacial Maximum (LGM) suggests that flow
was weaker by one third relative to today. This
decrease is important since the northward heat
transport is proportional to the strength of the
overturning circulation. As it passes through
the Florida Straits, the Gulf Stream is
constrained by the ocean margins and thus it has
clearly defined boundaries which allow us to
measure the total volume transport (30 Sv).
During the LGM, the sea level was lower by about
130 m, which reduced the shallowest Florida
Straits depth of 760m to 630m. It is possible
that this difference might have reduced the mass
transport through the Florida Straits during the
LGM or affected the vertical velocity profile of
the flow. In this study I calculate the
geostrophic velocity profile in the Florida
Straits using the density gradient between the
strait margins in the ocean model. I check my
velocity profiles against the velocity profile
from the regional ocean model (ROMS) of the
Caribbean and they match closely. I plan to use
this method to test the sensitivity of the
geostrophic transport estimate to the expected
change in sea level for the LGM.
Bathymetry in the Florida Straits
Section 1
Section 2
Latitude N
meters
Longitude W
In reconstructing geostrophic flow, we assume the
bottom velocity to be zero. However, the flow
can get so constricted in the Florida Straits,
that this assumption might not always be valid,
thus restricting the usefulness of the
geostrophic method in the reconstruction of
velocity profiles. For section number one, we
add the ROMS models bottom velocity to the
calculated geostrophic flow and find that it has
a negligible effect on the calculated profile.
The difference between the model velocity profile
and the calculated geostrophic velocity profile
is less than 8 of the actual velocity, with most
of the error at the edges (geostrophic flow stays
more to the right). The error could be caused by
edge effects in the interpolation process or by
physical factors, such as the wind-driven
circulation included in the ROMS model.
The model data is provided in columns, each
sampled at 30 equal intervals from the ocean
bottom to the surface. We interpolate the model
data, so that they are located at constant depth
across the flow cross-section and at vertical
intervals of 10 meters. The difference between a
pair of densities at equal depth represents the
vertical velocity shear at that depth. Assuming
zero velocity at the bottom, we calculate the
total volume transport through the strait.
Model velocity (m/s)
Calculated bottom velocity (m/s)
Section 1
Total volume transport 27.7 Sv Using the ROMS
model velocity
Total volume transport 30.7 Sv Using calculated
geostrophic velocity
Results The total volume transport calculated
from the ROMS model velocity (27.7 Sv) and from
the geostrophic velocity (30.7 Sv) agree with the
observed values of 30 Sv. The transport
calculated using only bottom density (29.5 Sv)
agrees with all of the above. This means that
the geostrophic method can be used together with
benthic foraminifera data to reconstruct past
volume transport of the Gulf Stream through the
Florida Straits.
Total volume transport 29.5 Sv Using the
interpolated bottom density
Conclusion We show that the flow in the
Florida Straits is in geostrophic equilibrium and
can be calculated using density values along the
margins of the Straits. This is important
because d18O data from benthic foraminifera in
sediments can be used to reconstruct past density
profiles along the margins, and thus to calculate
past geostrophic transport through the Florida
Straits. The drop in sea level during the LGM
can be modeled and investigated using this
method. In order to check the effect of the sea
level drop during the LGM, I will raise the
bathymetry of the model by 130 m, which simulates
the LGM conditions in the Florida Straits. The
model results will show whether the flow velocity
changes considerably at the surface or at the
bottom, and how this change affects the total
mass transport through the Straits.