Velocities |
The standard approach to interpreting IES data has been to make use of
the high correlation in particular regions of the ocean between
acoustic travel time and other integrated quantities to convert tau
to other dynamical parameters. This approach was presented in
the Dynamic Heights section.
The Gravest Empirical Mode (GEM) technique uses a
different approach to interpreting IES data. In this technique,
tau is used as a cross-stream axis variable. The basic structure
of the current system is reconstructed from hydrographic sections.
tau is calculated for each cast in every section and then another
variable such as temperature (T) or specific volume anomaly
is mapped as a function of measured pressure (p) and
calculated tau. The GEM technique uses the simplifying assumption
that this map contains all the T or specific volume anomaly variability.
In other words, the assumption is that for a given observable tau value in
this region, a particular water column profile of T or
specific volume anomaly will
be observed. This assumption is good for particular regions of the
ocean because of these regions' overall consistent dynamical
structures. With this assumption, each value of an IES measured
tau time-series can be simply used to select the proper
water-column profile from the hydrographicaly derived GEM map. Thus,
time-series of T and specific volume anomaly profiles are obtained.
This technique
(or an analog of it) has been applied successfully to the North
Atlantic Current (Meinen and Watts 2000), the Gulf Stream (He et al.
1998), and the Subantarctic Front (Watts et al. 2000).
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To calculate GEM's for the region, the hydrographic profiles along the
ASUKA line were used to map T and specific volume anomaly with
tau800 and
p. The data were interpolated and extrapolated in the horizontal
using a cubic smoothing spline and then smoothed in the vertical using
a fourth-order least-squares spline approximation. Seasonal
corrections were applied to the data before smoothing to prevent
seasonal bias in the data distribution from affecting the result.
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Temperature GEM for the ASUKA region. The blank region between the
upper and lower panels indicates a change of scale. Contour intervals
are 2°C (upper panel) and 1°C (lower panel).
Seasonal corrections were applied to tau800
and iteratively to T in the top 150 dbar before smoothing to make this GEM.
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Specific volume anomaly GEM for the ASUKA region. The blank region
between the upper and lower panels indicates a change of scale.
Contour intervals are 40 x 10-8m3/kg (upper panel) and
20 x 10-8m3/kg (lower panel). Seasonal corrections were
applied to tau800 and iteratively to specific volume anomaly
in the top 150 dbar before smoothing to make this GEM.
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