same order. Resolution of the mean flow on scales
of 500 km then demanded more than 200 floats,
with lifetimes of 2.5 years, in order to reduce the
eddy error to less than 1 mm s
91 (see Hogg and
Owens, 1999). Fiscal realities and technological
problems reduced that number to approximately
70 at each of the AABW and NADW surfaces.
For those floats that lasted the full 2.5 years we
show in Fig. 4.5.7 (see Plate 4.5.7, p. 300) their net
displacements. At neither level do they conform to
the Stommel schematic. Although the DWBC is
visible at both levels, interior displacements are
mainly zonal and possess small meridional length
scales. The implied circulation is also quite unlike
any of the modern interpretations of hydrographic
data, which are all dominated by meridional flows
in the interior (e.g. Reid, 1989; Zhang and Hogg,
1992; Speer and Zenk, 1993; Durrieu De Madron
and Weatherly, 1994).
This zonal interior is consistent with the notion
that little mixing is occurring away from the rough
topography: little mixing implies little upwelling,
no vortex stretching and zonal flows to conserve
the planetary vorticity component (ignoring possible complications from topography).
SECTION 4 THE GLOBAL FLOW FIELD
268
50° W
4 0 °
30°
20°
10°
10°N
5°
0°
5°
10°
15°
20°
25°
30°
35°
40°S
Fig. 4.5.6 The Deep Basin Experiment experimental plan. Symbols are identified in the legend. Solid circles are an
array of sound sources used for locating neutrally buoyant floats placed at three levels: the AAIW (ϳ800 m), NADW
(ϳ2500 m), and AABW (ϳ4000 m).Those with numbers also contained current meters at 2500 m depth.