Division of Meteorology and Physical Oceanography
Rosenstiel School of Marine and Atmospheric Science
Tech.Rept. 94-002
Pathways in the Deep Western Boundary Current Recirculation South of 30 North
by Kevin D.Leaman & Peter Vertes
I. Introduction/Background.
As the result of work carried out as part of the NOAA-funded STACS experiment in
the late 1980's, it became evident that the actual structure of the Deep Western
Boundary Current(s) observed east of the Bahamas (Abaco) differed in some signifi-
cant ways from what might have been expected based on earlier studies.
It was realized that deep subsurface (RAFOS) floats would be the most efficient
and illuminating way to study these deep currents (Rossby et al., 1986). In parti-
cular the floats would be well-suited to track possible recirculations in the 2000-
2500m velocity core where there is little freon signal. For this reason, in late
1989 the Rosenstiel School of Marine and Atmospheric Science (RSMAS) of the Univer-
sity of Miami established, with support of the National Science Foundation, an in-
house program to construct, ballast, launch and analyze data from RAFOS floats,
with the goal of seeding different parts of the Deep Western Boundary Current (DWBC)
with these floats. All floats were launched from a local Bahamian fishing boat
based in Marsh Harbor.
II. The RSMAS RAFOS Float Facility
b) Float construction
In order to develop a "learning curve" the first four floats deployed by RSMAS
were fabricated by Don Dorson & Jim Fontaine (URI) and ballasted at URI. Subsequent
floats were constructed locally and, once our tank became operational ( after float
#905) ballasted at RSMAS. RAFOS float boards were fabricated by Dorson (i.e. "non-
Forth, non-WOCE" boards). Float temperature & pressure sensors were calibrated using
precision resistors & a dead-weight pressure tester,respectively. To test, the first
two floats were launched on two-month drifts with three windows each day for range
determinations. Later floats were launched on six-month and then one-year drifts as
our skill increased. The one-year missions were, due to memory restrictions, limited
to one range window (for each of 3 sources) per day. Unlike Sea Scan (WOCE) floats,
these had no (low-voltage or excess pressure) bailout circuits.
Shallow (1200-1500m) and intermediate (2000-2500m) floats were fabricated ident-
cally with glass tubes provided by Mooney Bros. (manufactured by Corning). Tubes
are #7740 borosilicate glass with a length of 62" and I.D. = 3". Hemispheric glass
endplates were fused to one end of the tubes. The tube wall thickness was 5.2 mm.
Aluminum end plates (6061 material) were fabricated locally (Miami or Fort Lauder-
dale) by a precision machinist (Laszlo Nemeth). These end plates had a thickness of
0.5" and diameter of 4.0" and were anodized to prevent corrosion. Deep floats used
the same Bathysystems electronics. However, the glass supplier/manufacturer was Rad-
noti using Schott glass. These tubes were 200 cm long with O.D. of 9 cm and wall
thickness of .7 cm with hemispheric end caps fused to the tubes. Correspondingly,
the end caps (again 6061 aluminum, anodized) were heavier with thickness of 0.75"
and 4.0" diameter. Early floats in the series used Telonics PTT ARGOS transmitters,
whereas later floats (the last 24) used transmitters manufactured by ORE.
c). Float temperature and pressure calibrations.
Floats used standard YSI 44032 thermistors and Data Instruments EAF pressure trans-
ducers. Temperature was calibrated in a temperature bath at RSMAS using a Seabird
temperature probe. (Range 0-30 degrees).
Pressure was calibrated using a Chandler deadweight tester. All pressure sensors
were 7k psi range - however the deadweight tester had a maximum range of only 5k psi.
The deadweight tester had an accuracy of better than O.5 psi.
d). Sources
With one exception this experiment used standard sound sources manufactured by
Webb Research Inc., N. Falmouth, MA. These sources produce a nominal sound power
output of +183 db rel 1 microPascal, provide approximately 4300 transmissions with a
standard battery pack and have a maximum range of approximately 2250 km. See Table I
for details.
III. Summary of Float Deployments and Performance.
As noted above, it was originally intended to deploy a float at each of the
three levels: shallow (1200-1500m), intermediate (2000-2500m) and deep (approx.
3000m). This would be repeated every two months since other (current meter) data
suggested the tracks would be independent at this time separation. This would result
in 36 floats being deployed - twelve in each level. For reasons described below, the
final distribution of floats was somewhat different than this. Table IIa Table IIb
Table IIc summarize the important parameters for all float deployments. Note the
following correspondence between drift durations and expected total frames:
91 frames = 2 months; 254 frames = 6 months; 182 frames = one year. Also note that
the float numerical sequence is not always the same as the launch sequence.
a). Failure modes.
The absence of a bailout circuit for these floats makes it sometimes difficult to
determine an exact cause of failure. Seven floats failed entirely - i.e. they were
never heard from after deployment. We suspect that the cause of these failures can
be traced to float flooding due to leakage through the burn-wire release that was
initially used. In late 1991 we noticed several cases where floats had leaked during
ballasting. Investigation showed that almost all had leaked along the burn wire
(i.e. between the wire and the epoxy) of the release, but the manner of leaks was
otherwise not very consistent ( some leaked immediately, some after several days,
some at high pressure, some at low, etc.). In any case floats deployed after April
1992 were equipped with a new (Sea-Scan) release fitted with a facing O-ring & glass
(rather than epoxy) potting. It can be seen from Table II that significantly fewer
total losses occurred after this date. This was the primary failure mode. In a few
cases the PTT stopped transmitting prematurely after the float surfaced. In a few
other cases the acoustic data were of unexpectedly low quality. Such failures have
been attributed to leakage in the hydrophone connection. Many of the failures des-
cribed above have been seen in other RAFOS float programs ( see for example the
"Report of the RAFOS Float Technology Workshop", Woods Hole, MA, January 13-14,1994).
In three cases the PTT quit early.
The case of pressure deserves special mention. A wide variety of pressure behavior
is observed, ranging from floats whose pressure was stable through the record, to
those where pressure slowly increased suggesting slow sinking, to those with seem-
ingly bizarre behavior such as sinking over the first half of the record and rising
over the second half (#908). At the aforementioned technology workshop floats which
had problems maintaining steady depth were divided into "slow sinkers" and "fast
sinkers". Our "fast sinkers" would probably have never been detected since we had
no bailout circuits. Most of our sinking floats therefore come under the heading of
"slow sinkers," and for these the sinking rate was typically <100m/year. It is still
unclear if this is due to slow glass creepage or to extremely slow leaks in, for ex-
ample, the burn wire, or hydrophone, or RTV seal which holds the glass tube to the
end plate. Some other problems (e.g. noisy pressure data part-way through the record
as in #903) were also seen in other float records discussed at this workshop.
IV. Float Data.
The intermediate and deep floats in particular show that most of the floats do in
fact not drift straight down the boundary but rather tend to be ejected and recircu-
late into the boundary at least once. The most pronounced topographic effect appears
to be caused by an approach of an intermediate or deep float to San Salvador island
at about 24N latitude. The majority of these floats do recirculate into the interior
& tend at least for awhile to follow constant depth contours to the northeast. Some
of these continue into the interior while others appear to circulate back to the
west at about 26 - 27 N. These float tracks are the subject of a separate paper
(Leaman and Vertes, 1994).