Observations of the Florida and Yucatan Currents from a ...
Transcript of Observations of the Florida and Yucatan Currents from a ...
Observations of the Florida and Yucatan Currents from a Caribbean Cruise Ship
CLEMENT ROUSSET AND LISA M. BEAL
Division of Meteorology and Physical Oceanography, Rosenstiel School of Marine and Atmospheric Science,
University of Miami, Miami, Florida
(Manuscript received 16 February 2010, in final form 17 March 2010)
ABSTRACT
The Yucatan and Florida Currents represent the majority of the warm-water return path of the global ther-
mohaline circulation through the tropical/subtropical North Atlantic Ocean. Their transports are quantified and
compared by analyzing velocity data collected aboard the cruise ship Explorer of the Seas. From 157 crossings
between May 2001 and May 2006, the mean transport of the Florida Current at 268N was estimated to be 30.8 6
3.2 Sv (1 Sv [ 106 m3 s21), with seasonal amplitude of 2.9 Sv. Upstream, the Yucatan Current was estimated
from 90 crossings to be 30.3 6 5 Sv, with seasonal amplitude of 2.7 Sv. These two currents are shown to be
linked at seasonal time scales. Hence, contrary to former results, it was found that transports through the
Florida Straits and the Yucatan Channel are similar, with the implication that only small inflows occur through
minor channels between them.
1. Introduction
The Yucatan Channel and the Florida Straits carry the
upper limb of the meridional overturning circulation in
the tropical/subtropical North Atlantic Ocean and thus
have a significant impact on the global climate. The Florida
Current has been intensely studied, and submarine cable
measurements over more than 20 years show that its mean
transport at 278N is 32.3 6 3.2 Sv (1 Sv [ 106 m3 s21)
(Larsen 1992). This transport is fed from the south by
a main flow through the Yucatan Channel and by minor
flows through the Old Bahama and Northwest Provi-
dence Channels (Fig. 1).
The Yucatan Channel has been far less studied. Early
geostrophic transport estimates ranged from 23 to 33 Sv
(Schlitz 1973), but the classical geostrophic calculation
might not provide accurate estimates because there is no
definite level of no motion in the channel (Ochoa et al.
2001). Later on, by balancing the transport budget for
the Caribbean passages, a nominal transport of 28.5 Sv
was attributed to this channel (Johns et al. 2002). How-
ever, there was large uncertainty because of a lack of
observations in some passages. Two years of more recent
direct measurements across the channel (‘‘Canek’’ pro-
gram, August 1999–June 2001) gave a mean transport of
23.1 6 3.1 Sv (Candela et al. 2003). The authors noted
the large discrepancy between the Yucatan Channel and
Florida Strait transports and ventured the hypothesis
that it could be related to poorly known transports through
Old Bahama and Northwest (NW) Providence Channels.
However, sparse measurements estimated these flows at
only 1 or 2 Sv (Leaman et al. 1995; Atkinson et al. 1995),
implying that the discrepancy could be due to high in-
terannual variability in the various passages.
In this study, we compare the fluxes through the Florida
Strait at 268N and the Yucatan Channel using new mea-
surements. During 2001–06, the Royal Caribbean Cruise
Ship Explorer of the Seas, outfitted with two acoustic
Doppler current profilers (ADCP), collected ocean ve-
locity data throughout the intra-Americas seas. The two
ADCPs of frequencies 38 and 150 kHz penetrate to 1200
and 250 m, respectively. In this study, the 38-kHz ADCP
is used to analyze velocity structure, transport, and vari-
ability, whereas the 150 kHz is used for validating and
improving surface velocities. In addition to these mea-
surements, we use the Hybrid Coordinate Ocean Model
(HYCOM) in its global 1/128 data-assimilative configura-
tion (e.g., Chassignet et al. 2007) to estimate missing flows
where there are gaps in the data. A detailed description of
the ADCP data quality and final processing is given by
Beal et al. (2008).
Corresponding author address: C. Rousset, MPO, RSMAS,
University of Miami, 4600 Rickenbacker Causeway, Miami, FL
33149.
E-mail: [email protected]
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DOI: 10.1175/2010JPO4447.1
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2. Results
a. Time-mean transports
The Florida Current was better sampled than the
Yucatan Current, owing to the consistency and orien-
tation of the cruise tracks and the shallower depth of
the Florida Straits (Fig. 1). The calculation of flux es-
timates across the Florida Straits follows the technique
of Beal et al. (2008). In brief, 157 good crossings were
obtained over which tidal velocities using the Oregon
State University (OSU) tidal prediction model (Egbert
et al. 1994; Egbert and Erofeeva 2002) were estimated
and removed. The shallowest measurements were at
60 m, and analysis of the 150-kHz ADCP showed that
the most realistic surface extrapolation was to apply a
constant velocity above. A gap between the deepest cell
and the bottom equal to 13% of the total water depth
(caused by acoustic sidelobe interference) was filled
with a constant vertical shear extrapolation. The mean
transport was adjusted for heading-dependent gyro-
compass biases, applying two-thirds of the bias to the
eastbound tracks and one-third to the westbound tracks
(Beal et al. 2008). Figure 2 shows a summary of these
corrections. The mean Florida Current transport was
FIG. 1. Mean surface currents (0–212 m) from the Explorer of the Seas ADCP dataset superimposed on the bathymetry of the Caribbean
Sea. Sections A, B, C, and D cross the Yucatan Current, and the red arrows represent the Florida Current at 268N.
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then estimated to be 30.8 6 3.2 Sv, with a standard error
of 0.3 Sv.
For the Yucatan transport, four different cruise tracks
(A, B, C, and D) crossed the channel at varying incidence
angle, and our measurements do not penetrate to the sea
bed, making flux calculations more difficult (Fig. 1).
Transport was estimated as for the Florida Current, with
the following exceptions. First, the gap caused by the
acoustic sidelobe interference was filled by constant
vertical shear where large flows exist over the depth of
the Yucatan Current (700 m) and by a no-slip condition
below (700–1200 m). Second, we account for missing flow
at the coasts and at depths greater than 1200 m by cal-
culating the transports through each of the measured
sections in HYCOM and comparing them with their
equivalents across closed, full-depth sections. Third,
section D is treated somewhat differently because it
crosses the Yucatan Current upstream of the channel.
Our data and HYCOM show that the current here con-
tinues through the channel while an offshore anticyclonic
gyre recirculates to the south of it. Therefore, we account
for the varying width of the throughflow by adjusting the
distance along the track over which we integrate trans-
port, dependent on the position of the offshore flank of
the gyre. However, because the mean transport along the
rest of the entire section D (out to the Cayman Islands) is
close to zero, this adjustment does not change the mean
transport by more than 0.2 Sv relative to a fixed-length
calculation. The resultant transports lie between those
of the other sections. A summary of all of the extrapo-
lations and corrections is given in Fig. 2. By averaging the
90 crossings (11 in section A, 3 in section B, 23 in section C,
FIG. 2. An illustration of the five steps in our mean transport calculations for each of the four
sections A, B, C, and D across the Yucatan Current (colors as for Fig. 1), plus their average
(black line). The red line shows the same for the Florida Current. Starting with the processed
dataset (raw), the steps are 1) detiding, 2) extrapolation at the surface between 0 and
60 m (constant velocity layer), 3) deep extrapolation for sidelobe interference, 4) estimation of
missing flows at the coast and below 1200 m using the HYCOM 1/128 simulation, and 5) cor-
rection for the heading-dependent bias. Note that the transports of the Yucatan and Florida
Currents are within ;1 Sv both before and after extrapolations and corrections. Note also that
;80% of the total adjustment is performed by the surface extrapolation, which increases the
transport equally for the Florida and Yucatan Currents, whereas the rest of the adjustment
accumulates to less than 1 Sv. In particular for the Yucatan Current, the deep extrapolation for
sidelobe interference adds only 0.1 Sv to the mean transport, whereas the missing flow at the
coast and below 1200 m is 20.7 Sv, with one-half of this owing to the missing deep flows.
Moreover, note here that the heading-dependent gyrocompass bias in the Yucatan Current
is reflected in the comparison of the mean transport of 33 Sv from the 35 eastbound tracks and
28.9 Sv from the 55 westbound tracks (independent of the four different sections). And,
following the gyrocompass bias attribution established for the Florida Current (Beal et al. 2008;
using bottom-tracking data, which are not available here), the heading correction in the
Yucatan Current leads to a 20.2 Sv change in the global-mean transport (0.5 Sv if we assume
equal bias between the eastward and westward tracks).
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and 53 in section D), we find a mean Yucatan Current
transport of 30.3 6 8.8 Sv, with a standard error of 1.1 Sv.
Applying the Student’s t test gives 99% confidence that
the mean transport lies between 28.2 and 33.6 Sv.
Our mean Florida Strait transport is consistent with
the cable measurements, which include inflow from the
NW Providence Channel (Baringer and Larsen 2001).
The difference between cable transport at 278N and ours
at 268N implies a NW Providence Channel transport of
1.5 Sv, which is close to previous estimates [1.2 Sv in
Leaman et al. (1995); 0.9 Sv in Hamilton et al. (2005)].
However, our Yucatan Current transport differs by
6 standard errors from Canek (Sheinbaum et al. 2002),
and we find no discrepancy between the Florida and
Yucatan Current mean transports. This finding—that the
Florida and Yucatan Currents are similar—is indepen-
dent of the extrapolations and corrections required for
our transport calculation, as shown in Fig. 2. Our results
imply a 0.5 Sv mean flow through the Old Bahama
Channel (although this is within our estimated uncer-
tainty), which is smaller than a previous estimate of 1.9 Sv
(Atkinson et al. 1995). Old Bahama and NW Providence
Channels would then account for about 2 Sv together,
which is not strongly different from the generally ac-
cepted 3 Sv (Johns et al. 2002; Hamilton et al. 2005).
The small transport difference between the Yucatan
Channel and Florida Straits found in this study is also
reflected in the HYCOM simulation and in the 1/158
numerical experiment conducted in this region with a
z-coordinate ocean model (Jouanno et al. 2008). In sum-
mary, our measurements suggest a new paradigm for the
transport budget of the northern passages of the intra-
Americas seas. The mean transports of the Florida and
Yucatan Currents differ by less than 1 Sv, and hence flows
in minor passages are small.
We note that, at 8.8 Sv, the standard deviation of our
Yucatan Current transport is much larger than the 3 or
4 Sv expected from previous observational and numerical
studies (Candela et al. 2003). We investigated several
possible reasons for this discrepancy based on the fol-
lowing sampling errors: 1) fluctuations of the deep flow
below 1200 m that we do not measure, 2) missing flow at
the coast, and 3) the differing lengths of the four sec-
tions sampled. In the first case, both the Canek results
(Sheinbaum et al. 2002) and HYCOM show that fluc-
tuations of the deep flow have negligible impact on the
fluctuations of the total water column (#0.2 Sv), although
of themselves they can be large (standard deviation
2.2 Sv). For the second point, the model shows that
closed sections have a 20% reduction of the standard de-
viation. For the third, both HYCOM and our data show
that transport variability increases the farther south
and longer the section is. We interpret this as the larger
impact of the recirculating flow from the anticyclonic
gyre in the more southerly sections, which increases the
standard deviation of the transport by 25% without af-
fecting the mean. Overall, we account for about one-half
of our standard deviation in sampling errors and hence
estimate the oceanic standard deviation to be 4.5–5 Sv.
b. Vertical and horizontal structure of the current
The mean structure of the Yucatan Current is best
represented by section D, because it is almost perpen-
dicular to the isobaths and thus to the current direction,
although it is south of the channel (Fig. 1). The northward
flow is 170–180 km wide, and the core of the current is
located above the western shelf slope, close to the coast,
with a peak speed of 130 cm s21 at the surface (Fig. 3a).
During the same period, the Florida Current showed a
peak of 170 cm s21 (Fig. 3c). The part of the Yucatan
Current that flows over bathymetry deeper than about
2500 m (5–6 Sv according to the measurements) typically
recirculates south of Cuba to return across section D as
the southward flow between 848 and 85.58W in the center
of the Yucatan Basin. Analysis of the model shows that
this flow forms an anticyclonic gyre, as depicted also by
recent surface drifter trajectories (Richardson 2005).
Sections A, B, and C (Fig. 1) also capture some of this
southerly flow (Emilsson 1971; Ochoa et al. 2001) at their
eastern ends. The variability of the Yucatan Current at
section D (Fig. 3b) is surface intensified, with a maximum
located at the front between the current and the anti-
cyclonic gyre, indicating that the position of the front is
highly variable.
Figure 3d shows the transport per unit depth for the
Florida and Yucatan Currents. The Yucatan Current has
a deeper extension than the Florida Current. A compari-
son of the two transport-per-unit-depth profiles shows that
the Yucatan transport is 1.6 Sv less than the Florida
transport integrated from the surface to 450 m, whereas
it is 1.1 Sv stronger below. The required upwelling of
waters into the surface 450 m of the Florida Current,
plus the intensification of the flow, is consistent with
a combination of the tilting and uplifting of isotherms
between the Yucatan and Florida Currents (see Fig. 2 in
Leaman et al. 1987 and Fig. 18 in Candela et al. 2003, not
shown), plus the contribution of the Old Bahama Channel.
c. Time series and seasonal cycles
The time series from the individual crossings of the
Florida and Yucatan Currents are shown in Fig. 4a (upper
panels), together with the cable transports at 278N. The
direct correlations between these three time series are
very low (0.2–0.3) as a result of several factors. First, the
dominant short-period variability [,20 days in the Florida
Current (Lee et al. 1985) and ,40 days in the Yucatan
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Current (Abascal et al. 2003)] is aliased by low temporal
sampling. Second, the NW Providence and Old Bahama
Channel inflows lie between these sections and can have
substantial instantaneous fluxes. Third, there are high
sampling errors associated with individual Yucatan sec-
tions, as discussed above, which inflate the range of in-
stantaneous values to 12–49 Sv, as compared with 15–33 Sv
found during Canek (Sheinbaum et al. 2002).
By averaging our data over longer time scales, we ex-
pect better correlations. Figure 4a (bottom panel) shows
all three time series filtered using a 3-month Hanning
window. Correlations are larger (0.4–0.5), although dif-
ferences do persist, specifically from mid-2003 to early
2004 in the Yucatan Current. The correlations based on
weekly-mean datasets are the same as those described
here and based on daily interpolated datasets. With
90 crossings over 4 yr, the Yucatan Channel was occupied
a little less than twice per month, and we attempt to re-
solve its seasonal cycle and compare it with the Florida
Current. Transports were averaged month by month over
their respective periods of sampling in the two passages
(Fig. 4b). The two currents exhibit similar seasonal vari-
ability, with a maximum in summer preceded and followed
by a minimum in spring and autumn. The amplitudes of
their cycles are also similar, 2.9 Sv for Florida at 268N
and 2.7 Sv for Yucatan, despite the elevated sampling
error in the latter. Because of the relatively small number
of measurements in each month (respectively, 13 and
7.5 on average in the Florida and Yucatan Currents),
we note that the standard errors are the same order as
the annual cycle, and hence the statistical significance
of these results is marginal. However, our annual cycles
corroborate with numerical simulations (Johns et al. 2002;
Candela et al. 2003) and with the cable data [the Canek
program was not long enough to resolve the seasonal
cycle, as noted by Candela et al. (2003)], and this brings
subjective confidence to our results and suggests that these
three locations are strongly linked at seasonal time scales.
Thus, the wind forcing over the subtropical Atlantic Ocean,
the Caribbean Sea, and the Florida Straits, which is
thought to be responsible for seasonal fluctuations in
the Florida Current (Schott et al. 1988), also appears
FIG. 3. (a) The 4-yr-mean (2002–06) cross-sectional velocity at section D and (b) its standard deviation. (c) The
5-yr-mean (2001–06) cross-sectional velocity in the Florida Strait at 268N. Note that the x axis scaling is changed
between (a) and (c). (d) Mean transport per unit depth (Sv m21) of the Yucatan Current (combination of all four
sections; black line), and of the Florida Current at 268N (red line), together with their standard deviations (gray and
pink shading, respectively). The mean transports above and below the isodepth of 450 m are also indicated for both
passages.
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to have a dominant role in the Yucatan Current. It is worth
noting here that the average of the monthly-mean trans-
ports differs by only 0.2 Sv from the direct means reported
above, and therefore seasonal sampling biases are small.
3. Conclusions
Five years of ocean velocity data have been collected
aboard a cruise ship and used to characterize the flows
of the Yucatan and Florida Currents. Contrary to results
from Canek, we find that the mean transports through the
two passages are similar, which is in agreement with es-
timated flows in minor passages and the overall mass
budget for the region. We have also observed, for the first
time, that the two currents are strongly linked at seasonal
time scales.
Owing to the opportunistic sampling, our measurements
were examined carefully for errors, biases, and gaps (see
section 2). Except for the surface extrapolation (which
increases the transport equally for the Florida and Yucatan
Currents), these adjustments accumulate to less than
1 Sv (Fig. 2), which is not enough to account for the dif-
ference between the Yucatan transport reported here and
the 23.1 6 3.1 Sv reported previously. The disagreement
between these two estimates is puzzling, and a fuller in-
vestigation is part of an ongoing study. Because our ob-
servations do not overlap in time, there is the possibility
of interannual variability playing a role. However, cable
measurements in the Florida Current indicate interan-
nual variability of less than 2 Sv between the two time
periods.
Acknowledgments. The Explorer of the Seas shipboard
ADCP program is a collaboration between Royal Carib-
bean International, RSMAS at the University of Miami,
and AOML. This work was supported by the National
Science Foundation Grant OCE 0728897. The Florida
Current cable data are made freely available by AOML
on the Internet (www.aoml.noaa.gov/phod/floridacurrent)
and are funded by the NOAA Office of Climate Obser-
vations. We thank the reviewers for their help to improve
this manuscript.
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