Transport Variability of the Deep Western Boundary Current and The

ARTICLE IN PRESS

Deep-Sea Research I 51 (2004) 1397–1415 www.elsevier.com/locate/dsr

Transport variabilityof the Deep Western BoundaryCurrent and the Antilles Current off Abaco Island, Bahamas

Christopher S. Meinena,Ã, Silvia L. Garzolib, William E. Johnsc, MollyO. Baringer b aCooperative Institute for Marine and Atmospheric Studies, University of Miami, NOAA/AOML/PHOD, 4301 Rickenbacker Causeway, Miami FL 33149, USA bAtlantic Oceanographic and Meteorological Laboratory, National Oceanic and Atmospheric Administration, Miami FL 33149, USA cRosenstiel School of Marine and Atmospheric Science, University of Miami, Miami FL 33149, USA

Received 30 September 2003; received in revised form 6 July2004; accepted 15 July2004 Available online 15 September 2004

Abstract

Hydrography is combined with 1-year-long Inverted Echo Sounder (IES) travel-time records and bottom pressure observations to estimate the Deep Western BoundaryCurrent (DWBC) transport east of Abaco Island, the Bahamas (near 26.51N); comparison of the results to a more traditional line of current meter moorings demonstrates that the IESs and pressure gauges, combined with hydrography, can accurately monitor the DWBC transport to within the accuracyof the current meter arrayestimate at this location. Between 800 and 4800 dbar, bounded bytwo IES moorings 82 km apart, the enclosed portion of the DWBC is shown to have a mean southward transport of about 25 Sv (1 Sv ¼ 106 m3 sÀ1) and a standard deviation of 23 Sv. The DWBC transport is primarilybarotropic (where barotropic is defined as the near-bottom velocityrather than the vertical average velocity);geostrophic transports relative to an assumed level of no motion do not accuratelyreflect the actual absolute transport variability(correlation coefficient is 0.30). The IES-pressure gauge absolute transport within 1200–4800 dbar agrees well with the current meter absolute transports (upper integration limit based on shallowest current meter level); the standard deviation of the difference is 12 Sv and the mean difference is 0.2 Sv. The correlation coefficient between these two time series is 0.76. The northward flowing Antilles Current (AC) east of Abaco Island has a mean baroclinic transport of 6 Sv as estimated bythe IESs and a standard deviation of 3 Sv. The AC variations observed during 1996–1997 are uncorrelated with the Florida Current transport variations west of Abaco Island in the Florida Straits, however, the AC transport variations bear some resemblance to the historical estimates of the AC annual cycle. r 2004 Elsevier Ltd. All rights reserved.

ÃCorresponding author. Tel.: +1-305-361-4355; fax: +1-305-361-4412. E-mail addresses: [email protected] (C.S. Meinen), [email protected] (S.L. Garzoli), [email protected] (W.E. Johns), [email protected] (M.O. Baringer).

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1. Introduction Antilles Current (AC) in the upper 800–1000 m; the AC has a mean transport of about 5 Sv (e.g. A significant fraction, albeit definitelynot all, of Lee et al., 1996). The DWBC and AC have been the deep water masses in the world ocean are shown to varyon annual and semiannual time believed to have last interacted with the ocean scales (e.g. Rosenfeld et al., 1989). surface when theywere in the northern Atlantic Studyof the deep branch of the MOC is Ocean. The meridional overturning cell (MOC) complicated and expensive due to the presence of that moves these deep waters around the world is surface currents (e.g. AC, Gulf Stream, North not a stationarysystem, with changes occurring in Atlantic Current), varying pathways and signifi- the area of deep water formation and in the cant recirculations. One fact that is quite evident is amount of deep water exported from the northern that an efficient and inexpensive method for North Atlantic along the western boundary observing/monitoring the MOC flows is needed. (Dickson et al., 2002; Schott et al., 2004). The Arrays involving large numbers of moorings with picture is further complicated because the pathway multiple current meters are not financiallyprac- for deep water flow is not as simple as was once tical for spanning large portions of the basin over thought, with some observations and models periods of manyyearsto quantifythe MOC flow indicating that the deep outflow from the northern and its variability. The purpose of this paper is North Atlantic can occur awayfrom the western twofold. First, to investigate the feasibilityof boundaryin the interior ( Smethie et al., 2000; quantifying the DWBC transport using a combi- Schott et al., 2004) and studies using subsurface nation of inverted echo sounders (IESs) and deep floats and tracer hydrography suggest that the pressure gauges bycomparing the results to those deep flow along the western boundaryof the from a concurrent line of traditional current Atlantic near Canada maynot continue southward meter moorings at 26.51N off Abaco Island along the boundarybeyond the Southeast New- in the Bahamas. The second purpose is to pre- foundland Rise (Fischer and Schott, 2002; Rhein sent the time variabilityof both the DWBC et al., 2002). and the shallow northward flowing AC for the The path of the Deep Western Boundary period July1996–June 1997 and discuss these Current (DWBC) in the subtropical Atlantic has results in the wider context of the flow throughout been studied extensively; the DWBC meanders and the region. bifurcates under the Gulf Stream (Pickart, 1994) and then continues southward to the east of the Bahamas (Johns et al., 1997). Several long-term 2. Data mooring arrays have been placed along the DWBC path to investigate the spatial and temporal Four primaryinstrument data sets are used in variabilityof the flow: for example, near the this analysis; conductivity–temperature–depth Southeast Newfoundland Ridge (Schott et al., (CTD) profilers, deep pressure gauges, current 2004), east of the Bahamas (Lee et al., 1996)andin meter moorings (CMMs), and IESs. The CMMs the equatorial Atlantic (Fischer and Schott, 1997). and IESs were deployed in a line eastward along The longest time series of moored and hydro- 26.51N from Abaco Island in the Bahamas (Fig. 1) graphic observations has been at roughly26.5 1N, from October 1995 to June 1997 and from July east of Abaco Island, the Bahamas (Molinari et 1996 to June 1997, respectively. A complete al., 1998). Current meter studies at 26.51Nhave description of the moored current meter results shown that the southward flowing DWBC has a can be found in Zantopp et al. (1998) and Johns mean transport of 40 Sv (1 Sv ¼ 106 m3 sÀ1), et al. (submitted for publication); the latter also although as much as 27 Sv of that maybe presents a comparison between current meter- recirculating to the north within the interior of derived transports to transports derived from the basin (Lee et al., 1996). East of Abaco the dynamic height mooring data. In this paper, the DWBC is overlaid bythe northward flowing current meter data are used onlyto compare with ARTICLE IN PRESS

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Fig. 1. Location of the moored instruments and CTD observations used in this study. Small inset panel indicates study area relative to the North American continent. Lower panel shows the vertical distribution of instruments, with bottom topographyfrom the Smith and Sandwell (1997) dataset indicated in gray. Contours indicate the average meridional velocity from 11 CTD/Pegasus sections as presented in Lee et al. (1990); solid and dashed contours indicate 5 cm sÀ1 positive and negative intervals, respectively, while bold solid contour indicates zero flow. the IES-derived quantities. One CMM (site A) was Two-hundred sixtyCTD casts were obtained deployed on the shelf in about 1000 m of water along the Abaco region during the period with an IES located nearby( o5 km); the remain- 1985–2002 (Fig. 1); of these, 229 reached at least ing CMMs and IESs were deployed in deep water 3000 dbar (1 dbar ¼ 104 Pa) and an additional 22 at varying distances from the shelf (Fig. 1, lower reached at least 1000 dbar. The CTD casts were panel). Mooring sites B, C, and D were chosen to acquired fairlyevenlyover the 18 yearperiod, and span the location of the core of the DWBC based theywere spread fairlyevenlythroughout the year on previous studies (Leaman and Harris, 1990; Lee except for November and December, when few et al., 1996), while site E was chosen to sample the observations were made. In Section 3, how these flows further offshore. Site A was occupied in hydrography measurements were used to develop order to measure the shallow northward flowing the empirical characteristic relationships needed to current, often called the Antilles Current (AC, e.g. analyze the IES measurements is discussed. Lee et al., 1990; Leaman and Harris, 1990). Each An IES is about 0.6-m tall and is moored of the four deep CMMs was additionallyequipped roughly1 m off the ocean bottom. It transmits a with a bottom pressure gauge at the base of each 10 kHz sound pulse and measures the time (t) for mooring. the pulse to travel to the ocean surface and back ARTICLE IN PRESS

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(Watts and Rossby, 1977; Chaplin and Watts, once per day, at noon GMT. Finally, all time 1984). The raw IES records were first filtered with series presented herein have been smoothed with a a 40 h boxcar filter to remove most of the tidal 5-dayrunning-mean to remove higher-frequency signal. IES and CMM data were also subsampled signals. from their original hourlysampling rate down to

3. Methods GEM field of temperature [° C ] 0 22 All analyses of IES data depend upon empirical 18 14 10 characteristic relationships determined indepen- 1000 6 dently from hydrography. The data from CTD 4 casts are used to simulate the travel time that 2000 3.5 would be measured at anydepth z byusing the 3 3000 measured temperature ðTÞ and salinity ðSÞ along 2.5 with the empirical equation for sound speed in 2.4

Pressure [ dbar ] ð Þ 4000 2.3 water, c S; T; p ; via the following equation: Z 0 1 5000 0 2.2 tsim ¼ 2 dz ; (1) z c 3.928 3.932 3.936 3.940 where tsim is the simulated travel time and c is RMS scatter about GEM field [° C ] calculated via the sound speed equation of Del 0 1 Grosso (1974). With a tsim value associated with 0.3 each CTD proﬁle, it is possible to consider the 0.3 1000 relationships between t and the directlymea- 0.2 sim 0.15 0.1 sured values of T and S from the CTD casts. 2000 0.1 The earliest IES deployments were made in and near the Gulf Stream, where empirical relation- 3000 ships between tsim and the depth of the 12 1Cor 0.05 15 1C isotherm were used to estimate thermocline Pressure [ dbar ] 4000 depth from IES measurements (e.g. Rossby, 1969; 5000 0.05 Watts and Rossby, 1977; Watts et al., 1995). In 0.05 recent years, empirical hydrographic relationships 3.928 3.932 3.936 3.940 have been developed which can be combined with

Signal to noise ratio for GEM estimates of temperature 0 5 10 Fig. 2. Upper panel: GEM representation of temperature for 1000 15 10 the Abaco region. GEM ﬁeld calculated using the data from the 229 CTD casts obtained along the Abaco line. Solid, dashed, 5 2000 3 and dotted contours denote intervals of 2, 0.5, and 0:1 C; respectively. Vertical gray lines indicate CTD observations. 1 3000 Middle panel: Root-mean-squared (RMS) difference between the CTD measured T values and the temperatures which would 1 be predicted based on the CTD tsim value and the T GEM ﬁeld. Pressure [ dbar ] 4000 Solid, dashed, and dotted contours denote intervals of 0.5, 0.1, 1 and 0:05 C; respectively. Bottom panel: Signal-to-noise ratio of 5000 1 the observed peak-to-peak signal at each depth (from top panel) to the RMS scatter at each depth (from middle panel). 3.928 3.932 3.936 3.940 Solid contours are at intervals of 5, dotted contours are at τ Acoustic round trip travel time 3000 [ s ] intervals of 2. ARTICLE IN PRESS

C.S. Meinen et al. / Deep-Sea Research I 51 (2004) 1397–1415 1401 the IES measured t to estimate full water column series of full water column profiles of T; S; and d profiles of T; S; and specific volume anomaly d (Meinen and Watts, 2000; Watts et al., 2001b). (Meinen and Watts, 2000). These relationships are Basically, the tsim values calculated using each of referred to as the ‘Gravest Empirical Modes’, or the CTD casts are used to sort the T values, for GEMs, with separate GEM look-up tables for example, as a function of tsim and pressure p: T; S; and d: Vertical integration of the d profiles Smoothing splines are used to extract a look-up yields profiles of geopotential height anomaly, table of T on a regular grid of tsim and p: Fig. 2 which when differenced horizontallybetween shows the T GEM field determined from the 229 neighboring IESs yield profiles of the relative deep CTD casts, where the CTD tsim values were velocityusing the geostrophic (dynamic) method. calculated between the surface and 3000 dbar. This combination of hydrography-based GEM Prior to combining the IES time series of t with relationships and IES measurements has been the GEM fields such as that shown in Fig. 2 they successfullyapplied in a wide range of locations must first be ‘calibrated’ into time series of t3000 in different ocean regions, including the North (i.e. the travel time which would have been Atlantic Current (Meinen and Watts, 2000), the measured at 3000 dbar if the IESs were at that Kuroshio (Book et al., 2002), and the Antarctic level). This calibration was completed using CTDs Cirumpolar Current (Watts et al., 2001b; Meinen which were taken at the IES sites during the year- et al., 2002; Meinen and Luther, 2003). The results long deployment. The calibration was done as presented herein utilize both the simple earlier type follows; the IES measured t coincident to the CTD of technique for analyzing IES data and the newer, profile time was subtracted from each dayof the more involved, GEM technique. Both methods time series of t for that IES, and then the CTD 1 depend on the availabilityof sufficient hydrogra- measured t3000 was added to the full time series. phyto characterize the suite of variabilityob- This ‘calibration’ is valid because t variations are served in the region of interest. highlycorrelated in the vertical, with a slope that Before continuing, a few words are in order varies from one byonlya few percent as a function regarding the long term use of IESs for ocean of pressure, for depths below the main thermocline monitoring. The empirical relationships used in (e.g. Meinen and Watts, 1998). Once the IES- analyzing the IES data may change over long time measured t records are calibrated into t3000 (only scales (decades and longer) as T–S properties possible at sites B and D as the ‘calibration CTD’ change. Meinen and Watts (2000) tested the at site A onlyreached about 1000 dbar due to the impact of using hydrography from the 1990s to ocean depth at that location), the timeseries of build GEM fields of temperature and specific t3000 at each IES site are combined with the GEM volume anomalyversus using data from the field shown in Fig. 2 to produce a time series of 1960s–1980s and found little difference in the full-water-column T profiles. Similar GEM fields resulting GEM fields. This result, however, were created for S and d (not shown), allowing for onlyapplies to the region studied byMeinen the calculation of T; S; and d profiles at sites B and and Watts (the North Atlantic Current region D. Profiles of d were verticallyintegrated to obtain near 421N). Employing IESs in a long-term profiles of geopotential anomaly( F) via the monitoring situation will always require occa- standard method; sional (every1–2 years)hydrographic observations Z p to test for long-term trends in the empirical F ¼ d dp0: (2) relationships. 0

3.1. Multiple parameter relationship: obtaining DWBC transport 1When multiple CTDs are available at an IES site, as is common for most deployments, they can all be used to provide As mentioned earlier, hydrography can be used a calibration via this method and the resulting time series can be in combination with an IES to provide a time averaged to yield a more accurately calibrated time series. ARTICLE IN PRESS

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3.2. Absolute referencing of the geostrophic The DWBC transport is defined for the pur- velocities poses of this paper as the flow between 800 and 4800 dbar. The transport is observed within the Differencing the F profiles at sites B and D gap between the two fixed moorings at sites B and results in a time series of estimates of the D (see Fig. 1), although transport outside this gap geostrophic relative velocityperpendicular to the is discussed later as well. line between sites B and D. These velocities are, of course, relative to an assumed level of no motion 3.3. Single parameter relationship: obtaining upper as are all geostrophic velocities determined via the ocean baroclinic transport dynamic method. The common choice of a level of no motion for this region is 800 dbar, based on The AC is mostlyconfined to the upper 800 m of previous long-term absolute velocityobservations the ocean, with a velocitycore near 400 m, and from current meters (Lee et al., 1990, 1996)and inshore of site B (Lee et al., 1996). In addition to PEGASUS sections (Leaman and Harris, 1990). It calculating tsim; the T and S measurements from will be shown, however, that applying such an the CTD casts can also be used to calculate the assumption to the data would lead to radically potential energyanomaly,sometimes called the inaccurate estimates of the deep transport in this Fofonoff potential (Fofonoff, 1962), between the region. Rather than depend on a level of no ocean surface and 800 dbar. Fofonoff potential, w; motion assumption, the IES–GEM velocities were is determined via the following equation: combined with bottom pressure gauge data Z 1 p obtained at the same time in order to determine w ¼ p0d dp0; (3) absolute velocityprofiles. g 0 It is well known that the horizontal gradients where g is gravityand p is pressure.2 Horizontal between pairs of deep pressure gauges are propor- differences of w give the baroclinic transport within À1 tional to the deep absolute velocity. One well- the integration layer (transport =ðrf Þ ðw2 À w1Þ; known problem in using deep pressure gradients in Fofonoff, 1962).3 Therefore bydeveloping a this manner, however, is that the instruments relationship between tsim and w; the IES observa- cannot be geodeticallyleveled accuratelyenough tions can be used to estimate the geostrophic to discriminate between the non-dynamical geo- transport within the upper 800 dbar relative to an graphic pressure difference and the dynamical assumed level of no motion at 800 dbar. time-mean pressure offset associated with the deep When data from each of the 251 CTD casts mean geostrophic flows. As such, the gradient which reached 1000 dbar (Fig. 1) is used to between instruments cannot provide an estimate calculate the Fofonoff potential above 800 dbar of the time-mean deep velocity( Watts et al., (w800) and the simulated travel time above 2001a). Theycan, nonetheless, provide an estimate 1000 dbar (t1000), the close relationship between of the absolute velocityvariability.The measure- the two quantities is clearlyevident ( Fig. 3). We ments of the bottom pressure gauges at the bases cannot directlyapplythe linear fit from Fig. 3 to of the CMMs at sites B and D were differenced the IES records. Just as for the calibration of t3000 and scaled bythe Coriolis parameter to provide a discussed above for the GEM method, the IES time series of the velocityvariabilitybetween the time series of t must be ‘calibrated’ into a time two sites via geostrophy. The time-mean deep series of t1000 (the travel time which would have velocitywas estimated byusing the time-mean been measured at 1000 dbar if the IESs were at meridional velocityfrom the 4000 dbar current meter on the mooring at site C; 8:7cmsÀ1: This 2Comparing Eqs. (2) and (3) shows that w is roughly value is veryclose to the 8 cm s À1 average equivalent to the vertical integration of F: 3Note that w as derived here is onlythe equivalent of determined from two LADCP sections obtained baroclinic streamfunction when the integration limits are the during the experiment (Johns et al., submitted for ocean surface and the geostrophic reference layer (Sun and publication). Watts, 2002). ARTICLE IN PRESS

C.S. Meinen et al. / Deep-Sea Research I 51 (2004) 1397–1415 1403 ] 2 − 60 J m 5 − RMS scatter = 0.6 x 105 J m 2 55

1.302 1.304 1.306 1.308 1.31 1.312 1.314 Baroclinic streamfunction [ 10 Acoustic travel time τ [ s ] 1000

Fig. 3. Each symbol represents w800 vs. t1000 calculated with the T and S measurements of a single CTD; values for all 251 CTDs from the Abaco region are shown. Bold line represents a least-squares linear fit to the data. RMS difference from linear fit values indicated on plot. that level). This calibration was completed in the and therefore the scatter can be lessened byan same manner as the t3000 calibration discussed approach such as that shown in Fig. 3. Given the above. Once calibrated in this manner, each of the small size of the dynamic signals available in the IESs were combined with the slope and intercept Abaco region (full scale t range of 12 ms compared from the linear fit in Fig. 3 to produce time series to 55 ms for the North Atlantic Current for of w800 at sites A, B, and D. example, see Meinen and Watts, 2000), this It should be noted that there are no technical increased scatter about the GEM fields is unac- reasons whythe GEM field presented in Fig. 2 ceptable. As a result the GEM approach was only could not have been developed using t1000 rather applied to the deep IESs (sites B and D), providing than t3000; which would allow all three IESs to detailed vertical structure information at those utilize the GEM technique rather than just sites B sites, while the w method is applied to all three and D. There are, however, several practical IESs for studying the shallow transport. reasons whythis was not done. The main consideration is noise reduction. The choice of the maximum pressure for the simulated t 4. Results integrations represents a balance between max- imizing the number of CTDs from the region Bycombining the calibrated IES t time series which can be used in building the GEM fields with the T GEM field a time series of full water (onlythose CTDs reaching the maximum pressure column profiles of temperature is obtained at site can be used), while choosing a maximum depth B and another time series is obtained at site D. The encompassing as much of the baroclinic structure mean structures of temperature and salinityat sites as possible (Meinen and Watts, 2000). Because B and D, as estimated bythe IES–GEM observa- there is still appreciable shear below 1000 dbar in tions, are presented in Fig. 4 to illustrate typical T the Abaco region, using t1000 as the abscissa for and S values for this region. These time-mean the GEM fields would result in larger scatter about temperatures agree well with those of the moored the GEM field. Of course, this additional scatter is current meters; the CMMs did not have salinity also present in the single parameter correlations sensors against which the IES–GEM estimates such as that shown in Fig. 3. Values of w; however, could be compared. represent a vertical integration quantityrather Some of the current meters on the moorings than a point value like T on a particular p surface, were equipped with pressure gauges (1200 dbar at ARTICLE IN PRESS

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0 mooring motion at deeper levels based on that 500 observed at 800 and 1200 dbar, or byapplying a mooring design program to estimate mooring 1000 motion at these deeper instruments, a rough 1500 comparison can also be made of the deep 2000 temperature records to those estimated bythe IES–GEM combination (not shown). For instru- 2500 ments moored at and below 2000 dbar such 3000 comparisons reveal root-mean-square (RMS) dif- Pressure [ dbar ] 3500 ferences equal in magnitude to the observed temperature signals, indicating that the IES–GEM 4000 technique provides no skill in determining the Site B Site B 4500 Site D Site D temperatures at these depths and cannot observe small T changes such as those which herald the 5 10 15 20 25 35 35.5 36 36.5 arrival of recentlyventilated Labrador Sea Water Temperature [°C ] Salinity [ psu ] (Molinari et al., 1998). This is consistent with the Fig. 4. Time-mean vertical structure of T and S determined roughly1-1 signal-to-noise ratio observed in the from the IESs. Left panel shows the mean T profiles at sites B GEM field at those depths (Fig. 2, lower panel). and D, right panel shows the mean S profiles. The temperature variations are verysmall at these depths (p0:2 C) relative to the upper ocean temperature changes (X2 C), but a similar site B and 800 and 1200 dbar at site D), which magnitude difference between upper ocean signal measure the ‘blow-over’ of the moorings as they and deep ocean signal is also present in regions of are deflected bythe currents. These pressure the world ocean where the GEM technique has excursions can be quite large, exceeding a few shown much stronger predictive abilityin the deep hundred dbar at times. Following these pressure ocean (e.g. Meinen and Watts, 2000). This lack of gauges up and down in the water column, the correlation between deep temperature variability coincident T from the IES–GEM time series of T and t suggests that the deep temperature fluctua- profiles is extracted to yield a time series of T: The tions in the Abaco area are not correlated with comparisons of the directlymeasured tempera- changes in the thermocline layer. Because the tures from the moorings to those estimated by temperature variations in the upper ocean are this IES–GEM combination are generallygood much larger than those in the deep ocean, and (Fig. 5), with correlation coefficients ranging from because the IES measurement of t is an integral 0.81 to 0.89. There are some consistent small biases measurement, these uncorrelated T signals at of about 0.2 1C for all three comparisons, however depth cannot be extracted from a signal dominated the differences are well within the estimated error bythe upper ocean signal. Instead theyin essence bars (see Fig. 2, middle panel). Furthermore, manifest as enhanced scatter about the GEM field because the small biases are of the same sign and (lower panels in Fig. 2). As will now be shown, roughlythe same magnitude theywill have little to however, this does not implythat the IES–GEM no effect on the calculated velocities, which depend combination cannot provide reasonablyaccurate on densitygradients between two points. It is clear, velocityinformation at depth, because geostrophic nonetheless, that the IES–GEM combination is velocities are also integral quantities (being pro- picking up most of the variabilityobserved bythese portional to geopotential anomalygradients). three moored temperature sensors. Similar comparisons could not be made for the 4.1. Test of IES–GEM for DWBC application deeper temperature gauges because none of those sensors had pressure gauges alongside. Never- This studyrepresents the first attempt to apply theless, bymaking some assumptions regarding the GEM technique to IES data for the purpose of ARTICLE IN PRESS

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6 C ] °

3 Mean difference = −0.3°C ° Site B CM measured STD difference = 0.3 C Site B IES estimated Temperature at "1200 dbar" [ 2 N D J F M A M J J A S O N D J F M A M J 19951996 1997

14 C ] ° 12

6 Mean difference = −0.3°C Site D CM measured STD difference = 0.6°C Site D IES estimated Temperature at "800 dbar" [ 4 N D J F M A M J J A S O N D J F M A M J 19951996 1997

6.5 C ] ° 6

5.5

4.5 Mean difference = −0.2°C STD difference = 0.2°C 4 Site D CM measured Site D IES estimated

Temperature at "1200 dbar" [ 3.5 N D J F M A M J J A S O N D J F M A M J 19951996 1997

Fig. 5. Comparison of directlymeasured temperatures from instruments on moorings to the estimated temperature at the same levels from the IES–GEM combination. Comparisons are onlyshown for temperature gauges which were collocated with pressure gauges, so that the temperature at the same pressure could be extracted from the IES–GEM full water column temperature proﬁles. Upper, middle, and lower panels show the comparisons for the moored T and p instrument pairs at nominal depths of 1200 dbar at site B as well as 800 and 1200 dbar at site D, respectively. Mean and standard deviations of the differences (CMM–IES) during the coincident time periods are noted on each panel. interpreting a deep ocean current (the DWBC), as Differencing the F proﬁles at sites B and D opposed to an upper ocean current (e.g. Fig. 7 of results in a time series of estimates of the Meinen and Watts, 2000). Therefore a series of geostrophic relative velocityperpendicular to the comparisons between the velocityestimates from line between sites B and D. The relative velocities the IES–GEM data to direct measurements from cannot be directlycompared to observations from the CMMs deployed during the study will be current meters, because a current meter measures presented next. absolute velocity. If there is more than one current ARTICLE IN PRESS

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60 CM measured relative velocity ]

1 IES estimated relative velocity − 40

[ cm s 20 1200 0 minus V −20 − 400 Mean difference = −1.9 cm s 1 V − STD difference = 9.9 cm s 1 −40 N D J F M A M J J A S O N D J F M A M J

60 CM measured relative velocity ]

1 IES estimated relative velocity − 40

[ cm s 20 3000 0 minus V −20 − 400 Mean difference = −5.4 cm s 1 V − STD difference = 9.7 cm s 1 −40 N D J F M A M J J A S O N D J F M A M J 19951996 1997

Fig. 6. Comparison of directlymeasured velocities from instruments on moorings to the estimated velocities from the IES–GEM combination. Comparisons are of relative velocities; current meters moored at the indicated two nominal pressure levels (e.g. 400 and 1200 dbar for upper panel) were differenced to provide the graydashed lines, while IES–GEM estimated relative velocities were recalculated between the pressures measured coincident to the moored current meters. The 3000 dbar current meter was assumed not to blow over. Mean and standard deviation of the differences (CMM–IES) are noted in the panels. meter on a mooring, however, and if the pressures the study. The velocities measured by the current at which both current meters reside are known, the meters nominallymoored at 1200 and 3000 m were two current meter records can be differenced to separatelydifferenced with the velocitymeasured provide a relative velocitytime series between bythe current meter at a nominal depth of moving pressure surfaces. The relative velocities 400 dbar at site C to yield to relative velocity derived from the IESs can then be reevaluated proﬁles; one over 400–1200 dbar and the other between the same two pressures and compared to over 400–3000 dbar. IES relative velocitytime the current meter measurements. series were extracted over the same pressure range The current meters at nominal depths of 400 (where the level movements matched those from and 1200 m on the mooring at site C, located at the the moored pressure gauges) and were compared midpoint between the IESs at sites B and D, were to the current meter measurements (Fig. 6). The both equipped with pressure gauges. The deeper RMS differences between the current meter current meters were not equipped with pressure measured time series and the IES–GEM estimated gauges. Because moorings tend to blow over in time series are quite small and the variabilityseems such a waythat the deeper instruments blow over to agree well (correlation coefﬁcients of 0.63 and less than those higher on the mooring, and because 0.78 for 10 dayaverages of the velocities shown in the velocityshear at great depth is generallyvery the upper and lower panels of Fig. 6, respectively). small, it was assumed for simplicitythat the Even in the absence of measurement errors and 3000 m current meter did not blow over during methodological problems, one would not expect ARTICLE IN PRESS

C.S. Meinen et al. / Deep-Sea Research I 51 (2004) 1397–1415 1407 the agreement between these two velocities to be 0 perfect because the current meters measure velo- 500 cityat a discrete point while the IES–GEM derived velocities represent an average geostrophic velocity 1000 across the gap between sites B and D. The current meters will observe velocitychanges due to much 1500 smaller scale oceanic features (both temporal and 2000 spatial) than the geostrophic velocitybetween the IESs. As such, the agreement between the time 2500 series shown in Fig. 6 is quite good. The surface 3000 flows in the gap between sites B and D have been Pressure [ dbar ] shown previouslyto be fairlysmall in the mean 3500 (e.g. Leaman and Harris, 1990; Lee et al., 1996; 4000 Johns et al., 1997). This suggests that while the IES–GEM combination maynot be able to 4500 observe the small temperature variations that −20 −10 0 10 20 − occur in the deep ocean off Abaco, the technique Relative velocity [ cm s 1 ] does capture the majorityof the velocityvariations occurring at depths which correspond to the Fig. 7. Time-mean vertical structure of geostrophic velocity DWBC. The abilityto capture the velocity,but determined from the IESs. The plot shows the mean velocity profile between sites B and D relative to a level of no motion at not the deep temperature variations, results from 800 dbar. Cross-hatched region illustrates plus-minus one the fact that the velocityis an integral quantity standard deviation based on the observed temporal variability (proportional to gradients in F; which has been over the year of measurements. integrated from the surface to 3000 m for example), while temperature is a point measurement, and from the fact that the signal-to-noise ratio of indicate a stronger shear, about 3 cm sÀ1; at the the integrated F values is sufficientlygood that the base of the DWBC (Johns et al., submitted for velocityshear through the upper portion of the publication). This lack of vertical shear from the DWBC is well estimated. IES–GEM technique mayresult from the inability to capture the small temperature (and density) 4.2. Mean vertical structure in the DWBC variabilityoccurring below 2000 dbar because it is uncorrelated to the dominant thermocline varia- Fig. 7 shows the time-mean geostrophic velocity bility. Note, however, that the current meters structure relative to 800 dbar from the IES and deployed as part of this study had widely varying GEM data; the cross-hatched region represents mean vertical shears, with a difference larger than plus or minus one standard deviation of the 10 cm sÀ1 between the 2000 and 4000 dbar mean temporal variability. The velocity structure agrees meridional velocities measured at site B, but onlya fairlywell with the current meters, although the 2cmsÀ1 difference between the same levels at sites current meters indicate a more significant 5 cm sÀ1 C and D. Furthermore, the magnitude of the shear reduction in meridional flow at the base of the was highlyvariable over the 2 yearsthat the DWBC between 2000 and 4500 dbar (Johns et al., moorings were deployed (Johns et al., submitted submitted for publication), while the IES–GEM for publication). As such, some portion of the has a reduction of less than 1 cm sÀ1 between 3000 difference between the IES–GEM shear at the base and 4500 dbar. Geostrophic velocities determined of the DWBC and the current meter data may from the moored temperature sensors at sites B result purelyfrom horizontal sampling issues. and D (applying historical T–S data to get density Taken at face value, a 5 cm sÀ1 shear difference from T and to interpolate–extrapolate from a few might lead one to expect a transport bias of about sensors to a full-water-column profile of T) also 5 Sv vs. the IES 1 cm sÀ1 shear, however, it is ARTICLE IN PRESS

1408 C.S. Meinen et al. / Deep-Sea Research I 51 (2004) 1397–1415 shown below that the effect of this shear difference transport (using the deep pressure gradient refer- on the calculated DWBC transports between sites ence) and baroclinic transport (relative to B and D is in fact much smaller. This occurs 800 dbar) were calculated (Fig. 8a), and theyyield because the IES/GEM technique tends to under- record-length-mean transports of À25 Æ 9 and estimate the vertical shear both above and below À20 Æ 6 Sv, respectively(see appendix for deriva- the core of the DWBC somewhat, leading to a tion of the estimated error bars). The standard compensation in transport differences. deviations of the absolute (relative) transport was 21 Sv (15 Sv). This mean absolute DWBC trans- 4.3. DWBC transport port is somewhat smaller than the 30–40 Sv found in previous studies (Lee et al., 1990, 1996), perhaps As noted earlier, the DWBC transport was because the entire DWBC was not captured in the defined as the meridional flow between 800 and gap between sites B and D. Assuming the velocity 4800 dbar between sites B and D (Fig. 8a). structure inshore of site B was the same as between Transport here is simplythe integral of velocity sites B and D (see Fig. 1), an admittedly through this layer multiplied by the distance questionable assumption, an additional 5 Sv of between sites B and D (82 km). Both absolute southward flow could be contained within the

40 20 0 −20 −40 −60 Absolute transport DWBC transport [ Sv ] −80 Baroclinic transport (relative to 800 dbar) −100 A S O N D J F M A M J (a)

40 20 0 −20 −40 − 60 IES + pressure gauge Current meter mooring DWBC transport [ Sv ] −80 Dynamic height mooring −100 A S O N D J F M A M J (b) 1996 1997

Fig. 8. Transport of the DWBC offshore of Abaco Island. (a) Baroclinic transport, relative to an assumed level of no motion at 800 dbar, and absolute transport integrated between 800 and 4800 dbar and between sites B and D. (b) Absolute transport from the IES combined with the bottom pressure measurements compared to absolute transport integrated from the observations of the coincident current meter line. Because current meter data was onlyavailable at 1200 dbar and below, the transports in this panel are integrated onlyover 1200–4800 dbar. Also shown is the transport determined bycalculating dynamicheights from the temperature sensors moored alongside the current meters (Johns et al., submitted for publication), utilizing the same bottom pressure sensors for the barotropic reference as were used with the IES data. Current meter and dynamic height mooring estimates are dotted after February 1997 because after the loss of the upper portion of mooring B the remaining portion of the mooring ﬂopped over, and it provided data over onlya fraction of the DWBC layer.Units are Sverdrups (1 Sv ¼ 106 m3 sÀ1). ARTICLE IN PRESS

C.S. Meinen et al. / Deep-Sea Research I 51 (2004) 1397–1415 1409 unobserved ‘bottom triangle’. The average of 11 Fofonoff definition of near bottom velocityrather PEGASUS sections at the same location in the than being defined as a vertical average (Fofonoff, mid 1980s found an additional 8 Sv of mean 1962). This is consistent with earlier mooring southward DWBC flow located inshore of site B deployments in this area, which demonstrated the integrated between 800 and 3000 dbar (Leaman significant barotropic contribution to the DWBC and Harris, 1990). Wider arrays of current meters flow (Lee et al., 1990). The pressure gauges provide at 26.51N, extending from mooring site A to site E, the estimates of deep flow variability, which might have also indicated a larger southward transport, suggest that the DWBC flow could be monitored about À27 Sv of absolute DWBC transport (Lee et with bottom pressure gauges alone. However, the al., 1996). As will be discussed shortly, however, variabilityof the relative transports is roughly the coincident current meter transport from this 75% of the size of the variabilityof the absolute studyintegrated within the gap between sites B transport, indicating that the baroclinic changes and D agrees quite well with the IES + bottom observed bythe IES–GEM method are not pressure gauge values. The periods of northward negligible. deep flow between sites B and D (e.g. September A final test for the IES plus bottom pressure 1996, March–April 1997) are consistent with gauge method of determining the DWBC absolute northward flow that has been observed when the transport is to compare it against a more ‘tradi- DWBC meanders offshore (e.g. Hacker et al., tional’ measurement technique, the current meter l996). Further complicating the DWBC transport moorings. Determining transport from a CMM picture is the presence of large recirculation cells arraysuch as that deployedhere (see Fig. 1) which extend hundreds of kilometers into the requires assumptions to account for mooring interior (Johns et al., 1997). motion, for the extrapolation upward from the It is evident when comparing the absolute and uppermost current meter on the moorings, and for relative DWBC transports that, while the record- the interpolation between mooring sites. The latter length-means might be similar, the variabilityof is the most worrisome in an arraysuch as this, the two time series is completelydifferent. The where the deep meridional currents are essentially correlation coefficient between the two transport uncorrelated between moorings B, C, and D time series is only0.33, indicating that a linear fit (ro0:2 for the 5-dayrunning-mean data and between the two would onlyexplain 11% of the ro0:6 for 40-dayrunning-mean data), indicating total variance. The most obvious result of this the current was undersampled for all but the comparison, therefore, is that the use of a level of longest time scales. Nevertheless, the current meter no motion assumption in trying to assess the method is probablythe most widelyaccepted DWBC transport from anyindividual CTD standard for determining current transport, and as section will likelylead to a significant instanta- such it is the appropriate quantityto test the IES neous error. The RMS difference between the two plus bottom pressure gauge method. Fig. 8b shows time series is 24 Sv, which is approximatelyequal the absolute transport between 1200 and 4800 dbar to the time-mean absolute transport. The observed (the widest depth range available from the current variabilityis difficult to assess in terms of actual meters) integrated between sites B and D deter- DWBC variability, particularly the periods when mined from the current meters at sites B, C, and D the flow between 800 and 4800 dbar is northward, (Johns et al., submitted for publication) and because it is impossible to determine whether the compares it to the transport determined from the observed changes are due to the DWBC transport combined IES data and bottom pressure gauge pulsing and changing (e.g. Chave et al., 1997)or data. The time series of current meter transport whether the variabilityis due to the DWBC agrees fairlywell with that from the IES plus meandering into and out of the gap between sites pressure gauge; the RMS difference between the B and D (e.g. Hacker et al., 1996; Lee et al., 1996). two time series is 12 Sv (time-mean offset 0.2 Sv) The DWBC transport is stronglydetermined by and the correlation coefficient is 0.76 for the 5-day the barotropic flow, where barotropic here has the running-mean data. This good agreement is ARTICLE IN PRESS

1410 C.S. Meinen et al. / Deep-Sea Research I 51 (2004) 1397–1415 despite the fact that the IES–GEM method was 20 failing to reproduce the proper shear at the base of the DWBC as discussed earlier.4 Given the Æ8Sv 15 absolute dailytransport error bar estimated for the 10 IES plus bottom pressure gauge method presented herein and the Æ8 Sv dailytransport error bar 5 estimated for the current meter integration (Johns et al., submitted for publication), the differences in 0 transports are within the combined measurement − accuracies of the two systems. Johns et al. Antilles current transport [ Sv ] 5 A S O N D J F M A M J (submitted for publication) also calculated trans- 1996 1997 ports for the same gap between sites B and D using dynamic heights calculated using the T measure- Fig. 9. Transport of the AC offshore of Abaco Island. ments on the two moorings and canonical rela- Transport is baroclinic relative to an assumed level of no motion at 800 dbar and is integrated between the surface and tionships between T and density. They referenced 800 dbar. Values represent the transport between sites A and B the resulting relative velocities with the same (see Fig. 1), with the upper ocean transport between sites B and pressure gauges that were used for this study, D also being added when the net upper ocean transport however rather than use the 8:7cmsÀ1 time-mean between those two sites was northward. Units are Sverdrups 6 3 À1 velocityfrom the 4000 m current meter on (1 Sv ¼ 10 m s ). mooring C to provide the time-mean reference velocitytheyused the mean of two LADCP sections, which yielded a mean of 8 cm sÀ1: The between sites B and D is generallysouthward. 5 agreement between the ‘‘dynamic height mooring’’ This is consistent with the idea that the AC is transports and the IBS-based transports is excel- trapped close to the top of the continental slope lent (Fig. 8b), with a mean offset of 0.5 Sv and a (Olson et al., 1984; Rosenfeld et al., 1989; Leaman RMS difference of only5 Sv. These differences are and Harris, 1990). In periods when the upper also well within the associated error bars for the ocean transport between sites B and D was two methods. northward, it is hypothesized that the AC has shifted enough awayfrom the shelf to overwhelm 4.4. Antilles current (AC) transport the general weak southward flow typical for the upper ocean in this region (Leaman and Harris, Using the linear relationship in Fig. 3, time 1990). Hence, the transport of the AC was defined series of Fofonoff potential w (potential energy here as the upper ocean transport between sites A anomaly) integrated between the surface and and B, added to the upper ocean transport 800 dbar were determined for each of the IESs at between sites B and D onlyduring periods when sites A, B, and D. Differences in w between sites the latter is northward (Fig. 9). are proportional to the baroclinic transport above Based on the roughly1 yearof measurements 800 dbar relative to 800 dbar. The differences in w obtained in this experiment, the time-mean AC suggest that northward flow above 800 dbar occurs transport (relative to an assumed level of no primarilybetween sites A and B, while the flow motion at 800 dbar) was 6:1 Æ 1:7 Sv (see appendix for accuracydiscussion). This is larger than a previous absolute transport estimate of about 5 Sv 4Note that in late Januaryor earlyFebruarythe current meter mooring at site B lost its upper portion due to a wire 5Two of the three current meters on mooring A failed, and separation. After this period the nominal 1200 m current meter mooring B lost its upper portion prior to recovery, so there is on the mooring flopped over at times byas much as 900 dbar; limited direct velocitydata for comparison. The one function- the current meter transport integration is much less reliable ing current meter on mooring A and the upper ocean current after this point and therefore is denoted as a dotted line in meters on mooring C are consistent in sign with the transports Fig. 8b estimated from w: ARTICLE IN PRESS

C.S. Meinen et al. / Deep-Sea Research I 51 (2004) 1397–1415 1411 determined from a multi-year mooring deploy- in transport in March–April–Mayand Septem- ment that observed the AC at the same site in the ber–October and maximums in November–De- late 1980s and early1990s using essentiallyone cember and June (Lee et al., 1996), however the mooring (Lee et al., 1996). It also exceeds the agreement is not perfect. Perfect agreement is not 3.8 Sv absolute transport estimate from a series of expected, of course, due to the existence of 11 repeat snapshot PEGASUS sections which mesoscale and interannual variability. The annual spanned a slightlylonger 1985–1987 period ( Lea- cycle only represents 9.7% of the total variance of man and Harris, 1990). These mean transport the 1982–1998 Florida Current record, so anyone differences mayreflect the inadequacyof the year of Florida Current transport does not look assumed 800 dbar level of no motion, or they much like its long-term mean annual cycle either. mayreflect sampling biases, however, the differ- Longer deployments will be required to determine ences in mean transports are not statistically how much of the variabilityin AC transport is significant based on the estimated error bars. The occurring at the annual and semi-annual periods AC transport observed during the studypresented associated with seasonal forcing in order to assess here was highlyvariable at time scales of a few the importance of the apparent AC annual cycle days to a few months, with the strongest transport differences relative to historical estimates. occurring in October–November 1996 and the weakest transports occurring over the period between March and May1997. The standard 5. Summary and conclusions deviation of the baroclinic transport was 3.2 Sv, while the individual dailyvalues (after applying a 5 This study has examined a year-long deploy- dayrunning mean) are accurate to within about ment of IESs in the DWBC east of Abaco Island, 2 Sv (including a possible 1.5 Sv bias, see appen- the Bahamas, and has compared the inferred dix). The observed temporal variabilityis very transports to a traditional CMM array. Historical similar to that observed from a series of 11 hydrography and IES observations were used to PEGASUS sections, which yielded a standard quantifythe baroclinic transport (relative to deviation of 3.9 Sv for the AC transport at 800 dbar) associated with the northward flowing 26.51N(Leaman and Harris, 1990). AC east of Abaco Island. The 11-month mean The AC is believed to be forced as part of the relative transport of about 6 Sv was a bit larger broad Sverdrup circulation (Olson et al., 1984), than previous absolute estimates of 4–5 Sv, how- and as such it might be expected that the ever the differences are not statisticallysignificant. variations in AC transport might be correlated in Over the July1995–June 1996 observation period some waywith the variations in transport of the there was no statisticallysignificant correlation Florida Current to the west of Abaco Island at the between the AC and the Florida Current transport same latitude. Comparison of the AC transport to observed to the west of Abaco Island in the the measured absolute transport of the Florida Florida Straits (Baringer and Larsen, 2001), Current from the subsurface cable running from however the observed AC transports showed some Florida to Grand Bahama Island (Baringer and similarities to previous estimates of the observed Larsen, 2001) indicates that the two transport time seasonal cycle in the AC (Lee et al., 1996). series are not statisticallycorrelated (not shown). IES data was combined with bottom pressure This result is consistent with the PEGASUS gauge observations and the deep mean velocity section results of Rosenfeld et al. (1989), who also estimated from current meters to yield profiles of calculated transports for the Florida Current and absolute velocityover the full water column. These AC in the mid-1980s and found that the currents velocityprofiles were then used to estimate the were uncorrelated. absolute transport associated with the DWBC. The AC time series does have similarities to the The DWBC absolute transport (integrated be- AC transport annual cycle observed in previous tween 800 and 4800 dbar) was highlyvariable, with mooring experiments, which indicated minimums the standard deviation (23 Sv) roughlyequalling ARTICLE IN PRESS

1412 C.S. Meinen et al. / Deep-Sea Research I 51 (2004) 1397–1415 the time series mean (À25 Sv). With onlytwo IESs, for improving this manuscript. C. Meinen was it was not possible to determine what fraction of funded for this work under the auspices of the this variance was due to meandering of the DWBC Cooperative Institute for Marine and Atmospheric into and out of the array(e.g. Hacker et al., 1996; Studies (CIMAS), a joint institute of the Uni- Lee et al., 1996) as opposed to pulsation of the versityof Miami and the National Oceanic and DWBC itself (Chave et al., 1997). The IES–GEM Atmospheric Administration (NOAA), Coopera- method for determining the vertical shear profile tive Agreement #NA67RJ0149. S. Garzoli and M. was unable to reproduce the deep velocityshear Baringer were funded byNOAA, and the IES observed in current meter data at the base of the deployment was funded by NOAA/AOML. W. DWBC (Johns et al., submitted for publication), Johns and the CMM deployment was funded by nonetheless this did not result in large errors in the NSF Grant OCE-9811374, and byNOAA Grant IESþpressure gauge estimates of DWBC trans- NA37RJ0200 administered through CIMAS. port, with the two time series agreeing to within Æ 12 Sv for transports integrated between 1200 and 4800 dbar. This is within the combined error bars Appendix A. Transport error analysis for the two techniques. The CMM vs. IESþpres- sure gauge comparison indicated that the latter A.1. AC transport accuracy would allow for the monitoring of the DWBC transport variabilityat this location with about the IESs and the GEM technique on their own only same accuracy, but at a much lower cost, as a provide geostrophic velocities relative to an traditional ‘‘picket fence’’ of current meter moor- assumed level of no motion. As such, transports ings. As noted above, however, the deep vertical errors can result from errors in the IES–GEM velocitystructure would be less well determined by estimates and from errors due to the assumed level the IESþpressure gauge technique. Application of no motion. Without additional information elsewhere would require similar tests to those from other observations it is impossible to estimate presented herein in order to validate the accuracy the error due to the level of no motion assumption, of this method for estimating deep transports at and therefore the accuracydiscussion here must that location. A line of IESs with pressure gauges, focus onlyon the accuracyof the baroclinic AC commonlyreferred to as PIES, with occasional transport estimates. The errors in the AC bar- hydrographic cruises (every 1–2 years), represents oclinic transport estimate result from the accuracy a cost effective method for monitoring changes in of the IES measured t; the accuracyof the the DWBC transport east of Abaco over longer calibration of the IES measurements into t1000; time scales than has been possible in the past. and the scatter about the linear relationship in Fig. 3. The accuracyof the IES hourlymeasure- ment is roughly1 ms ( Chaplin and Watts, 1984; Acknowledgements Meinen and Watts, 1998), which when combined with the slope from Fig. 3 yields a w accuracyof The authors would like to thank the following Æ 1:4 Â 105 JmÀ2: people who were involved in collecting, calibrat- The calibration of the IES measured t into t1000 ing, and processing the hydrography and mooring is susceptible to three sources of error; the error in data used in this study: E. Johns, T. Kanzow, T. assuming the magnitudes of t variations are the Lee, R. Molinari, W.D. Wilson, Q. Yao, and R. same at 1000 dbar and at the pressure of the IES, Zantopp. This work would not have been possible the error in the CTD calculated t1000; and the without the excellent work of the crew and officers errors in combining the CTD simulated t1000 with of the NOAA Ship Malcolm Baldridge and the the temporallycoincident t measurement from the NOAA Ship Ronald H. Brown and we heartily IES. The first two can be estimated using hydro- appreciate their help. G. Goni and the anonymous graphic data, while the third can be estimated by reviewers provided a number of helpful comments looking at the temporal variabilityof the IES ARTICLE IN PRESS

C.S. Meinen et al. / Deep-Sea Research I 51 (2004) 1397–1415 1413 measurements over a 4–5 h period. The three A.2. DWBC transport accuracy sources result in potential errors of 0.5, 0.1, and 0.5 ms, respectively. Combining the three sources The DWBC transport was estimated using a of error which contribute to the conversion of tIES GEM approach, however, a similar approach to into t1000 in a square-root of the sum of squares the AC error analysis can be applied. The IES manner yields an accuracy of 0.7 ms, which measurement error and IES calibration errors are corresponds to a w accuracyof Æ 1:0 Â 105 JmÀ2: the same for both methods, the onlydifference The scatter about the linear relationship in Fig. would be a new calculation for the scatter around 3 could introduce an error of Æ 0:6 Â 105 JmÀ2: relationship between the CTD simulated t values This error is independent from the t calibration (in this case t3000 rather than t1000) vs. w: The RMS and t measurement errors, and so the total error in scatter about the w4000 vs. t3000 linear fit (not the IES estimated w is the square root of the sum of shown) is about 4:4 Â 105 JmÀ2: Combining the squares of the individual errors, which is 1:8 Â this with the other error sources, the baroclinic 105 JmÀ2: The transport estimates presented here- full water column hourlysample transport accu- in result from the difference in w at two sites scaled racywas roughly Æ 9 Sv. The dailyestimates of bydensityand the Coriolis parameter ( Fofonoff, DWBC baroclinic transport, after the application 1962);p theffiffiffi error in such a difference would be at of the 40 h boxcar filter and the 5 dayrunning most 2 times the error in w at one end, assuming mean, is 2.6 Sv (2.2 Sv random scatter, 1.5 Sv theyare independent errors, scaled bydensityand potential bias). the Coriolis parameter. Using a value of Finally, the barotropic transport of the DWBC 1030 kg mÀ3 for densityand 6 :5 Â 10À5 sÀ1 for the is based on the pressure gauges (for the time- Coriolis parameter, the resulting accuracyof an varying component) and the deep current meters hourlyestimate of the baroclinic transport be- (for the time mean). The accuracyof using the tween two IESs is about 4 Sv. Most of these point measurement current meter record at site C potential sources of error are random in nature; to provide an estimate for the horizontally onlythe error resulting from using the CTD integrated deep time-mean velocitybetween sites estimated t1000 to determine the mean offset from B and D is difficult to explicitlydetermine, tIES is a bias, and that error could be reduced with however a reasonable estimate would be to look multiple calibration CTDs if theywere available. at the differences between the deep mean velocities While this estimated accuracybound seems quite at moorings B, C, and D. The average difference large compared to the AC transports shown in Fig. between the three deep time-mean meridional 9, representing about 120% of the observed velocities and their mutual average was about standard deviation of the transport over the full 2cmsÀ1; which is also the average of the three record, note that this is an accuracyfor the hourly statistical standard errors of the mean. This transport estimates. Separating out the random corresponds to a potential transport bias of as and bias errors, and taking into account the 40 h large as 8 Sv. boxcar filter and the 5 dayrunning mean, the daily The accuracyof the pressure gauges has two AC baroclinic transports presented herein are components; the precision of the instantaneous accurate to within about 2 Sv. With an average pressure measurement and the accuracyof drift integral time scale (Emeryand Thomson, 1997 )of removal. The precision of the deep pressure 13 days from the IES records, the full records yield gauges is 0.001 dbar according to the manufac- about 12 degrees of freedom. So the measurement ture specifications; assuming the errors were of accuracycontribution to the error bar for the time- equal magnitude at sites B and D and that the mean AC transport is essentiallyequal to the bias errors were independent of one another this error of 1.5 Sv. The statistical standard error of the translates to an error in the deep velocityof about mean (Emeryand Thomson, 1997 ) is about 0.9 Sv, 0:2cmsÀ1: The second component of the pressure sothe total combined measurement and statistical accuracyrelates to how well the long-term drift error bar for the AC baroclinic transport is 1.7 Sv. can be removed from the record (Watts and ARTICLE IN PRESS

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Kontoyiannis, 1990). Pressure sensors tend to variations south of Japan determined from inverted echo exhibit long-period exponential drifts that can sounder measurements. Journal of Geophysical Research onlybe removed to within an accuracyof about 107 (C9), 3121 (doi:10.1029/2001JC000795). Chaplin, G.F., Watts, D.R., 1984. Inverted echo sounder 0.015 dbar, which would correspond to a velocity development. In: IEEE Oceans ’84 Conference Record, vol. À1 error of about 3:9cms : This error applies onlyat 1, pp. 249–253. the longest time scales, periods greater than 100 Chave, A.D., Luther, D.S., Filloux, J.H., 1997. Observations of days (Watts and Kontoyiannis, 1990), and is the boundarycurrent systemat 26 :5N in the subtropical largest during the initial few months of the record. North Atlantic Ocean. Journal of Physical Oceanography Because the pressure gauges used herein had been 27 (9), 1827–1848. Del Grosso, V.A., 1974. New equation for the speed of sound in in the water for a full year prior to the IES natural waters (with comparisons to other equations). deployment, this source of error will be neglected Journal Acoustical Societyof America 56 (4), 1084–1091. in the error analysis. With 82 km between sites B Dickson, B., Yashayaev, I., Meincke, J., Turrell, B., Dye, S., and D, and with a DWBC layer thickness of Holfort, J., 2002. Rapid freshening of the deep North approximately3600 m, the combined errors corre- Atlantic Ocean over the past four decades. Nature 4l6 (6883), 832–837. spond to an hourlyabsolute barotropic transport Emery, W.J., Thomson, R.E., 1997. Data Analysis Methods in accuracyof 8 Sv (1 Sv random scatter from the Physical Oceanography. Pergamon Press, Oxford. pressure gauge precision, 8 Sv potential bias from Fischer, J., Schott, F.A., 1997. Seasonal transport variabilityof the accuracyof the time-mean current meter the deep western boundarycurrent in the equatorial estimate). If the same number of degrees of Atlantic. Journal of Geophysical Research 102 (C13), 27,751–27,769. freedom are obtained in the barotropic signals as Fischer, J., Schott, F.A., 2002. Labrador sea water tracked by were in the baroclinic signals, the dailybarotropic profiling floats—from the boundarycurrent into the open transport accuracyafter the application of the 5 North Atlantic. Journal of Physical Oceanography 32 (2), dayrunning mean is 8 Sv ( o1 Sv random scatter, 573–584. 8 Sv potential bias). Fofonoff, N.P., 1962. Dynamics of ocean currents. In: Hill, M.N. (Ed.), The Sea: Ideas and observations on Progress in The absolute DWBC transport accuracywill be the Study of the Seas, vol. 1. Physical Oceanography, Wiley/ a combination of the barotropic and baroclinic Interscience, New York, pp. 323–395. components, in a square-root of the sum of Hacker, P., Firing, E., Wilson, W.D., Molinari, R., 1996. Direct squares sense because the two errors are indepen- observations of the current structure east of the Bahamas. dent. As such, the dailyabsolute DWBC trans- Geophysical Research Letters 23 (10), 1127–1130. port, after the application of the 5 dayrunning Johns, E., Fine, R.A., Molinari, R.L., 1997. Deep flow along the western boundarysouth of the Blake Bahama Outer mean, should be accurate to within 8 Sv (2 Sv Ridge. Journal of Physical Oceanography 27 (10), random scatter, 8 Sv potential bias). The time 2187–2208. mean DWBC transport measurement accuracy, Johns, W.E., Kanzow, T., Zantopp, R. Volume transports from again assuming the same number of degrees of dynamic height and current meters east of Abaco. Deep-Sea freedom as were determined from the IESs, would Research I, submitted for publication. Leaman, K.D., Harris, J.E., 1990. On the average absolute be 8 Sv (1 Sv random scatter, 8 Sv potential bias). transport of the deep western boundarycurrents east of The statistical standard error of the mean is 5 Sv, Abaco Island, the Bahamas. Journal of Physical Oceano- so the total accuracy(statistical plus measurement) graphy20 (3), 467–475. of the time-mean absolute DWBC transport is Lee, T.N., Johns, W., Schott, F., Zantopp, R., 1990. Western about 9 Sv. boundarycurrent structure and variabilityeast of Abaco, Bahamas at 26:5 N: Journal of Physical Oceanography 20 (3), 446–466. Lee, T.N., Johns, W.E., Zantopp, R.J., Fillenbaum, E.R., 1996. References Moored observations of western boundarycurrent varia- bilityand thermohaline circulation at 26 :5 Ninthe Baringer, M.O., Larsen, J.C., 2001. Sixteen years of Florida subtropical North Atlantic. Journal of Physical Oceano- current transport at 27N: Geophysical Research Letters 28 graphy26 (6), 962–983. (16), 3179–3182. Meinen, C.S., Luther, D.S., 2003. Comparison of methods of Book, J.W., Wimbush, M., Imawaki, S., Ichikawa, H., Uchida, estimating mean synoptic current structure in ‘‘stream H., Kinoshita, H., 2002. Kuroshio temporal and spatial coordinates’’ reference frames with an example from the ARTICLE IN PRESS

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