1. Introduction
The Florida Current is a highly sheared, energetic flow through confined topography. Such an environment has potential for strong mixing. Measurements from a transect across the Straits of Florida at 27°N characterize the levels and patterns of turbulence and shear in the current.
The background state of the global ocean is one of weak mixing. Measurements of viscous dissipation rates ε in the midlatitude Pacific pycnocline during the Patches Experiment (PATCHEX) yielded a diapycnal diffusivity, Kρ = 0.2ε/N2 (Osborn 1980), of only 4.3 × 10−6 m2 s−1 (Gregg and Sanford 1988). Similar microscale measurements during the North Atlantic Tracer Release Experiment (NATRE) yielded 1.1 × 10−5 m2 s−1 (Toole et al. 1994), consistent with the observed thickening of tracer streaks at the site (Ledwell et al. 1998). Presumably, turbulence at these sites was driven by internal wave shear, which for NATRE was at twice the spectral level of Garrett and Munk (1972, 1975; hereafter referred to as GM76). During PATCHEX, shear closely approximated GM76 conditions, for which the scalings of Gregg (1989) and Polzin et al. (1995) predict Kρ ≈ 5 × 10−6 m2 s−1.
Higher diffusivities occur where internal wave activity is enhanced by mesoscale features or by interactions with topography. For instance, the PATCHEX North dataset taken beneath a coastal jet off northern California—but well away from direct influences of topography, background shear, or atmospheric forcing—yielded Kρ = 3.9 × 10−5 m2 s−1. Near the steep slope of Fieberling Guyot between 700 and 1300 m, Polzin et al. (1995) found (6.7–12.4) × 10−5 m2 s−1; 10 times greater than that at similar depths farther from the guyot.
Stronger topographic effects lead to extreme values, such as 2.5 × 10−3 m2 s−1 in the Denmark Straits (Oakey and Elliott 1980) and 5.5 × 10−2 m2 s−1 in the hydraulically controlled flow through the Strait of Gibraltar (Wesson and Gregg 1994).
High mean shear can dominate turbulent production above the core of the Equatorial Undercurrent (EUC) in the central Pacific. Peters et al. (1988) found Kρ ranging from about 10−4 to over 10−3 m2 s−1. Parameterizations in terms of the mean Richardson number, Ri = N2/〈dU/dz〉2, account for much of the variability, particularly for Ri < 0.4. However, when the same site was reoccupied over two years later, mean shear was weaker and was exceeded by internal wave shear, and dissipation rates and diffusivities were 10 times lower (Peters et al. 1994) and were correlated to total, rather than mean shear (Peters et al. 1995).
The Florida Current is an excellent setting to study all of these mechanisms of enhanced mixing. The conditions of strong flow, nearby topography, and vertical and lateral shear are in sharp contrast to the remotely forced ocean pycnocline. Because the current is part of the Gulf Stream, it is important to investigate its levels of turbulence and mixing.
In this paper we relate observed turbulent dissipation and finescale shear to each other and to attributes of the background flow. First, the collection and treatment of the measurements are discussed (with some details in the appendix), and the background environment is described using these as well as historical observations. Patterns and levels of dissipation and shear are presented, and their relationship is tested against the internal wave scalings to assess the turbulent mechanisms. Five mixing regimes are identified, their distinguishing characteristics are compared, and their contributions to the Florida Current are analyzed.
2. Observations
The survey was conducted from the R/V Endeavor between 3 and 17 June 1990 along 27°N, where the current fills an 80-km-wide, 800-m-deep channel between Florida and Little Bahama Bank (Fig. 1). The primary instrument was the Multi-Scale Profiler (MSP), a freely falling dropsonde carrying CTD, velocity, and turbulence sensors (Winkel et al. 1994, 1996). It was deployed during daylight hours, from 0700 to 1900 local time (1100–2300 UTC). Most of the 80 MSP drops were made at seven stations spanning the channel (Table 1 and Fig. 2). Profiling routinely came to within 5–15 m of the bottom, judging from telemetry pinged to the ship to track and recover the MSP. Time between drops was 1.5–3 h, depending on station depth. Nighttime measurements by a SeaBird CTD (Seim et al. 1999) and continuous velocity profiles from the vessel's RD Instruments ADCP supplement the MSP data.
a. Measurements
Representative profiles from MSP drops are shown in Fig. 3. Finescale hydrography was measured by SeaBird temperature and conductivity sensors, finescale horizontal velocity by a two-axis Neil Brown acoustic current meter (ACM), and turbulent velocity by a pair of airfoil probes. The low noise of the ACM enables it to resolve fluctuations from scales of 1 km down to 0.5 m; however, it detects only relative flow, so the platform motion must be reproduced to yield oceanic velocity (Winkel et al. 1996). Absolute velocities, estimated by comparison with ADCP profiles, are generally accurate to within 0.1–0.2 m s−1. East and north components, u and υ, are essentially across- and along-channel. Viscous dissipation rates ε were computed at 2-m depth increments from airfoil spectra, and were then averaged in 10-m bins to form ε10 profiles.
Finescale velocity and density profiles are assumed to represent internal wave fluctuations superimposed on a subinertial background. As discussed in the appendix, the background flow and stratification were estimated by time-averaging of vertically smoothed drop profiles, yielding station-mean profiles 〈ũ〉, 〈
Fluctuation profiles u′ = u − 〈ũ〉 and υ′ = υ − 〈
Although the sampling was hardly synoptic, station-mean profiles are consistent with historical observations, and will be used to illustrate the subinertial background. For stations 1, 2, and 7, paired mean profiles were averaged together to form single profiles for display. For contour plots of mean hydrographic fields, CTD averages at 5-km intervals were included between MSP stations.
b. Large-scale structure
The bottom on the channel's west side is smooth at 27°N, gently sloping down across isobaths that parallel the current (Fig. 1). The channel shoals from 800 m off Miami to 740 m at the terminus of the straits, 40 km north of the survey site. Midchannel depth at 27°N is just over 750 m (Fig. 5). On the east side, the bottom between the 720-m and 620-m isobaths is bumpy and uneven, and then is fairly smooth where the cross-channel slope steepens to 0.025 at 500 m. The east bank rises sharply from 450 m, initially sloping up at 0.11 and reaching 0.32 toward the surface. The channel narrows from the south with the steep wall of Little Bahama Bank oriented 20° west of north, resulting in convergence in the cross-channel flow (Leaman et al. 1987).
The high-velocity core of the current is near the surface on the channel's west side, where northward speeds exceeding 1.6 m s−1 are attained (Fig. 5a). The velocity maximum stays near the surface to the west, but deepens eastward. Vertical shear is highest at the base of the core and on its west side. Toward the east, shear is more uniform but diminished in the deeper waters; above 150 m, its sense is negative.
The axis of the current divides anticyclonic flow on its east from cyclonic flow on its west. The lateral shear is characterized by the Rossby number, Ro = ∂x
Mean isotherms and isopycnals slope up toward the west (Figs. 5b,d), in the proper sense for thermal wind balance with the vertical shear. At any given depth, the east side is warmer than the west side. The salinity maximum between 100 and 200 m is saltier toward the east (Fig. 5c). Water colder than θ = 6°C or denser than σθ = 27.5 kg m−3 seldom penetrates the Straits of Florida north of Miami.
The temperature–salinity relation varies across the channel, with relatively fresher water at a given density on the west side (Fig. 6). Schmitz and Richardson (1991) find that the fresher water, including that overlying the salinity maximum, originates in the equatorial or southern Atlantic; the higher salinities are traced to subtropical water from the North Atlantic, which subducts under the southern water mass as they collide in the northern Tropics before entering the deeper passages of the Caribbean. For the 12°–24°C temperature range, most of the transport originates in the North Atlantic, including a thick layer of 18°C water. Leaman et al. (1995) conclude that much of the 18° water enters through Northwest Providence Channel (NWPC) and through Santaren Channel (east of Cay Sal Bank; Fig. 1) via Old Bahama Channel, bypassing the Caribbean route argued by Schmitz and Richardson (1991). Deeper, most of the water colder than 12°C comes from the South Atlantic, with the notable freshening influence of Antarctic Intermediate Water (AAIW) in midchannel.
c. Conditions during MSP survey
The Florida Current flows persistently through the survey site, but exhibits considerable variability in its strength and its structure. The Subtropical Atlantic Climate Studies (STACS) program of the 1980s employed a variety of measurement systems to monitor the current's heat and volume transport past this location (Molinari 1985). PEGASUS dropsonde profiles yielded a mean transport of 31.7 ± 3 Sv (1 Sv ≡ 106 m3 s−1) (Leaman et al. 1987), and moored current meters (Lee et al. 1985) and submarine cables (Larsen and Sanford 1985) gave similar results. Transport fluctuations on weekly scales were comparable to the yearly variation of 4 Sv. Velocity and temperature varied most strongly on the west side of the channel (near MSP stations 1 and 2), often in association with meandering of the current core (Leaman et al. 1987).
The total transport inferred from the MSP mean profiles is 31.6 ± 4 Sv (given uncertainties in absolute velocities and interstation interpolation), with cross-channel distribution similar to that for STACS shown in Fig. 8 of Leaman et al. (1989). Transport by temperature was also comparable, except in the two warmest bins: STACS reported an 8.6-Sv peak at 24.5°–27°C but no warmer flow, while MSP found only 3.9 Sv in this bin but another 3.8 Sv above 27°C (Fig. 7). The MSP survey resolved the peak in the layer containing 18° water, most of which was transported through the east half of the channel, consistent with STACS findings.
The subinertial background specific to this study had features not highlighted in the historical observations. Some of this is due to the finer vertical resolution of MSP, but mostly it relates to the established variability of the current. Of particular interest was an eastward shift in the current's axis that evolved throughout the occupations of the three western stations. Some relevant aspects of the hydrography will be described before discussing the meandering.
1) Interleaving
Interleaving of the fresher western water with the saltier midchannel water is evident near the salinity maximum, between 100 and 150 m, in many drops at stations 2 and 3. Centered about local minima in salinity profiles were layers 5–20 m thick, which appear in θ–S plots between the characteristic western and central-to-eastern water masses (as for station 2a in Fig. 6). Weaker intermingling was found above 100 m at all stations. Well below the salinity maximum, intrusions of slightly saltier water occurred at various depths in drops of stations 3–5. The deepest of these involved AAIW as the fresher water mass.
2) Boundary layers
Homogeneous or minimally stratified boundary layers extend down from near the surface in all drops. The maximum penetration varies with station (and drop) from 10 to 55 m (Fig. 5a). Data from these layers are excluded from subsequent analysis.
Bottom homogeneous boundary layers (HBLs) were found in many drops, but not all. Because MSP profiles stopped 5–15 m above the bottom, the thicknesses of these layers are uncertain. The shallowest HBL encounter at each station is indicated on Fig. 5a. At stations 1 and 7, little if any HBL was apparent, as stratification reached the bottom of all but a few drops. Station 2 had HBLs in some drops, but they were thinner than those at stations 3–5; the thickest, at over 60 m, occurred at station 4. A CTD survey during the experiment revealed a benthic front between stations 2 and 3, where the density of the HBL jumped abruptly by 0.2 kg m−3 (Seim et al. 1999).
Atop many of the observed HBLs were turbulent layers or interfaces, which were typically more stratified and sheared than the water above (and below). These turbulent stratified boundary layers (TSBLs) account for much of the mixing in the channel, as will be discussed later. The strongest occurred at station 3 within the deep shear region and at station 7 at the base of the east channel wall (Fig. 8).
3) Shift of current axis
Midway through the MSP cruise, from 9 to 11 June, the current underwent a 2-Sv drop in transport while its core moved 10 km east toward midchannel. According to submarine cable monitoring, transport averaged around 32 Sv just before the shift and 30 Sv just after (J. Larsen 1995, personal communication). Transects through the three western MSP stations yielded ADCP profiles that show the position and structure of the stream. Within the 1.5 m s−1 velocity contour, the flow was relatively pluglike, and that portion of the core was near station 2 on 7 June before moving east of station 3 by 11 June (Winkel 1998). Between these dates, variations are evident in the shear structure at and below the base of the core.
The evolving subinertial flow complicated the separation of data from the western stations into background and internal wave components. Forming two sets of mean profiles for stations 1 and 2—one for drops before 10 June and another for those on or after 10 June (Fig. 2)—removed much of the variability. Most dramatic was the change in northward velocity at station 2 (Fig. 4); although the flow was still evolving during the second occupation, the profiles are much more similar to one another than to those of the first occupation. Regarding station 3, in the two drops made after 10 June—and nearly two days apart—the deep shear and vigorous TSBL found in the earlier 10 drops were absent; these two were excluded from analysis.
Changes in density profiles and θ–S relations are consistent with the eastward shift. For stations 1–3, later profiles are denser at all depths than earlier profiles, as if the background structure had moved offshore. The later θ–S relation for station 2 is characteristic of the west Florida Current (Schmitz and Richardson 1991), whereas earlier drops exhibited interleaving with the saltier midchannel waters (Fig. 6). After 10 June, such interleaving was found at station 3 where θ–S curves from the later drops zigzag between the western fresh and the central salty characteristics.
Conditions also changed at eastern station 7 in the week between its two occupations. Mean density and velocity profiles differed, and water was fresher near the surface and saltier at middepth the second time around. Mixing and internal wave activity above the TSBL were much stronger after 10 June. There is no evidence to establish whether these changes are directly linked to the shift in the current, or instead to unrelated variability near Little Bahama Bank (see Leaman and Molinari 1987).
3. Patterns of turbulence and shear
Turbulent mixing in the Florida Current is represented by vertical diffusivities computed from MSP measurements of dissipation and stratification. Distributions are presented by contouring station-mean profiles across the channel. General tendencies are noted, and areas of strong or weak activity are emphasized. Patterns of turbulence are compared to those of finescale shear components to understand which processes were at work.
Station-mean profiles of diffusivity 〈Kρ〉 = 0.2〈ε10〉/〈
Over much of the section, mixing seems to be driven by the internal wave field, based on the similarity of observed dissipation rates with those predicted by the scalings of Gregg (1989) and Polzin et al. (1995). West of the core, the maximal mean shear was stabilized by high stratification, but it did influence turbulent production. Topography appears to affect internal waves and mixing at the easternmost station.
a. Vertical diffusivity and turbulent dissipation
Averaged over all MSP stations, Kρ = 6.7 × 10−5 m2 s−1, including TSBLs near the bottom, and avoiding homogeneous boundary layers at the surface and bottom. Such diffusivity implies moderate but not strong mixing by oceanic standards.
Turbulence was not distributed uniformly across the MSP section. Most of the high diffusivities occurred in the TSBLs, which occupy only 7% of the total record; their average Kρ = 4.2 × 10−4 m2 s−1 is 10 times the average in the remainder of the stratified water. Further variability is revealed in cross-sectional contour plots.
The highest diffusivities, Kρ > 10−4 m2 s−1, are mainly within 100 m of the bottom, but are also scattered at lesser depths (Fig. 9a). They are associated with elevated dissipation, ε10 > 10−8 W kg−1, found below 200 m within moderate stratification of
The high Kρ and ε10 near the bottom are due to TSBLs, which vary in character and thickness with station and from drop to drop. Some lie atop HBLs, others extend at least to the bottom of the drop. The strongest are marked by shear, stratification, and especially dissipation that are elevated in contrast to waters immediately above. Those of stations 3 and 7 are 30–70 m thick. (See the examples in Figs. 3 and 8). At station 2, they appear as 2–15-m-thick interfaces that cap mixed or weakly stratified bottom layers (i.e., where N < 0.002 s−1); the few detected at station 1 are similar but thinner. At stations 4–6, TSBLs tend to be 10–30 m thick, lying atop HBLs that are up to 60 m thick. Section maxima of ε10 > 10−7 W kg−1 occur in the HBLs of stations 4 and 5 and in the TSBL of station 3 (Fig. 9b).
b. Finescale shear
Patterns of finescale variance are examined to understand the observed distribution of turbulence and mixing. The subinertial background shear S̃2 can generate turbulence directly, or it can affect the downscale transfer by altering or reinforcing the internal wave field. Internal wave levels are represented by the fluctuating shear variance
1) Mean shear
The highest S̃2, in excess of 10−4 s−2, are found above 150 m adjacent to the west side of the core and in the TSBL of station 3 (Fig. 10a). For these regions, and also for the local maximum near 220 m at station 2, the northward component dominates the eastward. The maximum mean shears of 0.01–0.018 s−1 are comparable to those in the Gulf Stream off New England (Gregg and Sanford 1980) and above and below the EUC in the Pacific (Peters et al. 1995). Lesser S̃2 of (1–10) × 10−5 s−2 occur deeper at stations 1 and 2, within and under the central core, and at station 7 near the surface and the bottom; in the latter two areas, the east mean component contributes significantly. A vast region of S̃2 < 10−5 s−2 covers much of stations 4–7.
Areas of
2) Fluctuating shear variance
The basic pattern in Fig. 10b is that
Contouring
3) Total shear variance
Patterns of total shear variance (Fig. 10c) are similar to those of the fluctuating component, except where mean shear is high. The most notable difference is the larger expanse of
4) Comparison with GM76
Fluctuating shear variance in these data is generally 1.5–3 times the GM76 counterpart of
The shear deviates from GM76 assumptions of vertical symmetry and horizontal isotropy. Clockwise variance is 2–4 times the anticlockwise component over much of the section, and is the lesser only shallow at station 7 (Fig. 12). Cross-channel shear variance is more than twice along-channel in the upper 200 m of stations 5 and 6, and particularly at station 7 where it is up to 4 times higher. There are regions where along-channel shear variance is greater than cross-channel, but only in the TSBL of station 3 is it twice as high.
c. Internal wave scalings of dissipation
Both scalings are based on the analytic model of Henyey et al. (1986) of the downscale energy flux from internal waves evolving within a background shear consisting only of larger-scale internal waves. However, Polzin et al. (1996) estimate that subinertial shear contributes significantly to the modeled flux in flows with mean Fr2 exceeding 0.05 (i.e., Ri < 20). A simple way to include mean shear contributions in εiwG and εiwP is to use total Froude variances and spectra in place of the fluctuating quantities. This approach also establishes upper bounds for the scalings, in that it avoids underestimation of internal wave shear related to the uncertain removal of subinertial flow from the measurements.
Within 100-m intervals at each station, observed average
Ratios
The fair agreement, overall, of εiwP and εiwG with observed dissipation rates was not anticipated. The background conditions of strong flow, nearby topography, and vertical and lateral shear are in sharp contrast to the remotely forced ocean pycnocline that Henyey et al. (1986) assume in their simulations and model. Close matching of
4. Mixing regimes
The regional variability of turbulent and finescale characteristics in the flow can be examined by specifying five mixing regimes (Fig. 14). Shear instability appears to be the primary turbulent mechanism in all regimes; contrast occurs in the level and composition of the shear that drives the mixing.
Interior (N): This is the large region of moderate Kρ and
Fr′210 High mean shear zones (S): In these areas of maximum mean shear, dissipation rates exceed those anticipated from fluctuating shear; total shear variance is a better indicator here.
Current core (C): The weakest diffusivities of the section are found near the surface in the high-velocity axis of the current. The weak fluctuating shear slightly overpredicts ε.
East channel wall (E): Fluctuating shear is predominately cross-channel, suggesting that the nearby topography is affecting the internal wave field. Dissipation is sometimes weak and sometimes strong and tends to be underpredicted by the scalings.
TSBLs: The highest Kρ occur in these energetic layers, which are often interfaces or transition zones between HBLs below and interior waters above.
Determination of regime boundaries was guided by features in the station-mean profiles. Contrast among the regimes is enhanced by examination of parameters computed on designated vertical intervals. Most deliberately chosen were the interval bounds for all high mean shear zones (HMSZs), for the weakly mixing core, and for the TSBL deep at the east wall (Fig. 14).
Each analysis interval is characterized by quantities vertically averaged from 10-m station-mean profiles (such as
a. Interior
The interior lies in an active and variable subinertial environment, and has turbulence and finescale variance similar to a moderately forced oceanic pycnocline (Table 2). Interior
Observed dissipation rates are anticipated fairly well by the fluctuation scalings (Fig. 16). This result is robust, whether comparing εiwP (with Rω = 3) or εiwG. Of the 21 intervals, all but one have
Finescale shear in much of the interior differs from that assumed for the internal wave interactions parameterized by εiwG and εiwP. For fluctuating variance in the low-wavenumber band, east components generally exceed the north ones by 1–2 times, and clockwise components often more than double—and always exceed—the anticlockwise (Figs. 17c,d). Despite such variable composition in interior Froude spectra, Winkel (1998) shows that they are consistent in shape and level with the scaling of Polzin et al. (1995). Subinertial shear is significant in many intervals, especially in the six where mean
b. High mean shear zones
Mean shears here are the highest observed in the section (Fig. 15) and are predominately northward (Fig. 17b). Dissipation rates are high, and are 3.7–33 times those predicted by fluctuating shear (Table 2). Total shear variance does, however, provide a fair estimate of
Two intervals—from deep station 2 and the TSBL of station 3—are distinct in that their
The other four intervals are shallow at stations 1 and 2, where strong stratification moderates high
Fluctuating shear, regardless of its nature, is a poor measure of dissipation in this regime. Nor does the moderate to strong mean shear account for all the turbulent production. The best indicator is the total shear, when applied in the εiwG formulation. This suggests that the magnitude of the downscale energy flux is independent of whether the shear is composed of energetic internal waves in a quiet background (as assumed in the scalings), or of weaker waves in an energetic background (as in this regime).
c. Current core
The weakest diffusivities are found in the current core, where low dissipation occurs within high stratification (Fig. 15). Total shear is also among the weakest, with
d. East channel wall
Profiles at station 7 are within 2 km of the channel's east side, which for internal waves is virtually a wall owing to its steep slope. This regime is marked by its proximity to topography and, above the TSBL, by its predominately eastward shear variance (Figs. 12 and 17b,c). The strong turbulence on 13 June is in sharp contrast to the weak activity on 6 June (Table 2).
In the two middepth intervals of 13 June,
Finescale shear in the TSBL differs from that elsewhere at station 7. Northward mean shear contributes more than eastward to the high
In the remaining five east wall intervals, fluctuating shear is a fair indicator of dissipation, especially when εiwP is modified for high-frequency content (Winkel 1998). The combined adjustments C(Rω) via (3) are moderate, ranging from 0.9 to 3.4 for the inferred Rω of 3.4–1.3. Higher-frequency internal waves are a reasonable expectation near the east wall, for which the critical reflection frequency is around 0.1–0.3N. Individual drop profiles of shear exhibit rectilinear behavior, indicative of higher frequencies than in the more elliptical profiles of the interior (Winkel 1998).
e. Turbulent stratified boundary layers
These near-bottom layers account for nearly one-half of the section-averaged Kρ, although they occupy only 7% of the total record. Most prominent are TSBLs that exhibit higher dissipation, stratification and shear than waters immediately above. The TSBLs of stations 3 and 7 are summarized in Table 2. Some drops from other stations exhibit TSBLs that are similar to, though thinner than, those at stations 3 or 7. Others are less turbulent or less distinct. Because TSBLs vary in character and in depth range from drop to drop, inter-regime contrast is enhanced by computing attributes for each drop's TSBL, and then averaging appropriately to form station statistics. This approach increases mean ε, Kρ, and Fr2 by only 50%–100% for stations 3 and 7.
f. Discussion
The importance of mixing in the various regimes depends on the volume and properties of the affected water. Examination of transport in θ–S bins emphasizes that the interior is the dominant component, especially below 24°C (Fig. 18). To discuss contributions in terms of the source waters analyzed by Schmitz and Richardson (1991), transport and diffusivity are tallied by regime in four temperature bins (Table 3).
Surface boundary layers accounted for 3.2 Sv of the total 8.3 Sv warmer than 24°C; the remainder was in stratified flow concentrated near midchannel (Fig. 19). The weakly mixing core carried almost as much as the eastern interior. The combination of rapid flow and weak turbulence suggests that this water mass (beneath the SBLs) undergoes little modification as it transits the Straits of Florida.
The next deeper layer, 17°–24°C, contains North Atlantic water characterized by the θ–S curve from the salinity maximum through the 18°C water (Fig. 6). The MSP survey found such properties in 90% of the total 10.8 Sv, mostly in the eastern interior (Table 3). The remaining 10% occurred in the upper HMSZ (Fig. 18), consistent with reports of fresher, South Atlantic water on the channel's west side (Schmitz and Richardson 1991); some of this water intruded eastward into the core, weakly eroding its salinity maximum. There was a transport maximum east of midchannel between 17°C and 19.5°C (Fig. 19), comparable to that in Fig. 9a of Leaman et al. (1989). Diffusivities exceeding 10−4 m2 s−1 affected 0.7 Sv near the east wall, including 0.3 Sv of 17.5°–18.5°C straddling the upper TSBL; another 1.9 Sv of this 18° water flowed through the interior. Overall, Kρ was only twice the (1–2) × 10−5 m2 s−1 reported for the subtropical North Atlantic by Toole et al. (1994), so the Florida Current is not a hot spot for mixing of this water mass. The east wall could be an exception if the slower transit allows the stronger turbulence to affect the 18° water significantly.
Regarding the 12°–17°C layer, the interior accounted for 6.1 Sv of the dominant North Atlantic component. Around 1 Sv of South Atlantic water flowed through west of station 3, half in the interior and half in the deeper turbulent shear zone of station 2. Only 0.1 Sv trickled through the lower TSBL at the east wall, mixing with a mean diffusivity of 3.6 × 10−4 m2 s−1. The conclusion is the same as for the overlying layer: Given the predominant interior-regime conditions, the Florida Current is not an exceptional site for modifying this water mass.
Water in the deepest layer, 6°–12°C, comes mainly from the South Atlantic (Schmitz and Richardson 1991). Most of the transport was again in the interior, where turbulence was slightly stronger than in the warmer layers (Table 3). Antarctic Intermediate Water, categorized as the fresher 7°–10°C bins in Fig. 18, accounted for 1.4 Sv in the midchannel interior. Water colder than 7°C summed to 0.6 Sv, split evenly among midchannel flows in the interior, TSBLs, and bottom HBLs; the station 3 TSBL carried another 0.5 Sv of water above 7°C. This layer has the strongest mixing, but the smallest transport (Fig. 19). Also, much of this flow does not interact with the bottom in the deeper channel upstream of 25°N (Fig. 1), so the enhanced mixing in the TSBLs persists only for the 10-day transit through the shallower portion. Even so, the effects of friction and turbulent mixing are more important here than in the other layers.
5. Summary and conclusions
Regions of high, moderate, and low turbulent activity have been identified, and relationships between finescale variance and dissipation have been investigated. Influences of background shear have been examined by separating velocity profiles into station-mean and fluctuating components. To accentuate the observed variety of background conditions and turbulent and internal wave activity, five mixing regimes have been designated (Fig. 14). Most of the stratified flow, that is, 73% of the total 28 Sv, has the following characteristics of the interior regime.
Fluctuating shear variance is 1–4 times higher than GM76 internal wave levels, and has larger contributions from the clockwise and cross-channel components than from their counterparts.
Similarly moderate values of turbulent diffusivity, (0.7–7) × 10−5 m2 s−1, are consistent via internal wave scalings with observed patterns and levels of fluctuating shear.
Total shear is determined more by fluctuations than by background flow, even though mean Fr2 range up to 0.55.
Turbulence and finescale quantities show no significant dependence on mean shear.
The remaining regimes are associated with specific areas in the flow or the channel. The high velocity core of the current transports nearly 4 Sv, and zones of high northward mean shear adjacent to the core carry 2 Sv.
The core has predominately clockwise shear variance at GM76 levels, and the weakest diffusivity in the current at 3 × 10−6 m2 s−1.
High mean shear is persistent in the upper western waters, where strong N2 results in background Fr2 of only 0.4 and diffusivity is moderate.
Intermittent features beneath the core with mean Fr2 > 1 have some of the strongest turbulence, Kρ ≈ 4 × 10−4 m2 s−1.
Total shear, determined more by the mean than by fluctuations, reasonably predicts turbulence when adapted to the finescale parameterizations.
Near the east slope, finescale shear is predominately cross-channel and turbulence varies from nearly the weakest to nearly the strongest;
TSBLs are typically more sheared, stratified, and turbulent than the waters immediately above, and at (2–6) × 10−4 m2 s−1 account for nearly one-half of the section-averaged diffusivity;
Shear tends to underpredict strong turbulence near the east slope and in some TSBLs, but yields a fair estimate where mixing is weaker and for the thick TSBL below the core.
These observations provide evidence that the Florida Current is not a hot spot for mixing. Diffusivities were above 10−4 m2 s−1 in just 2 Sv, while the core carried 4 Sv of weakly turbulent water. The remainder of the flow—whether in the vast interior or in the upper shear zone—exhibited shear variance and mixing rates only a few times greater than those found in the open waters of the Atlantic Ocean.
Acknowledgments
We are grateful to the Captain and crew of the R/V Endeavor and to the MSP team of Bob Drever, Jim Carlson, Earl Krause, Bill Hess, and Pat McKeown for their expertise in the collection of these data. Harvey Seim, Ren-Chieh Lien, Eric D'Asaro, and Kurt Polzin provided useful discussions regarding internal waves and turbulence. Two anonymous reviewers made valuable comments that improved the manuscript. The Office of Naval Research funded the collection and analysis of these MSP measurements.
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APPENDIX
MSP Data Processing
During the Florida Straits experiment, the MSP fell at 0.31 ± 0.02 m s−1 and rotated once every 23 ± 2 m during its free-fall descent. Drops ended when the ballast release was triggered at a programmed pressure, or when the return from an onboard continuous acoustic transmission indicated proximity to the bottom. Several sensors were carried by the MSP to measure water properties and to detect platform motion and position. Winkel et al. (1994) provide specifications of the sensor systems and details of the data processing, and Winkel et al. (1996) discuss the treatment and quality of the velocity and shear measurements.
For each MSP drop, standard processing generated profiles of horizontal velocity (u and υ), potential temperature (θ), salinity (S), and potential density (σθ) on 0.1-m depth grids. Dissipation rates (ε and χ) were estimated every 2 m for data within 2.4-m windows centered at each grid point. Velocity profiles came from the ACM data and initially contained unknown, depth-independent offsets. Through comparisons with concurrent ADCP profiles corrected for ship motion, the offsets were estimated to produce absolute velocities. This was most successful at stations 1, 2, and 7, where ADCP bottom tracking was reliable. At each of the other stations (3–6), the ship's velocity was estimated from GPS fixes for a few drops, but poor daytime GPS coverage prevented such efforts for most drops.
Three types of operations were used to compute station-mean profiles and statistics. Given profiles A(z), the time-mean profile for a set of drops is denoted 〈A〉, vertical smoothing with a low-pass filter is denoted Ã, and the arithmetic average over a depth interval (typically 100 m) is denoted
The background stratification was estimated by averaging drop profiles of
The background flow is represented by 〈ũ〉 and 〈
Chart of the Straits of Florida. Dots mark the seven MSP stations. Channel bathymetry is shown with finer resolution north of 25°N than to the south. The three labeled banks rise steeply to depths shallower than 10 m within a few kilometers of their 200-m contours
Citation: Journal of Physical Oceanography 32, 11; 10.1175/1520-0485(2002)032<3269:POSATA>2.0.CO;2
Time and location of MSP drops in the Straits of Florida along 27°N. Distance is eastward from the origin at 80°W. The start of each day (i.e., 0000 UTC) is marked; local time is UTC − 4 h. A dashed line indicates when the current's axis apparently shifted, prior to drops on 10 Jun. Some drop numbers are shown for reference
Citation: Journal of Physical Oceanography 32, 11; 10.1175/1520-0485(2002)032<3269:POSATA>2.0.CO;2
Profiles from standard processing, MSP drop 0267 at station 3. Depth limits of the turbulent stratified boundary layer (TSBL) and homogeneous boundary layer (HBL) are shown
Citation: Journal of Physical Oceanography 32, 11; 10.1175/1520-0485(2002)032<3269:POSATA>2.0.CO;2
Northward velocity at station 2. Heavy lines are mean profiles 〈
Citation: Journal of Physical Oceanography 32, 11; 10.1175/1520-0485(2002)032<3269:POSATA>2.0.CO;2
(a) Mean northward velocity, with dashed lines marking maximum penetration of surface (SBL) and bottom (HBL) homogeneous layers. In subsequent figures, the 1.2 m s−1 isotach delineates the core of the current. Mean hydrographic fields are (b) potential temperature θ, (c) salinity, and (d) potential density σθ, and include nighttime CTD measurements between MSP stations
Citation: Journal of Physical Oceanography 32, 11; 10.1175/1520-0485(2002)032<3269:POSATA>2.0.CO;2
Mean θ–S relations by station. Heavier lines—labeled 1a, 2a, and 7a—distinguish earlier occupations (prior to 10 Jun) from the later ones—labeled 1b, 2b, and 7b. Dotted lines connect station numbers and correspondingly ordered θ–S curves. Lines of constant σθ are dashed. The small box bounds North Atlantic 18°C water
Citation: Journal of Physical Oceanography 32, 11; 10.1175/1520-0485(2002)032<3269:POSATA>2.0.CO;2
Transport in 2.5°C temperature bins. Estimates from MSP mean profiles (heavy line) are compared with STACS results (light line). Agreement improves if one-half of the 8.6-Sv peak of STACS is reassigned to the warmest bin, θ > 27°C (dashed line)
Citation: Journal of Physical Oceanography 32, 11; 10.1175/1520-0485(2002)032<3269:POSATA>2.0.CO;2
Boundary layer profiles from MSP drops (a) beneath the core and (b) near the east wall. The ranges of hydrography and velocity are larger in (a) than in (b)
Citation: Journal of Physical Oceanography 32, 11; 10.1175/1520-0485(2002)032<3269:POSATA>2.0.CO;2
Patterns of mixing and stratification, from station-mean profiles on 10-m grids: (a) diffusivity Kρ, homogeneous boundary layers excluded; (b) dissipation ε10; (c) squared buoyancy frequency
Citation: Journal of Physical Oceanography 32, 11; 10.1175/1520-0485(2002)032<3269:POSATA>2.0.CO;2
Patterns of 10-m first-differenced shear (east and north components combined): log10 contours of (a) mean squared S̃2, (b) fluctuating variance
Citation: Journal of Physical Oceanography 32, 11; 10.1175/1520-0485(2002)032<3269:POSATA>2.0.CO;2
Patterns of 10-m first-differenced Froude numbers, exclusive of homogeneous boundary layers: linear contours of (a) mean squared
Citation: Journal of Physical Oceanography 32, 11; 10.1175/1520-0485(2002)032<3269:POSATA>2.0.CO;2
Patterns for ratios of fluctuating shear components, summed from 0.02 to 0.07 cpm for 100-m spectral windows centered every 50 m (starting at 75 m)
Citation: Journal of Physical Oceanography 32, 11; 10.1175/1520-0485(2002)032<3269:POSATA>2.0.CO;2
Patterns for ratios of observed-to-scaled ε averaged or computed within 100-m windows centered every 50 m (from 75 m onward). (top) Comparison of fluctuating shear, via the spectral scaling of Polzin et al. (1995) given Rω = 3 for GM76 frequency content. (bottom) Comparison of total shear, via the scaling of Gregg (1989) computed with
Citation: Journal of Physical Oceanography 32, 11; 10.1175/1520-0485(2002)032<3269:POSATA>2.0.CO;2
Mixing regimes with boundaries of analysis intervals indicated. Regimes are outlined, with symbols marking interval midpoints: “N” for the interior (green), “C” for the current core (blue), “S” for the zones of high mean shear (red), and “E” for the east channel wall (brown). Slanted symbols deep at stations 3 and 7 indicate TSBLs. For stations 1, 2, and 7, first-occupation intervals are left of second-occupation ones
Citation: Journal of Physical Oceanography 32, 11; 10.1175/1520-0485(2002)032<3269:POSATA>2.0.CO;2
Interval statistics, vertically averaged from 10-m mean profiles, plotted with symbols from Fig. 14. (top) Mean shear squared vs buoyancy frequency squared; diagonals mark constant Fr2 =
Citation: Journal of Physical Oceanography 32, 11; 10.1175/1520-0485(2002)032<3269:POSATA>2.0.CO;2
Ratios of observed to scaled interval-averaged dissipation rates, plotted against interval-averaged
Citation: Journal of Physical Oceanography 32, 11; 10.1175/1520-0485(2002)032<3269:POSATA>2.0.CO;2
Comparisons of interval shear components, plotted against total variance
Citation: Journal of Physical Oceanography 32, 11; 10.1175/1520-0485(2002)032<3269:POSATA>2.0.CO;2
Transport per θ–S bins of 1°C by 0.1 ppt. Magnitude is proportional to line length, and mixing regime contributions are distinguished by colors, as indicated. Bin centers are marked by dots of transport-relative size
Citation: Journal of Physical Oceanography 32, 11; 10.1175/1520-0485(2002)032<3269:POSATA>2.0.CO;2
Contours of transport and area-averaged Kρ within bins of 2.5°C by 10 km. Contours for transport are heavy lines at intervals of 2 × 105 m3 s−1. Contours for Kρ change color every quarter decade, as shown by the colorbar. Only stratified water cooler than 27°C is included. Shading at bottom masks zero-transport bins
Citation: Journal of Physical Oceanography 32, 11; 10.1175/1520-0485(2002)032<3269:POSATA>2.0.CO;2
MSP Stations along 27°N
Regime statistics: averages or summations over each interval's bounds
Regime transports and diffusivities by temperature range
