Subtropical Atlantic Climate Studies Research Articles

Three-dimensional perspectives of the Florida current: transport, potential vorticity, and related dynamical properties

Abstract The Florida Current flows through the Straits of Florida, which starts as a zonal channel and turns to become a meridional channel. The spatial structure of the Florida Current and its transport, potential vorticity, and related dynamical properties are investigated using a three-dimensional, baroclinic, primitive equation model with a mesoscale-admitting (5.6 km) horizontal resolution and 25 vertical (sigma: terrain-following) levels. At 83°W, the Florida Current fills only a portion of the channel; however, due to the interaction with the shoaling bottom topography (from a maximum depth of over 2000 m at 83°W to less than 800 m at 27°N) and the narrowing Straits of Florida (from a maximum width of about 170 km at 83°W to about 110 km at 27°N), the Florida Current fills the entire channel at 27°N, and the potential vorticity distribution is altered. The specified transport of 28.6 Sverdrup (1 Sv = 10 6 m 3 s −1 ) from the Loop Current at the western boundary and the inflow from the Old Bahama Channel of 1.9 Sv converge into the meridional channel. With an additional inflow of 1.2 Sv from the Northwest Providence Channel, the simulated total transport of 31.8 Sv at 27°N is comparable to the STACS (Subtropical Atlantic Climate Studies) mean transport of 31.7 Sv. Both vertically and laterally integrated subsectional transports are examined at transects 83°W, 82°W, 81°W, 25°N, 26°N, and 27°N. The potential vorticity increases (decreases) on the cyclonic (anticyclonic) side of the Florida Current at 27°N compared to 83°W. The downstream variation of static stability, relative vorticity, and Froude number is also examined. While the vertical shear is strong only on the northern side at 83°W it is comparable on the both western and eastern sides downstream at 27°N, reaching to the bottom of the meridional channel. Large values of the Froude number exist only in the upper 300 m of the zonal channel, but they reach to the bottom of the meridional channel.

Statistical relationships between local sea level and weather with Florida‐Bahamas cable and Pegasus measurements of Florida Current volume transport

Straits of Florida sea level is studied as a measure of Florida Current volume transport because sea level provides an independent time series both (1) for detecting changes in the calibration of the Florida‐Bahamas submarine cable which has been operating since 1982, and (2) as a measure of the strength of the Gulf Stream System dating to the 1930s when cable measurements were not available. Accordingly, tide gauge records from Haulover Beach (Miami) and Lake Worth, Florida, and from Cat Cay and Settlement Point, The Bahamas, together with Miami weather and cable voltage, are correlated with each other and with discrete volume transport estimated from all Pegasus data taken during the intensive 1982–1984 Subtropical Atlantic Climate Studies (STACS) observations. Time domain linear correlation coefficients between these 132 Pegasus values and cable voltage observations, Cat Cay minus Haulover Beach sea level, and Haulover Beach sea level only, are 0.91, 0.55, and −0.76, respectively, but for the 1982–1988 time series, Cat Cay minus Haulover Beach is better correlated with cable observations (0.62) than is Haulover Beach alone (−0.44). Frequency domain modeling computations reveal that the sea level and cable data are organized in such a way that most of their energy lies below frequencies of 30−1 cycles per day (cpd). In a subseasonal band (up to and including the semiannual frequency) 49% of the cable energy and 59% of the Haulover Beach energy can be accounted for using 30‐day low‐passed data. A subseasonal frequency response function has been tentatively identified that relates both Florida sea level and Bahamas‐Florida sea level difference to cable observations and can successfully account for more than 60% of the observed cable variance (standard error is ±1×106 m3 s−1). Further, negative Florida sea level only and Bahamas‐Florida sea level difference have different phase relationships with cable observations; it is suggested that steric effects could be responsible for the phase shifts.

On the Average Absolute Transport of the Deep Western Boundary Currents East of Abaco Island, the Bahamas

Abstract Data obtained from a line of absolute current profiler (PEGASUS) and CTD stations at 26.5°N, east of Abaco Island in the northern Bahamas, are used to estimate the average transport, and overall level of variability, of the deep (below 800 m) southward flow associated with the so-called “Deep Western Boundary Current”(DWBC) at this latitude. From April 1985 to September 1987 a total of 11 sections were made along a line extending from near the island boundary to 85 km offshore as part of the Subtropical Atlantic Climate Studies (STACS) program. In all but two cases (when PEGASUS data were obtained over the total depth), absolute velocities were determined from 0 to about 3000 db while CTD derived relative velocities were determined over the total depth. Using the combined datasets, it is possible to reference the relative geostrophic velocity with PEGASUS data, thereby obtaining an estimate of the absolute southward transport of the deep flow. The average value of transport determined in this man...

Topographic Modification of the Florida Current by Little Bahama and Great Bahama Banks

Abstract The effect of local topography in modifying the structure and variability of the Florida Current is examined using shipboard acoustic Doppler and PEGASUS acoustic current profiler data. PEGASUS absolute velocity data were obtained during 16 cruises in the Florida Current at 27°N as part of the Subtropical Atlantic Climate Studies (STACS) program. The ensemble average of all PEGASUS velocity data shows that the effect of the constriction imposed on the mean Florida Current by Little Bahama Bank can be detected up to 30 km into the Straits of Florida. A simple model is proposed to explain how this effect can produce the subsurface maximum of northward flow commonly observed in the eastern Straits. PEGASUS and acoustic Doppler data obtained during the March 1984 STACS cruise are ~used to describe the temporal and spatial variability of the flow. It is shown that intermittent southward flow can exist in a band 10–15 km wide off Little Bahama Bank; one such event was detected during this cruise. The P...

Changing transport and curvature bias: A new type of meandering in the Florida Straits

Meanders in strong, inertial flows are generally asymmetric. Florida Current meanders with periods of 7–10 days and flows that tend to meander offshore when strong and onshore when weak, which we call type A, have been widely documented. We find a countertrend in the Florida Current data collected under the aegis of Subtropical Atlantic Climate Studies: a type B meandering where flows tend to meander offshore when weak and onshore when strong. On time scales of several months and depending on the strength of the flow, the tendency displays a largely one‐sided preference for either the onshore or the offshore component of the meander. In particular, the data reveal a negative curvature bias during the generally higher transports in spring and summer when most of the speed cores were inshore, and a positive curvature bias during lower transports in fall and winter when most speed cores were offshore. The data show that any use of cross‐channel sea level slope to estimate the transport would generally overestimate the weaker, but underestimate the stronger flows. Type B meander is interpreted as one of three types embodied in a meandering equation in which curvature changes are controlled primarily by the Coriolis effect of downstream acceleration and hence by that of changing transport. In this light the bimodal paths of the Kuroshio south of Japan is a type B meander.

Meandering and Transport Variations of the Florida Current

Abstract Current meter observations were collected from a three-dimensional array moored in the Florida Straits between December 1983 and June 1984 as part of the Subtropical Atlantic Climate Studies (STACS) program. Approximately one-fourth of the total subinertial velocity and temperature variance contained in these records is associated with meandering of the Florida Current on time scales ranging from several days to a few weeks. There approach to be no strong correlation between the occurrence of meanders and variations in Florida Current volume transport or local wind forcing. Utilizing frequency-domain empirical mode analysis we find the most coherent, energetic meandering signals within two limited frequency bands centered near periods of 12 days and 5 days. These meanders propagate downstream (northward) with phase speeds and wavelengths of approximately (28 km d−1, 340 km) and (36 km d−1, 170 km) respectively. Periodic waveforms composed from these modes indicate an asymmetric meander pattern wi...

Structure and Variability of the Florida Current at 27°N: April 1982–July 1984

Abstract Results of a two-year field experiment as part of the SubTropical Atlantic Climate Studies (STACS) program in the Straits of Florida are presented. Temperature and absolute ocean current observations were obtained by PEGASUS acoustic current profilers over 16 cruises during which repeated cross sections of the Florida Current were made at 27°N. Results are shown for the mean velocity and temperature fields, the perturbation horizontal kinetic energy and potential energy fields and for those energy conversion terms that could be computed directly from the data. The barotropic and baroclinic energy conversion terms, although small, indicate that the flow is stable for both types of perturbations. A 1arge part of the variability is contributed by short time scales (one week or less). The average and standard deviation of northward volume transport by the Florida Current during these cruises was (31.7 ± 3.0) × 106 m3 s−1. Barotropic and baroclinic contributions to the total heat flux across the North...

Comparison of surface currents measured by HF Doppler radar in the western Florida Straits during November 1983 to January 1984 and Florida Current transports

An evaluation of surface currents measured by HF radar during November 29, 1983, to January 31, 1984, with radar sites at Jupiter and Stuart on the Florida east coast is carried out in comparison with currents and transports measured by moorings and submarine cable. While an earlier analysis of currents measured in summer 1983 with radars located at Palm Beach and Jupiter (Schott et al., 1985) found significant northward shear in the northward radar currents about 20 km offshore leading to concerns about a possible bias in the radar currents, this effect was not observed in the second application farther north. It is possible that the shear in the summer 1983 field might have been real and related to the topography in the southern part of the 1983 radar field where no intercomparison current data had been available. Concerning the usefulness of radar currents as Florida Current transport indicators, which was the prime intention of their application in the context of the Subtropical Atlantic Climate Studies, this second study finds much more encouraging results than the one based on the observations of summer 1983. While the first study was inconclusive because only small transport fluctuations occurred during the summer 1983 observation period, this second study finds significant correlation. Florida Current transport fluctuations had a total range of 15 × 106 m3/s during the second observation period, and correlation with downstream radar currents, averaged zonally across the center of the radar field, was 0.85. Coherence was significant for periods longer than 5 days. Highest correlation with transport was found for radar currents farthest out, to the right of the axis of the stream.

Subtropical Atlantic Climate Studies (STACS) revisited

The objectives of the Subtropical Atlantic Climate Studies (STACS) are to identify those processes that contribute most to the transport of heat in the North Atlantic, to develop the technology to monitor these processes, and to obtain a better understanding of the dynamics of the circulation of the North Atlantic [Molinari, 1983].

High‐frequency Doppler radar measurements of the Florida Current in summer 1983

An oceanographic evaluation is carried out here of high‐frequency (HF) Doppler radar measurements of surface currents made by the NOAA Wave Propagation Laboratory June 26 to August 4, 1983, at the western side of the Florida Straits in the area between Jupiter and West Palm Beach in the context of the Subtropical Atlantic Climate Studies. These current measurements are compared with direct current measurements made at various positions in the area covered by the radar, and investigated for their potential as transport indicator. Means and standard deviations of the downstream current component compared well with those from PEGASUS and subsurface moored current measurements carried out in the northern part of the radar current field up to 35 km distance from the coast, but there seemed to exist a bias in the southern part of the current field measured by the radar, causing significant northward mean shear about 20 km offshore. Low‐frequency current fluctuations from the radar currents and near‐surface moored currents were coherent for the downstream but not the small cross‐stream component. Mean downstream components in a center strip of the radar current field, where data quality was found to be best, were compared with Florida Current transports as determined by cable and by moored current measurements, but transport fluctuations were small during the measurement period which fell into the summer maximum of the Florida Current. Coherence with cable transports was significant at the meander time scale of 5 days, but a longer period transport burst of 3×106 m3/s occurring during the time period was not identified in the surface current measurements. In summary, the HF radar as used in this application is useful to determine near‐surface patterns of eddies and meanders but doubtful for derived quantities like energy fluxes and vorticity without additional calibration.

Florida Current: Seasonal and Interannual Variability

Annual and interannual variations in the Florida Current, Caribbean, and subtropical Atlantic are investigated with the use of historical sea level differences and wind field data. Observational and model evidence suggests that the seasonal transport cycle of the Florida Current is locally forced, either upstream in the Caribbean or downstream over topography. Although at seasonal and shorter periods sea level or bottom pressure fluctuations on the left side of the Florida Current contribute almost all of the variance of sea level difference across the Florida Straits and hence transport, this relation does not seem to apply at interannual time scales. Using results from the Subtropical Atlantic Climate Studies, it is estimated from historical sea level data that interannual transport fluctuations of the Florida Current are only of order 1 x 10(6) cubic meters per second. Interannual fluctuations in the 2- to 3-year period range in the Florida Straits seem to be correlated with sea level differences across the Caribbean and the subtropical Atlantic but not with Sverdrup transport fluctuations in the subtropical Atlantic.

STACS: Subtropical Atlantic climate studies

STACS: Subtropical Atlantic climate studies