No abstract

For more than 30 years, the volume transport of the Florida Current at 278N has been regularly estimated both via voltage measurements on a submarine cable and using ship-based measurements of horizontal velocity at nine historical stations across the Florida Straits. A comparison of three different observational systems is presented, including a detailed evaluation of observational accuracy and precision. The three systems examined are dropsonde (free-falling float), lowered acoustic Doppler current profiler (LADCP), and submarine cable. The accuracy of the Florida Current transport calculation from dropsonde sections, which can be determined from first principles with existing data, is shown to be 0.8 Sv (1 Sv [ 10 6 m 3 s 21 ). Side-by-side comparisons between dropsonde and LADCP measurements are used to show that the LADCP-based transport estimates are accurate to within 1.3 Sv. Dropsonde data are often used to set the absolute mean cable transport estimate, so some care is required in establishing the absolute accuracy of the cable measurements. Used together, the dropsonde and LADCP sections can be used to evaluate the absolute accuracy and precision of the cable measurements. These comparisons suggest the daily cable observations are accurate to within 1.7 Sv, and analysis of the decorrelation time scales for the errors suggests that annual transport averages from the cable are accurate to within 0.3 Sv. The implications of these accuracy estimates for long-term observation of the Florida Current are discussed in the context of maintaining this key climate record.

The Florida Current (FC) is the name given to the Gulf Stream as it passes through the Straits of Florida from the southernmost Florida Keys to the northernmost Bahamas Islands (Figure 1). At 27°N, the FC has a mean transport of about 32 Sv (1 Sv = 10 6 m 3 s −1 ; e.g., Larsen & Sanford, 1985), and essentially fills the entire water column from the east coast of Florida to the west coast of Grand Bahama Island (Figure 2a). The FC carries the majority of the upper-ocean northward transport of warm and saline waters in the subtropical North Atlantic at this latitude, and thus accounts for the bulk of both the upper limb of the Atlantic meridional overturning circulation and the western boundary component of the subtropical gyre circulation (e.g., C. S. Meinen et al., 2010

Observations from five different systems provide a robust picture of the structure and variability of the Antilles Current, an important contributor to the oceanic flux budget, at 26.5°N during 2005–2015. The analysis includes three direct measurement technologies (current meters, shipboard acoustic Doppler current profilers, and lowered acoustic Doppler current profilers) and two geostrophy‐based measurement technologies (conductivity‐temperature‐depth profilers and pressure‐equipped inverted echo sounders). The direct systems are shown to produce weaker, and less variable, Antilles Current transport estimates than the geostrophy‐based systems. The record‐length‐mean geostrophic estimate for the Antilles Current is 4.7 Sverdrups (Sv; 1 Sv = 106 m3/s), and the daily temporal standard deviation is 7.5 Sv. The variations of the Antilles Current transport exceed those of the entire basin‐wide meridional overturning circulation, illustrating the impact of this unusual current. Seasonal variability shows a maximum northward transport in August–September; however, the seasonal component of the variability is weak, and aliasing of higher frequencies is still a problem even with 10.5 years of data. The dominant time scales of variability in the spectra are at 70 and 180 days, and there is indication of westward propagation of Rossby Wave‐like features into the region at a speed of 9 cm/s. There is no significant correlation between the Antilles Current transport variations and those of the Florida Current at 27°N, in phase or at lags/leads of up to 5 years, likely reflecting the varying coastal wave/wall jet time scales for information to pass from the basin interior through the Bahamas Islands.