Ocean velocity defines ocean circulation, yet the available observations of subsurface velocity are under-utilized by society. The first step to address these concerns is to improve visibility of and access to existing measurements, which include acoustic sampling from ships, subsurface float drifts, and measurements from autonomous vehicles. While multiple programs provide data publicly, the present difficulty in finding, understanding, and using these data hinder broader use by managers, the public, and other scientists. Creating links from centralized national archives to project specific websites is an easy but important way to improve data discoverability and access. A further step is to archive data in centralized databases, which increases usage by providing a common framework for disparate measurements. This requires consistent data standards and processing protocols for all types of velocity measurements. Central dissemination will also simplify the creation of derived products tailored to end user goals. Eventually, this common framework will aid managers and scientists in identifying regions that need more sampling and in identifying methods to fulfill those demands. Existing technologies are capable of improving spatial and temporal sampling, such as using ships of opportunity or from autonomous platforms like gliders, profiling floats, or Lagrangian floats. Future technological advances are needed to fill sampling gaps and increase data coverage.

The Florida Current (FC) contributes to Atlantic circulation by carrying the western boundary flow of the subtropical gyre and the upper branch of meridional overturning circulation. Repeated FC hydrographic (velocity, salinity, and temperature) sections during 1982–1987 and 2001–2015 characterize its water mass structure and associated transport variability. On average, FC volume transport comes from subtropical North Atlantic water (NAW, 44%), Antarctic Intermediate Water (AAIW, 14%), surface water (SW, 27%), and an indistinct source (Rem 15%), while salinity transport relative to the average salinity along 26°N comes from NAW (55%), AAIW (0.2%), SW (30%), and Rem (15%). From 1982–1987 to 2001–2015, NAW, AAIW, and Rem salinified by 0.03–0.16 g kg^-1 and increased the salinity anomaly transport by 3%. These patterns imply that advective salt transport by the FC (1) is sensitive to subtropical North Atlantic variability and (2) is partially decoupled from the volumetric pathway of the upper overturning branch.

The first continuous estimates of freshwater flux across 26.5°N are calculated using observations from the RAPID–MOCHA–Western Boundary Time Series (WBTS) and Argo floats every 10 days between April 2004 and October 2012. The mean plus or minus the standard deviation of the freshwater flux (FW) is –1.17 ± 0.20 Sv (1 Sv ≡ 10⁶ m³ s^-1; negative flux is southward), implying a freshwater divergence of –0.37 ± 0.20 Sv between the Bering Strait and 26.5°N. This is in the sense of an input of 0.37 Sv of freshwater into the ocean, consistent with a region where precipitation dominates over evaporation. The sign and the variability of the freshwater divergence are dominated by the overturning component (–0.78 ± 0.21 Sv). The horizontal component of the freshwater divergence is smaller, associated with little variability and positive (0.35 ± 0.04 Sv). A linear relationship, describing 91% of the variance, exists between the strength of the meridional overturning circulation (MOC) and the freshwater flux (–0.37 – 0.047 Sv of FW per Sverdrups of MOC). The time series of the residual to this relationship shows a small (0.02 Sv in 8.5 yr) but detectable decrease in the freshwater flux (i.e., an increase in the southward freshwater flux) for a given MOC strength. Historical analyses of observations at 24.5°N are consistent with a more negative freshwater divergence from –0.03 to –0.37 Sv since 1974. This change is associated with an increased southward freshwater flux at this latitude due to an increase in the Florida Straits salinity (and therefore the northward salinity flux).

The meridional overturning circulation (MOC) at 26.5°N in the Atlantic has a standard deviation of 4.9 Sv and contains large fluctuations at subannual periods. The geostrophic component of the MOC is believed to be influenced on subannual time scales by eddies and Rossby waves. To quantify this effect, the vertical structure and surface characteristics of westward propagating signals are studied using altimetric data and full-depth mooring measurements from the RAPID array at 26.5°N. Westward propagating features are observed in the western North Atlantic in both data sets and have periods of 80–250 days in the first baroclinic mode. These features are still observed by the RAPID moorings 20 km offshore of the western boundary. The western boundary also exhibits deep variability characterized by enhanced energy in higher baroclinic modes. The effect of eddies and Rossby waves on the geostrophic transport is quantified by representing their vertical structure with the first baroclinic mode. In total, 42% of the variance of the transbasin thermocline transport inferred from geostrophic calculations at 26.5°N can be attributed to first mode variability, which is associated with eddies and Rossby waves at periods of 80–250 days. The standard deviation of the transbasin thermocline transport due to eddies and Rossby waves is estimated to be 2.6 Sv.

A submarine cable across the Florida Straits yields a time series of volume and temperature transports using previously determined calibrations, and here a calibration is defined for salinity transport using data not yet compared to the cable. Since 2001, 32 transects were collected with conductivity-temperature-depth (CTDs) sensors and lowered acoustic Doppler current profilers (LADCPs). Calibrations for volume and temperature transports using CTD/LADCP data are consistent with previous studies. A salinity calibration is obtained by regressing salinity transport against volume transport, where salinity transport is calculated relative to the basin-averaged salinity at 26°N (S_ref = 35.156 psu). On average, the transect-derived salinity transport is 33.0 Sv psu (1 Sv ≡ 10⁶ m³ s^-1), has a standard deviation of 2.8 Sv psu, and has a 90th percentile range of 29.1–37.4 Sv psu. The cable-derived salinity transport has a root-mean-square error of 2.2 Sv psu compared to the CTD/LADCP transects. Inherent spatial fluctuations and their covariability in the Florida Straits are responsible for noise in the calibrations and for slight increases in accuracy from salinity to temperature to volume calibrations. Salinity fluctuations are strongest in middepth waters of intermediate salinity, where velocity is neither particularily fast nor variable. In contrast, temperature is highly stratified and warm near-surface waters coincide with fast and variable velocities. Temperature additionally exhibits seasonality near the surface, whereas no robust seasonality is found for salinity or velocity. Temperature and salinity transports are largely driven by volume transport, which in turn, because of a large average electrical conductivity, is closely related to the conductivity-weighted velocity that generates the cable-measured voltage.

A subsurface Lagrangian float that utilizes motional induction to calculate vertically averaged velocities was tested in the North Atlantic Current (NAC), taking advantage of existing cruises and infrastructure. The Electric Field Float (EFF) is a RAFOS float with horizontal electrodes that measures its own velocity by RAFOS tracking and calculates vertically averaged velocities when merged with the electrode system. The observations showed depth-averaged velocities that were fast in the core of the NAC (0.6 – 0.9 m s^-1) and moderate in adjacent recirculations and eddies (0.3 – 0.4 m s^-1). A float at 850 dbar moved at close to the depth-averaged velocity, while shallower floats followed surface intensified flow on top of the depth-averaged motion. Integral time scales of depth-averaged velocity (1.3 – 1.6 ± 0.4 d) are slightly shorter than time scales of float velocity (1.6 – 2.0 ± 0.3 d), while integral length scales of depth-averaged water velocity (35 ± 10 km for u, 18 ± 6 km for v) are slightly shorter than length scales of float motion (53 ± 12 km for u, 28 ± 6 km for v). Velocity spectra of depth-averaged velocity show significant variance at inertial periods. Quantitative and qualitative validation with multiple independent data sets confirms the accuracy of the instrument and sampling strategy in the NAC, advancing the limited observational knowledge of depth-averaged circulation in subpolar regions.

Hydrographic data from full-depth moorings maintained by the Rapid/-MOCHA project and spanning the Atlantic at 26° N are decomposed into vertical modes in order to give a dynamical framework for interpreting the observed fluctuations. Vertical modes at each mooring are fit to pressure perturbations using a Gauss-Markov inversion. Away from boundaries, the vertical structure is almost entirely described by the first baroclinic mode, as confirmed by high correlation between the original signal and reconstructions using only the first baroclinic mode. These first baroclinic motions are also highly coherent with altimetric sea surface height (SSH). Within a Rossby radius (45 km) of the western and eastern boundaries, however, the decomposition contains significant variance at higher modes, and there is a corresponding decrease in the agreement between SSH and either the original signal or the first baroclinic mode reconstruction. Compared to the full transport signal, transport fluctuations described by the first baroclinic mode represent <25 km of the variance within 10 km of the western boundary, in contrast to 60 km at other locations. This decrease occurs within a Rossby radius of the western boundary. At the eastern boundary, a linear combination of many baroclinic modes is required to explain the observed vertical density profile of the seasonal cycle, a result that is consistent with an oceanic response to wind-forcing being trapped to the eastern boundary.

Measuring oceanic electric fields to calculate water motion is a mature technique that has been proven operationally.

A case study across the Gulf Stream at Cape Hatteras demonstrates practical application of the technique.

The accuracy of the 1D theory is quantified for the effect of steep topography and horizontal velocity gradients.

The 1D approximation is almost entirely valid, and errors are small (1–3 cm/s) and correctable by an iterative method.

Motionally induced electric fields and electric currents in the ocean depend to first order solely on the vertical dimension. We investigate the significance of two-dimensional (2-D) perturbations that arise in the presence of sloping topography. The full electric response is calculated for a schematic geometry that contains a topographic slope, has a two-layer ocean with a layer of sediment beneath, and is described by five nondimensional parameters. When considered over the realistic ranges of topographic aspect ratio (the ratio of mean water depth to topographic width), topographic relief, sediment thickness, and sediment conductivity, velocity errors arising from 2-D perturbations are found to be less than a few percent of the dominant one-dimensional (1-D) signal. All errors depend on the topographic aspect ratio to the power of 1.9 and have linear dependence on topographic relief and the depth of the surface jet. Depth-uniform velocity errors are roughly proportional to the 1-D sediment conductance ratio, whereas depth-varying velocity errors are independent of sediment thickness or conductivity. Two-dimensional perturbations decay with a half width of 0.2–1 times the 1-D effective water depth. The magnitude of estimated errors is consistent with those found at a measurement location with strong 2-D perturbations. This study extends the first-order theory to the maximum expected aspect ratios for topography and finds small perturbations with simple dependencies. Overall, the 1-D approximation is found to be adequate for interpreting observations at all but the most extreme locations.

Motionally induced electric fields and electric currents in the ocean depend to first order solely on the vertical dimension. We investigate the significance of two-dimensional (2-D) perturbations that arise in the presence of horizontal velocity gradients. The full electric response is calculated for two schematic geometries that contain horizontal velocity gradients, have a two-layer ocean with a layer of sediment beneath, and are described by four nondimensional parameters. When considered over the realistic ranges of oceanic aspect ratio (the ratio of water depth to the width of velocity), sediment thickness, and sediment conductivity, velocity errors arising from 2-D perturbations are found to be less than a few percent of the dominant one-dimensional (1-D) signal. All errors depend on the aspect ratio to the power of 1.9 (1) for signals induced by the vertical (horizontal) component of the Earth's magnetic field. Depth-uniform velocity errors are proportional to the 1-D sediment conductance ratio, whereas depth-varying velocity errors are independent of sediment thickness or conductivity. Errors are weakly (proportionally) dependent on the jet depth for signals induced by the vertical (horizontal) component of the magnetic field. Two-dimensional perturbations decay away from the forcing region with a half width of 0.2–1 times the 1-D effective water depth. This study extends the first-order theory to the maximum expected aspect ratios for oceanic flow and finds small perturbations with simple dependencies on the nondimensional parameters.