Abstract

During the third intensive observational period of the Surface Wave Dynamics Experiment (SWADE), an aircraft-based experiment was conducted on 5 March 1991 by deploying slow-fall airborne expendable current profilers (AXCPs) and airborne expendable bathythermographs (AXBTs) during a scanning radar altimeter (SRA) flight on the NASA NP-3A research aircraft. As the Gulf Stream moved into the SWADE domain in late February, maximum upper-layer currents of 1.98 m s⁻¹ were observed in the core of the baroclinic jet where the vertical current shears were O(10⁻² s⁻¹). The SRA concurrently measured the sea surface topography, which was transformed into two-dimensional directional wave spectra at 5–6-km intervals along the flight tracks. The wave spectra indicated a local wave field with wavelengths of 40–60 m propagating southward between 120° and 180°, and a northward-moving swell field from 300° to 70° associated with significant wave heights of 2–4 m.

As the AXCP descended through the upper ocean, the profiler sensed orbital velocity amplitudes of 0.2–0.5 m s⁻¹ due to low-frequency surface waves. These orbital velocities were isolated by fitting the observed current profiles to the three-layer model based on a monochromatic surface wave, including the steady and current shear terms within each layer. The depth-integrated differences between the observed and modeled velocity profiles were typically less than 3 cm s⁻¹. For 17 of the 21 AXCP drop sites, the rms orbital velocity amplitudes, estimated by integrating the wave spectra over direction and frequency, were correlated at a level of 0.61 with those derived from the current profiles. The direction of wave propagation inferred from the AXCP-derived orbital velocities was in the same direction observed by the SRA. These mean wave directions were highly correlated (0.87) and differed only by about 5°.

Abstract

To assess the spatial structures and temporal evolutions of distinct physical processes on the West Florida Shelf, patterns of ocean current variability are extracted from a joint HF radar and ADCP dataset acquired from August to September 2003 using Self-Organizing Map (SOM) analyses. Three separate ocean–atmosphere frequency bands are considered: semidiurnal, diurnal, and subtidal. The currents in the semidiurnal band are relatively homogeneous in space, barotropic, clockwise polarized, and have a neap-spring modulation consistent with semidiurnal tides. The currents in the diurnal band are less homogeneous, more baroclinic, and clockwise polarized, consistent with a combination of diurnal tides and near-inertial oscillations. The currents in the subtidal frequency band are stronger and with more complex patterns consistent with wind and buoyancy forcing. The SOM is shown to be a useful technique for extracting ocean current patterns with dynamically distinctive spatial and temporal structures sampled by HF radar and supporting in situ measurements.

Abstract

Over a 29-day time series in July 1999, an ocean surface current radar (OSCR) in very high frequency (VHF) mode mapped the surface velocity field at 250-m resolution at 700 cells off Fort Lauderdale, Florida. During the experiment, autonomous underwater vehicles (AUVs), equipped with upward- and downward-looking 1.2-MHz acoustic Doppler current profilers (ADCPs), measured subsurface current structure over four to six radar cells during two mixed layer patterns on 9 and 27 July 1999. As these AUV sampling patterns were conducted over 500 m × 500 m and 500 m × 750 m areas, these missions required about 80–90 min (four radar sample intervals) to form four and seven synoptic snapshots, respectively.

Based on autocorrelation analyses of the profiler data, along-AUV-track subsurface profiles were averaged at 10-s intervals, mapped to a surface from 1.5–6.5 m, and compared to surface currents at more than 500 points for each snapshot. Comparisons between the surface and subsurface currents from the AUV revealed spatially averaged differences ranging from 4 to 26 cm s⁻¹ during these two experiments. The largest differences occurred when the surface and subsurface current vectors were orthogonal; otherwise, differences were O(10 cm s⁻¹). Scatterplots between 2-m and radar-derived surface currents indicated a consistent relationship with mooring data. From the seven spatial snapshots acquired during the second experiment, current profiles suggested a time-dependent oscillation that was corroborated by radar and moored ADCP data. Least squares fits of these profiles from sequential AUV snapshots to a simple model isolated an ∼9.2 ± 1 h oscillation where the along-shelf current was O(50 cm s⁻¹).

Spatially averaged current profiles from four and seven snapshots were subsequently time averaged to form a mean profile from each experiment. In the downwind directions, these mean profiles were compared to a wind-driven, logarithmic layer profile in the upper 6.5 m based on a 10-m surface winds. Regression analyses suggest a slope of ≈1.16 between the theoretical and observed mean profiles with a bias of about 3 cm s⁻¹. In this context, the averaged winds played a role in driving the coastal ocean circulation. These results further suggest that the spatial averaging by the radar is consistent when subsurface current variations are averaged over similar time and space scales.

Abstract

A dual-station high-frequency Wellen Radar (WERA), transmitting at 16.045 MHz, was deployed along the west Florida shelf in phased array mode during the summer of 2003. A 33-day, continuous time series of radial and vector surface current fields was acquired starting on 23 August ending 25 September 2003. Over a 30-min sample interval, WERA mapped coastal ocean currents over an ≈40 km × 80 km footprint with a 1.2-km horizontal resolution. A total of 1628 snapshots of the vector surface currents was acquired, with only 70 samples (4.3%) missing from the vector time series. Comparisons to subsurface measurements from two moored acoustic Doppler current profilers revealed RMS differences of 1 to 5 cm s⁻¹ for both radial and Cartesian current components. Regression analyses indicated slopes close to unity with small biases between surface and subsurface measurements at 4-m depth in the east–west (u) and north–south (υ) components, respectively. Vector correlation coefficients were 0.9 with complex phases of −3° and 5° at EC4 (20-m isobath) and NA2 (25-m isobath) moorings, respectively.

Complex surface circulation patterns were observed that included tidal and wind-driven currents over the west Florida shelf. Tidal current amplitudes were 4 to 5 cm s⁻¹ for the diurnal and semidiurnal constituents. Vertical structure of these tidal currents indicated that the semidiurnal components were predominantly barotropic whereas diurnal tidal currents had more of a baroclinic component. Tidal currents were removed from the observed current time series and were compared to the 10-m adjusted winds at a surface mooring. Based on these time series comparisons, regression slopes were 0.02 to 0.03 in the east–west and north–south directions, respectively. During Tropical Storm Henri’s passage on 5 September 2003, cyclonically rotating surface winds forced surface velocities of more than 35 cm s⁻¹ as Henri made landfall north of Tampa Bay, Florida. These results suggest that the WERA measured the surface velocity well under weak to tropical storm wind conditions.

Abstract

Ocean surface current measurements from high-frequency (HF) radar are assessed by comparing these data to near-surface current observations from 1 to 30 October 1994 at two moored subsurface current meter arrays (20 and 25 m) instrumented with vector-measuring current meters (VMCMs) and Seacat sensors during the Duck94 experiment. A dual-station ocean surface current radar (OSCR) mapped the current fields at 20-min intervals at a horizontal resolution of 1.2 km over a 25 km × 44 km domain using the HF (25.4 MHz) mode and directly overlooked these moorings. In response to wind, tidal, and buoyancy forcing over 29 days, surface current observations were acquired 95% of the time in the core of the OSCR domain, decreasing to levels of about 50% in the offshore direction.

Regression analyses between surface and subsurface measurements at 4 and 6 m indicated biases of 2–6 cm s⁻¹, slopes of O(1), and rms differences of 7–9 cm s⁻¹. Episodic freshwater intrusions of about 30 practical salinity units (psu) were associated with a coastally trapped buoyant jet superposed on tidal currents. This tidal forcing consisted of diurnal (K₁) and semidiurnal (M₂) tidal constituents where the surface and subsurface (4 m) speeds were 3 and 8 cm s⁻¹, and 2 and 7 cm s⁻¹, respectively. During the passage of a nor’easter, near-surface winds reached 14 m s⁻¹, which induced vertical mixing that caused weak stratification in the water column. An abrupt wind change following this event excited near-inertial (≈20.3 h) currents with amplitudes of about 20 cm s⁻¹ rotating clockwise with time and depth. Bulk current shears over 4- and 6-m layers were O(10⁻² s⁻¹) at the 25-m mooring where the correlation coefficients exceeded 0.8. Similar results were found at the 20-m mooring until the nor’easter when correlation coefficients decreased to 0.5 due to the superposition of storm-induced flows and the buoyant jet, causing the surface current to exceed 90 cm s⁻¹ over the inner to midshelf.

Abstract

A Systematically Merged Pacific Ocean Regional Temperature and Salinity (SPORTS) climatology was created to estimate ocean heat content (OHC) for tropical cyclone (TC) intensity forecasting and other applications. A technique similar to the creation of the Systematically Merged Atlantic Regional Temperature and Salinity (SMARTS) climatology was used to blend temperature and salinity fields from the Generalized Digital Environment Model and World Ocean Atlas 2001 at a 0.25° resolution. The weights for the blending of these two climatologies were estimated by minimizing residual covariances across the basin. Drift velocities associated with eddy variability were accounted for using a series of 3-yr sea surface height anomalies (SSHA) to ensure continuity between the periods of different altimeters. In addition to producing daily estimates of the 20° and 26°C isotherm depths, mixed-layer depth, and OHC, the model produces mapping errors from the optimal interpolation of the SSHA due to gaps in altimeter track coverage and sensor uncertainties.

Using SPORTS with satellite-derived sea surface temperature (SST) and SSHA fields from radar altimetry, daily OHC was estimated from 2000 to 2011 using a 2.5-layer model approach. Argo profiling floats, expendable probes from ships and aircraft, long-term Tropical Atmosphere Ocean (TAO) moorings, and drifters provide more than 267 000 quality controlled in situ thermal profiles to assess uncertainty in estimates from SPORTS. This carefully constructed climatology creates an accurate estimation of OHC from satellite-based measurements, which can then be used in TC intensity forecasts in the North Pacific Ocean and analysis of ocean thermodynamics. The SPORTS time and space series extends from 1998 to 2016, forming a 19-yr dataset by the end of 2016.

Abstract

Three drifting buoys were successfully air-dropped ahead of Hurricane Josephine. This deployment resulted in detailed simultaneous measurements of surface wind speed, surface pressure and subsurface ocean temperature during and subsequent to storm passage. This represents the first time that such a self-consistent data set of surface conditions within a tropical cyclone has been collected. Subsequent NOAA research overflights of the buoys, as part of a hurricane planetary boundary-layer experiment, showed that aircraft wind speeds, extrapolated to the 20 m level, agreed to within ±2 m s⁻¹, pressures agreed to within ±1 mb, and sea-surface temperatures agreed to within ±0.8°C of the buoy values. Ratios of buoy peak 1 min wind (sustained wind) to one-half h mean wind > 1.3 were found to coincide with eyewall and principal rainband features.

Buoy trajectories and subsurface temperature measurements revealed the existence of a series of mesoscale eddies in the subtropical front. Buoy data revealed storm-generated, inertia-gravity-wave motions superposed upon mean current fields, which reached a maximum surface speed > 1.2 m s⁻¹ immediately following storm passage. A maximum mixed-layer-temperature decrease of 1.8°C was observed to the right of the storm path. A temperature increase of 3.5°C at 100 m and subsequent decrease of 4.8°C following storm passage indicated a combination of turbulent mixing, upwelling and horizontal advection processes.

Abstract

The quality and vertical correlation scales of high-frequency (HF) radar-derived ocean surface current measurements from an ocean surface current radar (OSCR) are assessed by comparing surface to subsurface current observations from 11 June to 8 July 1993 at directional discus buoys D_W and D_E, each instrumented with a three-axis ultrasonic current meter at the 13.8- and 9.5-m depths, respectively. A dual-station OSCR mapped the current fields at 20-min intervals at a horizontal resolution of 1.2 km over a 30 km × 44 km domain inshore of the Gulf Stream using the HF (25.4 MHz) mode. Over a 27-day experimental period, surface current observations were acquired 97% of the time extending to the maximum theoretical range of 44 km. Linear regression analyses indicated a bias of 2–4 cm s⁻¹ and slopes of O(1). While there were periods when the daily averaged complex correlation coefficients were highly correlated (>0.8), periods of low correlation (<0.3) are explained in terms of vertical phase differences and a decoupling between surface and subsurface records.

Surface and subsurface current time series at the two mooring sites were decomposed into the tidal, mean (>48 h), near-inertial (20.7 h), and high-frequency (4.5 h) bands. Tidal analyses, based on the semidiurnal (K ₂, M ₂, L ₂, S ₂) and diurnal (K ₁, O ₁, P ₁, Q ₁) constituents, indicated maximum amplitudes of 5 cm s⁻¹ at D_W, whereas these amplitudes increased offshore to a maximum of 13 cm s⁻¹ at D_E. Net differences between the surface and subsurface tidal currents ranged between 2 and 5 cm s⁻¹ with the largest difference of 7.7 cm s⁻¹ for the K ₁, constituent at D_E. The tidal currents were removed from the surface and subsurface current time series and low-pass filtered at 48 h, bandpass filtered between 18 and 23 h, and high-pass filtered at 8 h. The mean current components were highly correlated (>0.9) over most of the record with small phase differences. Intrusions of the mean flow at 3–5-day intervals were correlated with bursts of near-inertial motions having amplitudes of 20 cm s⁻¹ at D_E and 15 cm s⁻¹ at D_W. The frequency of these motions was shifted 5%–10% above and below fduring these episodes of mean flow intrusions. The higher-frequency surface motions with amplitudes of 5–8 cm s⁻¹ oscillated at periods of 4.3–4.7 h but were directly out of phase with the subsurface currents, which caused the correlations to decrease below 0.3. Thus, temporal decorrelations appeared to be a result of high-frequency motions in the internal wave band between the inertial and Nyquist (1.5 cph) frequencies.

Abstract

Bathymetry, current, temperature, and depth (CTD) measurements using a small, mobile, autonomous underwater vehicle (AUV) platform are described. Autonomous surveys of a shallow water column off the east coast of Florida during December 1997 were carried out using a 2.13-m long, 0.53-m maximum diameter Ocean Explorer series AUV, equipped with a 1200-kHz acoustic Doppler current profiler (ADCP) and a CDT package. At a speed of 1–2 m s⁻¹, this AUV can perform preprogrammed missions over a period of several hours, collecting in situ oceanographic data and storing it on an onboard datalogger. The vehicle may also carry side-scan sonar or a custom small-scale turbulence measurement package or other instruments for subsidiary measurements. The versatility of the AUV allows measurement of oceanographic data over a substantial region, the motion of the platform being largely decoupled from that of any surface mother ship.

In the missions of 5 and 11 December 1997, “lawn mower pattern” AUV surveys were conducted over 1 km² regions on the east coast of Florida, north of Fort Lauderdale, at depths of 7 and 3 m, respectively, in a water column where the depth ranged from 10 to 32 m. During 5 December, the region was subjected to a cold front from the northwest. Local wind measurements show presence of up to 10 m s⁻¹ winds at temperatures of up to 10°–15°C below normal for the time of the year. The fixed ADCP indicates occurrence of significant internal wave activity in the region. The data collected using the mobile AUV are utilized to develop a map of the bottom topography and examine current, temperature, and density variations in the context of the background information from a fixed bottom–mounted ADCP and Coastal-Marine Automated Network buoys. The work described here is a significant step in the development of an autonomous oceanographic sampling network, illustrating the versatility of an AUV platform. The data collected during the missions described will form part of a bank for information on the impact of a cold front on shallow subtropical waters. The authors expect to repeat the missions during other such fronts.

Abstract

Empirical orthogonal function (EOF) analysis has been widely used in meteorology and oceanography to extract dominant modes of behavior in scalar and vector datasets. For analysis of two-dimensional vector fields, such as surface winds or currents, use of the complex EOF method has become widespread. In the present paper, this method is compared with a real-vector EOF method that apparently has previously been unused for current or wind fields in oceanography or meteorology. It is shown that these two methods differ primarily with respect to the concept of optimal representation. Further, the real-vector analysis can easily be extended to three-dimensional vector fields, whereas the complex method cannot. To illustrate the differences between approaches, both methods are applied to Ocean Surface Current Radar data collected off Cape Hatteras, North Carolina, in June and July 1993. For this dataset, while the complex analysis “converges” in fewer modes, the real analysis is better able to isolate flows with wide cross-shelf structures such as tides.