Search Results

You are looking at 1 - 10 of 36 items for

  • Author or Editor: D. Randolph Watts x
  • Refine by Access: All Content x
Clear All Modify Search
Stephan D. Howden
and
D. Randolph Watts

Abstract

Mesoscale alongstream speed changes of the Gulf Stream are diagnosed from an array of current meters at depths 400, 700, and 1000 m, near 68°W, during the development of steep [ratio of “amplitude” to “wavelength” O ] meanders. Speed maxima (jet streaks) are generally found between trough and crest axes in steep meanders with local speed minima near the trough and crest axes. Speed changes along streamlines can be quite dramatic. Speed changes along the jet axis, between jet streaks and local minima in excess of 0.60, 0.40, and 0.35 m s−1, are observed at depth 400, 700, and 1000 m, respectively. This is in comparison with peak speeds in a frontal coordinates system mean of 1.22, 0.67, and 0.28 m s−1, at depth 400, 700, and 1000 m, respectively, from a previous study.

The presence of the jet streaks can be explained kinematically as a superposition of the Gulf Stream and barotropic vortices. The development of these jet streaks in relation to the developing steep meanders differs from the canonical picture of jet streak/baroclinic wave development in the atmospheric jet stream in that the jet streaks in the Gulf Stream are predominantly fixed in place with respect to meanders as they steepen.

Full access
Che Sun
and
D. Randolph Watts

Abstract

A streamfunction EOF method is developed to identify long-term thermohaline variations within a strong baroclinic current. The temporal variability associated with meandering fronts and mesoscale eddies is removed by projecting hydrographic data into a baroclinic streamfunction space. The residual field, after removing the streamfunction mean field, is analyzed to find empirical orthogonal functions.

The method is applied to a time series of hydrographic sections across the Antarctic Circumpolar Current (ACC) south of Australia. The temperature variation in the surface layer (0–300 dbar) is dominated by a seasonal signal. In the subsurface water (300–3000 dbar), a separately calculated first EOF mode dominates the thermohaline variation and exhibits two phases. In the strengthening phase both salinity and temperature in the Subantarctic Mode Water increase and the ACC section is characterized by less Antarctic Intermediate Water and higher salinity at the core of the Circumpolar Deep Water. The water masses vary conversely in the relaxing phase. The authors call this mode the ACC pulsation mode and hypothesize that it is related to the ACC barotropic transport and is a response to the large-scale wind stress variation. Observations of westerly winds and ACC transport appear to support the hypothesis as they all display semiannual periods nearly in phase with higher coherence to the south.

Full access
Stuart P. Bishop
and
D. Randolph Watts

Abstract

From 2004 to 2006 an observational array of current- and pressure-recording inverted echo sounders (CPIES) were deployed as part of the Kuroshio Extension (KEx) System Study (KESS). KESS observed a transition from a weakly meandering (“stable”) to strongly meandering (“unstable”) state (Qiu and Chen). As the KEx made this transition, potential vorticity (PV) observed within the southern recirculation gyre (SRG) rapidly increased from January to July 2005. In this study, the authors diagnose eddy PV fluxes (EPVFs) in isentropic coordinates within the subtropical mode water (STMW) layer from the CPIES data to determine the role of mesoscale eddies in this rapid increase of PV.

The rapid increase in PV within the SRG coincided with enhanced cross-front EPVFs and eddy PV flux convergence upstream of a mean trough in the KEx path and adjacent to the SRG. The enhanced cross-front EPVFs were the result of the formation of a cold-core ring (CCR) and the interaction of the jet with a preexisting CCR. Eddy diffusivities are diagnosed for the unstable regime with values that range from 100 to 2000 m2 s−1. The high eddy diffusivities during the unstable regime reflect the nature of mesoscale CCR formation and CCR–jet interaction as efficient mechanisms for stirring and mixing high PV waters from the north side of the KEx into the low PV waters of the SRG where STMW resides. This mechanism for cross-frontal exchange can explain observed increases in the STMW PV in the SRG over the 16 months of KESS observations.

Full access
Jae-Hun Park
and
D. Randolph Watts

Abstract

This paper investigates the internal tidal energy distribution in the southwestern Japan/East Sea using vertical round-trip travel time (τ) data from 23 pressure-sensor-equipped inverted echo sounders (PIES). The τ records are analyzed by bandpass filtering to separate time-dependent variability of the semidiurnal and diurnal bands. The semidiurnal internal tides exhibit a horizontal beam pattern of high energy, propagating into the open basin. They originate from a restricted portion of the shelf break where the Korea Strait enters the Ulleung Basin. The generation appears to occur at ∼200-m water depth near 35.5°–35.7°N and 130°–131°E, where the slope of bottom topography matches that of the wave characteristics, coinciding with the location where the semidiurnal barotropic cross-slope tidal currents are strongest. Maximum vertical displacement of the thermocline interpreted as a long-wave first baroclinic mode from the measured τ is about 25 m near the generation region. Annual and monthly variations of the propagation patterns and generation energy levels are observed, and these are closely associated with changes in the mesoscale circulation and stratification. Eastward (westward) refraction is observed when a warm (cold) eddy crosses the path of internal tide propagation. Moreover, when the generation region is invaded by cold eddies that spoil the match between shelf break and thermocline depth, the internal tidal energy level decreases by a factor of about 2. A simple geometric optics model is proposed to explain the observed horizontal refraction of the beam of semidiurnal internal tides in which stratification and current shear play essential roles. In contrast, diurnal internal tides are observed to be trapped along the continental slope region around 36°N.

Full access
Robert S. Pickart
and
D. Randolph Watts

Abstract

The methodology for converting the travel time measurement of the inverted echo sounder (IES) into an amplitude of the first baroclinic dynamical mode, A 1, is presented. For a Gulf Stream IES record the so-generated A 1(t) time series is used to compute a vertical profile of first mode temperature versus time by perturbing a basic state temperature profile. The basic state is constructed by averaging together historical CTD data collected near the IES site. Similarly the first mode amplitudes are used to perturb a basic state dynamic height profile, and, using neighboring IESs, a profile of alongstream geostrophic velocity is obtained at the same location. The resulting IES-derived temperatures and velocities compare favorably to independent current meter results, exhibiting most of the variability observed in both the current meter temperature and alongstream velocity.

Full access
D. Randolph Watts
and
Harilaos Kontoyiannis

Abstract

Sixteen records from seven Digiquartz deep-ocean bottom pressure sensors have been in deployments of 3–12 month duration under the Gulf Stream in depths of 3300 to 4400 m. Particular attention is given (i) to characterizing any observed drift in their calibration in relation to their construction (bellows or Bourdon-tube) and to their prior history of pressurization, and (ii) to estimating and removing this drift from the records. Bellows-type sensors exhibited significant drift (0.2 to 0.85 db) in all of their deployments. Bourdon-tube sensors had less drift in their first deployment (0 to 0.45 db), and in subsequent deployments had either no drift or a small drift with different shape that may have been due to clock-frequency drift. An exponential decay with time [∼exp(−αt)] was found to best represent the drifts, such a curve was fit in a least-squares sense to each pressure record and then subtracted from it Careful attention is given to estimating the uncertainty of the residual “dedrifted” records, which is 0.02 dh for records that are at least a year long; the stability over a few days and resolution of these measurements is better than 0.001 db. As a consistency cheer, neighboring pairs of bottom pressure records are used to calculate geostrophic currents from their differences, and the comparison with directly observed currents confirms that the error in drift removal may be less than 0.02 db. Typical amplitudes of the deep-ocean tidal- and derided signals are respectively 0.7 and 0.13 db in this region, so that we infer that this methodology is suitable for studies requiring knowledge of deep-ocean dynamic pressures even for subtidal mesoscale periodicites.

Full access
Christopher S. Meinen
and
D. Randolph Watts

Abstract

The addition of an accurate pressure sensor to the inverted echo sounder (IES) has allowed for the development of a new method for calibrating the IES’s acoustic travel-time record without the need for coincident conductivity–temperature–depth (CTD) or expendable bathythermograph profiles. Using this method, the round-trip travel-time measurement of the IES can be calibrated into various dynamic quantities with better accuracy than was possible with previous methods. For a set of four IES records from the Newfoundland Basin, the estimate of the accuracy of the geopotential height anomaly (integrated between 100 and 4000 db) calibrated from the IES measurements was reduced from 0.65 to 0.52 m2 s−2, which is a substantial reduction toward the intrinsic scatter of the geopotential height anomaly versus travel-time relationship for this region (0.42 m2 s−2). The addition of the pressure sensor to the IES results in reduced errors and eliminates the need for coincident CTD measurements. Moreover, the pressure sensor provides a complementary dataset recording the changes of the barotropic pressure field.

Full access
D. Randolph Watts
and
H. Thomas Rossby

Abstract

Inverted Echo Sounders (IES) were deployed during MODE at seven ocean bottom stations to acoustically monitor depth variations of the main thermocline. The IES transmits pulses of 10 kHZ sound and records the time τ for the echo to return from the ocean surface; τ varies by a few milliseconds in response to vertical displacements of the temperature and salinity profiles in the water column. The acoustic travel time is inherently an integral measurement, which is insensitive to finestructure in the vertical but is dominantly influenced by vertical displacements which are coherent throughout the water column. Thus the IES performs as a natural “matched filter” for the most fundamental internal displacement mode. A perturbation analysis on the dynamic height (D), the total heat content (Q) and the acoustic travel time (τ) integrals shows that all three are dominated by displacements of the main thermocline. The proportionality is unique when a single mode of internal displacements is dominant.

Comparisons with MODE hydrographic data near each instrument show that the measured travel times may be rescaled into dynamic height (ΔD) records with an uncertainty of only ±1 dyn cm, which is comparable to the best of hydrographic measurements. Time series of τ show that internal waves on the main thermocline in this “mid-ocean” location have larger amplitude than is generally appreciated: ΔD can change by 2–3 dyn cm in 2–3 h, thereby aliasing a measurement taken at a single instant in time. Differences between the low-pass filtered IES dynamic height records from pairs of sites are compared, via the thermal wind relationship, with the observed current shear across the main thermocline, as determined from current meter and SOFAR float records; the agreement is good within the limitations imposed an estimating the current streamfunctions from a sparse network of current meters. Thus the IES records can be used to extend the mapping of the baroclinic velocity field.

Full access
Hyun-Sook Kim
and
D. Randolph Watts

Abstract

Contours of the main thermocline (12°C isotherm depth: Z12) topography objectively generated from Inverted Echo Sounder observations in the Gulf Stream may be treated as a baroclinic, geostrophic streamfunction ψ. As preliminary steps, the authors developed techniques to estimate geostrophic velocity V ψ = k×(g */f)∇Z12 and geostrophic vorticity ζψ =(g */f)∇2Z12. In doing this the authors also determined a reduced gravity g * = 1.53 cm s−2 by least-squares fitting the estimated V ψ to observed V bc , velocities (i.e., fine-tuned for the ψ at 400 m relative to 3000 m).

Accuracy of the objective maps of ψ is investigated by comparing V ψ, against V bc directly measured from tall current meter moorings and ζψ against vorticity ζ p separately estimated using a “rigid-stream” velocity section and path curvature. The V ψ, values, with speeds up to 100 cm s−1, are correlated with V bc at the 99% confidence level, and the two measurements differ by only 10 cm s−1 rms error. Vorticity estimates, ranging between −0.44f and +0.64f, also show excellent agreement between ζψ and ζ p within 0.77 × 10−5 s−1 (or 0.09f) rms difference. This study carefully documents the objective error-estimation techniques for these fields that, bemuse they are derivatives of the measured ψ field, are sensitive to noise. The objective estimates of error in V ψ and ζψ agree in each case with the rms differences from observations.

The authors also illustrate the consequent utility of the objective ψ maps by applying quasigeostrophic calculations to diagnose ageostrophic motions Va , in strong events in the Gulf Stream. The authors found Va /V ψ to have peak values of 0.2–0.6. Roughly equal contributions to Va came in events of large curvature or with high temporal rate of exchange. Vertical stretching ∂w/∂z at the 400-m level was estimated using the quasigeostrophic vorticity equation, finding downwelling as large as −4 mm s−1 downstream of meander crests, and upwelling as large as 3 mm s−1 downstream of meander troughs.

Full access
Kathryn A. Kelly
and
D. Randolph Watts

Abstract

Profiles of the Gulf Stream sea surface height (SSH) from the Geosat altimeter were compared with thermocline depth measurements from inverted echo sounders (IES) for a period of about 19 months. The correlation between SSH anomalies and thermocline depth anomalies at about 68°W was 0.92. To distinguish between agreement in the location of the Gulf Stream jet and agreement in the strength of the jet, separate comparisons were made for jet location and the SSH difference across the Gulf Stream. Based on an error function fit to each Geosat and IES profile, the correlation between the jet centers from the two types of measurements was 0.97, and the correlation between the amplitudes of the error functions was 0.76. Although the altimeter measures surface geostrophic velocity, the high degree of vertical correlation in the Gulf Stream velocity profiles allows the altimeter to monitor transport fluctuations, as well as changes in jet position. Based on this assessment of the accuracy of the altimeter, the authors suggest that a correlation between the height difference across the Gulf Stream and the jet position between 48° and 65°W reflects large-scale changes in the structure of the recirculation gyres.

Full access