D. Randolph Watts

Vertical structure and transport on a transect across the North Atlantic Current near 42°N: Time series and mean

A new method is presented for combining historical hydrography with the acoustic travel time measurements from an inverted echo sounder (IES) to obtain time series of full water column profiles of temperature T and specific volume anomaly. Hydrography is used to demonstrate that profiles constructed from the IES measurement account for more than 90% of the T and δ variance through the main thermocline range. Horizontal gradients between neighboring pairs of instruments, when combined with measurements from bottom pressure sensors and deep current meters, provide accurate estimates of absolute geostrophic velocity profiles. A section of IESs and current meters across the North Atlantic Current (NAC) along World Ocean Circulation Experiment line ACM6 near 42.5°N is analyzed using these methods, and the Eulerian mean temperature and absolute velocity sections are calculated. The mean transport for the combined NAC and northward flow within the inshore portion of the Mann Eddy is 146±13 (× 106 m3 s−1). The temporal standard deviation of the northward transport is 41×106 m3 s−1, arising not only from changes in the transport of the NAC but also from lateral shifts of the currents and eddies captured by the fixed span of moorings. Assumption of a level of no motion at the bottom and at 2000 dbar underestimates the true absolute transport by 33 or 60%, respectively. Historical measurements indicate that 50–60×106 m3 s−1 circulates within the Mann Eddy adjacent to the North Atlantic Current, thus 86–96×106 m3 s−1 appears to be throughput at the ACM6 line. Because the throughput observed on the ACM6 line considerably exceeds the combined historical estimates of northward and eastward outflow from the Newfoundland Basin of ≈ 30×106 m3 s−1, these measurements suggest a substantial (≈ 50 × 106 m3 s−1) recirculation or southward flow within the Newfoundland Basin in addition to the historical recirculation within the Mann Eddy.

A Two-Dimensional Gravest Empirical Mode Determined from Hydrographic Observations in the Subantarctic Front

Abstract South of Australia, where the baroclinicity in the Subantarctic Front extends almost to the seafloor, the geopotential height of the sea surface (ϕ) and the vertical acoustic travel time (τ) exhibit a tight empirical relationship to each other and to the entire vertical structure of temperature T(p), salinity S(p), and specific volume anomaly δ(p). Measurements of τ provide proxy estimates of the profiles TG(τ, p), SG(τ, p), and δG(τ, p), based on a two-dimensional “gravest empirical mode” (GEM) representation of the vertical structure fitted to 212 hydrographic stations. A seasonal model was fitted in the near-surface layers using additional historical data. At each depth in the range 150–3000 dbar, more than 96% of the variance in both the T and δ fields is captured by the GEM representation. During the Subantarctic Flux and Dynamics Experiment (Mar 1995–Mar 1997), inverted echo sounders (IES) and current meters were moored in a 2D array along the WOCE SR3 transect south of Australia. At each I...

Measuring Dynamic Heights with Inverted Echo Sounders: Results from MODE

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. Compa...

Deep-Ocean Bottom Pressure Measurement: Drift Removal and Performance

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” ...

A circumpolar gravest empirical mode for the Southern Ocean hydrography

Historical hydrographic data from the Southern Ocean are projected into a baroclinic stream function space, generating a three-dimensional gravest empirical mode (GEM) parameterized by pressure, geopotential height, and longitude. The low-dimensional GEM projection is intrinsically steady, vertically coherent, and equivalent barotropic. The GEM fields capture more than 97% of the total density and temperature variance in the Antarctic Circumpolar Current region.

Eddy–Mean Flow Interaction in the Gulf Stream at 68°W. Part I: Eddy Energetics

Abstract From June 1988 to August 1990 twelve tall, high-performance, current meter moorings measured the Gulf Streams velocity and temperature fields at nominal depths of 400 m, 700 m, 1000 m, and 3500 m along three lines centered at 68°W. The overall eddy variability during the 26-month experiment was dominated by six large amplitude trough formation events, with each event lasting approximately one month. To determine the source(s) of energy for this eddy variability, the eddy energy budget is evaluated. Traditionally, the baroclinic conversion of mean potential energy to eddy potential energy is defined as a downgradient heat flux. However, because a nondivergent heat flux can be downgradient even under conditions in which there is no barolinic conversion occurring, a better, more dynamically correct definition of a baroclinic conversion rate is a downgradient horizontally divergent heat flux. The horizontally divergent heat flux component is estimated to be approximately half the full horizontal hea...

Observations of the Subtropical Mode Water Evolution from the Kuroshio Extension System Study

Abstract Properties and seasonal evolution of North Pacific Ocean subtropical mode water (STMW) within and south of the Kuroshio Extension recirculation gyre are analyzed from profiling float data and additional hydrographic and shipboard ADCP measurements taken during 2004. The presence of an enhanced recirculation gyre and relatively low mesoscale eddy variability rendered this year favorable for the formation of STMW. Within the recirculation gyre, STMW formed from late-winter convection that reached depths greater than 450 m near the center of the gyre. The lower boundary of STMW, corresponding to σθ ≃ 25.5 kg m−3, was set by the maximum depth of the late-winter mixed layer. Properties within the deep portions of the STMW layer remained largely unchanged as the season progressed. In contrast, the upper boundary of the STMW layer eroded steadily as the seasonal thermocline deepened from late April to August. Vertical eddy diffusivity responsible for this erosion was estimated from a budget analysis of ...

Mapping Circulation in the Kuroshio Extension with an Array of Current and Pressure Recording Inverted Echo Sounders

Abstract The Kuroshio Extension System Study (KESS) aimed to quantify processes governing the variability of and the interaction between the Kuroshio Extension and the recirculation gyre. To meet this goal, a suite of instrumentation, including 43 inverted echo sounders equipped with bottom pressure gauges and current meters [current and pressure recording inverted echo sounders (CPIES)], was deployed. The array was centered on the first quasi-stationary meander crest and trough east of Japan, which is also the region of highest eddy kinetic energy. KESS was the first experiment to deploy a large quantity of these new CPIES instruments, and it was unique in that the instruments were deployed in water depths (5300–6400 m) close to their limit of operation. A comprehensive narrative of the methodology to produce mesoscale-resolving four-dimensional circulation fields of temperature, specific volume anomaly, and velocity from the KESS CPIES array is provided. In addition, an improved technique for removing p...

Gulf Stream path and thermocline structure near 74°W and 68°W

The SYNoptic Ocean Prediction (SYNOP) experiment had the goal of providing a physical understanding of energetic mesoscale eddy processes in the Gulf Stream. In the SYNOP Inlet Array off Cape Hatteras and in the Central Array near 68°W modred observations were collected from October 1987 through August 1990. The Inlet Array measured the surface path and bottom currents where the Gulf Stream leaves the continental margin to enter the deep water regime; small amplitude propagating and growing meanders characterized the variability there. The Central Array measured velocity and temperature (as a proxy for density) at four levels in the water column, as well as the upper and deep level streamfunctions, all with mesoscale resolution. Near 70°W the path envelope exhibited a relative node, confined within a 40-km band 55% of the time. Near 68°W the path envelope was over 3 times as wide, due to several elongated (“steep”) meander troughs and relatively steep meander crests. The crests typically propagated downstream without much growth. The troughs often stalled near 68°W, steepened, and persisted for one to several months. Two cases evolved into “S-shaped” paths and subsequently formed rings. Even the time-averaged fields showed a small trough in the mean path and thermocline structure. Whereas meanders of 20- to 60-day periods had similar spectral levels throughout 70°–67°W, meanders with long periods (>85 day) accounted for the local minimum in variance at 70°W. Bottom pressure and velocity observations revealed repeated periods of intense (swirl speeds > 0.30 m s−1) abyssal eddies; the time-averaged deep currents exhibited a mean cyclone centered 30 km offshore and downstream of the upper layer mean trough. The cross-stream slope of the thermocline steepened linearly with path curvature, consistent with gradient wind balance. Structures are illustrated in the mapped fields consistent with baroclinic instability wherein troughs steepen and rings form.

Oscillations of Dynamic Topography in the Eastern Equatorial Pacific

Abstract For 14 months in 1980–81, surface dynamic height was monitored with inverted echo sounders at five sites from 0 to 9°N along 110°W. These records show that the SEC/NECC equatorial current system was well-developed during the boreal summer and fall, but weak and irregular during winter and spring when westward flow associated with the NEC extended as far south as 6°N. Superimposed on the mean dynamic topography of this region are energetic 20-to-80-day oscillations, longer periods being associated with higher latitudes. Near the equatorial ridge (∼5°N), these oscillations have predominantly monthly periods, and amplitude of ∼10 dyn cm comparable to the mean dynamic-height difference across the NECC. The broad in-phase meridional extent of these monthly oscillations implies that the principal mode of ridge variation is vertical undulation rather than meridional meandering, producing large in-phase monthly modulations in transport of the SEC and NECC. Oscillations or the equatorial ridge are correla...