Karen L. Tracey

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.

Mapping Abyssal Current and Pressure Fields under the Meandering Gulf Stream

Abstract Using bottom pressure measurements and current meter measurements at 3500-m depth at 12 sites under the Gulf Stream near 68°W daily optimally interpolated (OI) maps have been constructed for June 1988 to August 1990. Prior to mapping, the pressure records were leveled (referenced to the same absolute geopotential) using the current measurements under the assumption that their mean fields are geostrophic. The leveled pressures were subsequently used together with the current measurements in a multivariate, nondivergent OI mapping procedure. This procedure significantly reduced the mapping errors, because both pressure and its (geostrophic) gradient are specified as inputs. The mapped bottom pressure and current fields have typical mean-square errors of only 0.0005 db2 and 4 cm2 s−2, compared to typical signal variances of 0.0035 db2 and 80 cm2 s−2. The daily maps of abyssal pressure are used to identify the characteristic space–time structures of dynamical processes. Examples are shown in which de...

Gulf Stream flow field and events near 68°W

The SYNoptic Ocean Prediction (SYNOP) experiment was designed to provide an accurate understanding of the energetic mesoscale processes in the Gulf Stream. The Central Array measured velocity and temperature throughout the water column, with horizontal extent large enough nearly to span the meander envelope and Eulerian mean structure of the jet at 68°W. The 55- to 70-km mooring spacing resolved mesoscale eddy interactions with the Gulf Stream, and the 26-month duration allowed stable estimation of long-term mean fields. Six steep meander troughs propagated into or developed within the array, each lasting around 30-60 days, thus impressing a small mean trough near 68°W in the predominantly eastward currents at jet level (1000 m and above). At the deep level (3500m) the mean flow was southwest at the shoreward sites shallower than 4300 m, but it flowed cyclonically around a mean low-pressure anomaly affecting all the deeper offshore sites. The eddy kinetic energy per unit mass (E K ) decreased by a factor of about 2.5 with each depth increment from 400 to 700 to 1000 m but was only a factor of 2 smaller at 3500 m than at 1000 m. Values of E K in the upper central jet (400 m) were 100 to 230 mJ kg -1 and were 4-13 mJ kg -1 at 3500 m. Overall, E K in the upper 1000m at 68°W was higher than previously published values at 55°W. Two extended case studies of meander propagation through the array demonstrate the development and intensification of deep cyclonic and anticyclonic flows beneath the Gulf Stream. The cyclonic flow at 3500m, associated with amplifying meander troughs, often exceeded 0.35 ms -1 , which was much larger than the typical 0.05 ms -1 deep mean velocities.

Program Studies the Kuroshio Extension

The Kuroshio Extension system links to North Pacific climate through its role in subtropical-subpolar exchange, the formation and distribution of mode waters, and the intensification of the extratropical storm track across the North Pacific. The Kuroshio Extension System Study (KESS) offers a window into these processes through integrated measurements of the ocean and atmosphere and through modeling efforts (Figure 1). The northward flowing waters of the Kuroshio western boundary current leave the Japanese coast to flow eastward as a free jet—the Kuroshio Extension. The Extension forms a vigorously meandering boundary between the warm subtropical and cold northern waters.

Baroclinic Transport Time Series of the Antarctic Circumpolar Current Measured in Drake Passage

AbstractThe first multiyear continuous time series of Antarctic Circumpolar Current (ACC) baroclinic transport through Drake Passage measured by moored observations is presented. From 2007 to 2011, 19 current- and pressure-recording inverted echo sounders and 3 current-meter moorings were deployed in Drake Passage to monitor the transport during the cDrake experiment. Full-depth ACC baroclinic transport relative to the bottom has a mean strength of 127.7 ± 1.0 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) with a standard deviation of 8.1 Sv. Mean annual baroclinic transport is remarkably steady. About 65% of the baroclinic transport variance is associated with time periods shorter than 60 days with peaks at 20 and 55 days. Nearly 28% of apparent energy in the spectrum computed from transport subsampled at the 10-day repeat cycle of the Jason altimeter results from aliasing of high-frequency signals. Approximately 80% of the total baroclinic transport is carried by the Subantarctic Front and the Polar Front. Partition...

A Multi-Index GEM Technique and Its Application to the Southwestern Japan/East Sea

Abstract This paper demonstrates a new gravest empirical mode (GEM) technique that constructs multi-index lookup tables of temperature (T) and specific volume anomalies (δ) using historical hydrocasts as a function of three indices: round-trip travel time (τ) from sea floor to the surface, sea surface temperature, and pressure. Moreover, the historical hydrocasts are separated into non-mixed-layer (NML) and mixed-layer (ML) groups, and a single GEM field is constructed for each group. This is called the MI-GEM technique. The appropriate dates for MI-GEM fields are determined by the monthly distribution of the number of NML and ML profiles in the historical hydrocasts, which are also well correlated with the strength of the winds during the 2 yr of observations. The T and δ profiles that are determined by this MI-GEM technique capture 92% and 88% of the T and δ variances in the depth range of 0–200 db. These values reduce by about one-third of the unexplained error variance of the residual GEM, which was r...

Mean Antarctic Circumpolar Current transport measured in Drake Passage

The Antarctic Circumpolar Current is an important component of the global climate system connecting the major ocean basins as it flows eastward around Antarctica, yet due to the paucity of data it remains unclear how much water is transported by the current. Between 2007 and 2011 flow through Drake Passage was continuously monitored with a line of moored instrumentation with unprecedented horizontal and temporal resolution. Annual mean near-bottom currents are remarkably stable from year to year. The mean depth-independent, or barotropic transport, determined from the near-bottom current meter records was 45.6 Sv with an uncertainty of 8.9 Sv. Summing the mean barotropic transport with the mean baroclinic transport relative to zero at the seafloor of 127.7 Sv gives a total transport through Drake Passage of 173.3 Sv. This new measurement is 30% larger than the canonical value often used as the benchmark for global circulation and climate models.

Propagation of Kuroshio Extension Meanders between 143º and 149ºE

A two-dimensional array of current- and pressure-recording inverted echo sounders provided synoptic measurements of the upper and deep fluctuations in the Kuroshio Extension between 1438 and 1498E with mesoscale resolution. Downstream-propagating meanders with periods of 3‐60 days were always present between June 2004 and September 2005. Propagation speeds were estimated by two methods: spectral analysis of path displacements and complex empirical orthogonal functions (CEOF) analysis of along-path anomalies. The two methods produced similar results. Phase speeds increased smoothly from 10 km day 21 (0.12 m s 21 ) for meanders with wavelengths and periods [l, T] 5 [420 km, 40 days] to 35 km day 21 (0.41 m s 21 ) for [l, T] 5 [220 km, 6 days] meanders. This empirically derived dispersion relationship is indistinguishable from that obtained for Gulf Stream meanders downstream of Cape Hatteras. The deep ocean was populated with remotely generated, upstream-propagating eddies composed of a nearly depthindependent current structure.Upper meandersand deep eddies jointly spun up when they encountered each other with the deep eddy offset about a quarter wavelengthahead of the upper meander. Subsequently, as the upper and deep features moved past each other and the vertical offset changed, intensification ceased.

Determining geostrophic velocity shear profiles with inverted echo sounders

It is known that vertical acoustic travel time (τ), round-trip from the seafloor to the sea surface as measured by inverted echo sounders (IESs), can be interpreted in terms of dynamic height. This relationship is generalized and quantified in this paper for dynamic height (ΔDi, j) integrated between a variety of pressure limits (pi, pj) which span different portions of the main thermocline. The generalized form of the ΔDi,j(τ) relationships is nonlinear; the conventional linear relationship is valid when the limits of integration span the entire main thermocline. Velocity and temperature records are shown to be highly correlated vertically, indicating that the variability of the Gulf Stream and adjacent eddy field is dominated by the lowest baroclinic mode, which is called a gravest empirical mode (GEM). A “parallel isotherms model” is used to approximate the GEM and to develop analytic expressions for the observed ΔDi,j(τ) relationships in the Gulf Stream. The analytic expressions represent the observations well, with noise-to-signal variance ratios that are typically 1%. Using these new ΔDi,j(τ) relationships, the baroclinic velocity structure can be determined geostrophically by measuring the horizontal gradients with laterally separated τ measurements from IESs. Baroclinic velocities determined from a two-dimensional array of IESs in the Gulf Stream during 1988–1990 agree with velocity shears directly measured by current meters. The rms velocity difference between these two measurements of velocity shear at 400 dbar relative to 1000 dbar was 12 cm s−1 in the presence of typical currents of 50 cm s−1. Ageostrophic motions (at both mesoscale and submesoscale), measured by the current meters but not by the IESs, contribute most of the velocity differences.