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William J. Schmitz Jr.

Abstract

Latitudinal distributions of estimates of eddy kinetic energy and off-diagonal component of the horizontal Reynolds stresses are demonstrated to have similar shapes at corresponding (generally western) locations, but at much different depths, in the mid-latitude North Atlantic and North Pacific Oceans. This comparison is based on direct current measurements at abyssal depths in the North Atlantic along 55°W, and on indirect estimates of upper-level velocity differences in the vertical for the North Pacific between the Izu (or Izu-Ogasawara or South Honshu) Ridge and the Shatsky Rise. These sections span the more energetic segments of the Kuroshio Extension and Gulf Stream systems.

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William J. Schmitz Jr.

Abstract

Fourteen moorings were deployed across the midlatitude North Pacific 165°E to 152°W, for approximately 2 years during 1983–85. Ten mooring sites had previously been occupied at similar latitudes (30°–40°N nominal) for roughly two years (1980–82) along 152°E. Taken together, these observations form the basis for the first systematic basinwide zonal exploration of the eddy field based on moored instrument techniques in the midlatitude North Pacific along the Kuroshio Extension System and North Pacific drift. Eddy kinetic energy (KE ) at abyssal depths decays sharply moving east from 152°E, and has decreased by a factor of 4 by 165°E. There is a plateau in abyssal KE of about 10 cm2 s−2 across the Emperor Seamounts from 165° to 175°E. Abyssal KE drops to roughly 5 cm2 s−2 at 175°W and 1 cm2 s−2 at 152°W, for a total decay of a factor of about 50 across the midlatitude North Pacific. Upper level KE decreases by a total of roughly two orders of magnitude (approximately 103 to 101) from 152°E to 152°W.

The most energetic sites at 152° and 165°E have essentially the same vertical structure (shape), with the deep and near surface amplitudes at 152°E being 4 and 3 times higher, respectively. In fact, the same type of vertical profile for KE is appropriate as a first approximation across the entire midlatitude North Pacific, with amplitudes generally decreasing eastward and away from the Kuroshio Extension. Distributions of KE with frequency are typically peaked somewhat at the mesoscale near the Kuroshio Extension, and generally become more “red” proceeding east and/or toward lower energy areas, although examples of essentially every type of partitioning are available. The KE values at 165°E are generally the most stable from year-to-year that have ever been measured in energetic regions of the open ocean, at all depths.

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William J. Schmitz Jr.

Abstract

Unexpectedly large and stable abyssal mean flow have recently been observed along 165°E, between 31° and 41°N. These results are based on two deployments of a moored array for approximately a year each. Time-averaged currents at 4000 m are about 5 ± 0.5 cm s−1 to the southwest near 41°N and 6 ± 0.9 cm s−1 to the northwest near 33°N, on opposite of the Kuroshio Extension. These mean flows were reproduced between array deployments to within a fraction of a cm s−1 and few degrees (True). At 41°N the abyssal mean kinetic energy is several times larger than eddy kinetic energy so that the flow visually does not reverse. At 33°N, near the southern edge of the Kuroshio Extension, the abyssal mean and eddy fields are of roughly the same amplitude. These result along 165°E are in contrast to similar observations along 152°E, where the abyssal zonal mean flows are notably less stable than at 165°E. However, estimates of the latitudinal and two-years averaged zonal flow components are approximately the same at both longitudes, typically in the range of 1.4 to 1.9 cm s−1 to the west. The new currents described here have not be observed previously and are not a feature of any proposed circulation scheme with which the author is familiar.

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William R. Holland
and
William J. Schmitz

Abstract

All available observations indicate that the most energetic time-dependent currents are located in the vicinity of intense large-scale oceanic current systems. This characteristic is also a basic property of eddy-resolving gyre-scale numerical models. An initial detailed intercomparison of two-layer eddy-resolving numerical experiments with observation focused on the largest scales of horizontal structure in patterns of abyssal eddy kinetic energy, and on time scales. The numerical experiments examined generally had relevant temporal and meridional scales, but not necessarily realistic zonal scales. The model eddy field did not penetrate as far from the western boundary as observed distributions, by a factor of 2 to 3.

The present study examines the physical processes that govern the model zonal penetration scale and suggests reasons for the previous discrepancy. It is demonstrated that a subtle balance exists between the complex instability processes that tend to tear the jet apart (restricting its zonal penetration) and the tendency for inertial processes to carry the intense current right across the basin. It would seem that any factor that changes the nature of the instability of the thin Gulf Stream jet will alter the penetration scale. In these models this means not only changing physical parameters and including different physics, but also changing such model dependent factors as vertical resolution. Earlier work suggested the need for enhanced vertical resolution to give realistic zonal penetration, but it is now clear that all stabilizing/destabilizing effects conspire together to give a particular penetration scale.

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William J. Schmitz Jr.
and
J. Dana Thompson

Abstract

An adiabatic, primitive equation, eddy-resolving circulation model has been applied to the Gulf Stream System from Cape Hatteras to east of the Grand Banks (30°–48°N, 78°–45°W). A two-layer version of the model was driven both by direct wind forcing and by transport prescribed at inflow ports south of Cape Hatteras for the Gulf Stream and near the Grand Banks of Newfoundland for the deep western boundary current. The mean upper-layer thickness was sufficiently large for interface outcropping not to occur. Numerical experiments previously run at 0.2° horizontal resolution (∼20 km) had some realistic features, but a key unresolved deficiency was that the highest eddy kinetic energies obtained near the Gulf Stream were too low relative to data by a factor of about 2, with inadequate eastward penetration.

A unique set of new numerical experiments has extended previous results to higher horizontal resolution, all other conditions being held fixed. At 0.1° horizontal resolution, eddy kinetic energies in the vicinity of the Gulf Stream realistically increase by a factor of roughly 2 relative to 0.2°. The increase in eddy activity is a result of enhanced energy conversion from mean flow to fluctuations due to barotropic and baroclinic instabilities, with the nature of the instability mixture as well as eddy energy changing with increased resolution. One experiment at 0.05° horizontal resolution (∼5 km) yielded kinetic energies and key energy transfer terms that are within 10% of the equivalent 0.1° case, suggesting that convergence of the numerical solutions has nearly been reached.

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William J. Schmitz Jr.
and
W. Brechner Owens

Abstract

It is demonstrated that the outcome of an intercomparison between data and the vertical distribution of eddy kinetic energy predicted by a previously developed numerical model of the MODE area is frequency dependent. In the range of periods from 50 to 150 or even to 400 days (one definition of the temporal mesoscale, the scale that the model was designed to simulate), the comparison is quite good. For periods in the range of 5 to 50 days, the agreement is poor. For periods longer than 400 days, the comparison is indeterminate. Earlier conclusions concerning the relation of model results to the MODE data should be qualified by stipulating frequency range, and future intercomparisons for any model in all regions should be conscious of the desirability of doing so across common frequencies.

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Terrence M. Joyce
and
William J. Schmitz

Abstract

The meridional structure of the zonal flow in the Kuroshio Extension is investigated using a combination of data from hydrographic sections and moored current meter arrays. We emphasize 165°E, between 30° and 42°N, where high quality and very stable current measurements at 150 and 4000 m extend over a two-year period from October 1983 to October 1985. Hydrographic (CTD/O2) sections were occupied during the initial deployment and a second time when the array of six moorings was reset in 1984. The deep currents were extremely reproducible from one year to the next and revealed a pattern of weak eastward flow at 4000 m under the axis of the Kuroshio with strong westward flow on either flank. When combined with the hydrographic data, the total transport of the eastward flowing Kuroshio Extension was estimated to be 57.0 ± 3.7 Sv (Sv = 106 m3 s−1), essentially the same as when referenced to the broom (57.0 ± 2.0 Sv). South of 34°N, the velocities were westward at all levels, with a net transport of −85.1 Sv; north of 37°N the flow in the upper kilometer was eastward (22 Sv) near the axis of the Oyashio, or subarctic front, and westward elsewhere, yielding a net transport of −34.6 Sv. The net transport across the entire section from 30° to 42°N was westward and equal to −62.7 ± 12.3 Sv.

New methods of estimating transport when combining direct current and hydrographic data are illustrated where compatibility with dynamic height estimates is required. Observations of dynamic height variability across the 165°E array using the current meters suggested that the mean currents at 4000 m were consistent with the dynamic height range observed hydrograpically. However, the yearly averaged velocities al 150 m under-sampled the eastward upper level flow. Results are also compared to previously published work at 152°E and with the new data at 175°W. At 152°E, previous estimates of zonal transport over a similar latitude range yield −31 ± 16 Sv when current meter and hydrographic date were combined; our study suggests −31 ± 31 Sv. The section-averaged zonal transport changes sign across the Emperor Seamounts, becoming positive at 175°W, where the hydrographic and yearly averaged array data are totally consistent.

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Pearn P. Niiler
,
William J. Schmitz
, and
Dong-Kyu Lee

Abstract

Three new deep hydrographic sections taken in July 1980, May 1981 and May 1982, between 29–41°N along 152°E across the high eddy-energy area of the Kuroshio Extension are used to compute the relative geostrophic transport as a function of depth. The mean eastward geostrophic transport relative to the bottom through this section is 57×106 m3 s−1 (≡57 Sv). These sections were occupied across an array of ten moorings deployed from mid-1980 to mid-1982. Using the 22-month average directly measured currents at 1200 and 4000 m depth for reference levels in the least-square sense, the absolute transport during the 1980–82 period is estimated to be 31±16 (×108 m3 s−1) to the west. This lower bound on the uncertainty of the net transport estimate is based on the uncertainty of the measured mean currents. At 55°W, we use July 1976 and July 1977 deep hydrographic sections to compute the bottom-relative geostrophic transport across the high eddy-energy area of the Gulf Stream. Between 42 and 32°N, it is 32 Sv−1 to the east. Using 24-month average directly measured currents at 4000 m for a reference level, the absolute transport during the 1976–77 period is estimated to be 47±36 Sv to the west. Climatological, wind-driven Sverdrup mass divergence requires a net eastward transport of 17 Sv in the Pacific and 10 Sv in the Atlantic. Thus, the net westward circulations must be maintained by eddy or bottom or thermohaline interaction processes.

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Xiaobiao Xu
,
Peter B. Rhines
,
Eric P. Chassignet
, and
William J. Schmitz Jr.

Abstract

The oceanic deep circulation is shared between concentrated deep western boundary currents (DWBCs) and broader interior pathways, a process that is sensitive to seafloor topography. This study investigates the spreading and deepening of Denmark Strait overflow water (DSOW) in the western subpolar North Atlantic using two ° eddy-resolving Atlantic simulations, including a passive tracer injected into the DSOW. The deepest layers of DSOW transit from a narrow DWBC in the southern Irminger Sea into widespread westward flow across the central Labrador Sea, which remerges along the Labrador coast. This abyssal circulation, in contrast to the upper levels of overflow water that remain as a boundary current, blankets the deep Labrador Sea with DSOW. Farther downstream after being steered around the abrupt topography of Orphan Knoll, DSOW again leaves the boundary, forming cyclonic recirculation cells in the deep Newfoundland basin. The deep recirculation, mostly driven by the meandering pathway of the upper North Atlantic Current, leads to accumulation of tracer offshore of Orphan Knoll, precisely where a local maximum of chlorofluorocarbon (CFC) inventory is observed. At Flemish Cap, eddy fluxes carry ~20% of the tracer transport from the boundary current into the interior. Potential vorticity is conserved as the flow of DSOW broadens at the transition from steep to less steep continental rise into the Labrador Sea, while around the abrupt topography of Orphan Knoll, potential vorticity is not conserved and the DSOW deepens significantly.

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William J. Schmitz, Jr.
,
James F. Price
,
Philip L. Richardson
,
W. Brechner Owens
,
Douglas C. Webb
,
Robert E. Cheney
, and
H. Thomas Rossby

Abstract

SOFAR (sound fixing and ranging) floats deployed for engineering tests during 1977–79 yield the first long-term quasi-Lagrangian observations in the subsurface Gulf Stream System. The character of these float tracks supports the premise that the Gulf Stream is a persistent, large-scale, vertically coherent jet at depths (approximately) within and above the main thermocline. where mean and eddy kinetic energies are roughly the same and lateral motions of the Stream are clearly delineated. A float track at thermocline depth is visually coherent with the track of a concurrently launched surface drifter over the larger horizontal scales traversed during the first few months of their trajectories. Below thermocline depths, fluctuation or eddy kinetic energies are normally larger than the mean and a persistent Gulf Stream is difficult to detect. However, deep motions that are visually coherent with upper level flows may be observed for an intermediate range of space and time scales.

Eddy kinetic energies based on the float data are compatible with existing Eulerian estimates to the extent comparable. The consistency of a quasi-Lagrangian eddy kinetic energy estimate in the vicinity of the thermocline, roughly 400 cm2 s−2, the first such observation to our knowledge, is indirect but relatively convincing. Zonal and meridional variances for the float data are also in line with existing Eulerian results. Estimates of the frequency distribution of eddy kinetic energy for the longest float trajectory available are nearly identical to comparable Eulerian results at frequencies less than about a cycle per 20 days.

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