William J. Schmitz

On the North Atlantic circulation

A summary for North Atlantic circulation is proposed to replace the circulation scheme hypothesized by Worthington in 1976. Divergences from the previous model are in thermohaline circulation, cross-equatorical transport and Florida Current sources, flow in the eastern Atlantic, circulation in the Newfoundland Basin, slope water currents, and flow pattern near the Bahamas. The circulation patterns presented here are consistent with the majority of of published accounts of flow components. 77 refs., 14 figs., 3 tabs.

On the sources of the Florida Current

Abstract In our opinion roughly 13 Sv or 45% of the transport of the Florida Current is of South Atlantic origin, as compensation for the cross-equatorial flow of North Atlantic Deep Water. Of the 8.9 Sv moving through the Straits of Florida with temperatures above 24°C in the upper 100m of the water column, 7.1 Sv is composed of comparatively fresh water coming through the southern Caribbean passages from the tropical South Atlantic. Saltier surface water, 1.8 Sv, enters from the North Atlantic through Windward Passage, as does most of the 18° Water in the Florida Current. A South Atlantic contribution for the uppermost layer is clear-cut because the surface water in the open Atlantic north of the Caribbean is comparatively cold and salty and intrudes south as Subtropical Underwater or Salinity-Maximum Water below a comparatively warm and fresh layer 50-100 m thick, which could hardly he transported from the North Atlantic. Of the 13.8 Sv transported through the Caribbean in the 12-24°C temperature range, 13.0 Sv is of North Atlantic origin, with about 0.8 Sv of comparatively fresh South Atlantic water on the western side of the Florida Straits having entered the Caribbean on the southern side of St. Vincent and St. Lucia Passages. Of the 6 Sv transported by the Florida Current in the 7-12°C temperature range, 5 Sv appears to originate in the South Atlantic. Our estimate of the 13 Sv of South Atlantic and 16 Sv of North Atlantic origin for the total transport of 29 Sv for the Florida Current, along with partitioning in the aforementioned temperature ranges, is approximately consistent with open ocean sections along 24°N and with several previous investigations. We have formed a new estimate of the transport into five key Caribbean passages, yielding 28.8 Sv for the temperature range appropriate to the Straits of Florida off Miami, in close agreement with independent transport measurements for the Florida Current. The five passages and their contributions are: Grenada (7.7 Sv), St. Vincent (7.9 Sv), St. Lucia (3.8 Sv), Dominica (2.6 Sv), and Windward (6.8 Sv). Breakdowns of these passage transport estimates into broad classes by temperature range agree to within about 2 Sv in comparison with similar quantities for the Florida Current. Anegada Passage may transport 0.5 Sv of water that exits through the upper 200 m or so of the Florida Current, and the mid-depth (5-12°C) flow in this passage and in the general vicinity of the Caribbean deserves further examination.

Dynamics of the Kuroshio/Oyashio current system using eddy-resolving models of the North Pacific Ocean

A set of numerical simulations is used to investigate the Pacific Ocean circulation north of 20°S, with emphasis on the Kuroshio/Oyashio current system. The primitive equation models used for these simulations have a free surface and realistic geometry that includes the deep marginal seas, such as the Sea of Japan. Most of the simulations have 1/8° resolution for each variable but range from 1/2°, 1.5-layer reduced gravity to 1/16°, six layer with realistic bottom topography. These are used to investigate the dynamics of the Kuroshio/Oyashio current system and to identify the processes that contribute most to the realism of the simulations. This is done by model-data comparisons, by using the modularity of layered ocean models to include/exclude certain dynamical processes, by varying the model geometry and bottom topography, and by varying model parameters, such as horizontal grid resolution, layer structure, and eddy viscosity. In comparison with observational data, the simulations show that the barotropic mode, at least one internal mode, nonlinearity, high “horizontal” resolution (1/8° or finer), the regional bottom topography, and the wind forcing are critical for realistic simulations. The first four are important for baroclinic instability (eddy-mean energetics actually show mixed barotropic-baroclinic instability), the wind curl pattern for the formation and basic placement of the current system, and the bottom topography for the distribution of the instability and for influences on the pathways of the mean flow. Both the Hellerman and Rosenstein (1983) (HR) monthly wind stress climatology and 1000-mbar winds from the European Centre for Medium-Range Weather Forecasts (ECMWF) have been used to drive the model. East of about 150°E, they give a mean latitude for the Kuroshio Extension that differs by about 3°, approximately 34°N for HR, 37°N for ECMWF, and 35°N observed. The subarctic front is the northern boundary of the subtropical gyre. It is associated with the annual and April–September mean zero wind stress curl lines (which are similar), while the Kuroshio Extension is associated with wintertime zero wind stress curl. This means that part of the flow from the Kuroshio must pass north of the Kuroshio Extension and connect with the Oyashio and subarctic front. Realistic routes for this connection are flow through the Sea of Japan, a nonlinear route separated from the east coast of Japan, and bifurcation of the Kuroshio at the Shatsky Rise. In addition, the six-layer simulations show a 3-Sv meridional overturning cell with southward surface flow and northward return flow centered near 400 m depth. Baroclinic instability plays a critical role in coupling the shallow and abyssal layer circulations and in allowing the bottom topography to strongly influence the shallow circulation. By this means, the Izu Ridge and Trench and seamounts upstream and downstream of these have profound influence on (1) the mean path of the Kuroshio and its mean meanders south and east of Japan and (2) on separating the northward flow connecting the Kuroshio and the Oyashio/subarctic front from the east coast of Japan. Without the topographic influence, the models show an unrealistic northward current along the east coast of Japan. In essence, the topography regulates the location and strength of the baroclinic instability. The baroclinic instability gives eddy-driven deep mean flows that follow the f/h contours (where f is the Coriolis parameter and h is the depth of the water column) of the bottom topography. These abyssal currents then strongly influence the pathway for subtropical gyre flow north of the Kuroshio Extension and steer the mean meanders in the Kuroshio south and east of Japan. This is corroborated by current meter data from the Kuroshio Extension Regional Experiment (World Ocean Circulation Experiment line PCM 7). The meander path south of Japan depends on the occurrence of baroclinic instability west of the Izu Ridge; otherwise, a straight path occurs. The pathway shows little sensitivity to the Tokara Strait transport over the range simulated (36–72 Sv in yearly means). However, interannual increases in wind forcing or Tokara Strait transport give rise to a predominant meander path, while decreases yield a predominant straight path. Resolution of 1/8° in an ocean model is comparable to the 2.5° resolution used in atmospheric forecast models in the early 1980s based on the first internal mode Rossby radius of deformation. Model comparisons at 1/8° and 1/16° resolution and comparisons with current meter data and Geosat altimeter data show that 1/16° resolution is needed for adequate eastward penetration of the high eddy kinetic energy associated with the Kuroshio Extension.

Deep cross-equatorial flow in the Atlantic measured with SOFAR floats

Neutrally buoyant SOFAR floats at nominal depths of 800, 1800, and 3300 m were tracked for 21 months in the vicinity of tropical boundary currents in the Atlantic near 6°N and at several sites near 11°N as well as along the equator. Trajectories at 1800 m show a swift (>50 cm/s), narrow (100 km wide), southward flowing deep western boundary current (DWBC) extending from 7°N to the equator. The average transport per unit depth in the DWBC was estimated to be 13.8 × 103 m2/s. Coupling this value with mean velocities measured in the DWBC by current meters gave a volume transport of 15 × 106 m3/s between depths of 900 m and 2800 m. Approximately 6 × 106 m3/s recirculated northward between the DWBC and the Mid-Atlantic Ridge, leaving 9 × 106 m3/s as cross-equatorial transport. No obvious DWBC nor swift equatorial current was observed by the 3300-m floats; a low mean velocity at this depth lay between F-11 and higher velocity cores above and below. The 1800-m trajectories also suggest that at times (February-March 1989) the North Atlantic Deep Water in the DWBC turned eastward and flowed along the equator and at other times (August-September 1990) the DWBC crossed the equator and continued southward. The velocity near the equator, calculated by grouping floats in a box along the equator, was eastward at 4.1 cm/s from February 1989 to February 1990 and westward at 4.6 cm/s from March 1990 to November 1990. Thus the amount of cross-equatorial flow in the DWBC appeared to be linked to low-frequency variability of the structure of the equatorial current system. Floats in Antarctic Intermediate Water at 800 m revealed a northwestward western boundary current, although flow patterns were complicated. Three floats that significantly contributed to the northwestward flow looped in anticyclonic eddies that translated up the coast at 8 cm/s. Six 800-m floats drifted eastward along the equator between 5°S and 6°N at a mean velocity of 11 cm/s; one reached 5°W in the Gulf of Guinea, suggesting that the equatorial currents at this depth extended at least 35°–40° along the equator. Three of these floats reversed direction near the end of the tracking period.

On the transport of the Florida current

Abstract This paper contains results from the first series of transport measurements made with free instrument techniques. The series consists of the equivalent of 50–60 transects across four sections of the current made over a period of about 3 years. Based on averages of our data, and a comparison of these averages with previous data, we submit 32 ± 3 × 10 6 m 3 /sec as the steady-state volume transport of the Florida Current. Monthly or bi-monthly averages lie within this bound and are reproducible. The total fluctuation bound is ± 12 × 10 6 m 3 /sec. We estimate tidal amptitudes of 3·5 ± 1 × 10 6 m 3 /sec for each of M 2 , K 2 , O 1 and 1·5 ±1 × 10 6 m 3 /sec for S 2 , in excellent agreement with past analysis of potential measurements across the Key West-Havana cable. However, in contrast with some previous investigations, in particular with the past analysis of cable data, there is no evidence in our data for total transport fluctuations at non-tidal periods of amplitudes larger than 10% of the mean.

The Sverdrup circulation for the Atlantic along 24°N

The 30 Sv transported by the Florida Current through the Straits of Florida off Miami could consist of a wind-driven contribution of 17 Sv along with a thermohaline component of 13 Sv. The latter might flow from the South Atlantic as upper layer compensation for a net lower layer cross-equatorial flow southward. The Sverdrup transport along 24°N in the interior North Atlantic east of 55°–60°W is about 17 Sv, potentially matching the wind-driven component of the Florida Current. The circulation in the vicinity of 24°N and west of 55°–60°W up to the Bahama Banks contains energetic, shorter spatial scale flows, where there is a similarity between the regional pattern of the Sverdrup transport contours and the scales and structure of the observed C-shaped pattern of dynamic height.

Preliminary field results for a Mid-Ocean Dynamics Experiment (MODE-0)☆

Three arrays of moored instruments were placed in the western part of the Sargasso Sea in 1971–1972 to provide pilot data for a Mid-Ocean Dynamics Experiment (MODE-I). Current, current-temperature, temperature-pressure, and acoustic positioning sensors were deployed on these moorings. The acoustic positioning instrumentation, in combination with conductivity, temperature, and pressure sensors, was also used in free-fall mode to obtain 12 vertical profiles of temperature and horizontal currents with a vertical resolution of 20 m over a 36-h period during deployment of the first array. These observations were collectively designed to provide estimates of energy levels and space and time scales for mesoscale motions. For frequencies less than 1 cycle day−1, velocity and temperature records are dominated by 50–100-day fluctuations, with apparent horizontal spatial scales of the order of 100 km. The vertical structure of the mesoscale motions appears to be dominated by the barotropic and first few baroclinic modes. Estimates of kinetic energy from current meter records were found to depend upon the type of mooring used. Records from moorings with surface buoyancy yield kinetic energies that are higher than those from moorings with subsurface buoyancy. This effect occurs over the entire frequency spectrum. A special purpose experiment, with current meters at the same depths on the two different mooring types and separated horizontally by only a few hundred meters, yielded the same type of result. The vertical and horizontal displacements of a mooring with subsurface buoyancy at 500-m depth (water depth of about 5400 m) observed over a 4-day duration during the retrieval of the third array were ±1 and ±50 m, respectively. Pressure measurements at other depths on this mooring yielded the same ±1 m bound on the magnitude of vertical excurssions. The vertical displacements obtained from a 412-month pressure record at 2000-m depth for a similar mooring configuration were ±6 m.

Zonal Penetration Scale of Model Midlatitude Jets

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 ...

Cyclones and westward propagation in the shedding of anticyclonic rings from the loop current

The modern Sea Surface Height database using altimetric observations from satellites contains very informative views of the Loop Current and its eddy field in the Gulf of Mexico. The synopsis for this book considers, among other topics, the state of the art in identifying the observed characteristics of a suite of local, regional, and remotely influenced phenomena associated with the composite process of the shedding of anticyclonic (warm-core) current rings from the Loop Current (LC). This composite process is observationally more complex than has been documented in the past. The present article contains a descriptive exploration of various databases, with emphasis on Sea Surface Height (SSH) maps. New findings are combined with a review of existing knowledge relative to some of the component phenomena involved in the diverse process of eddy-shedding from the LC. The shedding of anticyclones by the LC is well known to essentially involve its penetration into, as well as the westward propagation of warm-core eddies away from, the eastern Gulf of Mexico. Another basic element of the composite eddy-shedding process, in need of more attention and further development at the present time, is the specific role in the initial detachment of warm-core eddies from the LC by peripheral cyclonic (cold-core) features that move clockwise around the boundary of the LC. In addition, the modern SSH database is found to contain many examples of several kinds of reattachment between eddies and the LC, along with a variety of views of necking-down, leading to a second and less well publicized but frequently observed mode of detachment. Also, temporary blocking of the penetration of the LC into the Gulf of Mexico by cyclones (along with their arrangement in a peripheral shield around an extended LC) is documented by consideration of ∼ 15 examples (details are in the appendix on CD at the back of this volume).

Zonal Velocity Structure and Transport in the Kuroshio Extension

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 −8...