Schmitz William J.

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Schmitz

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William J.

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Observations of energetic low frequency current fluctuations in the Charlie-Gibbs fracture zone

(Woods Hole Oceanographic Institution, 1979-02) Schmitz, William J. ; Hogg, Nelson G.

Relatively energetic low frequency fluctuations in horizontal currents are found to exist below the thermocline in the northern trough of the Charlie-Gibbs Fracture Zone. For example, deep eddy kinetic energy levels there are about twice as large as those observed at similar relative depths in the MODE-I region. Eddy kinetic energies are about 2-10 times larger than mean kinetic energies. The vertical distribution of eddy kinetic energy is frequency dependent, increasing toward the thermocline for the longer time scales and intensifying toward the bottom at higher frequencies. In addition to the expected mean westward motion of Norwegian Sea Overflow Water through the northern trough of the fracture, rather consistent mean southward flow is observed at a depth immediately above the overflow.
Observed and numerically simulated kinetic energies for MODE eddies

(Woods Hole Oceanographic Institution, 1980-11) Schmitz, William J. ; Owens, W. Brechner

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.
On the deep general circulation in the western North Atlantic

(Woods Hole Oceanographic Institution, 1978-04) Schmitz, William J.

Very large (5-10 cm s-1) long-term averaged zonal flows have been observed near 4000 m depth in the vicinity of a recently hypothesized (Worthington, in press) horizontally restricted subtropical gyre in the deep western North Atlantic. The Reynolds stresses associated with low frequency fluctuations may play a significant role in the dynamics of this deep mean flow, possibly inducing a significant downstream increase in transport of the Gulf Stream, perhaps driving the deep gyre.
On the world ocean circulation. Volume II, the Pacific and Indian Oceans/a global update

(Woods Hole Oceanographic Institution, 1996-12) Schmitz, William J.

This is the second and final volume of a report that describes some of my investigations over the last 35 years or so into low-frequency ocean current structures, a topic which I will call the World Ocean Circulation (WOC). The material presented constitutes my final report to the Office of Naval Research, and their support over the years is deeply appreciated. I was also fortunate to have been partially supported by the National Science Foundation during my career and, for some of the preparation of this report, by the Clark Foundation. Volume I was focused on the North Atlantic Ocean, after a global scale summary. This volume (II) will consider first the Pacific and Indian Oceans, concentrating on interbasin circulations, meridional cells, and mesoscale eddy fields. Then, there is an exceptionaly brief discussion of the Southern Ocean(s) for background only, followed by a global summary. Lately, I have worked intensely on intergyre and interbasin exchanges, including an inter-comparison of some of the properties of the eddy field in the World's Oceans (Schmitz, W.J., Jr., Reo. Geophys.,33,151-173,1995; J. Geophys. Res., 101,16,259-16,271,1996). Volume II contains not only an update of the global picture, but also new representations of the transport structure of various components of the meridional overturning cells for each ocean. In summary, several similarties as well as dissimilarities between different oceans relative to both their general circulation and their mesoscale eddy field are shown to be associated with interbasin exchanges. This report is meant to be an informal, occasionaly anecdotal, state-of-the-art summary account of the World Ocean Circulation. Seemingly simple questions about how ocean currents behave, such as where various brands of sea water are coming from and going to, have been exciting research topics for many years. This report is not remotely about "all" of the WOC, it is simply a set of comments about what I have looked into during the preparation of this document. I do believe that the results in this report, although presented in a personal way, are consistent with community wisdom. The document is intended to be readable by non-specialists who have a basic scientifc/technical background, especially in other oceanographic areas or meteorology or the geophysical disciplines, not only by specialists in physical oceanography.
On the world ocean circulation. Volume I, some global features/North Atlantic circulation

(Woods Hole Oceanographic Institution, 1996-06) Schmitz, William J.

This is the first volume of a "final report" that summarizes, often in a speculative vein, what I have learned over the past 35 years or so about large-scale, low-frequency ocean currents, primarily with support from the Office of Naval Research (ONR). I was also fortunate to have been partially supported by the National Science Foundation and, during the preparation of this report, by the Clark Foundation. This report is meant to be an informal, occasionally anecdotal, state-of-the-art summary account of the World Ocean Circulation (WOC). Seemingly simple questions about how ocean currents behave, such as where various brands of sea water are coming from and going to, have been exciting and difficult research topics for many years. This report is not remotely about "all" of the WOC, it is simply a set of comments about what I have looked into. I believe that the results in this report, although presented in a personal way, are consistent with community wisdom. The report is intended to be readable by non-specialists who have a basic scientific/technical background, especially in other oceanographic areas or meteorology or physics or the geophysical disciplines, not just by specialists in physical oceanography. Anyone wishing to get spun up on the observational basis for the WOC could use this report and associated reference lists as a starting point. Volume I concentrates on the North Atlantic Ocean although there is preliminary discussion of global features. Highlights of this global summary are a new type of composite schematic picture of the World Ocean Circulation in its "upper layers" (Figure I-I) and new summaries (Figures 1-12, 21,91) of the global "thermohaline" circulation.
Spreading of Denmark Strait overflow water in the western subpolar North Atlantic : insights from eddy-resolving simulations with a passive tracer

(American Meteorological Society, 2015-12) Xu, Xiaobiao ; Rhines, Peter B. ; Chassignet, Eric P. ; Schmitz, William J.

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.
Weakly depth-dependent segments of the North Atlantic circulation

(Woods Hole Oceanographic Institution, 1980-12) Schmitz, William J.

Time-averaged horizontal currents obtained from long-term moored instruments deployed in the western North Atlantic over the Sohm Abyssal Plain along 55W exhibit two segments of weakly depth-dependent flow: one, near 36N, predominantly westward and narrow or jet-like (~ 200 km wide or less); the second primarily eastward, located near 37.5N, about 200-300 km south of the mean position of the axis of the Gulf Stream (its width cannot be estimated quantitatively with the data available because only one mooring with adequate vertical coverage is clearly located in this flow regime, but an upper bound of roughly 200 km seems plausible). In both cases, long-term mean zonal currents between 600 and 4000 m depths (nominal) vary in amplitude from only 6 to 10 cm s-1 (approximately). The vertical structure of the westward recirculation varies with horizontal position, being both surface and bottom intensified. The possibility exists that the identification of these weakly depth-dependent flow regimes may point to one way of increasing the transport of the Gulf Stream. That is, flow with weak vertical shear is added offshore of the more baroclinic segment of the Stream, and possibly recirculated accordingly. This notion is generally consistent with all previous investigations which find the weakest vertical shears at the offshore edge of the Stream, wherever and however examined, and in particular with the addition of transport to the Florida Current over the Blake Plateau, after emerging from the Straits of Florida (Richardson, Schmitz, and Niiler, 1969). The horizontal patterns of the two weakly depth-dependent flow regimes found at 55W may be quite complex, containing variability on comparatively short and intermediate scales, associated to some extent with bottom topography. A specific example of the effect of bottom topography on the 55W data has been presented by Owens and Hogg (1980). It is hypothesized that the observations described here may indicate the presence of a previously unknown, weakly depth-dependent smaller scale gyre recirculating within the subtropical gyre, with the former confined between the New England Seamounts and the Grand Banks of Newfoundland. It should be emphasized that other horizontal and vertical structures may be characteristic of different locations in the recirculation of the North Atlantic. Eddy kinetic energy (Schmitz, 1978) and the off-diagonal component of Reynolds' stress are also to some extent weakly depth-dependent in each of the weakly depth-dependent mean flow regimes noted above, relative to more mid-ocean locations. At one site in particular, the off-diagonal component of the Reynolds' stress is found to be essentially depth-independent. The observation of weak depth-dependence in association with relatively strong abyssal currents for the recirculation regime could in principle help rationalize (Schmitz, 1977; Stommel, Niiler and Anati, 1978; Wunsch, 1978) some of the difficulties in geostrophica\ly balancing (at the leading order of approximation!), according to Worthington (1976), the North Atlantic Circulation in this type of region. Estimates of contributions to momentum balances (based on the available moored instrument data) involving horizontal gradients of the Reynolds' stresses, or of the momentum transport by the time-averaged flow, are typically at least an order of magnitude less than the Coriolis force associated with the zonal (or downstream) mean flow component, and possibly also the meridional (or cross-stream) flow component at most locations, thereby precluding violation of geostrophy at leading order by these effects. Geostrophic terms associated with estimates of the curvature of the Reynolds' stresses and/or mean momentum flux could be significant at the next order of approximation in the immediate vicinity of the Gulf Stream or near topographic features. Niiler (1979) has developed a model of an eddy-driven mean flow, where the eddy-terms in the vorticity equation are locally significant only in the Gulf Stream, but with a basin-wide mid-ocean flow driven in response to the noncompensated eddy-induced pressure gradient at the offshore edge of the region where eddy effects are locally significant dynamically. Two recent hydrographic sections across the Gulf Stream and recirculation along SSW were found to be in mass balance geostrophically, relative to the bottom (McCartney, Worthington and Raymer, 1980).
A dynamical interpretation of low frequency motions near very rough topography : the Charlie Gibbs Fracture Zone

(Woods Hole Oceanographic Institution, 1981-04) Hogg, Nelson G. ; Schmitz, William J. ;

As a sequel to Schmitz and Hogg (1978), nine-month moored observations of current and temperature from the Charlie Gibbs Fracture Zone are further described, and then interpreted in terms of low frequency quasigeostrophic motions. Large vertical and horizontal changes are observed in the variance of both mean and fluctuating components. It is demonstrated that these changes could be associated with the (complex) nature of the topography. With regard to the mean flow, it is shown through an advective model that the topography is sufficiently steep to force this motion to closely follow isobaths. Time-dependent motions for periods from 2 to 96 days are described using the technique of empirical orthogonal functions. The most energetic mode is always bottom trapped, with flow oriented along isobaths at lower frequencies and approaching equipartition of along- and cross-isobath motions at higher frequencies. At the lowest frequencies a second mode which increases upward in energy is also judged significant, while for periods shorter than 3.6 days the second mode is again highly bottom trapped. We interpret these motions using linear wave theory. There is relatively close correspondence between theory and observation when the effects of both large- and small-scale topographic features are included in the model calculations. In addition to the usual topographic wave, the abrupt slope changes on the north wall allow for a baroclinic fringe mode with a ncar bottom node at low frequencies and small-scale bottom corrugations force highly bottom trapped waves above the smooth slope cut-off frequency.
Observations of the vertical distribution of low frequency kinetic energy in the western North Atlantic

(Woods Hole Oceanographic Institution, 1978-11) Schmitz, William J.

Eddy or low frequency (periods greater than a day) kinetic energy per unit mass (KB) in the western North Atlantic is observed to increase approaching the Gulf Stream system along 55W longitude from the interior of the subtropical gyre, by a factor of 30 in the thermocline and by over two orders of magnitude at 4000 m depth. KE at 4000 m depth decreases moving up the Continental Rise from the Gulf Stream. The relative vertical distribution of KE is found to be less depth dependent near the Gulf Stream (with increasing KE?) than in the interior of the gyre. The vertical structure in the MODE-I region is an intermediate case. The shape of the frequency spectrum for KE varies with depth and with geographical position; spectral estimates in the thermocline from the MODE-I area (28N, 70W) are dominated by longer time scales, but at a site near the Gulf Stream (37.5N, 55W), where spectral shape does not vary strongly in the vertical, the thermocline spectrum is more weighted toward the temporal mesoscale (~50-100 days) that dominates at 4000 m depth in the MODE-I area. Spectral estimates at depth appear to be weighted toward shorter time scales moving from the Gulf Stream up the Continental Rise, Slope, and Shelf. If the frequency spectrum for KB is divided into two pieces with periods less than 100 days in one and greater than 100 days in the second, then: (1) The relative vertical KB distribution near the Gulf Stream does not differ significantly from band to band, whereas in the MODE-I region the lower frequencies are much more depth dependent than the higher. (2) The relative vertical KE distribution for periods less than 100 days is not appreciably dissimilar at the two locations, although KE level and total vertical structure are distinct. (3) The relative vertical KE distribution at the lower frequencies in the MODE-I region does not differ significantly from the vertical distribution of total KB at a site farther into the interior of the gyre. Time averaged currents at all depths where data are available show a relatively narrow (~200 km) and comparatively depth-independent return flow for the Gulf Stream system along 55W; transporting roughly 70 X 106 m3 s-1 westward, with about 25 X 106 m3 s-1 relative to the bottom and approximately 45 X 106 m3 s-1 associated with the "bottom transport."
SOFAR float trajectories from an experiment to measure the Atlantic cross equatorial flow (1989-1990)

(Woods Hole Oceanographic Institution, 1992-08) Richardson, Philip L. ; Zemanovic, Marguerite E. ; Wooding, Christine M. ; Schmitz, William J. ; Price, James F.

Neutrally buoyant SOFAR floats at nominal depths of 800, 1800, and 3300 m were tracked for 21 months in the vicinity of western boundary currents near 6N and at several sites in the Atlantic near 11N and along the equator. Trajectories at 1800 m show a swift (>50 cm/sec), narrow (100 km wide) southward-flowing Deep Western Boundary Current (DWBC) extending from 7N to the equator. At times (February-March 1989) DWBC water turned eastward and flowed along the equator and at other times (August-September 1990) the DWBC crossed the equator and continued southward. The mean velocity near the equator was eastward from February 1989 to February 1990 and westward from March 1990 to November 1990. Thus the cross-equatorial flow in the DWBC appeared to be linked to the direction of equatorial currents which varied over periods of more than a year. No obvious DWBC nor swift equatorial current was observed by 3300 m floats. Eight-hundred-meter floats revealed a northwestward intermediate level 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/sec. Six 800 m floats drifted eastward along the equator between 5S and 6N at a mean velocity of 11 cm/sec; one reached 5W in the Gulf of Guinea, suggesting that the equatorial current extended at least 35-40° along the equator. Three of these floats reversed direction near the end of the tracking period, implying low frequency fluctuations.
Large cyclonic rings from the northeast Sargasso Sea

(Woods Hole Oceanographic Institution, 1978-12) McCartney, Michael S. ; Worthington, L. Valentine ; Schmitz, William J.

Expendable bathythermograph observations have revealed large cold core cyclonic current rings to the east of 60°W in a region that mechanical bathythermograph observations (Parker, 1971) indicated to be devoid of rings. As a class these rings are larger than typical Gulf Stream rings that form and drift west of 60°W. The typical diameter (15°C at 500 m) there is around 100 km, while the eastern Sargasso rings are 200 km and more in diameter. Several of these eastern rings were observed on each of four cruises in the northern Sargasso Sea in 1974 and 1975. The overall picture of the region east of 60°W obtained was a very noisy one, dominated by large‐diameter, large‐amplitude eddies. One of the eastern rings was seen in all four cruises and was observed to drift westward for over 730 km at an average speed of 4.4 km/d, starting at 56°30′W and 34°40′N and passing north of Bermuda. The character of the dissolved oxygen anomalies in the cores of the eastern rings suggests a possible formation region at the eastern end of the Sargasso Sea gyre, around 40°W. Hence the eastern rings may have already been a year old when first observed in November 1974. A single deep hydrographic section showed the center of the deep circulation to lie considerably further southwest than the near‐surface circulation center, although this could be a distortion due to a large seamount. Moored current meter data suggest a level of no motion within eastern rings at about 2000 m, giving a weak anticyclonic circulation of 4 × 106 m3/s below that level, compared with the 45 × 106 m3/s cyclonic circulation above 2000 m. On several occasions, smaller‐scale upward displacements of the thermal structure were seen at the sides of eastern rings. It is not known whether these represented interactions with smaller rings or some breakdown of the circular symmetry.
SOFAR float trajectories in the tropical Atlantic 1989-1992

(Woods Hole Oceanographic Institution, 1994-09) Richardson, Philip L. ; Zemanovic, Marguerite E. ; Wooding, Christine M. ; Schmitz, William J.

Neutrally buoyant SOFAR floats at nominal depths of 800 m, 1800 m, and 3300 m were tracked acoustically for 3.7 years in the vicinity of the western boundary and the equator of the Atlantic Ocean. Trajectories and summaries from the whole experiment are shown along with detailed trajectories from the second setting of the listening stations, October 1990 to September 1992. Some highlights are mentioned below. Trajectories at 1800 m revealed a swift narrow southward flowing deep western boundary current (DWBC) extending from 7°N across the equator. Two floats directly crossed the equator in the DWBC and went to 10°S. Two other floats left the DWBC near the equator and drifted eastward. Three floats entered the DWBC from the equatorial current system and drifted southward. No obvious DWBC or swift equatorial currents were observed by the 3300 m floats. The 800 m floats plus some surface drifters measured seven anticyclonic eddies as they translated northwestward along the coast of South America in a band from the equator to 12°N. One of the floats (28) entered the Caribbean where tracking stopped. This float was again tracked as it drifted across the mid-Atlantic Ridge and entered the Canary Basin near 34°N 28°W after a gap of 2.7 years. We infer that this float went westward though the Caribbean and northeastward in the Gulf Stream. Float 17 drifted northward from 10°N to 22°N in an eastern boundary current off the coast of West Africa. Floats between 6°N-6°S (roughly) drifted long distances zonally in the equatorial current system.

Schmitz William J.

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