Nelson G. Hogg

The heton, an elementary interaction between discrete baroclinic geostrophic vortices, and its implications concerning eddy heat-flow

Among the interactions of two discrete baroclinic geostrophic vortices in a two-layer system there is one class of interaction that is non-trivial; when the two vortices are of opposite sign and in different layers, and close enough together, they transport heat. Because this particular interaction can transport heat, we propose to call it the heton. It is a tilted baroclinic pair. In the Northern Hemisphere it transports heat to the left of the direction toward which its top tilts. Two warm or two cold hetons repel one another when outside the radius of deformation. A warm and a cold heton attract one another. A simple two-heton engine that exhibits vortex splitting, loss of available potential energy, and meridional heat transport is presented.

On the stratified Taylor column

We analyse the effects of small, circularly symmetric topography on the slow flow of an inviscid, incompressible, diffusionless, horizontally uniform, baroclinic current and show that the vertical influence depends primarily on three parameters: a stratification measure S (the square of the ratio of buoyancy frequency times height scale to Coriolis parameter times length scale), a topographic parameter β (ratio of scaled topographic height multiplied by scaled bottom current to Rossby number e) and the scaled upstream shear u ′ 0 ( z ) (the dimensional upstream shear divided by the ratio of the r.m.s. upstream flow speed to height scale). Investigating a linear stratification model we find that the topographic effect is depth independent if S [lsim ] e and a Taylor column, as indicated by the appearance of closed streamlines above the bump, exists when β > 2. Moderate stratification ( S ∼ 1) causes the flow to be fully three-dimensional and the Taylor column to be a conical vortex whose height depends on β S and u ′ 0 ). The results are compared with Daviess (1971, 1972) experiments. Our results tend to support the Taylor column theory of Jupiters Great Red Spot but effects due to variations in the Coriolis parameter with latitude have been (unjustifiably) ne glected. Using typical values for the earths oceans we find that Taylor columns of significant height could be found there. Some pertinent observations from the ocean are discussed.

The northern recirculation gyre of the gulf Stream

Abstract Results from two recent field programsin the western North Atlantic are presented with particular emphasis on the deep circulation. New long-term moored current measurements show that the flow north of the Gulf Stream and east of the New England Seamount Chain is toward the west from 500 m to the bottom with very little depth dependence. Nearly 40 × 106 m3s−1 is transported to the west near 63°W, and half of this recirculates back to the east over the Seamount Chain to add a strong component to the deep Gulf Stream between the Chain and the Grand Banks. We call this current the “Northern Recirculation Gyre” in contrast with a similar feature to the south of the Stream popularly known as the “Worthington Gyre” ( Worthington , 1976, The Johns Hopkins Oceanographic Studies, 6, 110 pp.). The new gyre is similar to that proposed by Hogg (1983, Deep-Sea Research, 30, 945–961) but somewhat smaller in scale. Its relationship to the Gulf Stream and the Deep Western Boundary Current is made explicit by the new measurements. Tracer measurements show that the Northern Recirculation Gyre exchanges water properties with the Deep Western Boundary Current where the two are in close proximity along the northern boundary. The relatively high values of oxygen and freon, so imparted, are then advected to the interior where the gyre carries water eastward under the Gulf Stream. Beneath the thermocline these tracer fields are practically homogenous within the gyre, perhaps a reflection of the expulsion process described by Rhines and Young (1983, Journal of Fluid Mechanics, 133, 133–145). An advective-diffusive model is used to interpret some slight differences between the various tracer distributions.

A note on the deep circulation of the western North Atlantic: its nature and causes

Abstract Based on presently available long-term moored current meter measurements, the abyssal circulation in the western North Atlantic is hypothesized to have two components. One is above the 4000-m isobath and transports about 10 × 106 m3 s−1 of water along the whole of the continental rise and slope; this is the classical thermohaline Deep Western Boundary Current. The second component is further offshore and takes the form of two or more gyres each recirculating as much as 40 × 106 m3 s−1 with little direct influence of the New England seamounts although in their neighborhood. The circulation scheme is in disagreement with other schemes, principally in the nature of the recirculating gyres. The smaller and more southern of the recirculating gyres was suggested by Schmitz (1977, Journal of Marine Research, 35, 21–28). Calculations based on POLYMODE Array 2 current meter data indicate that both lateral and vertical eddy momentum fluxes are important in driving the gyres. The vertical or ‘thickness’ flux appears to be dominant in the most intense region near the Gulf Stream. It is suggested that the principal role played by the New England seamounts is to intensity the eddy field thereby inducing a divergence of the Reynold stresses.

On the transport of the gulf stream between cape hatteras and the grand banks

Abstract Direct velocity observations from four locations in the Gulf Stream between Cape Hatteras and the Grand Banks are analysed to determine the baroclinic and barotropic transport of the Stream. Three of the sites are moored arrays where a method similar to that proposed by Hall and Bryden (1985, Geophysical Research Letters , 12 , 203–206) and Hall (1986, Journal of Physical Oceanography , 16 , 1814–1818) is used; the fourth is the Pegasus site of Halkin and Rossby (1985, Journal of Physical Oceanography , 15 , 1439–1452). The baroclinic transport is found to change little from site to site while the barotropic changes substantially. Total transport reaches 150 Sv at 60°W and remains at that strength to 55°W, the farthest east that moored information is available. These results, augmented by additional information, are used to deduce a scheme for the total transport steamfunction in the western North Atlantic.

Oceanic observations of stratified Taylor columns near a bump

Abstract Analyses of two hydrographic sections and records from moored instruments show that the flow over a 400-m high seamount, centred at 36°N, 55°W in the recirculation region of the Gulf Stream system, resembles a stratified Taylor column. Potential density sections indicate a bottom intensified uplifting over the seamount with a vertical scale of approx. 3 km. Time series of relative vorticity deduced from the current meter records show a significant correlation with the bathymetry and a similar vertical scale as seen in the density structure. Estimates of the vorticity balance nominally 1000 m above the bottom demonstrate that the balance is between the advection of relative vorticity and vertical vortex stretching which is consistent with Taylor column behavior. Objective stream function maps at the same depth show closed contours near the seamount for most of the nine-month duration of the moored array. The results suggest that the bathymetry distorts the larger scale mean circulation so as to produce large amplitude, small scale variability which should be taken into account before one extrapolates isolated measurements from moored instruments.

Determining the strength of the deep western boundary current using the chlorofluoromethane ratio

Abstract The dilution of a passive tracer during deep water formation and the subsequent advection/mixing in a deep boundary current is modeled with application to chlorofluoromethanes (CFMs) in the North Atlantic. Two different types of boundary currents are considered: a uniform flow and a simple shear flow. In each case the core of the flow mixes with surrounding water, which continually accumulates CFMs. In an extreme case the coupled systems predicts that the CFM ratio in the current is unaltered from the ratio of its source water (save for a time lag). More realistic cases however suggests that the ratio is not a conserved quantity, but is substantially altered in both the overflow basin and boundary current. Matching the model results to CFM data collected near the Grand Banks gives a predicted (average) core speed of 5–10 cm s−1 for the Deep Western Boundary Current, and provides a constraint on the transport and diffusivity of the flow as well.

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 ...

Western boundary currents

The past decade has seen considerable progress toward clarifying the mean circulation of the Worlds oceans. At the same time we have come to realize that the specification of a mean circulation is difficult as there is energy at all time scales which are quantifiable and the spectrum is usually red. The western boundary currents (WBCs) of the oceans are the principal conduits for communication between the equatorial regions, where heat is added to the oceans and the polar regions where it is removed. Understanding how these current systems work is fundamental to understanding the earths global climate engine. Several substantial observational programs focussed on WBCs have been undertaken in recent years especially within the North and South Atlantic. These include studies of the Brazil-Malvinas Confluence and the South Atlantic Ventilation Experiment (SAVE) within the South Atlantic, the Western Tropical Atlantic Experiment (WESTRAX) in the tropics, and the Subtropical Atlantic Climate Study (STACS) and Synoptic Ocean Prediction experiment (SYNOP) in the subtropics. We shall concentrate in this review with a summary of the results from these programs but will also briefly cover new findings from other parts of the globe. Efforts connected with programs of the World Ocean Circulation Experiment in the Pacific and the South Atlantic (particularly the Deep Basin Experiment) are still underway and can be expected to make substantial contributions to the knowledge of WBCs in the future. The nature of this review also compells us to emphasize recent U.S. research but we will incorporate results from the international community, as well, especially in regions of the globe where the U.S. has done little. The reader may also wish to consult recent reviews by Ierley [1990] and Huang [1991], which discuss many issues relevant to WBCs in subtropical gyres.

Circulation and Variability at the Southern Boundary of the Brazil Basin

As a contribution to the WOCE Deep Basin Experiment, an array of current meters with individual record lengths exceeding ii years was set across the southern boundary of the Brazil Basin between early 1991 and early 1996. The array spanned the Santos Plateau, the Vema Channel, and the Hunter Channel, all areas believed to be important for transport of Antarctic Bottom Water between the Argentine and Brazil Basins. From the combination of geostrophic velocities computed from hydrographic stations and those directly measured, the total transport of bottom water (potential temperature below 2 degrees C) is estimated to be about 6.9 Sv (Sv = 10(6) m(3) s(-1)) northward, with about 4 Sv coming through the Vema Channel and the remainder through the Hunter Channel. Properties of the eddy field are also discussed. Eddy energy levels and their spatial distribution are similar to comparable regimes in the North Atlantic. Integral timescales vary from a few days to several weeks with distance from the Brazil Current and the western boundary. The eddy heat Bur is in the same direction as the heat advection by the mean flow but considerably smaller.