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James F. Price

Abstract

The structure and dynamics of a rain-formed mixed layer (ML) are studied using hourly STD and profiling current meter casts. Because the fluid beneath the rain-formed ML was vertically homogeneous, the ML buoyancy g′ and horizontal velocity difference δV were easily observable as the ML depth h increased by entrainment from 7 to 18 m in a period of 8 h. The overall Richardson number of the mixed layer g′(h + d/2)/δV 2 ≈ 0.7 during that period, where d is the thickness of the transition layer at the base of the ML. The entrainment rate was consistent with that of a laboratory surface half-jet (Ellison and Turner, 1959).

The transition layer thickness was an appreciable fraction of the ML thickness, roughly dh/3. The density profile through the transition layer was linear and symmetric, and the overall Richardson number of the transition layer gdV 2 ≈ 1/4.

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James F. Price

Abstract

The upper ocean response to a moving hurricane is studied using historical air-sea data and a three-dimensional numerical ocean model. Sea surface temperature (SST) response is emphasized. The model has a surface mixed-layer (ML) that entrains according to a velocity dependent parameterization, and two lower layers that simulate the response in the thermocline.

The passage of Hurricane Eloise (1975) over buoy EB-10 is simulated in detail. SST decreased 2°C as Eloise passed directly over EB-10 at 8.5 m s−1. Model results indicate that entrainment caused 85% of the irreversible heat flux into the ML; air-sea heat exchange accounted for the remainder. The maximum SST response was predicted to be −3°C and to occur 60 km to the right of the hurricane track. This is consistent with the well-documented rightward bias in the SST response to rapidly moving hurricanes. The rightward bias occurs in the model solution because the hurricane wind-stress vector turns clockwise with time on the right side of the track and is roughly resonant with the ML velocity. High ML velocities cause strong entrainment and thus a strong SST response.

Model comparisons with EB-10 data suggest that a wind-speed-dependent drag coefficient similar to Garratt's (1977) is appropriate for hurricane conditions. A constant drag coefficient 1.5 × 10−3 underpredicts the amplitude of upwelling and the SST response by ∼40%.

Numerical experiments show that the response has a lively dependence on a number of air-sea parameters. Intense, slowly moving hurricanes cause the largest response. The SST response is largest where cold water is near the sea surface, i.e., where the initial ML is thin and the upper thermocline temperature gradient is sharp.

Nonlocal processes are important to some aspects of the upper ocean response. Upwelling significantly enhances entrainment under slowly moving hurricanes (≲4 m s−1) and reduces the rightward bias of the SST response. Horizontal advection dominates the pointwise ML heat balance during the several-day period following a hurricane passage. Pressure gradients set up by the upwelling do not play an important role in the entrainment process, but are an effective mechanism for dispersing energy from the ML over a 5–10 day time scale.

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James F. Price

Abstract

The ocean's baroclinic response to a steadily moving storm is analyzed using a numerical model for an inviscid, multi-layered fluid. This first part of a two-part study gives a detailed account of the response to a rapidly moving hurricane, while parameter dependence is examined in the second part. A central theme of both parts is the coupling between wind-forcing, the surface mixed layer, and the thermocline.

The baroclinic response is made up of a geostrophic component and a three-dimensional wake of inertial-internal waves which is emphasized. These waves initially have large horizontal spatial scales set directly by the storm. Their along-storm track wavelength is the storm translation speed times the wave period, which is typically five percent less than the local inertial period. Their cross-track scale is the storm scale. If the storm is intense as it is here, finite amplitude effects soon produce a double inertial frequency wave and smaller spatial scales.

An important qualitative result is that the vertical penetration scale is large compared to the thermocline thickness. The initial isopycnal displacement is almost uniform through the thermocline, and the associated pressure field couples the mixed layer to the entire thermocline. Vertical energy propagation is thus very rapid new the storm track, O(100 m day−1), and largely responsible for a rapid post-storm decay of mixed-layer inertial motion (e-folding in ∼5 inertial periods).

Measurements made by buoy EB-10 in the wake of Hurricane Eloise provide a semi-quantitative check on the model results. The model-computed decay of mixed-layer inertial motion and its blue shift are roughly consistent with the EB-10 measurements. A large vertical penetration scale is evident in both the measured velocity and the isopycnal displacement.

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Jiayan Yang and James F. Price

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This paper examines the role of potential vorticity (PV) balance in source- and sink-driven flows between two basins. As shown in previous studies, PV advection into a basin, say a positive PV advection, requires a negative frictional torque to maintain a steady PV balance. This sense of torque may be provided by a cyclonic boundary current within the basin. The PV advection through a channel is due almost entirely to advection of planetary PV, f/H, where f is the Coriolis parameter and H is the column thickness. Therefore a localized change of depth, and thus H in the channel, directly affects the PV transport and will result in a basinwide change of the circulation pattern. For example, if the channel depth is made shallower while holding the transport fixed, the PV advection is then increased and the result may be a strong recirculation within the basin, as much as two orders of magnitude greater than the transport through the channel. When the basins are connected by two channels at different latitudes or with different sill depths, the throughflow is found to be divided between the two channels in a way that satisfies the integral constraint for flow around an island. The partition of the flow between two channels appears to be such as to minimize the net frictional torque. In still another set of experiments, the large-scale pressure difference (layer thickness) between the basins is specified and held fixed, while the throughflow is allowed to vary in response to changes in the frictional torque. The interbasin transport is strongly influenced by the length of the boundary or the magnitude of the viscosity in the sense that a greater PV frictional torque allows a greater PV transport and vice versa. This result is counterintuitive, if it is assumed that the throughflow is determined by viscous drag within the channel but is a straightforward consequence of the basin-scale PV balance. Thus, the important frictional effect in these experiments is on the basin-scale flow and not on the channel scale.

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James R. Valdes and James F. Price

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The authors have designed and deployed a neutrally buoyant sediment trap (NBST) intended for use in the upper ocean. The aim was to minimize hydrodynamic flow interference by making a sediment trap that drifted freely with the ambient current. The principal design problem was to make the NBST descend to and stay near a prescribed depth. For a variety of reasons, the most success has been with NBSTs that were autoballasted by means of a microprocessor-controlled volume changer. Autoballasting NBSTs has demonstrated an ability to hold a prescribed depth to within 10 m.

There have been two successful, concurrent deployments of NBSTs and conventional surface-tethered sediment traps (STSTs) at the Bermuda Atlantic Times Series site. During both periods the observed flow past the STSTs was low, about 0.05 m s−1, so that hydrodynamic effects on the STSTs would have been minimized. Comparisons of the trap results (described in a companion paper by Buesseler et al.) indicate that the total mass of collected material was generally similar in the two traps. Other variables, including the composition of the material and the fraction contributed by swimmers, were markedly different (swimmers are small animals that enter a trap intact and presumably alive). These are intriguing results but could not be conclusive since there is no absolute standard for such measurements. Future field work that includes comprehensive geochemical sampling will be required to learn which sediment trapping method yields the more useful observations.

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Michael A. Spall and James F. Price

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The outflow through Denmark Strait shows remarkable mesoscale variability characterized by the continuous formation of intense mesoscale cyclones just south of the sill. These cyclones have a diameter of about 30 km and clear signatures at the sea surface and in currents measured near the bottom. They have a remnant of Arctic Intermediate Water (AIW) in their core.

The authors’ hypothesis is that these cyclones are formed by stretching of the high potential vorticity (PV) water column that outflows through Denmark Strait. The light, upper layer of the outflow, the East Greenland Current, remains on the surface in the Irminger Sea, while the dense overflow water descends the east Greenland continental slope. The midlevel waters, mostly AIW, could thus be stretched by more than 100%, which would induce very strong cyclonic relative vorticity.

The main test of this new hypothesis is by way of numerical experiments carried out with an isopycnal coordinate ocean model configured to have a marginal sea connected to a deep ocean basin by a shallow strait. An outflow is produced by imposing buoyancy forcing over the marginal sea. If the buoyancy forcing is such as to produce a single overflow layer (analogous to the overflows through the Strait of Gibraltar and the Faroe Bank Channel), then the resulting overflow is slightly time dependent. If the buoyancy forcing is such as to produce both a deep overflow and a midlevel outflow (analogous to the AIW), then the resulting outflow is highly time dependent and develops intense midlevel cyclones just south of the sill where the dense overflow water begins to descend the continental slope. The cyclones found in the numerical solutions have time and space scales set by the midlevel outflow transport, the bottom slope, and the deep stratification. Their scales and structure are roughly consistent with the cyclones observed south of the sill in Denmark Strait.

High PV outflow through Denmark Strait is a result of the large-scale wind and buoyancy forcing over the Norwegian–Greenland Sea and Denmark Strait’s location on a western boundary. So far as we know, this configuration and this specific form of mesoscale variability are unique to Denmark Strait.

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Shinichiro Kida, James F. Price, and Jiayan Yang

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The oceanic response to overflows is explored using a two-layer isopycnal model. Overflows enter the open ocean as dense gravity currents that flow along and down the continental slope. While descending the slope, overflows typically double their volume transport by entraining upper oceanic water. The upper oceanic layer must balance this loss of mass, and the resulting convergent flow produces significant vortex stretching. Overflows thus represent an intense and localized mass and vorticity forcing for the upper ocean. In this study, simulations show that the upper ocean responds to the overflow-induced forcing by establishing topographic β plumes that are aligned more or less along isobaths and that have a transport that is typically a few times larger than that of the overflows. For the topographic β plume driven by the Mediterranean overflow, the occurrence of eddies near Cape St. Vincent, Portugal, allows the topographic β plume to flow across isobaths. The modeled topographic β-plume circulation forms two transatlantic zonal jets that are analogous to the Azores Current and the Azores Countercurrent. In other cases (e.g., the Denmark Strait overflow), the same kind of circulation remains trapped along the western boundary and hence would not be readily detected.

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Molly O’Neil Baringer and James F. Price

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Field data taken in the Gulf of Cadiz have been analyzed to describe some aspects of the momentum andenergy balance of the Mediterranean outflow. A crucial component of the momentum balance is the total stress(entrainment stress and bottom drag), which has been estimated from a form of the Bernoulli function evaluatedfrom density and current observations.

For the first 60 km west of the Camarinal Sill the outflow was confined within a narrow channel on thecontinental shelf. At about 70 km downstream the outflow crossed over the shelf–slope break and began todescend the continental slope. The buoyancy force increased substantially, and the outflow underwent a geostrophic adjustment, albeit one heavily influenced by mixing and dissipation. The current direction turned 90degrees to the right at a near-inertial rate. In this region, the estimated geostrophic velocity greatly underestimatedthe actual current, and the estimated curvature Rossby number was about 0.5. Current speeds were in excessof 1 m s−1 and the total stress was as large as 5 Pa. The entrainment stress, estimated independently fromproperty fluxes, reached a maximum of about 1 Pa, or considerably smaller than the inferred bottom stress.

By about 130 km downstream, the current was aligned approximately along the local topography. The currentamplitude and the estimated stress were then much less, about 0.3 m s−1 and 0.3 Pa. The entrainment stress wasalso very small in this region well downstream of the strait. This slightly damped geostrophic flow continuedon to Cape St. Vincent where the outflow began to separate from the bottom.

Bottom stress thus appears to be a crucial element in the dynamics of the Mediterranean outflow, allowingor causing the outflow to descend more than a kilometer into the North Atlantic. In the regions of strongestbottom stress the inferred drag coefficient was about 2 − 12 (× 10 −3) depending upon which outflow speedis used in the usual quadratic form. Entrainment stress was small by comparison to the bottom stress, but theentrainment effect upon the density anomaly was crucial in eroding the density anomaly of the outflow. Theobserved entrainment rate appears to follow, roughly, a critical internal Froude number function.

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Shinichiro Kida, Jiayan Yang, and James F. Price

Abstract

Marginal sea overflows and the overlying upper ocean are coupled in the vertical by two distinct mechanisms—by an interfacial mass flux from the upper ocean to the overflow layer that accompanies entrainment and by a divergent eddy flux associated with baroclinic instability. Because both mechanisms tend to be localized in space, the resulting upper ocean circulation can be characterized as a β plume for which the relevant background potential vorticity is set by the slope of the topography, that is, a topographic β plume.

The entrainment-driven topographic β plume consists of a single gyre that is aligned along isobaths. The circulation is cyclonic within the upper ocean (water columns are stretched). The transport within one branch of the topographic β plume may exceed the entrainment flux by a factor of 2 or more.

Overflows are likely to be baroclinically unstable, especially near the strait. This creates eddy variability in both the upper ocean and overflow layers and a flux of momentum and energy in the vertical. In the time mean, the eddies accompanying baroclinic instability set up a double-gyre circulation in the upper ocean, an eddy-driven topographic β plume. In regions where baroclinic instability is growing, the momentum flux from the overflow into the upper ocean acts as a drag on the overflow and causes the overflow to descend the slope at a steeper angle than what would arise from bottom friction alone.

Numerical model experiments suggest that the Faroe Bank Channel overflow should be the most prominent example of an eddy-driven topographic β plume and that the resulting upper-layer transport should be comparable to that of the overflow. The overflow-layer eddies that accompany baroclinic instability are analogous to those observed in moored array data. In contrast, the upper layer of the Mediterranean overflow is likely to be dominated more by an entrainment-driven topographic β plume. The difference arises because entrainment occurs at a much shallower location for the Mediterranean case and the background potential vorticity gradient of the upper ocean is much larger.

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Molly O’Neil Baringer and James F. Price

Abstract

Hydrographic and current profiler data taken during the 1988 Gulf of Cadiz Expedition have been analyzed to diagnose the mixing, spreading, and descent of the Mediterranean outflow. The θS properties and the thickness and width of the outflow were similar to that seen in earlier surveys. The transport of pure Mediterranean Water (i.e., water with S ≥ 38.4 psu) was estimated to be about 0.4 × 106 m3 s−1, which is lower than historical estimates—most of which were indirect—but comparable to other recent estimates made from direct velocity observations.

The outflow transport estimated at the west end of the Strait of Gibraltar was about 0.7 × 106 m3 s−1 of mixed water, and the transport increased to about 1.9 × 106 m3 s−1 within the eastern Gulf of Cadiz. This increase in transport occurred by entrainment of fresher North Atlantic Central Water, and the salinity anomaly of the outflow was consequently reduced. The velocity-weighted salinity decreased to 36.7 psu within 60 km of the strait and decreased by about another 0.1 before the deeper portion of the outflow began to separate from of the bottom near Cape St. Vincent. Entrainment appears to have been correlated spatially with the initial descent of the continental slope and with the occurrence of bulk Froude numbers slightly greater than 1. In the western Gulf of Cadiz, where entrainment was much weaker, Froude numbers were consistently well below 1.

The outflow began in the eastern Strait of Gibraltar as a narrow (10 km wide) current having a very narrow range of θS properties. The outflow broadened as it descended the continental slope of the northern Gulf of Cadiz and reached a maximum width of 80 km in the western Gulf of Cadiz. The descent of the outflow was very asymmetric: The southern (offshore) edge of the outflow descended about 1000 m from Gibraltar to Cape St. Vincent, while the northern (onshore) edge of the outflow descended only a few hundred meters. The northern, onshore side thus remained considerably higher in the water column and thus entrained relatively warm North Atlantic Central Water. This caused the outflow to develop horizontal θS variability and, by about 140 km downstream, the across-stream variation in temperature on an isopycnal was more than 2°C.

Much of the volume transport in the western Gulf of Cadiz was contained within two preferred modes or cores. The deeper, offshore core had a central σ θ = 27.8 kg m−3, and the shallower onshore core, which was still in contact with the bottom in the Gulf of Cadiz, had a central σ θ = 27.5 kg m−3. These two cores develop as a result of the spreading and horizontally varying entrainment noted above, combined with topographic steering.

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