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Melinda M. Hall

Abstract

Thirty-six XBT temperature profiles have been used in a parametric model introduced by Hendry to model the Gulf Stream's thermal structure at 65°W between 200 and 1200 dbar, with an rms residual error of 0.56°C. Velocity has been computed geostrophically relative to 1200 dbar, and has been included in calculating potential vorticity analytically from the model. The resulting potential vorticity section for 65°W has been compared with the analogous result from Hendry's parametric model at 59°W, as well as observed potential vorticity sections from 68° to 55°W. There is a significant feature in the potential vorticity structure at 65°W not found at 59°W-namely, a relative minimum in potential vorticity along isopycnals, centered at the Gulf Stream's axis and 350 dbar. The modeled potential vorticity sections are consistent with the observation including the downstream disappearance of this feature. The dynamical implications of these results are briefly discussed.

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Melinda M. Hall

Abstract

A curent meter mooring, instrumented from the bottom into the thermocline, was deployed in the Gulf stream at 68°W for a year. Data from the uppermost instrument indicate the Gulf Stream moved back and forth across the mooring site, so that the horizontal as well as vertical structure of the Stream may be deduced. The two key points to the success of the analysis are: 1) the well-defined relationship between temperature and cross-stream distance in the thermocline, enabling the use of the former as a horizontal coordinate; and 2) a daily-changing definition of Gulf Stream flow direction based on the shear between the thermocline and 2000 m depth. Time-series of daily-rotated velocities may be used to calculate empirical orthogonal functions for the along- and cross-stream vertical structures, which are decoupled and are respectively baroclinic and barotropic. Using the inferred horizontal coordinate one can estimate masss, momentum and kinetic energy fluxes agree well with historical data. Bryden's method has been used to calculate vertical velocities from the temperature equation; the resulting time-series of w are visually coherent throughout the water column and their vertical amplitude structure looks like that of a first baroclinic mode. The rms vertical velocities are large [O(.05 cm s−1)], and these as well as other estimates have been used to explore the validity of the quasi-geostrophic approximation at the mooring site. The Rossby number for the thermocline flow is about 0.3, and for the deep flow is ≤ 0.1.

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Melinda M. Hall

Abstract

A simplistic interpretation of eddy heat fluxes from a two-year current meter mooring deployment in the Kuroshio Extension leads to the conclusion that the eddy field is denying at 152°E, contradicting observations from the surface to 300 m that indicate the region to be one of steady or growing eddy energy. Thus, a simplified version of the method used by Hall to construct the velocity field of the current from the moored data has been used to examine the baroclinic and barotropic energy conversions in the cyclonic and anticyclonic portions of the current, for both geographic and ‘stream’ coordinates. Although the error bars are large, in stream coordinates significant conversions of mean to eddy potential energy occur on the anticyclonic side of the current at both 350 and 625 dbar, with smaller average conversions of eddy to mean energy over the cold portion. Barotropic conversions in this coordinate system are small, but qualitatively the calculated Reynolds stresses agree with previous observations showing that (uv′)/∂y < 0 across the current, so that on average they converge mean momentum. For geographical coordinates, integrated energy balances still suggest overall decay of eddy energy, though not as strong as that found in the “simplistic” interpretation. Reynolds stresses are much stronger than for stream coordinates, and are still convergent, resulting in relatively large apparent conversions of eddy to mean kinetic energy in this coordinate system. Comparison with a similar energetic analysis by Rossby in the Gulf Stream at 73°W shows that: 1) the effects of going from geographical to stream coordinates are similar for the two currents, and 2) at locations that are geographically comparable for the two currents, very different energetic regimes prevail. Dynamical differences are also reflected in the vertical velocity structure. It is hypothesized that external factors, such as the nature of the underlying deep flow, may influence the western boundary current systems in the two oceans in an important way.

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Pearn P. Niiler and Melinda M. Hall

Abstract

A current meter mooring maintained for over three years at 28°N, 152°W, in the eastern North Pacific has yielded velocity and temperature data throughout the water column, with particularly good thermocline resolution The flow is characterized by weak primarily westward mean velocities, with a superimposed eddy field having rms velocities ranging from 10 cm s−1 in the upper thermocline to 3 cm s−1 at 1000 m depth. The eddy energy is divided into two main bands: the low frequency eddies have spatial scales of 250–300 km and periods of 100–200 days, propagate southwestward, and have slightly more zonal than meridional energy. The high frequency eddies also propagate southwestward, have spatial scales of 150–175 km and periods of 40–80 days, and are strongly meridionally oriented. Vertical EOF structure calculated in the frequency domain suggests that the low frequency eddies are more wavelike (linear) in nature than are the high frequency. The entire band appears to derive energy baroclinically from a secularly varying background flow; as a function of time, the eddy heat flux tends to be down the very low frequency varying temperature gradient. Some interesting points of comparison are found with eddies in a three-layer nonlinear model of the eastern North Pacific recently described by Lee.

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Melinda M. Hall and Nick P. Fofonoff

Abstract

Two CTD sections across the Gulf Stream at 68° and 55°W were acquired in late March of 1988 within 11 days of one another as part of an effort to look at downstream changes in the current. Using complementary current meter measurements, sections of total barotropic and geostrophic baroclinic velocity are constructed and used to calculate transport in potential density classes. Potential vorticity sections are presented for both locations, including the effects of planetary, stretching, and relative vorticity. The data are also used to examine the core properties of recently formed 18°Water at the two sections. It is found that: 1) water parcels in the exposed surface layers experience downstream density and potential vorticity changes consistent with surface forcing; 2) thermocline Gulf Stream transport is conserved downstream and below the exposed layers is conserved within individual density classes; 3) subthermocline Gulf Stream transport increases modestly at levels above the sill depth of the New England Seamounts but quadruples at levels below that; 4) the calculated potential vorticity structure is consistent with the transport distribution and historical observations and displays several distinct layers; and 5) transport and potential vorticity distributions together suggest that five active layers and steep bottom topography are required to fully describe downstream evolution of the Gulf Stream as an open-ocean eastward jet.

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Melinda M. Hall, Michael McCartney, and J. A. Whitehead

Abstract

A moored array at the equator in the western basin of the Atlantic provides a 604-day time series of abyssal currents and temperatures spanning the full breadth of the Antarctic Bottom Water (AABW) flowing from the Brazil Basin to the Guiana Basin. Mean AABW transport is estimated to be 2.0 Sv (Sv ≡ 106 m3 s−1), comprising organized westward flow of 2.24 Sv and return flow of 0.24 Sv. The low-frequency variability is dominated by a quasi-annual transport cycle of amplitude 0.9 Sv and a 120-day period of amplitude 0.6 Sv. Maximum transports occur in September–October, while minimum transports occur in February–March. Allowing for this quasi-annual cycle and extrapolating the 604-day record to a full two years adds about 7% to the estimated mean AABW transport. The array also provides limited sampling in the overlying lower North Atlantic Deep Water (LNADW), where a southern boundary intensified flow of LNADW gives the strongest recorded mean speed through the array, 9.9 cm s−1 into the Brazil Basin. The LNADW records also have a quasi-annual cycle with strong LNADW flow episodes occurring in April–May. Time series of temperature indicate that the LNADW/AABW transition layer rises and falls in synchrony with the quasi-annual AABW transport cycle (uplifted transition layer during strong AABW transport periods). An observed overall warming trend appears to be accompanied by a decline in AABW transport.

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Michele Y. Morris, Melinda M. Hall, Louis C. St. Laurent, and Nelson G. Hogg

Abstract

One of the major objectives of the Deep Basin Experiment, a component of the World Ocean Circulation Experiment, was to quantify the intensity and spatial distribution of deep vertical mixing within the Brazil Basin. In this study, basin-averaged estimates of deep vertical mixing rates are calculated using two independent methodologies and datasets: 1) vertical fluxes are derived from large-scale temperature and density budgets using direct measurements of deep flow through passages connecting the Brazil Basin to surrounding basins and a comprehensive hydrographic dataset within the basin interior and 2) vertical mixing rates are estimated from finescale bathymetry and hydrographic data using a functional relationship between turbulent dissipation and bathymetric roughness, deduced from localized measurements of ocean microstructure obtained during the Deep Basin Experiment. The space–time mean estimates of vertical mixing diffusivities across representative surfaces within the Antarctic Bottom Water layer fell in the range κ ∼ 1–5(× 10−4 m2 s−1) and were indistinguishable from each other within the estimation uncertainties. The mixing rates inferred from potential temperature budgets update, and are consistent with, earlier estimates that were based on less data. Mixing rates inferred from budgets bounded by neutral surfaces are not significantly different from the former. This implies that lateral eddy fluxes along isopycnals are not important in the potential temperature budgets, at least within the large estimation uncertainties. Unresolved processes, such as cabbeling and low frequency variability, which complicate inference of mixing from large-scale budgets, have been considered. The agreement between diffusivity estimates based on a modeled relationship between bathymetric roughness and turbulent dissipation, with those inferred from large-scale budgets, provides independent confirmation that the mixing rates have been accurately quantified.

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