Melinda M. Hall

Antarctic Bottom Water Flux in the Equatorial Western Atlantic

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 s21), 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 quasiannual 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 s21 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.

Multiple deep gyres of the western North Pacific: A WOCE section along 149°E

The top to bottom large-scale ocean circulation in the northwest Pacific is described using a World Ocean Circulation Experiment (WOCE) onetime hydrographic section along 149°E between Papua New Guinea and Japan. The circulation is quantified using a combination of geostrophic and lowered acoustic Doppler current profiler velocity estimates. At the northern end of the section the flow regime is distinct in that the deep flow largely reflects that at the surface: the Kuroshio jet and its northern and southern recirculations have deep expressions. South of 25°N, the deep and bottom water flows do not mirror the surface flows, and the circulation assumes a highly baroclinic structure. Below the depth of local North Pacific ventilation the flow in the upper deep waters (800-2500 m) alternates in sign roughly every 10° of latitude revealing a set of deep clockwise gyres with significant transports of 40 Sv (1 Sv = 10 6 m 3 s -1 ) for a tropical gyre (south of 6°N) and 20 Sv in a subtropical gyre (6° - 24°N). These gyres provide a pathway for South Pacific influences to reach 22°N (the location of a strong water mass front) through exchange along the western boundary. Maps of properties on density surfaces suggest that the zonal extent of the upper deep water gyres found along 149°E is basin wide. Below 2500 m, flow across the section is isolated from the Philippine Sea by the Izu-Ogasawara-Mariana Ridge and the flow regime and property distribution reflect this: Lower Circumpolar Water flows west in a deep western boundary current near 10°N and coalesces at the Izu-Ogasawara-Mariana Ridge with a tongue of North Pacific Deep Water also flowing west near 15°N. About 4 Sv of a mixture of these waters flows east again near 25°N, associated with an abyssal water mass front. North of the front, the water properties are laterally homogeneous on density surfaces in the strongly recirculating gyres associated with the deep Kuroshio system.

Abyssal mixing in the Brazil Basin

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 ; 1‐5(3 1024 m2 s 21) and were indistinguishable k 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.

Horizontal and Vertical Structure of the Gulf Stream Velocity Field at 68°W

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...

Hemispheric asymmetry of deep water transport modes in the western Atlantic

Subtropical studies of the Atlantic meridional cold water flow show a hemispheric contrast in the dominant southward transport mode below 2000 m; in the North Atlantic, lower deep water (LDW) (1.8° ≤ θ ≤ 2.4°C) dominates with small transport of middle deep water (MDW) (2.4° ≤ θ ≤ 3.2°C), while in the South Atlantic, the opposite is observed. We use numerous observations in the western basins of the tropics to show that the transition occurs rapidly near the equator in the western Atlantic. A meridional section in the central Brazil Basin suggests zonal flows are responsible for the transition. LDW transport from the Guiana Basin (north of the equator) flows eastward in the northern Brazil Basin and is inferred to continue on through the Romanche Fracture Zone into the eastern Atlantic. An opposing flow of MDW from the eastern tropical Atlantic flows toward the western boundary, where it bifurcates to supply MDW to the Deep Western Boundary Current (DWBC) of the Brazil Basin, as well as to feed the northward flow of MDW in the Guiana Basin offshore of the DWBC. The magnitude of each of these oppositely directed flows is roughly 7 Sv. We furthermore speculate that they are connected predominantly by upwelling from LDW to MDW within the low-latitude eastern basin. The overall deep water transport system below 2000 m in the western basins of the mid- and low-latitude Atlantic is thus found to comprise the following three distinct components. (1) A strong DWBC transport of LDW with associated recirculation dominates the Guiana Basin north of the equator. (2) In the northern Brazil Basin (just south of the equator) a narrow eastward flow absorbs the LDW and carries it eastward, while a somewhat broader westward flow imports MDW into the western basin. (3) This MDW flow then bifurcates, with the southward branch causing the MDW dominance in the Brazil Basin, where the MDW dominated DWBC and associated recirculations are the third component of the deepwater transport system.

Energetics of the Kuroshio Extension at 35°N, 152°E

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 obser...

Downstream Development of the Gulf Stream from 68° to 55°W

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 th...

Low-Frequency Eddy Variability at 28°N, 152°W in the Eastern North Pacific Subtropical Gyre

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 fro...

Synthesizing the Gulf Stream Thermal Structure from XBT Data

Abstract Thirty-six XBT temperature profiles have been used in a parametric model introduced by Hendry to model the Gulf Streams 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 Hendrys 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 Streams 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.

Circulation in the eastern North Pacific: results from a current meter array along 152°W

Abstract Data from four, 2–3 year long current meter records, at 28°N, 35°N, 39°N and 42°N, along 152°W in the eastern North Pacific, are used to describe the variability found in mesoscale period ( 200 days) motions. Energy in the mesoscale energy band of 40–200 day periodicity is found in the upper ocean at each location, generally decreasing to the north and with depth. The long period flow is not coherent among these locations. Record length mean velocities at 3–4 separate depths were used to provide estimates of reference level velocities for vertical profiles of geostrophic currents derived from historical hydrographic data. The vertical profile of measured east-west vertical shear agrees well with the geostrophically computed value; the north-south measured vertical shear is not in as good agreement. Assuming a vorticity balance of fwz=βv, and with w(z=0) as the Ekman pumping, the vertical velocity profiles were also calculated at 28°N and 42dgN. Using these three-dimensional referenced vertical profiles of mean currents, an examination of the mean advection of density in the thermocline revealed significant residuals in the net three-dimensional advection of density (or heat and salt) above 850 m at 28°N and above 240 m at 42°N. These results are relatively independent of the reference level velocities.