Georges L. Weatherly

Estimating groundwater discharge into the northeastern Gulf of Mexico using radon-222

Abstract Submarine groundwater discharge (SGD) may provide important chemical constituents to the ocean, but the dispersed nature of this process makes locating and quantifying its input extremely difficult. Since groundwater contains 3–4 orders of magnitude greater radon than seawater, 222Rn may be a useful tracer of this process if all other sources of radon to bottom waters can be evaluated. We report development of a SGD tracing tool based on radon inventories in a coastal area of the northeastern Gulf of Mexico. We evaluated factors that influence the concentration of radon in the water column (i.e., production-decay, horizontal transport, and loss across the pycnocline) using a linked benthic exchange-horizontal transport model. Total 222Rn benthic fluxes (≥2420 dpm m−2 day−1) measured with in situ chambers are of the magnitude required to support measured sub-pycnocline 222Rn inventories, while estimates of molecular diffusion show that this input is relatively small (≤230 dpm m−2 day−1). Using this model approach, together with measurements of the radon inventory, we estimated a regional subsurface fluid flow ranging from 180 to 710 m3 sec−1 into the 620 km2 study area. This discharge, equivalent to an upward advective velocity of approximately 2–10 cm day−1 dispersed over this entire study area, is equivalent to approximately 20 first magnitude springs.

On the Structure and Dynamics of the Oceanic Bottom Boundary Layer

Abstract The Mellor and Yamada (1974) Level II turbulence closure scheme is used to study the oceanic bottom boundary layer (BBL). The model is tested against observations of the BBL obtained on the western Florida Shelf reported in Weatherly and Van Leer (1977) and in turn conclusions about the BBL made in that paper are tested against the model. The agreement between the model and the observations is good. The predicted and observed BBL thickness is ∼10 m which is appreciably less than 0.4 u*/f ≈ 30 m, where u* is the friction velocity and f the Coriolis parameter. The reason for the discrepancy is attributed to the BBL being formed in water which initially was stably stratified and characterized by a Brunt Vasaila frequency N0. It is suggested that the oceanic BBL thickness should be identified with the height at which the turbulence generated in the BBL goes to zero and on dimensional grounds it is proposed that this thickness is A u*/f(1 + N02/f2)½, where A is a constant. The Level II model indicates...

On deep-current and hydrographic observations from a mudwave region and elsewhere in the Argentine Basin

Near-bottom current measurements from both sides of a mudwave, in an extensive mudwave region of the mid-Argentine Basin, are examined for evidence of lee waves. Such waves appear to exist when the across-crest flow exceeds a critical value [predicted by Flood (1988), Deep-Sea Research, 35, 973–983] of about 9 cm s−1. The observations agree with the predictions of Blumsack and Weatherly (1989, Deep-Sea Research, 36, 155–172) in three of four cases. However, if it is assumed that the flow perturbations induced by lee waves are shifted slightly downcurrent from that predicted by the Blumsack and Weatherly model, then there is agreement in all four cases. The mudwave region is characterized as having a strong (∼ 10 cm s−1), steady (eddy kinetic energy < mean kinetic energy) current. The westward flow is consistent with the anticyclonic abyssal gyre centered about the Zapiola Drift inferred by Flood and Shor (1987, Deep-Sea Research, 35, 973–983). Near-bottom flow measurements made further to the west, in the region of confluence of the Brazil and Malvinas Current Extensions, reveal relatively energetic fluctuations, and equatorward flowing deep western boundary currents along the continental slope. Along the continental rise a poleward flow is observed, consistent with that inferred by Reid (1989, Progress in Oceanography, 23, 149–244). Too-cold bottom layers were found to be common only on the lower continental slope (4000 m depth) and at the mid-basin, mudwave site.

Storms and flow reversals at the HEBBLE site

Abstract We examine a 20 month-long abyssal current meter record obtained from the HEBBLE site (about 40°27′N, 62°22′W). As expected the westward-flowing abyssal western boundary current described by Richardson et al. (1981) and Weatherly and Kelley (1982) generally passes over this site. We call this current the Cold Filament Current (CFC) and conclude that it is appreciably stronger than suggested by Weatherly and Kelley (1984) (∼ 15 cm s −1 as compared to ∼ 10 cm s −1 ). After comparing the abyssal current record with charts showing the surface location of the Gulf Stream and rings, we conclude that the majority of abyssal storms at the HEBBLE site are due to the presence of the CFC passing by and the absence of a meander or ring overhead. The presence of a ring or meander overhead apparently results in the CFC being displaced out of the HEBBLE site (as suggested by Kelley, 1984); at such times the near-bottom flow appears to respond to the barotropic component of the overlying ring or meander.

Benthic storms in the Argentine Basin

Abstract The Argentine Basin is a region of large lateral gradients and extremely high concentrations and integrated loads of particulate matter (PM) in the abyssal nepheloid layer. The region also has large lateral variations in currents and surface and abyssal eddy kinetic energy. To investigate a suggested link between PM concentrations and eddy kinetic energy, transmissometers were attached to current meters moored at 10 m above the seafloor at five sites from the Argentine continental slope (1970 m) to the central basin (5045 m) from April 1987 to March 1988. Benthic storms (periods of high PM concentrations often associated with high current velocities) were recorded at all sites. The intensity of benthic storms (based on both frequency and elevated PM concentration) increased with distance from the margin, despite frequent periods where currents exceeded the expected erosion velocity near the margin. A good correlation was found between abyssal eddy kinetic energy (AEKE) and variations in PM concentration, except at the central basin site. At that site, a three month period of very high PM concentrations occurred with only low to moderate currents. The high PM concentrations may have resulted from the rapid input of low-density, organic-rich phytodetritus from a plankton bloom in surface waters.

A Numerical Study of Time-Dependent Turbulent Ekman Layers over Horizontal and Sloping Bottoms

Abstract A numerical study is made of a time-dependent turbulent Ekman bottom boundary layer. Parameters for the model were chosen to simulate conditions near the bottom of the Florida Current in the Straits of Florida. The model used is that of Lykosov and Gutman. It allows the coefficient of turbulent viscosity v to vary with time t and height z and permits the effects of an imposed stable stratification and sloping bottom to be included. The variation of v with t and z is not preset but is determined in the course of solving the problem subject to the turbulent energy equation, the similarity arguments of Komolgoroff, and a mixing length hypothesis of Zilitinkevich and Laykhtman. The results of this preliminary study are compared to the authors observations. The agreement is good for the friction velocity values as well as for the mean total Ekman veering. However, most of the computed Ekman veering occurred above the logarithmic layer while most of the measured veering occurred within the logarithmic...

Two Views of the Cold Filament

Abstract Two views of the Cold Filament, first described by Weatherly and Kelley, are presented. The first is a local view near 40°N, 62°W. There its upslope edge is found to be a front which by benthic standards is large (its downslope edge was not sampled). What distinguishes this benthic front from others is that it is a permanent feature in the abyssal ocean. Above the Cold Filament, relatively murky detached bottom layers were observed and tracked to where they separated from the bottom at the benthic front. Apparently these detached layers entrain overlying water (a density jump at their base apparently restricts entrainment of underlying water) primarily during the detachment process with comparably less entrainment thereafter. The second view, a regional one, comes from examining historical hydrographic sections. These indicate that the Cold Filament extends from the Newfoundland Ridge westward then southward to 24°N and possibly to ∼20°N along the base of the continental rise. The Cold Filament i...

A Numerical Study of the Interaction between a Deep Cold Jet and the Bottom Boundary Layer of the Ocean

Abstract A two-dimensional (x-z) primitive equation model is used to study the interaction between a deep cold jet on a sloping bottom and the bottom boundary layer (BBL) of the deep ocean. Two closure schemes are used: a standard second order turbulence closure (SOTC) scheme (the level 2 1/2 model of Mellor and Yamada), and a new eddy viscosity closure scheme (K-model). The latter is a computationally simple model that produces very similar eddy viscosity and velocity fields as the more complicated SOTC-model while saving about 20% of the computational time. The results of the numerical simulations compare favorably to observations from the base of the North Atlantic continental rise where the cold jet known as the Cold Filament (CF) is found. The interaction between the CF and the BBL is found to be dominated by cross-isotherm Ekman flow, resulting in an asymmetry effect with different dynamics at each one of the fronts associated with the CF. Some of the unusual characteristics of this region are expla...

Eulerian Measurements of the North Atlantic Deep Water Deep Western Boundary Current at 18°S

Abstract An 18-month time series of moored current meter observations near 18°S in the Atlantic is used to study the deep western boundary current (DWBC) of North Atlantic Deep Water (NADW). This flow is taken to extend from about the shelf break seaward about 200 km and downward from the σ2 = 36.7 isopycnal (at about 1200-m depth) to the σ4 = 45.8 isopycnal (at about 3600-m depth). The mean transport is estimated at 39 ± 20 × 106 m3 s−1. Of the ∼20 × 106 m3 s−1 uncertainty about 80% is due to the uncertainty of the measured velocities due to the 18-month duration of the study and the remainder to choices in filling in data gaps and specifying boundaries of the DWBC and to data dropouts. The DWBC is embedded in a flow that extends downward through the underlying Antarctic Bottom Water (AABW) to the bottom, upward into the overlying Antarctic Intermediate Water (AAIW) at least to 900-m depth, and has a width about 200 km. An expected recirculation just seaward of the DWBC was not found and is attributed to...

Springtime hydrography of the southern Middle Atlantic Bight and the onset of seasonal stratification

The Ocean Margins Program was a major multi-disciplinary observational effort in the southern Middle Atlantic Bight (MAB), focusing on the transformation, retention, and export of biogenic materials from the shelf. The observational effort peaked in the spring and summer of 1996 with four ship-based surveys, augmented by an array of 26 moorings supporting 126 temperature and 118 salinity sensors. The data from the cruises and moored array are used to describe how the springtime hydrographic evolution takes place in an area in which five water masses locally vie for dominance and that is subject to strong wind stress, heat flux, and offshore forcing. The results show that the region is subject to large-scale intrusions from both the north and south, which materially affect the timing and development of stratified conditions. The intrusions from the north are wind driven and provide cold, moderately saline, unstratifed water, delaying the development of stratified conditions. Intrusions also occur from the south, where warmer and generally more saline waters from the South Atlantic Bight are driven into the area by alongshore winds and/or intrusions of Gulf Stream waters pushed shoreward by Gulf Stream frontal eddies. In 1996, an intrusion of saline water from the south, combined with a reversal of the alongshore winds from the north, slowed the southward flow of cold MAB water, and subsequently caused low-salinity Virginia Coastal Waters to spread out from the coastal plume over the denser water from the north. With the reduced alongshelf flow and an initial stratification provided by the low-salinity coastal water, solar insolation and sensible heat fluxes were then able to warm the surface layer, permanently establishing the seasonal thermocline/halocline for the area not directly impacted by intrusions from the Gulf Stream and South Atlantic Bight.