Search Results

You are looking at 1 - 9 of 9 items for

  • Author or Editor: Georges L. Weatherly x
  • Refine by Access: All Content x
Clear All Modify Search
Georges L. Weatherly

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 author's 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 layer. The results suggest, as do the observations, that turbulent Ekman bottom layers varying on time scales of order the local inertial period are not quasi-stationary. Allowing the bottom to be inclined at a small angle transverse to the flow is found to modify significantly the temperature profile near the bottom, leading at times either to the formation of a homogeneous layer of depth order 10 m or to conditions marginally suitable for the formation of convectively mixed layer of comparable depth.

Full access
Georges L. Weatherly

This is a report of an experiment designed to study the bottom boundary layer of the Florida Current at a representative site in the Straits of Florida. The objectives of the experiment were 1) to determine the bottom frictional stress τ 0, and 2) to determine whether the bottom boundary layer is a turbulent Ekman layer. A typical value of the bottom stress τ 0 was found to be ~0.2 dyn cm−2. A mean veering of ~10° in the correct sense was observed in the logarithmic layer. No mean veering was observed above the logarithmic layer; this is believed to be a consequence of the strong modulation of the bottom current by the diurnal tide. The implication of τ 0≈0.2 dyn cm−2 is considered in a simplified model of the Gulf Stream current system; this analysis suggests that, dynamically, the role of bottom friction is rather small.

Full access
Anastasia Romanou and Georges L. Weatherly

Abstract

The response of the turbulent buoyant bottom Ekman layer near a temperature front over uniform topography is studied here. The background stratification is variable across the slope; the upper slope is either neutrally or stably stratified at one-half of the gradient of the lower slope region. In case 1, a time-dependent, spatially uniform, along-isobath interior current with zero mean causes residual circulation across the boundary layer and net detachment of the fluid from the boundary layer. For forcing with time scales longer than the shutdown time scale [τ 0 = f/()2; e.g., as defined by McCready and Rhines, where f is the Coriolis parameter, N is the Brunt– Väisälä frequency in the lower slope region, and α is the bottom slope], it is shown that the front represents an area of strong mean flow convergence and subsequent net detrainment of boundary layer fluid into the interior and is also a region of significant relative vorticity generation by the mean field. The residual circulation occurs in the stratified region. However, its direction and magnitude are subject to the order at which the downwelling and the upwelling phases occur because the lower and upper parts of the boundary layer respond differently to the two phases. The results are sensitive to the choice of background diffusivity. Tidal forcing produces significant differentiation in the results only when superimposed to the low-frequency current. The mean circulation then has much weaker downslope and along-slope components to the right of the front (i.e., seaward of the front). The strength of the detrainment at the front is found to be the same as in the low-frequency forcing case. In case 2, constant southward current causes convergence in the boundary layer, upwelling into the interior, vertical displacement of the isopycnals, and, through the thermal wind balance, a southward jet in the interior. This jet, which is the result of boundary layer dynamics and the presence of a front, could relate and explain the shelfbreak jet. As is shown here, a possible mechanism for the formation of an along-isobath jet (not just a shelfbreak jet) is the convergence in the bottom boundary layer, which, according to buoyant Ekman layer theory, may occur in the presence of one at least of the following: a front that intersects the bottom of constant inclination or constant stratification and a shelfbreak.

Full access
Y. Hsueh and Georges L. Weatherly

Abstract

Because continental margins are inclined, the boundary layer over them is tied to bottom density as well as to the barotropic pressure gradient. Through the no-flux condition at the sea floor, the bottom boundary layer, in turn, constrains the interior density field. The barotropic pressure gradient and the baroclinic velocity are thus coupled. Theoretical analysis in the framework of a model of linear dynamics demonstrates this coupling and resolves it.

Full access
Tal Ezer and Georges L. Weatherly

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 explained with the aid of the numerical experiments. These are: velocity profiles significantly different from those obtained by classical Ekman dynamics, unstable BBLs and detachment of bottom layers. Spatial variations in the characteristics of the BBL which are often neglected in deep-ocean studies are found to be significant in this region.

Full access
Georges L. Weatherly and Edward A. Kelley Jr.

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 is populated to be a part of an abyssal western boundary current in the North American Basin associated with a southern source of Antarctic Bottom Water.

Full access
Francisco J. Sandoval and Georges L. Weatherly

Abstract

A synthesis of WOCE (and other) hydrographic data shows that the deep western boundary current of Antarctic Bottom Water has a double-core structure, and that it is differentially modified during its northward transit through the Brazil Basin. At 25°S, it descends about 200 m while continuing equatorward as a paired, double-core current over the relatively broad, sloping topography of the western region. The downslope core is faster, 4–6 cm s−1 versus 3 cm s−1, and is coincident with the high-density core. Equatorward of 19°S the cores begin to diverge and by 15°S, the velocity cores have separated and the high-speed pattern has inverted. Between 13° and 11°S the colder, slower-flowing branch (around 3 cm s−1) deepens by 350 m, with a noticeable decrease in the vertical stratification of overlying water characteristics. Subsequently, it continues to the equator toward the Romanche–Chain Fracture Zones. The warmer, faster-flowing branch (around 8 cm s−1) proceeds to the equator along the continental slope, eventually becoming the flow entering the equatorial channel.

Full access
Georges L. Weatherly and Paul J. Martin

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 Vasäilä 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 + N 0 2/f 2)½, where A is a constant. The Level II model indicates that this is a good approximation over the range 0 ≤ N 0/f ≲ 200 provided A ≈ 1.3. Other features common to the predicted results and observations are 1) the vertical profiles of temperature and current direction which are very similar, with most of the direction changes (Ekman veering) occurring at the top of the BBL where the density stratification is largest, 2) a jet-like structure in some of the speed and direction profiles; and 3) appreciably more total Ekman veering than expected for a comparable BBL formed in neutrally stratified water.

The one-dimensional BBL formed under an along-isobath current in a stably stratified ocean is investigated for the case when the bottom is inclined relative to the horizontal isotherms. It is found that the BBL may no longer have the signature of a simple, vertically well-mixed layer because of Ekman-veering-induced upwelling (downwelling) of cooler (warmer) water in the BBL.

The profile of down-the-pressure gradient velocity component in the BBL is found to closely resemble the downslope flow of a heavier fluid discussed in Turner (1973). The Froude number stability criteria given in Turner (1973) when applied to the Level II model results suggest that the BBL formed in a stably stratified ocean is, in a Froude number sense, stable or marginally stable on continental margins while it is unstable in the deep ocean.

Full access
Georges L. Weatherly, Yoo Yin Kim, and Evgeny A. Kontar

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 σ 2 = 36.7 isopycnal (at about 1200-m depth) to the σ 4 = 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 the data showing that a previously assumed level of no motion in the region is indeed not such a level. The current does not follow local topography and flow to the south but rather to the southeast, perhaps due to blocking effects of the Trindade–Vitoria Seamount Chain about 200 km south of the mooring array. The current exhibits a seasonal variability of amplitude about 10 × 106 m3 s−1 with maximum poleward transport occurring in February–March and minimum transport around September. The seasonal variability is nearly barotropic and appears to be due to the seasonal wind stress variability in the tropical South Atlantic. The AABW beneath the DWBC transports ∼4 × 106 m3 s−1 poleward (comparable in magnitude of the transport of the equatorward-flowing DWBC of AABW, which is found to the east). Although the net AAIW flow above the DWBC is poleward (transport ∼8 × 106 m3 s−1), the data suggest a strong equatorward flow of AAIW just seaward of the shelf break.

Full access