James R. Luyten

The Ventilated Thermocline

Abstract A simple theoretical model for the oceanic thermocline and the associated field of current is presented. The model consists of a finite but arbitarily large number of inviscid, homogeneous fluid layers each with a different density. The dynamical balances everywhere are Sverdrupian. IN regions where the Ekman pumping is negative (downward) the surface density is specified, i.e., the position of the outcrop of density interfaces is specified. This outcropping of density layers allows deep motion to be excited by the ventilation provided by Ekman pumping even in latitudes far south of the outcrop where the layer is shielded from direct influence of the wind. Analytical solutions are presented in the case where the density-outcrop lines are coincident with latitude circles. The solutions are not self-similar and important sub-domains of the solution are defined by critical potential vorticity trajectories which separate the ventilated from the unventilated regions in the lower thermocline. These cri...

Equatorial Currents at Semi-Annual Period in the Indian Ocean

Abstract Moored records of zonal velocity in the western equatorial Indian Ocean are dominated by motion of semi-annual period. The motion is coherent over an array spanning more than 1000 km along the equator, 160 km across the equator, and 500 m in the vertical The signal has an amplitude of 0.15 m s−1. It has zonal, vertical and meridional length scales which are long compared to the dimensions of the array, and it shows upward propagation of phase. Behavior is characteristic of an eastward propagating equatorial Kelvin wave and a westward long equatorial Rossby wave. The vertical wavelength is an estimated 5000 m, implying an equatorial trapping scale of 400 km. Zonal wavelengths are ∼8000 km for the equatorial Rossby wave and 24000 km for the equatorial Kelvin wave. A downward energy flux, estimated to be 3 × 1016 erg s−1, most likely represents propagation away from surface forcing by the zonal wind.

Intensive surveys of the Azores Front: 1. Tracers and dynamics

The hypothesis that fronts are sites of active subduction is examined using density, temperature, salinity, and horizontal velocity data from a trio of surveys of the Azores Front done in May 1991 and March 1992. These surveys were made using a SeaSoar equipped with a conductivity-temperature-depth profiler and a shipboard acoustic Doppler current profiler. The potential density and potential vorticity indicate that dense water from the north side of the front may be sliding down beneath the surface outcrop. This apparently subducting isopycnal has a great deal of temperature and salinity variability. Horizontal velocity is nearly parallel to isopycnals, indicating that the time rate of change and vertical advection must be small. The thermal wind balance is observed to be valid, especially in the region of the largest horizontal density gradients. Shear at the base of the mixed layer is likely due to near-inertial motions. The potential vorticity is dominated by the planetary vorticity, except at the front, where vertical shears (the tilting term) become large. The tilting term acts to reduce the magnitude of the potential vorticity at the front, in agreement with simple theoretical models. The magnitude of the tilting term is similar to the total vorticity in the seasonal thermocline.

The Local Dynamics of Eddies in the Western North Atlantic

The local dynamics of oceanic mesoscale eddies is a subject of enormous scope and detail. The scope of this paper, on the other hand, is restricted to a particular experiment, the POLYMODE Local Dynamics Experiment (LDE). The LDE is unique among mesoscale experiments to date in its high sampling density and variety of measurements, and thus is well suited to considerations of eddy dynamics in one locale. This uniqueness is, of course, one of degree not kind. Other mesoscale mapping experiments are POLYGON (USSR), MODE, Tourbillon (France), and the USSR component of POLYMODE. We, the authors, are the experimentalists and analysts of the LDE.

Gyres Driven by Combined Wind and Buoyancy Flux

Abstract The combined effects of buoyancy forcing and wind in a gyre-scale steady ocean circulation are modeled using discrete layers with an interfacial flux, not necessarily vertical. The equations for vorticity conservation of the geostrophic flow in this system are fully nonlinear, involving a Jacobian for the layer thicknesses. These equations are written in a form which can be solved by the method of characteristics. The form of these equations invites the interpretation that the geostrophic baroclinic flow is driven by buoyancy and steered by the wind. Two examples are solved and discussed, a subtropical gyre with heating, and a subpolar gyre with cooling. In each case, there are distinct regimes of flow, depending upon whether the characteristics originate at the eastern or western boundaries of the model. A simple geometrical argument illustrates that the difference between these two regimes, the direct and indirect cells, depends upon the sign of the true vertical velocity relative to the interf...

Multiple Equilibrium States in Combined Thermal and Saline Circulation

Abstract Structure and stability of the multiple equilibria of the thermohaline circulation am studied using 2 × 2 and 3 × 2 box models. Thermohaline catastrophe is a shallow phenomenon and its time evolution consists of three stages: the search stage, the catastrophic stage, and the adjustment stage. A 3 × 2 box model is introduced to permit fitting a somewhat less truncated form of freshwater forcing. Although the total number of possible modes of circulation increases quickly as the number of boxes is increased, the grand thermal mode and grand saline mode remain. Some multiple solutions are found to be statistically improbable, and they belong to the so-called minor modes. With the more realistic freshwater forcing appropriate for the present-day North Atlantic, our simple 3 × 2 model predicts that the thermohaline cimulation is near a critical mate, because a small increase in freshwater flux can cause the system to collapse from the present thermally dominated circulation to an intermediate mode of ...

The Sverdrup circulation for the Atlantic along 24°N

The 30 Sv transported by the Florida Current through the Straits of Florida off Miami could consist of a wind-driven contribution of 17 Sv along with a thermohaline component of 13 Sv. The latter might flow from the South Atlantic as upper layer compensation for a net lower layer cross-equatorial flow southward. The Sverdrup transport along 24°N in the interior North Atlantic east of 55°–60°W is about 17 Sv, potentially matching the wind-driven component of the Florida Current. The circulation in the vicinity of 24°N and west of 55°–60°W up to the Bahama Banks contains energetic, shorter spatial scale flows, where there is a similarity between the regional pattern of the Sverdrup transport contours and the scales and structure of the observed C-shaped pattern of dynamic height.

Evidence for bottom-trapped topographic Rossby waves from single moorings

Abstract Current-meter observations near 39°N, 70°W, on the continental rise, provide evidence that the motions with periods of 1 to 2 weeks are dominated by baroclinic topographic Rossby waves which decay upward from the bottom. Temperature and up-slope velocity are coherent and in quadrature at these frequencies, as predicted. The kinetic energy structure versus depth is consistent with horizontal wavelengths of 100 to 200 km. The spectra drop abruptly for periods shorter than a week, the shortest period the model says the slope and stratification around Site D can maintain. The principal axis of the velocity shifts from nearly perpendicular to the isobaths at 1-week period to nearly along the isobaths at long periods, in satisfactory quantitative agreement with the model.

Transient Gulf Stream Meandering. Part I: An Observational Experiment

Abstract The results from an observational experiment on the mesoscale space-time variability of the Gulf Stream are reported. Various techniques, including aerial surveys, ship trackings of the 15C isotherm at 200 m, drogues and moored current meters were used and are compared, to give estimates of the variability of the motion over a wide range of scales. A two-week time series of daily tracks of the Stream near 70W are used to interpolate instantaneous paths over 2° of longitude. These paths provide the first detailed information on the small-scale variability of the path indicator of the Gulf Stream northeast of Cape Hatteras. Similarly, the long time series of triweekly aerial surveys provides a detailed picture of the evolution of a large-scale meander.

Equatorial deep jets in the Atlantic Ocean

We report here a vertical profile of velocity measured in the equatorial Atlantic (0°00′N, 30°22′W) which reveals short vertical scale zonal jets with amplitudes of 10–20 cm s−1 over the upper 2500 m, alternating in the east-west direction with depth. Particularly prominent was an eastward jet centered at a depth of 1000 m with an amplitude of 28 cm s−1.