The MODE Site revisited

In the 1970\u27s, an intense physical oceanographic effort was focused on the MODE Area (centered at 28N, 70W) to study mesoscale eddies and their effect on the larger-scale longer-term (or general) interior ocean circulation. At that time there was considerable discussion as to the typicality of these results. It has become clear that the time-dependent field is horizontally inhomogeneous; eddies have a geography related to the general circulation. In some areas, significant temporal inhomogeneity (nonstationarity) has been observed, but this issue has not yet been clarified for the MODE Area. Recently, a collective experiment to study the effect of fronts on mixed layer dynamics was carried out near the subtropical front in the North Atlantic. This note summarizes the (subsidiary) mean flow and eddy-based results from two subsurface moorings set as support for the main experiment, focusing on MODE Center (28N, 70W) where instruments at several levels from 150 to 4000 m were deployed for 20 months. Abyssal mean flow increased by an order of magnitude. The previously most stable eddy-field observable, abyssal eddy kinetic energy, changed by more than 50% from the measurement period in the 1970\u27s to that in the 1980\u27s. Eddy kinetic energy and mean flow in the thermocline, expected to be the least reproducible observables, hardly changed. The directionality of the thermocline eddy field is notably different, essentially reversed from the 1970\u27s, with the meridional twice as large as the zonal variance in the 1980\u27s. The spectral distribution in the thermocline is less red, with the opposite tendency at abyssal depths. In summary, the MODE Area is neither particularly representative of the rest of the ocean nor are the MODE results of the 1970\u27s quantitatively representative of measurements there ten to fifteen years later. It does seem possible, however, that many of the differences observed could be rationalized in terms of comparatively small-scale horizontal excursions of larger-scale flow regimes, notably the subtropical frontal zone

On the deep general circulation in the western North Atlantic

Very large (5-10 cm s-1) long-term averaged zonal flows have been observed near 4000 m depth in the vicinity of a recently hypothesized (Worthington, in press) horizontally restricted subtropical gyre in the deep western North Atlantic. The Reynolds stresses associated with low frequency fluctuations may play a significant role in the dynamics of this deep mean flow, possibly inducing a significant downstream increase in transport of the Gulf Stream, perhaps driving the deep gyre

SOFAR float trajectories associated with the Newfoundland Basin

The connection between the current systems and water masses in the Newfoundland Basin and the North Atlantic Subtropical Gyre has been controversial for some time. SOFAR floats have now been launched in the Newfoundland Basin, and others observed to move there. Floats were typically deployed in pairs at nominal depths of 700 m (thermocline level) and 2000 m (deep). A thermocline level float set in the North Atlantic Current near 45N drifted to about 50N before recirculating, whereas a deep float deployed concurrently eventually moved south and west around the Grand Banks. A float pair deployed in the interior of the Newfoundland Basin moved in the mean consistent with a recirculation that transports roughly 15 to 20 × 106 m3 s−1 above 2000 m depth. Three thermocline level floats have drifted from west of the Grand Banks into the Newfoundland Basin, whereas deep floats have not yet been observed to do so. A crude estimate of the implied transport above 2000 m depth branching from the Gulf Stream System into the North Atlantic Current is 15 to 20 × 106 m3 s−1 The evidence points to branching above but not below 2000 m. whereas the recirculation in the Newfoundland Basin penetrates below 2000 m in the float data.The float pair deployed in the interior of the Newfoundland Basin exhibited significantly less energetic eddy motion than the float pair deployed in the vicinity of the North Atlantic Current. The estimated eddy kinetic energy in the thermocline in the North Atlantic Current is comparable to that observed for the Gulf Stream, whereas the deep eddy kinetic energy is notably less. The general structure of the eddy field (and mean flow) points to the type of horizontal or geographical inhomogeneity from strong current to interior regimes similar to that observed for the subtropical gyre

Abyssal eddy kinetic energy in the North Atlantic

Both eddy and mean fields have similar zonal and meridional scales of geographical variation in the mid-latitude North Atlantic. The first map of the North Atlantic that contained estimates of the intensity of the abyssal eddy field is now several years old, and in the interim the relevant data base has increased roughly six-fold. Contemporary charts are presented, containing these more recent observations, along with some new styles of data presentation. The basic picture of maximum abyssal eddy kinetic energy near the fully developed Gulf Stream is consistently substantiated, along with a two order or magnitude latitudinal decay (from about 102 to roughly 1 cm2 s–2) into the interior of the subtropical gyre west of the Mid-Atlantic Ridge. Results obtained in the last few years lead to the first relatively clear-cut identification of the zonal scales of variation of eddy intensity near the Gulf Stream. Eddy kinetic energy levels at abyssal depths near Cape Hatteras, in the vicinity of the Grand Banks, and east of the Mid-Atlantic Ridge are down by 1–2 orders of magnitude from maxima near the Gulf Stream at intermediate longitudes in the western North Atlantic. Preliminary contour maps of observed abyssal eddy kinetic energy are presented, albeit in schematic form

Observations of low-frequency current fluctuations on the continental slope and rise near Site D

An array of near-bottom current meters was set on the continental rise and slope near Site D in the autumn, 1970. At the junction of the slope and rise and over the slope the low-frequency (less than 1 c.p.d.) flow was predominantly along depth contours, and flow across depth contours was dominated by higher frequencies. The low-frequency kinetic energy decays in both directions away from a mooring position near the slope-rise junction...

On the dynamics of the Florida Current

This paper contains selected results from the first in a series of new efforts directed toward establishing an observational basis for testing hypotheses on the dynamics of the Florida Current. These efforts are based on the application of a recently developed free-instrument method for the collection of density and velocity field data at several sections across the Current...

A preliminary comparison of selected numerical eddy-resolving general circulation experiments with observations

Results from several numerical experiments based on one class of eddy-resolving gyre-scale models have been compared kinematically with observations. The model oceans are forced by a simplified steady wind stress distribution in basins with idealized coastlines and bottom topography...

A technique for the direct measurement of transport with application to the Straits of Florida

A method is described by which the volume transport per unit width of a water column is obtained from measurements of the run time, depth, and horizontal d efl ection of a freely falling instrument. Measurements of the north-south component of transport in the Straits of Florida using this technique gave 35.5 ± 1.2 x 106 m3/sec on August 16 and 17, 1964. The surface currents and east-west components of transport are also given

Observations of energetic low frequency current fluctuations in the Charlie-Gibbs Fracture Zone

Relatively energetic low frequency fluctuations in horizontal currents are found to exist below the thermocline in the northern trough of the Charlie-Gibbs Fracture Zone. For example, deep eddy kinetic energy levels there are about twice as large as those observed at similar relative depths in the MODE-I region. Eddy kinetic energies are about 2-10 times larger than mean kinetic energies...

An example of long-term variability for subsurface current and hydrographic patterns in the western North Atlantic

An example of long-term variability along 55W, perhaps interannual, for current and temperature distributions during mid-1975-1977 is presented. The existence of significant energy in the 55W data set at time scales longer than mesoscale (50-150 days) has been clear for , some time, but this is a first description of the latitudinal and vertical configuration of this low-frequency variability...