ALBERT

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 1985

Description: From October, 1982 to October, 1983 a current meter mooring reaching from the bottom into the thermocline was deployed for the first time in the Gulf Stream at 68°W. The temperatures, pressures, and velocities at the uppermost instrument indicate the Gulf Stream moved back and forth across the mooring site, so that the entire Stream was sampled in time; hence the data may be used to examine horizontal as well as vertical structure of the Stream. 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 long- and cross-stream vertical structures, which are decoupled and are respectively baroclinic and barotropic. Using the inferred horizontal coordinate one can estimate mass, momentum and kinetic energy fluxes for four individual events when the entire Stream swept by the mooring. The results agree well with historical data. Bryden's (1980) method has been used to calculate vertical velocities from the temperature equation; the resulting time-series of w are visually coherent throughout the water column and their vertical amplitude structure is reminiscent of that for a two-layer system. The rms vertical velocities are large (0(.05 cm/s)), and these as well as other estimates have been used to explore the validity of the quasi-geostrophic approximation at the mooring site. The Rossby number for the thermocline flow is about 0.3, and for the deep flow is ≤ 0.1. The entire data set may also be used to construct a horizontal and vertical profile of velocity in the Gulf Stream, from which a cross-section of the mean potential vorticity can be produced. The latter shares many common feature with cross-sections from past work for a nearby site, as well as analogous data from a three-layer numerical model, thus suggesting that they are robust features of Gulf Stream-like currents. These features are, in particular, a strong jump from low to high values crossing the Stream from south to north; and a change in the sign of the potential vorticity gradient on isothermal surfaces for T 〉 12°C. To complement the analysis of the observational data, a set of diagnostic calculations has been performed on an eddy-resolving qeneral circulation model, to provide a complete picture of the kinetic energy budgets of the free jet and its environs. It is found that the downstream convergence of kinetic energy in the decelerating jet is balanced primarily by an ageostrophic flow against the pressure gradient, which in turn implies some conversion of kinetic to available potential energy in the region. Energetic analysis of the observations as well as the numerical data suggests barotropic and baroclinic instabilities may be equally important to the kinetic energy budgets in the Stream. Because there is but one mooring, the dynamics governing the fluctuations remain elusive. Nonetheless, a kinematic framework is proposed, which is consistent with the data and accounts for a variety of unusual features that arise in the original analysis (for example, distinct asymmetries in the four Gulf Stream crossings, and the rather large vertical velocities). It is sugqested that the data we are now capable of collecting is proffering fundamentally new attributes of the Gulf Stream, which must be included and accounted for in future theoretical work.

Description: Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 109 (2004): C11008, doi:10.1029/2003JC002103.

Description: In July–August 1997, a hydrographic/Acoustic Doppler Current Profiler (ADCP)/tracer section was occupied along 52°W in the North Atlantic as part of the World Ocean Circulation Experiment Hydrographic Program. Underway and lowered ADCP (LADCP) data have been used to reference geostrophic velocities calculated from the hydrographic data; additional (small) velocity adjustments provided by an inverse model, constraining mass and silicate transports in 17 neutral density layers, yield the absolute zonal velocity field for 52°W. We find a vigorous circulation throughout the entire section, with an unusually strong Gulf Stream (169 Sv) and southern Deep Western Boundary Current (DWBC; 64 Sv) at the time of the cruise. At the northern boundary, on the west side of the Grand Banks of Newfoundland, we find the westward flowing Labrador Current (8.6 Sv), whose continuity from the Labrador Sea, east of our section, has been disputed. Directly to the south we identify the slopewater current (12.5 Sv eastward) and northern DWBC (12.5 Sv westward). Strong departures from strictly zonal flow in the interior, which are found in the LADCP data, make it difficult to diagnose the circulation there. Isolated deep property extrema in the southern portion, associated with alternating bands of eastward and westward flow, are consistent with the idea that the rough topography of the Mid-Atlantic Ridge, directly east of our section, causes enhanced mixing of Antarctic Bottom Water properties into overlying waters with distinctly different properties. We calculate heat and freshwater fluxes crossing 52°W that exceed estimates based on air-sea exchanges by a factor of 1.7.

Description: This work was supported by NSF grants OCE95-29607, OCE 95-31864, OCE98-18266, and OCE-0219644.