ALBERT

Description: Author Posting. © Elsevier B.V., 2012. The definitive version was published in Deep Sea Research Part I: Oceanographic Research Papers 72 (2013): 72–87, doi:10.1016/j.dsr.2012.11.005.

Description: Nearly every spring since 1990, hydrographic data have been collected along a section in the Labrador Sea known as AR7W. Since 1995, lowered acoustic doppler current profiler (LADCP) data have also been collected. In this work we use data from six of these sections, spanning the time period 1995 through 2008, to determine absolute velocity across AR7W and analyze the main features of the general circulation in the area. We find that absolute velocity fields are characterized by strong, nearly barotropic flows all along the section, meaning there is no “level of no motion” for geostrophic velocity calculations. There is strong variability from year to year, especially in the strength of the boundary currents at each end; nevertheless, combining data from.all 6 sections yields a well-organized velocity field resembling that presented by Pickart and Spall (2007), except that our velocities tend to be stronger: there is a cyclonic boundary current system with offshore recirculations at both ends of the line; the interior is filled with virtually uniform, top-to-bottom bands of velocity with alternating signs. At the southwestern end of the section, the LADCP data reveal a dual core of the Labrador Current at times when horizontal resolution is adequate. At the northeastern end, the location of the recirculation offshore of the boundary current is bimodal, and hence the apparent width of the boundary current is bimodal as well. In the middle of the section, we have found a bottom current carrying overflow waters along the Northwest Atlantic Mid-Ocean Channel, suggesting one of various possible fast routes for those waters to reach the central Labrador Sea. We have used the hydrographic data to compute geostrophic velocities, referenced to the LADCP profiles, as well as to compute ocean heat transport across AR7W for four of our sections. For all but one year, these fluxes are comparable to the mean air–sea heat flux that occurs between AR7W and Davis Strait from December to May (O(50–80 TW)), and much larger than the annual average values (O(10–20 TW)).

Description: This material is based upon work supported by the National Science Foundation under Grant No. OCE-0622640. Igor Yashayaev is supported by the ocean climate monitoring program of the Department of Fisheries and Oceans Canada.

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.

Description: Current and temperature measurements from Vector Averaging Current Meters (VACMs) deployed from September 1992 to June 1994 as part of the Deep Basin Experiment (DBE) measuring the trans-equatorial water flow are presented. Salinity and temperature measurements from Conductivity/Temperature/Depth (CTD) casts taken during the mooring deployment and recovery cruises are also presented. Six mooring sites were occupied with a total of 24 vector averaging current meters and 4 Aanderaa current meters. Three nominal depths (3900, 4100 and 4300 m.) were occupied on each mooring. Three of the 6 moorings had current meters at additional depths. Basic data from the vector averaging current meters are presented both in statistical tables and graphically as histograms, scatter plots, progressive vector diagrams and spectral diagrams. One day Gaussian filtered plots are shown in composite displays of variables versus time. Temperature and salinity profies and e/s plots for 22 CTD stations are presented.

Description: Funding was provided by the National Science Foundation through Grant No. OCE-9105834.