Monitoring the Gulf Stream Flow and Dynamics Northwest of Bermuda


OCCI Funded Project: 2004


Much of the growing observing system for the North Atlantic Ocean circulation is directed at bulk measures of its state, for example the integral heat content (Argo floats and XBTs), the strength of the deep western boundary current and the variations of the water masses carried by it (the moored array that has been built up around the station W mooring), and the strength of the Gulf Stream System (the Stream plus the adjacent recirculation cells, as indexed by pair of time series hydrographic stations W and S bracketing the system). In the western mid-latitude North Atlantic, the strength of the Gulf Stream is greatly amplified by the action of mesoscale variability driving strong compact recirculation cells north and south of the Stream. This proposal is to make measurements that reveal the changes of strength and structure of a cross section of the Gulf Stream, isolating that component of the Gulf Stream System, and simultaneously the low-frequency modulation of eddy variability that is held responsible for setting the degree of intensification of the Gulf Stream. The latter represents a first step towards moving beyond "just" bulk measures of state to measures of the internal dynamics that force that state. Over the past 30 years an image of the geography of mesoscale (nominal monthly) variability has been built up from moored current measurements. These showed, for example, the strong concentration of eddy kinetic energy at the Stream axis, and defined its degree of decay with depth and with distance from the axis, and validated the theoretical concepts tying the degree of Gulf Stream intensification to the strength of the eddy driving of circulation. Such distributions (flow strengths and pathways and eddy variability) have been treated as steady state fields, constructed by combining into maps the results of measurements made at different times and sites over several decades. Theory and model results indicate that mesoscale eddy variability should exhibit interannual - decadal variability, a variability that results in the modulation of the strength of the Gulf Stream and its recirculations. Such intrinsic variability must be measured and understood in order to understand the relation of Gulf Stream System variability to the strength of the Meridional Overturning Circulation. I propose here to begin a monitoring measurement of the distribution of eddy kinetic energy (and other important statistics) at the axis of the Stream between stations W and S near sites where limited duration mooring; provide historical data on the Gulf Stream flow and eddy variability (1983 and 1988-90). A single mooring will be placed at the Stream axis instrumented at multiple levels between the upper thermocline and the sea floor, measuring both the vertical and horizontal structure of the Stream - the latter through the Stream's meandering to and fro across the mooring. Looking backward in time: Has the depth and cross-Stream structure (currents and eddy statistics) changed since those earlier measurements? Looking forward in time: Are the changes in bulk strength of the Gulf Stream System and the deep western boundary current to its north, that will be measured, respectively, by the stations W and S pair and the station W array, mirrored in the changes of strength and structure of the Gulf Stream itself, including both the strength and structure of flow of the Stream and the structure and strength of the eddy forcing within and near the Stream. Are they consistent with the evolving understanding of the internal dynamics of circulation intensification in the western subtropics?