|Investigating the characteristics and consequences of interannual variability in the Northwest Atlantic's Deep Western Boundary Current|
T. Joyce, R. Curry, M. McCartney (WHOI) and W. Smethie (LDEO)
The overturning circulation of the Northwest Atlantic Ocean at mid-latitude involves poleward transport of warm water by the Gulf Stream and equatorward flow of colder intermediate and deep waters. Comprehension of how these limbs of the global current system and their associated regional recirculations vary on decadal time scale is incomplete. In particular, we lack understanding of how interannual variations in air-sea exchange and water mass modification at high latitudes are transmitted equatorward, and what impacts or feedbacks such signals may have for the Atlantic-wide circulation. Limiting advance in understanding is the lack of long, well resolved records to document interannual signals in water properties, stratification and transport of the Deep Western Boundary Current (DWBC) system anywhere north of the region east of the Bahamas. Importantly, anomalies created at subpolar latitudes may be profoundly altered or even blocked by the Gulf Stream. Conversely, subpolar anomalies may influence the position, strength, and/or stability of the Stream, and in turn affect patterns of air-sea exchange throughout the North Atlantic. These basic questions motivate the present proposal to observe the DWBC adjacent to, but upstream of its Gulf Stream crossunder point off Cape Hatteras. Companion field programs by fellow U.S., Canadian, British and German investigators will sample the DWBC both north and south of our study region, allowing a comprehensive investigation of the mechanisms and rates of North Atlantic Deep Water export to lower latitudes and the relationships (if any) to the upper ocean circulation. The present study will document temperature, salinity, tracer and velocity variations of the DWBC upstream of its Gulf Stream crossunder point by maintaining a moored array over the slope south of Woods Hole, and occupying a hydrographic section along this line semi-annually. The array (named Station W in memory of L. Valentine Worthington) will quantify changes in DWBC water properties, stratification (potential vorticity) and transport. The high-spatial-resolution sampling possible from ship will help verify that the array resolves the interannual signals (as well as return water samples for shoreside tracer analyses). A subset of the moored observations will be made available in real time for assimilation studies and float calibration. The full data set will be made available as instrumentation is recovered for model validation and study of regional processes by any interested investigators. In addition to raw observations, value-added products such as time series of core properties and transport by water mass will be produced and distributed. We will encourage other researchers to build on our infrastructure to augment the fields being sampled. Equally important, we will explore whether a subset of our present array is sufficient to index water property and transport variations in this area, setting the stage for a long-term ocean observing system.