A central component of the CLIMODE observational program involves winter upper ocean sampling in conjunction with direct air-sea flux measurements. The specific goals of this effort are to document and quantify the processes responsible for cross-frontal exchange and mixing sketched schematically in Fig.2. The specific hypothesis that we wish to explore here is that diapycnal processes are significant and act to diminish the formation rate from that inferred solely from air-sea buoyancy flux.
We plan initial field observations on a 2 week cruise in February of 2006 and in February/March of 2007 with 2, back-to-back, three week cruises. This late winter period spans the seasonal peak in cumulative ocean buoyancy loss by air-sea exchange and is when EDW formation is "most active". Cross frontal exchange and mixing have been observed least often during these times (late winter) and environmental conditions (strong baroclinic current). Consequently we have the least confidence in their model representations. During these winter cruises, a compound spar buoy (supporting atmospheric boundary layer sensors and water column profiling to 500 m depth) will be repeatedly deployed in the Gulf Stream (GS) within a larger-scale array of surface drifters. In our main field campaign in 2007, high-resolution SeaSoar surveys will be made of the Gulf Stream front. And while the Lagrangian systems are carried downstream through the EDW formation region, repeated cross sections of the GS front will be made using loose-tethered microstructure instruments and expendable profilers. The spar system will be deployed on the southern flank of the GS where there is weak vertical shear. It will log and relay GPS positions to the research vessel in real time to guide the shipboard sampling. Based on observed GS speeds and past drifter/model data analyses, we believe the system will remain in the Stream and transit the study area in about 10 days. In light of the seasonal climatology and frequency of atmospheric storms, the planned sampling should capture the full range of winter atmospheric forcing conditions. Satellite-tracked surface drifting buoys (drifters) provide in situ, concurrent measurements of sea surface temperature and near-surface horizontal velocity. Satellite fixes of drifter position will be obtained at a rate of 6-8 per day.