| Summary: | During 1997–2000, our research group participated in a major field study to determine the absolute circulation of the Subpolar North Atlantic. We successfully tracked 50 isopycnal RAFOS floats which measured pressure, temperature, and dissolved oxygen. The floats were ballasted for the 27.5 σ θ density surface, which is found in the lower subtropical thermocline waters of the North Atlantic. A comparison of our field results to historical hydrography for the region shows that the oxygen sensors provided an accuracy of about 5% over periods exceeding one year. One of our goals was to understand the conspicuous difference between the hydrographically measured temperature and oxygen fields in the Subpolar North Atlantic. To accomplish this goal, we have developed a horizontal, constant-density, advection-diffusion model that is based upon (1) RAFOS float observations of the absolute circulation and (2) climatological fields of temperature, salinity, and oxygen. A key feature of the model is the time-dependent ventilation in the Irminger Basin and Labrador Sea. We represent ventilation by imposing water-transformation rates typical for the region. The model adequately predicts the observed temperature, salinity, and oxygen fields when ventilated waters become saltier at a rate of 0.2 psu/yr, warmed by a corresponding amount to preserve constant density, and instantly saturated in oxygen. Another goal was to use RAFOS floats to study in situ biological processes simultaneous with the physical context in which they occur. To accomplish this goal, we have developed a vertical, one-dimensional model that is based upon (1) RAFOS float observations of the near-surface waters of the northern North Atlantic and (2) satellite-derived observations of meteorological and biological fluxes of heat, salt, momentum, and oxygen. We find that the model accurately reproduces the physical context: the model predicts the float observations of temperature to within 1°C for an entire year. The model also reproduces the observed oxygen variability: (1) a ∼3% oxygen supersaturation in the wintertime mixed layer, driven by an air-sea gas flux, and (2) a drop from 105% saturation to 92% saturation over a four month period as the float subducted below the mixed layer, driven by net community respiration at depth.
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