North Atlantic Subtropical Underwater and Its Year-to-Year Variability in Annual Subduction Rate during the Argo Period

Subtropical underwater (STUW) and its year-to-year variability in annual subduction rate are investigated using recently available Argo data in the North Atlantic. For the period of observation (2002-14), the mean annual subduction rate of the STUW is 7.3 +/- 1.2 Sv (1 Sv = 10(6) m(3) s(-1)) within the density range between 25.0 and 26.0 kg m(-3). Once subducted, the STUW spreads in the subtropical gyre as a vertical salinity maximum. In the mean, the spatial changes in temperature and salinity of the STUW tend to compensate each other, and the density of the water mass remains rather stable near 25.5 kg m(-3) in the southwestern part of the subtropical gyre. The annual subduction rate of the STUW varies from year to year, and most of this variability is due to lateral induction, which in turn is directly linked to the variability of the winter mixed layer depth. Through modulation of surface buoyancy, wind anomalies associated with the North Atlantic Oscillation are primarily responsible for this variability. Sea surface salinity anomalies in the formation region of the STUW are conveyed into the thermocline, but their westward propagation cannot be detected by the present data.