Subsurface quasi-decadal fluctuations in the north Atlantic

Abstract

Analysis of hydrographic time series data at ocean station S (Panulirus) and along the Labrador continental margin is used to investigate the subsurface structure associated with the North Atlantic quasi-decadal SST fluctuation. Quasi-decadal fluctuations of both temperature and salinity are confined to the upper water column that is ventilated by the seasonal cycle. In the subtropical gyre, they are independent of the variations of the gyre baroclinic transport. On the Labrador continental margin, the quasi-decadal fluctuations are confined to the upper portion of the Labrador Current. The quasi-decadal fluctuation appears to be surface forced; in the subtropical gyre, this is predominantly by latent heat flux. In the subpolar gyre, latent and sensible heat flux alone cannot account for the in-phase variation of temperature and salinity. Instead, the variation involves upper-ocean convective mixing whose strength is modulated by the density of the winter-cooled surface water. It has been noted that the cold phase of the North Atlantic quasi-decadal fluctuation is associated with anomalously strong wind stress. In the subtropical gyre, this corresponds to greater heat flux and vertical mixing, but in the western subpolar gyre, it corresponds to weaker vertical mixing because of lower surface salinity. This suggests that precipitation, river runoff, and perhaps freshwater from the Arctic are greater in the western subpolar gyre when the wind stress is greater.