NADW transformation at the western boundary between [formula omitted] and [formula omitted

Abstract

The variability of North Atlantic Deep Water (NADW) properties in the western tropical Atlantic between 66 ∘ W / 20 ∘ N and 60 ∘ W / 10 ∘ N is investigated based on repeated hydrographic sections. The water at the western boundary carries a CFC maximum, indicating the direct transport of newly formed NADW within the Deep Western Boundary Current towards the tropics. The offshore extension of this region with young water varies from 50 km at 16 ∘ N to 500 km at 10 ∘ N , depending on the width of the continental slope. The mean properties for the different NADW components reflect the changes in the water mass formation regions, especially the Labrador Sea. The greatest signal can be seen in the density range of the Labrador Sea Water (LSW), where a continuous freshening between 2000 and 2004 is observed. This is attributed to the arrival of low-salinity LSW formed in the Labrador Sea after 1988. In addition to the decreasing salinity in the LSW, the two lower NADW components also reveal a freshening trend, as has been observed in the subtropical and subpolar North Atlantic [Dickson. B., Yashayaev, I., Meincke, J., Turrell, B., Dye, S., Holfort, J., 2002. Rapid freshening of the deep North Atlantic Ocean over the past four decades. Nature 416, 832–837]. This tendency is not restricted to the boundary current region, but extends over the whole western tropical Atlantic. The spreading time of NADW from the Labrador Sea to the tropical Atlantic at 16 ∘ N is estimated by the salinity signal to be 10–13 years, corresponding to a mean velocity of 2.0–2.5 cm/s. The dilution factor of NADW determined from the CFC difference between the Labrador Sea and the tropics is 4–5 for all components, assuming an admixture of old, CFC-free waters. The NADW properties at 16 ∘ N on isopycnals show large spatial variability along the section. Profiles with high concentration of CFCs and low salinity in the LSW layer occur not only at the western boundary, but also in the interior, where they alternate with profiles influenced by older, CFC-poor water. At 10 ∘ N , the properties especially in the Upper NADW are horizontally homogenized. This is caused by the change of the shape of the continental slope, which is very steep at 16 ∘ N and gradually declining at 10 ∘ N , leading to a broadening of the upper portion of the boundary current between the two sections. The NADW at 10 ∘ N is a composite of the water from the western boundary at 16 ∘ N and about one third admixture of water from the interior. During the spreading from 16 ∘ N to 10 ∘ N , the uppermost portion of the Upper Labrador Sea Water becomes fresher and poor in CFCs by vertical mixing with the overlying Upper Circumpolar Deep Water. The resulting diffusivity K ρ is of the order of 10 - 4 m 2 / s .