G. Forget, G. Maze and J. Marshall
Journal of Physical Oceanography
The seasonal cycle in the volume and formation rate of Eighteen Degree Water (EDW) in the North Atlantic are quantified over the three-year period from 2004 to 2006. The EDW layer is defined as all waters which have a temperature between 17°C and 19°C. The study is facilitated by a synthesis of various observations — principally Argo profiles of temperature and salinity, sea surface temperature and altimetry — using a general circulation model as an interpolation tool. The winter increase in EDW volume occurs most intensively in February, peaking at about 8.6Svy, where 1Svy ≈ 3.15 × 1013m3 corresponding to a 1Sv = 106m3s−1 flow sustained for one year. This largely reflects winter EDW formation due to air-sea heat fluxes. Over the remainder of the year, newly created EDW is consumed by air-sea heat fluxes and ocean mixing, which roughly contribute 2/3 and 1/3, respectively. We estimate a net annual volume increase of 1.4Svy per year, averaged over the three year period. It is small compared to the amplitude of the seasonal cycle (8.6Svy) and annual formation due to air-sea fluxes (4.6Svy). The overall EDW layer volume thus appears to fluctuate around a stable point during the study period. Our estimate of the full EDW volume budget is provided along with an uncertainty estimate of 1.8Svy, and largely resolves apparent conflicts between previous estimates.