Constraining Evaporation Rates Based on Large-Scale Sea Surface Transects of Salinity or Isotopic Compositions

A Lagrangian model is constructed for a surface column of initial height h(0) that propagates at an average speed u and is subject to excess (i.e., net) evaporation of q m/year. It is shown that these parameters combine to form an evaporation length, L = uh(0)/q, which provides an estimate for the distance the column must travel before evaporating completely. While these changes in the surface water level due to evaporation are compensated by entrainment of water into the overall column, the changes in either near-surface salinity or isotopic compositions are retained and can be measured. Observations of surface salinity and isotopic compositions of δ¹⁸O and δD along 1,000- to 3,500-km long transects are used to estimate values of L in the Red Sea, Mediterranean Sea, Indian Ocean, and Gulf Stream. The variations of salinity, δ¹⁸O and δD in all four basins are linear. As anticipated, the estimated value of L is smallest in the slowly moving and arid Red Sea and is greatest in the fast-moving Gulf Stream.