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. Plain Language Summary: a conceptual Lagrangian model is developed for constraining rates of evaporation in the ocean. The model is applied to routinely measured variables in the ocean's surface such as salinity and isotopic compositions. Our results show that all variables yield very similar estimates of the model's parameter, which bolsters the use of surface transects of salinity to constrain calculated values of parameters relevant to rates of evaporation. Key Points: A Lagrangian model is developed for changes in salinity and isotopic composition of a moving water column that undergoes net evaporationThe model is successfully applied to four oceanic transectsThe observations provide reliable estimates of the model's new single‐parameter – evaporation length [ABSTRACT FROM AUTHOR]