SURFACE EVAPORATION DURING THE CENTRAL EQUATORIAL PACIFIC EXPERIMENT - A CLIMATE-SCALE PERSPECTIVE

This study is directed to evaluating the feedback between evaporation (F-L) and sea surface temperature (T-s) in the equatorial Pacific Ocea n by looking at the components that control dF(L)/dT(s), the variation of evaporation with T-s. First eddy correlation evaporation estimates obtained during long (similar to 1000-1500 km), low-level (30 m) trav erses of the central equatorial Pacific by research aircraft during th e Central Equatorial Pacific Experiment (CEPEX) are analyzed. From thi s limited dataset, extension to climate space- and timescales is made by comparing the aircraft measurements to bulk aerodynamic estimates t o F-L using mean values from both the aircraft and Tropical Atmosphere -Ocean buoys. Variation of surface evaporation with T-s is shown to be affected not only to surface saturation humidity deficit and its depe ndence on T-s, but also by variations of wind speed with T-s. Dependin g on the relative importance of the two contributions, surface evapora tion can either increase or decrease with T-s. Intercomparison between the aircraft data and the buoy data indicates that the humidity defic it effect is dominant during CEPEX, and in low T-s, where surface wind s are only weakly related to T-s: the effect of wind speed variation w ith T-s is much more important in the 2-yr buoy data for T-s greater t han or equal to 301 K. The discrepancy between the evaporation feedbac k in CEPEX and that from the 2-yr buoy data is shown to be largely due to oversampling of high winds and high evaporation during CEPEX for 3 02 less than or equal to T, < 303 K. The long-term buoy data show that for T-s < 301 K, dF(L)/dT(s) = +9 W m(-2) K-1, while for 304 K > T-c greater than or equal to 301 K, dF(L)/dT(s) = -13 W m(-2) K-1. Further more, observations of F-L are well below the values necessary for evap oration to be the primary limiting factor in the regulation of T-s in the equatorial Pacific.