Surface Evaporation during the Central Equatorial Pacific Experiment: A Climate-Scale Perspective

Abstract

This study is directed to evaluating the feedback between evaporation (FL) and sea surface temperature (Ts) in the equatorial Pacific Ocean by looking at the components that control dFL/dTs, the variation of evaporation with Ts. First eddy correlation evaporation estimates obtained during long (?1000?1500 km), low-level (30 m) traverses of the central equatorial Pacific by research aircraft during the Central Equatorial Pacific Experiment (CEPEX) are analyzed. From this limited dataset, extension to climate space- and timescales is made by comparing the aircraft measurements to bulk aerodynamic estimates of FL, using mean values from both the aircraft and Tropical Atmosphere?Ocean buoys. Variation of surface evaporation with Ts is shown to be affected not only by surface saturation humidity deficit and its dependence on Ts, but also by variations of wind speed with Ts. Depending on the relative importance of the two contributions, surface evaporation can either increase or decrease with Ts. Intercomparison between the aircraft data and the buoy data indicates that the humidity deficit effect is dominant during, CEPEX, and in low Ts, where surface winds are only weakly related to Ts; the effect of wind speed variation with Ts is much more important in the 2-yr buoy data for Ts ≥ 301 K. The discrepancy between the evaporation feedback 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 302 ≤ Ts < 303 K. The long-tem buoy data show that for Ts < 301 K, dFL/dTs = +9 W m?2K?1, while for 304 K > Ts ≥ 301 K, dFL/dTs = ?13 W m?2K?1. Furthermore, observations of FL are well below the values necessary for evaporation to be the primary limiting factor in the regulation of Ts in the equatorial Pacific.