THE RELATIONSHIP BETWEEN SEA-SURFACE TEMPERATURE AND LATENT-HEAT FLUX IN THE EQUATORIAL PACIFIC

Moored buoy data from the equatorial Pacific are analyzed to investigate the relationship between sea surface temperature (SST) and latent heat flux from the ocean. It is found that at low SST the latent heat flux increases with SST; at high SST the latent heat flux decreases with increasing SST, a relationship that cannot be explained by thermodynamic considerations alone, Analysis of the wind speeds and humidity differences between the surface air and the saturation humidity at the sea surface temperature indicates that while at low SST the humidity difference primarily determines the latent heat flux, and at high SST a sharp decrease in wind speed is mostly responsible for the low latent heat flux. A mechanism that leads to low latent heat flux at high SST is suggested; it involves the interaction between convection and the large-scale circulation. The longitudinal distribution of SST, wind speed, humidity difference, and latent heat flux is found to be similar to that in previous studies. In the eastern Pacific, SST is the lowest, the wind speed is large, and the humidity difference is low; in the western Pacific, SST is the highest, whereas the wind speed is low and the humidity difference is large. Latent heat flux increases from the eastern Pacific westward, reaching a maximum in the central Pacific, and then decreases toward the western Pacific warm pool. Through analyses of the data on different timescales, we found that the atmospheric processes leading to low latent heat flux over warm SST were mainly operative on seasonal timescales (periods longer than 90 days). On shorter timescales (periods of 30-90 days), the influence of intraseasonal Madden and Julian waves was evident. On this timescale, the relationship between SST and latent heat flux was characterized by a 10-day lag between atmospheric forcing (primarily related to winds) and the local oceanic response in the western and central Pacific. In the eastern Pacific cold tongue, SST and latent heat flux variations were nearly in phase on this timescale, indicating an atmospheric response to oceanic forcing. For periods less than 30 days, SST variations associated with tropical instability waves were likewise shown to be important in forcing latent heat flux variations in the eastern Pacific cold tongue.