On the annual cycle of latent heat fluxes over the equatorial Pacific using TAO buoy observations

In this paper, we describe the annual cycle in the latent heat flux (LHF) and its associated bulk variables (sea surface temperature, wind speed, humidity difference) over the equatorial Pacific. The in-situ, daily-averaged TAO buoy observations between 8 degrees N and 8 degrees S during the period 1992-1996 form the database. LHF was computed using a modified bulk parameterization scheme to account for active convection and low wind speed frequently observed in the western Pacific. Harmonic analysis was used to help quantify the phase and amplitude of the annual and semiannual cycles. The annual cycle of LHF was found to be conspicuous in two regions, namely, the northeastern and western/central Pacific. For the former region, the maximum LHF occurs in boreal summer and early fall, when surface wind speeds are strong and the temperature difference between sea surface and air near the bottom of the atmospheric boundary layer is large. For the western/central Pacific, maximum LHF occurs in boreal winter, when the winter monsoon is strong. In contrast to the aforementioned two regions, the annual cycle in LHF in the equatorial cold tongue is weak and low LHF prevails throughout the year. Also noted in this study is a westward propagation of the maximum LHF region from the northeastern Pacific around July to the western Pacific by the following March. We also ascertained the relative importance of dynamic and thermodynamic processes in regulating the month-to-month variations of the LHF along two meridional transects, one in the eastern and another in the western Pacific. In the eastern Pacific, except to the north of the cold tongue, variations in humidity difference (i.e., thermodynamic process) seem to be of primary importance to the annual variations in LHF. On the other hand, variations in wind speed (i.e., dynamic process) are more important to the LHF in the western/central Pacific.