Episodic Trade Wind Regimes over the Western Pacific Warm Pool

Abstract

The western Pacific warm pool experiences the greatest rainfall of any oceanic region on earth. While the SST is everywhere high over the warm pool, there is great spatial and temporal variability in rainfall. Sounding data from the recent TOGA COARE are used to document this variability. In particular, the vertical distributions of heating and moistening at different phases of the 30–60 day or intraseasonal oscillation are determined for different areas within the warm pool. While heating and moistening distributions near the equator over the warm pool are often similar to those observed over the western Pacific within the convectively active ITCZ, these profiles do not prevail at all times. In particular, during westerly wind bursts and suppressed, light-wind periods, heating and moistening distributions over the COARE Intensive Flux Array frequently resemble those observed in the trade wind belts. Such profiles are characterized by relatively large negative values of apparent moisture sink Q2 in the lowest 2–3 km, reflecting the important moistening effects of shallow, mostly nonprecipitating cumulus clouds. A maximum in moistening commonly occurs in the lower part of the cloud layer during the westerly wind bursts, indicating many “forced” cumuli that are extensions of boundary-layer turbulence. During suppressed, light-wind periods the moistening peak shifts to the upper part of the cloud layer, indicating a larger proportion of “active” trade cumulus at that time (consistent with higher SSTs and weaker vertical wind shear). A spectral cloud diagnostic model is used to determine the properties of the shallow cloud fields. Computed profiles of mass fluxes, entrainment, detrainment, and heat and moisture balances during these periods generally resemble those for the western Atlantic trade wind regime. These findings lead to the conclusion that, in association with the intraseasonal oscillation, the western Pacific warm pool boundary layer periodically develops trade-wind-like characteristics with abundant shallow cumulus, and these episodic “tradelike” regimes are frequent enough to impact the seasonal-mean distributions of boundary layer heating and moistening.