Multiscale Variability of the Atmospheric Mixed Layer over the Western Pacific Warm Pool

Abstract

Sounding data from Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) have provided a first opportunity to document the variability of the atmospheric mixed layer over the western Pacific warm pool on timescales ranging from diurnal to intraseasonal. Six-hourly sounding data from four sites—the atoll Kapingamarangi and R/Vs Moana Wave, Shiyan 3, and Xiangyanghong 5—are used to determine the mixed layer depth and its thermodynamic properties. Almost three-quarters of the soundings at these four sites exhibited well-mixed structures: nearly constant profiles of potential temperature and specific humidity capped by a ∼150-m-deep entrainment zone. The majority of the remaining soundings were modified by precipitation and their associated downdrafts. It is estimated that approximately 40%–50% of the total soundings in COARE were influenced by precipitation downdrafts. The mean mixed layer depth at the four sites was 512 m with large variations on multiple timescales. Mean depths decreased across the warm pool from west to east, consistent with the west-to-east increase in precipitation averaged over the 4-month Intensive Observing Period. Significant modulation of the mixed layer occurred on the timescale of the Madden–Julian oscillation (MJO): the mean depth was 562 m during the undisturbed, light-wind period prior to the strong westerly wind burst (WWB) associated with the December MJO; it decreased to 466 m during the heavy-rain period of the WWB, reflecting numerous, recovering precipitation downdraft wakes; and then increased to 629 m during the late stages of the WWB when precipitation had ended. Dry intrusions over the warm pool caused the mixed layer to deepen at times to 800 m and more. Since the surface buoyancy flux typically did not increase at these times, the deepening is linked to a suppression of shallow cumulus clouds by the dry air (reduced between-cloud subsidence) as well as a general reduction in the overall shower activity and associated precipitation downdrafts. Dry intrusions also acted to enhance radiative cooling in the mixed layer. A diurnal cycle in the mixed layer depth was observed, with maximum amplitude in undisturbed (mostly clear), light-wind conditions. The mixed layer was deepest and warmest in the afternoon in direct response to 1) an afternoon peak in the surface buoyancy flux, which, in turn, arose from the large diurnal cycle in SST (up to 2°–3°C) on light-wind days, and 2) absorption of solar shortwave radiation. Thus, the atmospheric mixed layer over the warm pool during undisturbed conditions behaves like that over land, albeit with a weaker diurnal cycle amplitude, but sufficient to generate an afternoon maximum in shallow cumulus clouds and precipitation. This pattern is distinct from the typical early morning maximum in rainfall during disturbed conditions over tropical oceans. Diagnosis of the mixed layer net radiative cooling rate during light-wind conditions indicates a large diurnal range, from ∼−1.5 K day−1 at midday to nearly −3 K day−1 at night.