Convective signals from surface measurements at DOE ARM Tropical Western Pacific sites

Madden-Julian Oscillation (MJO) signals have been detected using highly sampled observations (1-minute resolution) from the U.S. Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Climate Research Facility (ACRF) Tropical Western Pacific (TWP) Manus and Nauru sites. With downwelling shortwave radiative fluxes and derived shortwave fractional sky cover, and the statistical tools of wavelet, cross wavelet, and spectrum power, we report finding major convective signals and their phase change from surface observations spanning the periods from 1996 to 2006 for Manus and from 1998 to 2006 for Nauru. Our findings are confirmed with the satellite-retrieved values of precipitation from the Global Precipitation Climatology Project (GPCP) and interpolated outgoing longwave radiation (OLR) satellite measurements from the National Oceanic and Atmospheric Administration (NOAA) for the same location and period, though these products inherently represent large spatial scale. During the 1997-98 strong El Niño years, the MJO signals over both sites are weak. This is consistent with our current understanding. Overall, the MJO signal is much stronger and more persistent over Manus than over Nauru. We further composite 21 MJO events spanning the extended boreal winter period (November through the following April) from 1996 to 2006 for the Manus site, chosen using the NOAA Climate Prediction Center's MJO index over 140E. We find a strong modulation of the diurnal cycle by the MJO over Manus. In particular, our major findings are: 1) Daily average cloudiness is in phase with the MJO peak while the diurnal amplitude is out of phase; 2) The peak MJO phase is associated with enhanced early morning and suppressed late afternoon deep convection; 3) The convective precipitation lags the enhanced cloudiness in the morning by 3-6 hours; 4) Strong pre- conditioning of low and mid-level cloudiness is detected at about a 5-day lead of the MJO peak; and 5) The surface westerly wind burst (WWB) is strongly modulated by the MJO phases with afternoon maximums at peak MJO phases.