Horizontal and Vertical Structure of Easterly Waves in the Pacific ITCZ

Outgoing longwave radiation (OLR) and low-level wind fields in the Atlantic and Pacific Intertropical Convergence Zone (ITCZ) are dominated by variability on synoptic time scales primarily associated with convectively coupled easterly waves during boreal summer and fall. This study uses spectral filtering of observed OLR data to capture the convective variability coupled to Pacific easterly waves. Filtered OLR is then used to isolate easterly waves in winds, temperature and humidity fields from TAO/TRITON and TAO/EPIC buoys, radiosondes, and gridded reanalysis products. Our analysis shows that while some of the Pacific easterly waves originate in the Atlantic, most of the waves appear to form and strengthen within the Pacific. Pacific easterly waves have wavelengths of 3300-5500 km and phase speeds of 9-13 m s-1. A warm, moist boundary layer is observed ahead of the convective wave, with moisture lofted quickly through the troposphere by deep convection, followed by a cold, dry signal behind the wave. The waves are accompanied by substantial cloud forcing and surface latent heat flux signals in buoy observations. In the central Pacific the horizontal structure of the waves appears as meridionally-oriented inverted troughs, while in the east Pacific the waves are oriented southwest-northeast. Both are tilted slightly eastward with height. Although these tilts are consistent with adiabatic barotropic and baroclinic conversions to eddy energy, energetics calculations imply that the Pacific easterly waves are driven primarily by convection. This differs from African easterly waves, where the barotropic and baroclinic conversions dominate.