Energetics of East Pacific Easterly Waves during Intraseasonal Events

Abstract

Abstract The background atmospheric state of the east Pacific warm pool in which easterly waves develop varies dramatically on intraseasonal time scales. East Pacific intraseasonal variability is well known to modulate local convective and circulation patterns. Westerly (easterly) intraseasonal phases are associated with westerly (easterly) low-level wind and positive (negative) convective anomalies. This study investigates the perturbation available potential energy (PAPE) and perturbation kinetic energy (PKE) budgets of easterly waves composited during westerly, easterly, and neutral intraseasonal phases, respectively. The mechanisms, magnitudes, locations, and vertical structures of easterly waves are shown to strongly vary as a function of intraseasonal phase. Easterly waves draw energy from low-level barotropic conversion, regardless of phase, although the location and magnitude of the conversion varies strongly. During neutral and westerly intraseasonal phases, the generation of PAPE associated with perturbation diabatic heating that is subsequently converted to PKE is the dominant energy source for easterly waves. A novel and previously unrecognized result is the detection of strong barotropic generation of PKE at midlevels during westerly intraseasonal phases. This previously unidentified source of PKE at midlevels is in part due to strong intraseasonal modulation of the background midlevel winds.