A moist linear baroclinic model: Coupled dynamical-convective response to El Niño

Abstract

A newly developed, linear baroclinic model (LBM) and its application to the tropical ENSO teleconnection is described. The model, based on primitive equations linearized about the observed, zonally varying basic state in northern winter, involves linear schemes for the cumulus convection, and surface sensible and latent heat fluxes, referred to as the moist LBM. This enables us to solve a steady-state response of the coupled dynamical-convective system to a given SST anomaly, which is fairly different from the conventional dry LBM. Linear representation of the convection is acceptable for a realistic range of SST anomalies, reproducing well the Rossby wave train computed in the conventional LBM forced by a tropical heating. The moist LBM is used to examine the formation mechanisms of an anomalous low-level anticyclone near the Philippines that links El Niño with the Asian winter monsoon. Given that the conventional LBM simulates the Philippine Sea anticyclone as a Rossby response to the anomalous diabatic cooling associated with the weakened convection over the Maritime Continent, causes of the convective suppression are examined. Moist LBM experiments forced by observed El Niño SST anomalies indicate that a basinwide warming of the Indian Ocean, in addition to SST anomalies in the Pacific, has a considerable impact in weakening the convection over the Maritime Continent through a modulation of the Walker circulation. Observational analysis supports this idea and further suggests that the lagged Indian Ocean response to El Niño contributes to determining when the Philippine Sea anticyclone is developed. The moist LBM identified a positive wind-evaporation feedback at work between the Philippine anticyclone and the western Pacific SST anomaly, which might also contribute.