The equatorial 30-60 day oscillation and the Arakawa-Schubert penetrative cumulus parameterization

Abstract

Characteristics of the equatorial intraseasonal oscillation are studied with the use of a general circulation model which includes the Arakawa-Schubert (abbreviated as AS hereafter) model of penetrative cumulus convections. The AS model is modified by introducing the minimum value of cumulus entrainment rate of the environmental air, μmin, as μmin=α/D, where D is the depth of the planetary boundary layer (PBL) and α is a non-negative constant. The introduction of a positive α in the AS model suppresses the activity of deep penetrative cumulus in the area where D is not sufficiently thick, which allows, in turn, an accumulation of moist air in the large-scale low level convergence zone. This process is essential in maintaining the equatorial 30-60 day oscillations, and also in simulating the Pacific subtropical high during the northern summer. Experiments are performed by changing α from 0 (i.e., the original AS model), to ∞ (i.e., no penetrative cumulus convection) under an aqua-planet condition. When α =0, the 30-60 day oscillation does not appear in the tropics. Instead, there exists a quasi-10 day eastward propagating oscillation with zonal wavenumber 1, which resembles a neutral Kelvin wave. Moist air is not accumulated in the ‘low level east-west convergence longitude associated with the flow of zonal wavenumber 1 (LLCL)' due to the rapid response of the AS model to the evaporation and moisture flux convergence by small scale motions and also due to the resulting upward transport of water vapor by penetrative cumuli to the west of the LLCL before the moist air can be accumulated in the LLCL. When α =0.1, a quasi-30 day eastward propagating oscillation with zonal wavenumber 1 grows in the model, with the moist air and the major heating found around the LLCL. The change in the heating is mostly due to the increase of middle-level convection (i.e., moist convection between adjacent vertical layers within the free atmosphere) and to the decrease of deep penetrative cumuli to the west of the LLCL. Overall characteristics of the mode are close to the observed ones. When α =∞, a quasi-45 day eastward propagating oscillation grows in the model. The structure of the heating associated with the oscillation is similar to that of the quasi-30 day oscillation. Associated with the increase of α, the static stability decreases in the low latitudes. The maximum level of the zonally averaged heating lowers due to the suppression of deep penetrative cumulus and the increase in both the middle-level convection and large-scale condensation. The maximum amplitude level of the heating associated with the equatorial intraseasonal oscillation also lowers from 300mb for α =0 to 500mb for α =0.1–∞. These changes seem to provide favorable conditions for the occurrence of the equatorial intraseasonal oscillation, in agreement with linear stability studies of a CISK model.