Mechanisms of Poleward Propagating, Intraseasonal Convective Anomalies in Cloud System–Resolving Models

Abstract

Abstract An envelope of convection that propagates both poleward and eastward accounts for the largest fraction of intraseasonal variance of the tropical atmosphere during boreal summer. Here the mechanisms of poleward propagating convective anomalies are examined in a nonhydrostatic model with zonally symmetric boundary conditions, integrated on a beta plane at resolutions high enough to explicitly represent moist convection. When the domain has a narrow zonal dimension of 100 km or less, the model produces a quasisteady intertropical convergence zone (ITCZ). Meridionally propagating transients are produced for some prescribed sea surface temperature distributions, but these transients are shallow, vanish at finer resolutions, and have a structure that bears little resemblance to that of observed poleward propagating anomalies. This is in sharp contrast to previous studies that obtained robust poleward propagating anomalies in axisymmetric models using parameterized moist convection, and it suggests that the anomalies seen in those models may be caused by deficient representations of dynamics or subgrid-scale physics. Robust poleward propagating anomalies are obtained when the high-resolution, nonhydrostatic model is integrated in a wider domain with a zonal dimension near 1000 km. Diagnostics suggest that poleward propagation in this wide domain results from the convectively coupled beta drift of low-level vorticity anomalies. Deep near-equatorial ascent produces low-level cyclones that migrate poleward through the process of beta drift; Ekman pumping in these drifting cyclones then humidifies the free troposphere ahead of the initial deep ascent, shifting the convection poleward. The moist static energy budget and model sensitivity tests suggest that these anomalies can be viewed as moisture modes destabilized through a moisture–radiation feedback. Wind–evaporation feedback also seems to contribute to the instability of these anomalies, but because it enhances surface fluxes on the equatorward side of the anomalies, it also reduces their propagation speed. These results suggest a novel mechanism for the poleward propagation of intraseasonal convective anomalies and illustrate the need to evaluate theoretical models that use parameterized convection against cloud system–resolving models.