Equatorial modons in dry and moist-convective shallow-water systems on a rotating sphere

The adjustment of equatorial pressure anomalies is studied using high-resolution numerical simulations within one- and two-layer shallow-water systems on a rotating sphere, which include a simple self-consistent parameterization of the dynamical effects of condensation (the so-called moist-convective shallow-water equations). A systematic generation of localized, moving eastward along the equator cyclonic pairs, termed the equatorial modons, is observed in the one-layer model for sufficiently intense initial perturbations. The modons are strongly enhanced by the moist convection. In the two-layer model with a relatively small depth ratio, baroclinic equatorial modons are also observed. In both the one-layer model and the baroclinic mode of the two-layer model, the emergence of a transient Gill pattern preceding the generation of modons is demonstrated. It is found that the breaking of circumnavigating moist Kelvin waves, generated in the adjustment process, induces extensive easterly jets, which weaken the eastward nonlinear advection tendency of the modons. Finally, an inductive generation of the modons over oceanic warm-pools, modelled as local humidity maxima, is discovered, and explained on the basis of conservation of moist potential vorticity. © 2021 Cambridge University Press. All rights reserved.