Impact Of Shallow Convective Momentum Transport On Large-Scale Dynamics

Abstract

In the tropics, the character of the trade-winds is decisive for setting convergence and the large-scale circulation. Nevertheless, the vertical transport of momentum by shallow convection (SCMT) and yet its impact on trade- winds has not been investigated in depth yet. With this study, we aim to contribute in understanding the isolated effect SCMT has on the large scale circulations, setting the strength of the circulation and favouring zones of deep convection. Six climate models participated by each conducting three aquaplanet simulations, referring to a simplified climate model in which the entire planet’s surface is covered by ocean, useful for studying fundamental atmosphere-ocean dynamics in the absence of complex land surface interactions. In three distinctive simulations, the models either turn off or exaggerate their SCMT scheme, alternatively by altering an approximate convective momentum scheme. We hypothesized that given the typical tropical wind profile, north-easterly flow at the surface and south-westerly flow aloft, the depth of mixing is determinative for the long-term reaction in setting the circulation. By deeper types if mixing, the scheme will likely bring opposite winds to the surface, affecting both the magnitude and direction of the trade-winds. A first analysis reveals that certain confine the mixing of surface friction to the sub-cloud layer, showing a clear cut-off of induced tendency between the mixed layer and the cloud layer. It is those models that favour a local response only in the shallow overturning circulation, opposite to the models that actively mix momentum between the mix-layer and cloud-layer and show adjustments in the deep overturning circulation. Further building earlier work showing that the horizontal advection of moist static energy (MSE) in the sub-tropics drives tropical convection, we analyzed to what extent enhanced SCMT leads to a change in atmospheric cooling rates by di- or convergence of MSE. Apparent differences by enhanced SCMT are observed in the in cooling rates by advection of MSE between the models that show strong responses in the deep overturning versus the models that do not.