Abstract
There is compelling evidence that atmospheric moisture may either increase or decrease mid-latitude eddy kinetic energy (EKE). We reconcile these findings by using a hierarchy of idealized atmospheric models to demonstrate that moisture energizes individual eddies, but makes the large-scale conditions in which they form less favorable for eddy growth. For climates similar to present-day, the latter effect wins out, and moisture weakens mid-latitude eddy activity. The model hierarchy includes a moist two-layer quasi-geostrophic (QG) model and an idealized moist general circulation model (GCM). In the QG model EKE increases when moisture is added to simulations with fixed baroclinicity, closely following a previously derived scaling. But in both models moisture decreases EKE when environmental conditions are allowed to vary. We explain these results by examining the models’ Mean Available Potential Energy (MAPE) and by calculating terms in the models’ Lorenz Energy Cycles. In the QG model the EKE decreases because precipitation preferentially forms on the poleward side of the jet, releasing latent heat where the model is relatively cold and decreasing the MAPE, hence the EKE. In the moist GCM the MAPE primarily decreases because the mid-latitude stability increases as the model is moistened, with reduced meridional temperature gradients playing a secondary role. Together, these results clarify moisture’s role in driving the mid-latitude circulation and also highlight several drawbacks of QG models for studying moist processes in mid-latitudes.
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