A “tropical pipe” model of stratospheric transport

A conceptual model of global stratospheric transport is described, based on the assumption of rapid isentropic mixing within midlatitude “surf zones” but weak mixing into the tropics. Thus the tropical region is isolated from middle latitudes, and trace species budgets there are balances between mean upwelling and local chemical sources and sinks. In middle latitudes, where long‐lived species are assumed to be in “slope equilibrium,” the budgets are more complex, being influenced by isentropic mixing, mean downwelling, entrainment from the tropics, and local chemistry. The one‐dimensional vertical flux‐gradient relation obtained in previous studies in which mixing was assumed to be global is lost in this model. Tracer correlations are compact separately in each region, with substantial differences between tropical and midlatitude relationships. The result discussed by Plumb and Ko linking the slope of the correlation diagram to net global fluxes (and thus to lifetimes) is valid in this model only at the midlatitude tropopause.