Role of vertical and horizontal mixing in the tape recorder signal near the tropical tropopause

摘要

Nearly all air enters the stratosphere through the tropical tropopause layer(TTL). The TTL therefore exerts a control on stratospheric chemistry andclimate. The hemispheric meridional overturning (Brewer–Dobson) circulationspreads this TTL influence upward and poleward. Stratospheric water vaporconcentrations are set near the tropical tropopause and are nearly conservedin the lowermost stratosphere. The resulting upward propagating tracertransport signal of seasonally varying entry concentrations is known as thetape recorder signal. Here, we study the roles of vertical and horizontalmixing in shaping the tape recorder signal in the tropical lowermoststratosphere, focusing on the 80 hPa level. We analyze the tape recordersignal using data from satellite observations, a reanalysis, and achemistry–climate model (CCM). By modifying past methods, we are able to capturethe seasonal cycle of effective vertical transport velocity in the tropicallowermost stratosphere. Effective vertical transport velocities are found tobe multiple times stronger than residual vertical velocities for thereanalysis and the CCM. We also study the tape recorder signal in anidealized 1-D transport model. By performing a parameter sweep, wetest a range of different strengths of transport contributions by verticaladvection, vertical mixing, and horizontal mixing. By introducing seasonalityintothe transport strengths, we find that the most successful simulation of theobserved tape recorder signal requires verticalmixing at 80 hPa that is multiple times stronger compared to previous estimates in the literature. Verticalmixing is especially important during boreal summer when vertical advectionis weak. Simulating the reanalysis tape recorder requires excessive amountsof vertical mixing compared to observations but also to the CCM, which hintsat the role of spurious dispersion due to data assimilation. Contrasting theresults between pressure and isentropic coordinates allows for further insightsinto quasi-adiabatic vertical mixing, e.g., associated with overshootingconvection or breaking gravity waves. Horizontal mixing, which takes placeprimarily along isentropes due to Rossby wave breaking, is captured moreconsistently in isentropic coordinates. Overall, our study emphasizes the roleof vertical mixing in lowermost tropical stratospheric transport, whichappears to be as important as vertical advection by the residual masscirculation. This questions the perception of the as amanifestation of slow upward transport as opposed to a phenomenon influencedby quick and intense transport through mixing, at least near the tape head.However, due to the limitations of the observational dataset used and thesimplicity of the applied transport model, further work is required to moreclearly specify the role of vertical mixing in lowermost stratospherictransport in the tropics.