Extratropical stratosphere‐troposphere mass exchange associated with isentropic mixing: A 1992–2005 climatology derived from advection‐diffusion calculations

Abstract

Tropopause folding and subsequent mixing are the primary vehicles for the stratosphere‐troposphere mass exchange in the extratropics, but to isolate the irreversible transport associated with these processes is difficult. In this paper, a potential vorticity (PV)‐like passive tracer, constructed from advection‐diffusion calculations, is used to characterize the extratropical tropopause layer and the mass fluxes through it. The calculations are driven by the Met Office Stratospheric Analysis winds interpolated on 20 isentropic surfaces (285–380 K) for the period of 1992–2005. The mass fluxes associated with isentropic mixing are computed in the tracer coordinate, thus excluding the deformation of the tropopause itself from transport. The fluxes are partitioned into stratosphere‐to‐troposphere (STT) and troposphere‐to‐stratosphere (TTS) components using the formalism of Nakamura. The STT flux outweighs the TTS flux by approximately 2 to 1 in both hemispheres. These fluxes are several times greater than the well‐known net annual mean tropopause flux (∼1010 kg s−1), and they increase sharply downward, acting to detrain mass from the tropopause layer. However, the observed mass of the tropopause layer is nearly steady throughout the year, suggesting that isentropic mixing is offset by other nonadvective processes on a seasonal timescale. Both the annual mean values and the seasonal variations of the mass fluxes are markedly greater in the Northern Hemisphere than in the Southern Hemisphere. Consistent with the previous studies, the Northern Hemisphere fluxes below 340 K maximize in spring and minimize in fall, whereas an almost opposite seasonality is observed above 340 K.