Abstract
Abstract. In an earlier study of troposphere-to-stratosphere transport (TST) via the tropical tropopause layer (TTL), we found that the vast majority of air parcels undergoing TST from the base of the TTL enter the extratropical lowermost stratosphere quasi-horizontally and show little or no regional preference with regards to origin in the TTL or entry into the stratosphere. We have since repeated the trajectory calculations – originally limited to a single Northern Hemisphere winter period – in a variety of months and years to assess how robust our earlier findings are to change of timing. To first order, we find that the main conclusions hold, irrespective of the season, year and phase of the El Niño Southern Oscillation (ENSO). We also explore: the distribution of TST between the Northern and Southern Hemispheres; the sensitivity of modelled TST to the definition of the tropopause; and the routes by which air parcels undergo transport exclusively to the stratospheric overworld. Subject to a dynamical definition of the tropopause, we identify a strong bias towards TST in the Southern Hemisphere, particularly during the Northern Hemisphere summer. The thermal tropopause, defined according to the World Meteorological Organization, lies above the dynamical tropopause throughout the extratropics. Inevitably, on switching to the thermal definition, we calculate much less transport across the tropopause, particularly in the subtropics, which could be important with regards to interpretation of processes affecting ozone chemistry in the extratropical lowermost stratosphere (ELS). In contrast to the rather homogeneous nature of TST into the ELS, we find that transport to the overworld takes place from relatively well-defined regions of the TTL, predominantly above the West Pacific and Indonesia, except for an El Niño period in which most transport takes place from regions above the East Pacific and South America.
Highlights
In a recent study (Levine et al, 2007), we used trajectory calculations to investigate the routes by which air parcels undergo troposphere-to-stratosphere transport (TST) from the base of the tropical tropopause layer (TTL), during a baseline Northern Hemisphere winter period (January 2001)
We present the results for 2001 first as this “baseline” year is relatively unperturbed by the El Nino Southern Oscillation (ENSO)
Besides supporting our earlier findings, the study of a variety of months and years has revealed seasonal variations in the amount of TST and the distribution of TST between the Northern and Southern Hemispheres; we will discuss these variations in Sects. 4.1 and 4.2 respectively
Summary
In a recent study (Levine et al, 2007), we used trajectory calculations to investigate the routes by which air parcels undergo troposphere-to-stratosphere transport (TST) from the base of the tropical tropopause layer (TTL), during a baseline Northern Hemisphere winter period (January 2001). One of the main findings of our earlier model study was that the vast majority of air parcels, approximately 85%, which undergo TST on short timescales (less than or equal to four weeks), enter the extratropical lowermost stratosphere (ELS), that part of the stratosphere which lies below 380 K in potential temperature, by quasi-isentropic transport across the subtropical tropopause. This challenges the tacit assumption that TST mainly takes place into the overworld, that part of the stratosphere which lies above 380 K. We found that TST from the base of the TTL (transport into the overworld and ELS) was remarkably homogeneous, with a similar fraction of air undergoing TST from each longitudinal region of the TTL to each longitudinal region of the stratosphere.
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