Seasonal ozone variations in the isentropic layer between 330 and 380 K as observed by SAGE II: Implications of extratropical cross‐tropopause transport

Abstract

Abstract. To provide observational evidence on the extratropical cross-tropopause transport between the stratosphere and the troposphere via quasi-isentropic processes in the middleworld (the part of the atmosphere in which the isentropic surfaces intersect the tropopause), this report presents an analysis of the seasonal variations of the ozone latitudinal distribution in the isentropic layer between 330 K and 380 K based on the measurements from the Stratospheric Aerosol and Gas Experiment (SAGE) II. The results from SAGE II data analysis are consistent with (1) the buildup of ozone-rich air in the extratropical middleworld through the large-scale descending mass circulation during winter, (2) the spread of ozone-rich air in the isentropic layer from midlatitudes to subtropics via quasi-isentropic transport during spring, (3) significant photochemical ozone removal and the absence of an ozone-rich supply of air to the layer during summer, and (4) air mass exchange between the subtropics and the extratropics during the summer monsoon period. Thus the SAGE II observed ozone seasonal variations in the middleworld are consistent with the existing model calculated annual cycle of the diabatic circulation as well as the conceptual role of the eddy quasi-adiabatic transport in the stratosphere-troposphere exchange reported in the literature.

Influence of isentropic transport on seasonal ozone variations in the lower stratosphere and subtropical upper troposphere

“…Although the seasonal phase of the isentropic ozone STE is similar in both hemispheres, it is noted that the estimated annual ozone fluxes (both from S→T and from T→S) are more than 50% greater in the NH than in the SH (Table 4). For the S→T flux, this is mainly because the ozone mixing ratios in the LS in the NH are greater than in the SH [ P. H. Wang et al , 1998], and the transport of reduced springtime ozone air from high latitudes in the SH is likely to be one of the reasons that ozone mixing ratios in the SH lower stratosphere are lower than those in the NH. There may also be a contribution from a greater number of Rossby wave breaking events in the NH than in the SH [ Postel and Hitchman , 1999] and the subtropical UT and extratropical LS are therefore better mixed in the NH.…”

“…While diabatic transport tends to sharpen the latitudinal ozone gradients at midlatitudes by dumping ozone‐rich air at high latitudes and transferring ozone‐poor air upward at the equator, the effect of isentropic transport is to reduce this latitudinal ozone gradient by exchanging ozone quasi‐horizontally between the subtropics and the extratropics. Measurements taken by the Stratospheric Aerosol and Gas Experiments (SAGE) II, for example, have shown a decrease of ozone in the extratropical LS accompanied by ozone increase in the subtropical UT in late spring/summer [ P. H. Wang et al , 1998]. This suggests the possibility that isentropic transport could be a significant contributor to the ozone budget in the subtropical UT by spreading the stored ozone‐rich air from the extratropical LS.…”

Atmospheric tracers during the 2003–2004 stratospheric warming event and impact of ozone intrusions in the troposphere

“…As a consequence, less ozone was transported poleward (see Figs. 5c,d and 6c,d) and downward into the extratropical UTLS region, which may have reduced the wintertime buildup of ozone in the middleworld (Wang et al, 1998). .…”

“…Chen (1995) also suggests that the strong downward flux across the 100 hPa level in the extratropical lower stratosphere of the northern winter (DJF) (Holton, 1990;Rosenlof and Holton, 1993) can be interpreted as the mass being transported downward from the "overworld" into the "middleworld". Analyses of the isentropic cross-tropopause ozone transport using SAGE II observations (Wang et al, 1998) suggests that the winter buildup of ozone-rich air in the extratropical "middleworld", between the isentropic surfaces of 330 K and 380 K, should be attributed primarily to the wave-driven diabatic mass circulation during fall-winter-spring seasons, rather than to transport along isentropic surfaces. At least, there appears to be strong connections between the activity of upward planetary waves and the wintertime ozone distributions (Fusco and Salby, 1999;Salby and Callaghan, 2007).…”

Seasonal cycles and variability of O<sub>3</sub> and H<sub>2</sub>O in the UT/LMS during SPURT

“…Much of this decrease is presumably caused by the change in tropopause height and transport mechanisms. On the basis of observations of SAGE II, Pan et al (1997) and Wang et al (1998) assumed that isentropic cross-tropopause inflow of tropospheric air into the LMS influences the seasonal cycle of O 3 (and H 2 O, see Sect. 3.2) in that atmospheric region, especially during summer.…”