Observations of subtropical air in the european mid‐latitude lower stratosphere

Abstract

Evidence of two wave-breaking events, during which subtropical air was transported into the northern mid latitudes, is presented. The first event took place in January 1997 when ozone mixing ratios of the order of 120 parts per billion (lo9) by volume (p.p.b.v.) were measured in the lower stratosphere above Aberystwyth (52.4'N, 4.1"W). The amount of subtropical air involved was 13 x loi4 kg. A second, more dramatic, wavebreaking event took place in March 1997 involving 36 x lOI4 kg of air. This case contained a region with potential vorticity 5 2 potential vorticity units (1 PVU = 1 x K kg-'m2s-I), consistent with ozone mixing ratios of 80 p.p.b.v. observed at Aberystwyth at 365 K. Trajectory analysis showed that this air was advected northwards from near the subtropical tropopause. The extent to which irreversible transport into the mid-latitude lower stratosphere followed both events is evaluated using trajectory analysis and potential-vorticity evolution.

Seasonal and inter-annual variations in troposphere-to-stratosphere transport from the tropical tropopause layer

“…in February, April, June, August, October and December). However, we explore a number of additional months, including December 1997, in order to cover, comprehensively, the key meteorological seasons, including the Northern Hemisphere winters of 1997(El Niño) and 1998/1999.…”

“…Allam and Tuck, 1984;Holton et al, 1995) and observational evidence of its effects on the chemical composition of the mid-latitude lower stratosphere (e.g. Vaughan and Timmis, 1998;O'Connor et al, 1999). However, trace gas observations in the lowermost stratosphere have been used to infer seasonal variations in the amount of TST into the ELS (see Randel et al, 2001;Prados et al, 2003;Hoor et al, 2004 and, where an increase in the concentration of water vapour or carbon monoxide, accompanied by a decrease in the concentration of ozone, is indicative of an increase in TST.…”

Temperature climatology and trend estimates in the UTLS region as observed over a southern subtropical site, Durban, South Africa

“…Theoretical studies, transport models and in situ observations have shown that at subtropical latitudes, vertical and horizontal exchange processes are expected and have been observed (Grant et al, 1994;Holon et al, 1995;Vaughan and Timmis, 1998;O'Connor et al, 1999). Furthermore, recent observational studies have shown that in the southern subtropics, within the UTLS region, many dynamical processes, such as subtropical tropopause breaks (Baray et al, 1998), planetary-scale tropopause fold events (Baray et al, 2000) and isentropic horizontal exchanges in the low stratosphere through the southern subtropical barrier Portafaix et al, 2003) take place.…”