Signatures of stratosphere to troposphere transport near deep convective events in the southern subtropics

Abstract

A climatology of tropospheric ozone profiles associated with tropical convection in the southwestern part of the Indian Ocean and over South Africa is presented. Then case studies of stratospheric‐tropospheric exchange are documented using radiosoundings, ozone lidar, satellite and ECMWF global model data. In three distinct cases of varying tropical convection intensity (depression and cyclone Guillaume near Reunion in February 2002 and convection near Irene in November 2000), strong interaction between convection‐induced upper level circulation, jet front systems and Rossby Wave Breaking induces stratosphere to troposphere exchanges. Stratospheric filaments in the upper troposphere evident in the ECMWF analyses are in good agreement with ozone, humidity and temperature vertical profile observations. For the Guillaume case study near Reunion, filaments and subsidence occur in both cases (depression on 15 February and cyclone on 19 February 2002). On 15 February, a moderate enhancement of ozone in the free troposphere is observed and on 19 February, a 100 ppbv ozone peak is recorded. In the Irene case study, a large upper level depression coming from the stratosphere, fed by a filament wrapped around the convective area in the Mozambican channel, induces an ozone peak of larger magnitude (170 ppbv). Secondary ozone sources (jet front system in the Atlantic and biomass burning in South America) could further amplify this ozone enhancement. The radiosounding indicates a strong ozone enhancement in the upper troposphere, without a signature of pumping from the lower layers, in contrast to the Guillaume case.

Tropospheric ozone climatology at two Southern Hemisphere tropical/subtropical sites, (Reunion Island and Irene, South Africa) from ozonesondes, LIDAR, and in situ aircraft measurements

“…The difference observed is comparable to the difference between the two ozone climatological profiles obtained in the presence and absence of tropical convection (Fig. 4 of Leclair de Bellevue et al, 2006). While ozonesonde measurements can be performed in broad weather conditions, LIDAR measurements can be performed only during nights and with clear sky conditions.…”

“…Moreover, summer corresponds to the rainy season at Reunion and convection exerts a complex role on ozone formation and its vertical distribution. Uplift of ozone poor air masses into the troposphere in convective systems results in low values of ozone in the upper troposphere, while deep convection in the vicinity of STE processes along the subtropical jet stream can be associated with ozone enhancement in the mid-troposphere (Leclair de Bellevue et al, 2006). Figure 5 shows evidence of a lower ozonopause at Irene during austral winter.…”

“…-For Reunion, deep convection along the edge of the subtropical jet where stratosphere to troposphere exchange occurs is at a maximum in DJF (Leclair de Bellevue et al, 2006). …”

“…For this region, the dynamical mechanisms influencing tropospheric ozone distribution have been documented, including intrusions of stratospheric air masses via tropopause folds (Baray et al, 1998), Rossby wave breaking (Postel and Hitchman, 1999) and mid latitude cut off lows . A study of the effect of tropical convection on tropospheric ozone has also been reported (Leclair de Bellevue et al,2006) and showed some differences between the two sites, Reunion and Irene. In addition, photochemical sources have been identified: biomass burning activity in Southern Africa and Madagascar during austral spring associated with long range transport of air masses are a cause of tropospheric ozone increase over Reunion Island (Baldy et al, 1996;Taupin et al, 2002).…”

Tropical upper tropospheric humidity variations due to potential vorticity intrusions

“…Rossby wave breaking at mid-latitudes leads to transport of stratospheric air with high PV in to tropical upper troposphere (De Bellevue et al, 2006). These PV intrusions can be identified from relatively high PV value (> 2 PVU) at 350 K isentropic level (∼ 200 hPa over tropics) (Waugh and Polvani, 2000).…”