Influence of stratosphere‐troposphere exchange on tropospheric ozone over the tropical Indian Ocean during the winter monsoon

Abstract

Ozone (O3) and relative humidity (RH) soundings, launched over the Indian Ocean during the 1998 winter monsoon (February–March), were analyzed. In the marine boundary layer (MBL), O3 mixing ratios were relatively low (10–20 ppbv) except close to the Indian subcontinent (40–50 ppbv) where profiles were strongly influenced by pollution. Sometimes, relatively low O3 levels were observed in the upper troposphere. These were associated with deep convection in regions where MBL O3 levels were also low. In the midtroposphere (300–500 hPa), O3 maxima (60–90 ppbv) were often found with low RH. A remarkable new finding of this study is that in more than a third of the profiles, laminae with very high O3 mixing ratios (up to 120 ppbv) were observed just below the tropical tropopause (between 100 and 200 hPa). Back trajectory analyses showed that these layers originated in the vicinity of the subtropical jet stream (STJ). We hypothesize that stratosphere‐troposphere exchange (STE) near the subtropical jet by either shear‐induced differential advection or clear‐air turbulence (CAT) caused the midtropospheric maxima (STE followed by descent) and the upper tropospheric laminae. Another new finding is that stratospheric intrusions were not only found near the STJ but also deep within the tropics. Given the thickness of the midtroposphere intrusions (typically 3–5 km) and the very high O3 mixing ratios of the upper tropospheric laminae, it seems that STE plays an important role in the tropical tropospheric O3 budget, at least over the Indian Ocean during the winter monsoon.

On the origin of tropospheric O<sub>3</sub> over the Indian Ocean during the winter monsoon: African biomass burning vs. stratosphere-troposphere exchange

“…Furthermore, Folkins et al (1999) used temperature and humidity profiles to show that convection generally cannot reach altitudes above 14 km over tropical oceans. Zachariasse et al (2000) reported similar uppertropospheric layers with enhanced O 3 during the First Field Phase (FFP) of INDOEX, which took place during February and March 1998. They used trajectory analysis and potential vorticity fields derived from the European Center for Medium-Range Weather Forecast (ECMWF) to conclude that STE along the northern hemispheric subtropical jet (NH- The mid-tropospheric O 3 maxima are often associated with (very) low relative humidity (Peshin et al, 2001;Zachariasse et al, 2001).…”

“…Figure 3a and b show vertical profiles from the 1998 IN-DOEX IFP O 3 soundings. An analysis of these profiles based on backtrajectory calculations and potential vorticity can be found in Zachariasse et al (2000). For the details of the location and time of launch of these soundings see Table 2.…”

“…The INDOEX measurements show the presence of free-tropospheric layers with enhanced O 3 mixing ratios (de Laat et al, 1999;Mandal et al, 1999;Zachariasse et al, 2000;Peshin et al, 2001;Zachariasse et al, 2001). These layers can be found throughout the free troposphere.…”

Changes in surface ozone levels due to convective downdrafts over the Bay of Bengal

“…Surface sensible and latent heat fluxes are considered the main sources of q E in the atmosphere [Betts et al, 1992], while the primary sink is radiative cooling of the troposphere. Consequently, the tropical lower troposphere is characterized by a decrease in q E and an increase in O 3 with height [Folkins et al, 1999;Zachariasse et al, 2000;Betts et al, 2002]. Independent meteorological parameters such as temperature and pressure may not be conserved in downdrafts, while q E is near conservative.…”

Investigation of the short-time variability of tropical tropospheric ozone

“…Because of the low frequency of in situ measurements (weekly or less), it has been hard to characterize the day-to-day variability of tropospheric ozone. Several studies based on INDOEX measurements (De Laat et al, 1999;Zachariasse et al, 2000) have dealt with short-term variability of ozone though mainly north of the equator and in the Indian Ocean, when the convective activity is maximum, and biomass burning in Southern Africa is minimum (FebruaryApril) (Cahoon et al, 1992;Barbosa et al, 1999;Arino et al, 2001).…”