The impacts of dynamics and biomass burning on tropical tropospheric Ozone inferred from TES and GEOS-Chem model

Junhua Liu (jliu@seas.harvard.edu)1, Jennifer Logan1

1 School of Engineering and Applied Sciences, Harvard University

AbstractAbstract

5. DISCUSSION and CONCLUSION5. DISCUSSION and CONCLUSION3. Metrological Difference between 05 and 063. Metrological Difference between 05 and 06

We use a 2-year simulation with the GEOS-CHEM global three-dimensional model of tropospheric chemistry to interpret observations of tropical troposphere ozone from the Tropospheric Emission Spectrometer (TES) instrument. We show here that the model reproduces the observed tropical ozone maximum in the middle troposphere fairly well over the southern hemisphere biomass burning season in 2005 and 2006, except for September and October of 2005, when model is too low. 2005 is a weak La Niña year while 2006 is an El Niño year. Examination of meteorology associated with the different climate patterns in these two years appears to explain the variation of the tropospheric ozone distribution in the model, namely higher ozone in 2006. In 2005, an amplified subtropical north-south trough was persistent in the middle troposphere over the Atlantic, associated with a dipole structure in the anticyclones, which inhibits the east-west transport of biomass burning emissions by the equatorial easterlies from western Africa and the long-range transport from South America in the westerlies from being carried across the Atlantic. These processes, along with subsidence over the ocean, cause the ozone maximum in the tropical and south Atlantic in October . During the El Niño year 2006, intensified easterly winds 10ºS and westerly winds at south of 18ºS, combined with reduced wind speeds in between, were favorable for accumulation of ozone in the middle troposphere.

1. Climate variation between 05 and 061. Climate variation between 05 and 06

2. TES and Model Comparisons2. TES and Model Comparisons

ACKNOWLEDGEMENTSACKNOWLEDGEMENTSThis work was supported by the NASA ACMAP program.

4. Ozone mixing ratio and flux at 430 hPa4. Ozone mixing ratio and flux at 430 hPa

• July and August: model captures the observed ozone distribution and magnitudes of the high ozone region. The difference between the model and measurements are within ±15ppb.

• In the eastern equatorial Pacific Ocean, there are relatively low ozone amounts in this region, caused by the persistent upward motion with the ITCZ area.

• September and October: Ozone in the middle troposphere is largely underestimated by the model in 2005 over the ozone maximum region, while in 2006 the model reproduces well the observations, except over Africa.

• Blue color indicates descent while red shows ascent.

• 2005: an anticyclone located in the Amazon basin. An amplified trough was persistent in the south Atlantic associated with a dipole structure in the anticyclones (compared to 2006)

• 2006: El Niño contributed to weaker subtropical anticyclones and weaker easterly trade wind in the lower troposphere, while in the middle troposphere there were easterly winds at 10ºS and stronger westerly wind south of 18ºS, with very weak flow in between.

Sep 2005 Sep 2006

Oct 2005 Oct 2006

2005: Stronger recirculation of ozone along the south Atlantic region, generating a bipolar maximum in October.

Vertical patterns of GEOS-Chem simulated ozone flux along 14° S in October

1) Higher NOx in Africa in July/Aug. 2006 from lightning, => more ozone in 2006 from Africa

2) Higher NOx over S. America in Sept/Oct 2006, perhaps from lightning

3) Higher NOx, in addition to differences in circulation, causes higher model ozone in 2006

• Meteorological difference associated with different phase of ENSO patterns between these two years helps explain the model’s interannual variation of the tropospheric ozone distribution, but differences in NOx also contribute

• Future work will examine NOx sources from lightning and biomass burning in the two years.

• Future work will investigate the reasons of divergence between model and observations.

• Global ENSO SST index is the average SST anomaly equatorward of 20S/20N minus the average SST poleward of 20S/20N, and it captures the low-frequency part of the El Niño / Southern Oscillation(ENSO) phenomenon.

• A weak cold phase (weak La Niña) begun developing during September and October of 2005.

• By the beginning of September 2006, rapid warming in the upper-ocean along the equatorial zone in the Pacific basin indicated that an ENSO warm event (El Niño) had begun to develop.

H H

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2006: constant and stronger easterly wind along 10°S and westerly wind in the subtropics, with a relatively stable region in between, which is favorable for building up the ozone maximum.

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