2. Background • We use the GEOS-Chem global chemical

transport model that simulates tropospheric ozone chemistry, with parameterized stratospheric O3 chemistry.

•  The model is driven by meteorological fields from the NASA Global Modeling and Assimilation Office (GMAO). Here we use the MERRA fields from GMAO.

• We also use ERA-Interim meteorological fields from the European Centre for Medium-range Weather Forecasts (ECMWF).

• The 27-year mean July Strat O3 field is greatest over SIO.

• The greatest variability in O3 is also found over the SIO.

Increase in stratospheric O3 from late 1980s to early 2000s in the GEOS-Chem model.

3. Geopotential Height, 200hPa (Z200) •  Geopotential height with zonal

mean removed shows two high pressure centers called the South Indian Ocean High complex.

•  SIOH complex can be combined or separate systems.

•  Average July geopotential height at 200hPa over South African and Australian maxima in the high-low years plot.

•  Strong, significant correlation with stratospheric O3 burden in UT.

(Above) Average of low stratospheric ozone years (1988, 1990, 1991) subtracted from average of high stratospheric ozone years (1997, 2001, 2002)

•  July geopotential height over SAH (right) and AH (left).

•  Each component of SIOH has some positive, significant correlation to stratospheric O3 burden.

•  SAH component is strongest suggesting greatest influence on stratospheric O3 coming from closer to South Africa.

Analysis of the Interannual in Ozone over the South Indian Ocean

Abstract. Recent observational studies (Clain et al., 2009; Thomson et al., 2014) have found found a positive trend in ozone (O3) over the South Africa and Reunion Island. We use the GEOS-Chem model and meteorological reanalyses to study this trend over the South Indian Ocean between 1980 and 2006, during the austral winter season (JJA). We relate this trend to the variability in the meteorology over the South Indian Ocean. In particular, we believe that the trend is, in part, due to increase transport of O3 from the stratosphere, linked to changed in the South African High.

MODEL SIMULATIONS

1. Introduction

Nikhil Seth, Dave Mackenzie, Dylan Jones

Centre for Global Change Science, University of Toronto

• Ozone acts as a greenhouse gas in the mid-upper troposphere and can negatively impact air quality at the surface.

• Regional sources of tropospheric ozone include transport of ozone precursors from surface pollution and transport of ozone from the stratosphere.

4. Comparison of MERRA and ECMWF

5. Sea Surface Temperatures 7. Conclusions

• Recent studies have found significant positive trend in JJA O3 in the upper troposphere (UT) over SIO, but there are few observations in the region.

•  (Left) Comparison of MERRA and ECMWF using MATLAB.

•  (Right) Comparison of SIOH timeseries in MERRA and ECMWF.

•  This agreement means we can use MERRA and ECMWF interchangeably.

•  SSTs are a source of atmospheric waves that can alter UT circulation.

•  Correlation and significance maps show no notable correlations between sea surface temperatures and stratospheric O3 burden to UT.

6. Potential Vorticity and Stratospheric Intrusions

July timeseries of strat O3 contribution to trop over the SIO

GEOS-Chem O3 and Horizontal winds for July 2007 at 9km

•  PV is a dynamic tracer that is indicative of stratospheric air. •  Strong, significant correlations in SIOH complex region, promoting further

study.

Averaging PV over the SAH and AH shows a very strong correlation with the stratospheric O3 burden to the troposphere. There is more of a contribution from the SAH than the AH, due to the higher correlation coefficient.

Cross section of daily averaged PV shows an example of what would be considered a stratospheric intrusion. I wanted to count the number of intrusions events each July to compare to the stratospheric O3.

•  Intrusions are most correlated between 0 and 50E •  Intrusions added every 10 longitudes (20, 30, 40, 50E). This reasonably

accounts for the stratospheric O3 over the SIO.

•  Strong, significant correlation between geopotential hieght and PV at 200 hPa suggesting link between changes in circulation and stratospheric intrusions.

•  Stratospheric intrusions and stratospheric O3 burden timeseries strongly related along longitudes 0-50E. This suggests intrusions events west of SIO are affecting the stratospheric O3 over the SIO.