Quantifying the regional and global tropospheric ozone variability driven by ENSO using the GEOS-5 Ozone Assimilation

Tropospheric ozone is a critical component of Earth's radiative budget, particularly in the upper troposphere. In the lower troposphere, ozone is a pollutant and regulations have been made to control emissions of its precursors. Quantification of the natural variability of tropospheric ozone is essential to understanding the anthropogenic impact on both global and regional scales. NASA's Aura satellite was launched in late 2004 to study the atmosphere's chemistry and dynamics. On this platform, the Ozone Monitoring Instrument (OMI) measures total column ozone and the Microwave Limb Sounder (MLS) measures stratospheric ozone profiles. These observations have been used in the Goddard Earth Observing System version 5 (GEOS-5) assimilation system to provide 6 hourly, global analyses of atmospheric ozone. This 2° x 2.5° resolution analysis spans 2005 through 2012.

In this presentation, we will first demonstrate the fidelity of the analysis tropospheric column ozone (TCO) relative to ozonesonde observations. The analysis TCO is persistently biased low by ~1-5 DU depending upon latitude primarily due to a high bias in the MLS lower stratospheric observations. However, the bias and root-mean-square differences from sonde observations are significantly reduced compared to previous residual methods of subtracting the stratospheric column from the total column. We will then quantify the TCO variability forcing by ENSO and examine the regional differences and latitudinal extent of this forcing. We will show that the ENSO signature dominates the regional, short-term increasing trends in the tropics and middle latitudes. However, a small, yet significant, increasing trend remains throughout much of these latitudes when the ENSO signature is removed from the TCO time series.