Tropical and Midlatitude Tropospheric Column Ozone Response to ENSO in GEOS-5 Assimilation of OMI and MLS Ozone Data

Abstract:

Nine years of ozone observations from the Ozone Monitoring Instrument and Microwave Limb Sounder have been assimilated into the Goddard Earth Observing System Version 5 data assimilation system. We investigate the magnitude and spatial distribution of the influence by the El Niño Southern Oscillation (ENSO) on tropospheric column ozone (TCO) in the tropics through the middle latitudes. The tropospheric response in the tropics agrees well with previous studies. A newly identified two-lobed response symmetric about the Equator in the western Pacific/Indonesian region is consistent with the large-scale vertical transport. The ozone response is weaker in the middle latitudes, but significant explained variance of the TCO is found over several small regions, including the central United States. However, the sensitivity of TCO to the Niño 3.4 index is significant over a large area of the middle latitudes. The sensitivity maxima and minima coincide with anomalous anti-cyclonic and cyclonic circulations. The associated vertical transport is consistent with the sign of the sensitivity. ENSO related changes to the mean depth of the tropospheric column have little impact on the TCO response in the tropics but can contribute significantly to the midlatitude response. Comparisons to a 22-year chemical transport model simulation demonstrate that these results from the nine-year assimilation are representative of the longer-term. This investigation brings insight to several seemingly disparate prior studies of the El Niño influence on tropospheric ozone in the middle latitudes. In addition, these results are valuable as a process-oriented assessment of the tropospheric response in model simulations.