Driving mechanisms for the El Niño–Southern Oscillation impact on stratospheric ozone

Abstract. While the impact of the El Niño–Southern Oscillation
(ENSO) on the stratospheric circulation has been long recognized, its
effects on stratospheric ozone have been less investigated. In particular,
the impact on ozone of different ENSO flavors, eastern Pacific (EP) El
Niño and central Pacific (CP) El Niño, and the driving
mechanisms for the ozone variations have not been investigated to date. This
study aims to explore these open questions by examining the anomalies in
advective transport, mixing and chemistry associated with different El
Niño flavors (EP and CP) and La Niña in the Northern Hemisphere in
boreal winter. For this purpose, we use four 60-year ensemble members of the
Whole Atmospheric Community Climate Model version 4. The results show a
significant ENSO signal on the total column ozone (TCO) during EP El Niño
and La Niña events. During EP El Niño events, TCO is significantly
reduced in the tropics and enhanced at middle and high latitudes in boreal
winter. The opposite response has been found during La Niña.
Interestingly, CP El Niño has no significant impact on extratropical TCO,
while its signal in the tropics is weaker than for EP El Niño events.
The analysis of mechanisms reveals that advection through changes in
tropical upwelling is the main driver for ozone variations in the lower
tropical stratosphere, with a contribution of chemical processes above 30 hPa. At middle and high latitudes, stratospheric ozone variations related to
ENSO result from combined changes in advection by residual circulation
downwelling and changes in horizontal mixing linked to Rossby wave breaking
and polar vortex anomalies. The impact of CP El Niño on the shallow
branch of the residual circulation is small, and no significant impact is
found on the deep branch.
