[A-AS21_30AM2] Stratospheric Processes And their Role in Climate

[AAS21-11] The role of the mid-latitude oceanic front in the ozone-induced climate change in the Southern Hemisphere

Fumiaki OGAWA¹, Nour-eddine OMRANI², *Kazuaki NISHII¹, Hisashi NAKAMURA¹, Noel KEENLYSIDE³ (1.RCAST, University of Tokyo, 2.GEOMAR, University of Kiel, 3.Geophysical Institute, University of Bergen)

Keywords:Ozone hole, Annular mode, Oceanic front

The Southern Hemisphere Annular Mode (SAM) is the dominant mode of low-frequency atmospheric variability in the extratropical Southern Hemisphere, exerting substantial impacts on regional distributions of temperature and precipitation. Its multi-decadal trend in the troposphere observed in late 20th century has been related to the lower-stratospheric changes induced by the ozone depletion. Known as a manifestation of meridional shift of the eddy-driven polar-front jet (PFJ), which is collocated with the storm-track, the SAM variability may be sensitive to the near-surface baroclinicity associated with the midlatitude oceanic frontal zone. In the present study, aqua-planet atmospheric general circulation model experiments are conducted with two different zonally symmetric profiles of sea-surface temperature (SST) whose frontal gradient in midlatitudes is retained or eliminated. A comparison of the tropospheric response to the assigned stratospheric ozone depletion between the two SST profiles reveals critical importance of the frontal SST gradient for the intensified stratospheric polar vortex, which is due to the ozone depletion, in triggering and keeping positive phase of the tropospheric SAM in late spring through summer. We also reveal that the SAM trend in late 20th century simulated in CMIP3/5 models is sensitive to the position and intensity of the mid-latitude oceanic frontal zone. Specifically, a model that simulates the zonal-mean frontal zone at higher latitude tends to simulate the maximum positive trends in the zonal-mean westerlies and midlatitude precipitation also at higher latitudes than another model with the oceanic frontal zone at lower latitude. This relationship is more obvious in a subset of the models with the relatively strong oceanic fronts.