Subtropical Anticyclone Forcing of the Antarctic Stratosphere

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A new hypothesis regarding the forcing of the stratosphere is that subtropical monsoon anticyclones affect the winter long-wave pattern, and that this explains the Southern Hemisphere (SH) "ozone croissant" as well as SH stratospheric variability related to the El Nino/Southern Oscillation (ENSO). The studies build on earlier work by the Principal Investigator's group, which showed that the Southeast Asian monsoon outflow creates the wave 1 asymmetry in SH midlatitude ozone known as the "ozone croissant." During austral winter and spring, Southeast Asian convection maintains two anticyclones in the upper troposphere/lower stratosphere: the Tibetan High and the Australian High. Outflow around the Australian High into the Southern Indian Ocean forms the entrance to the Australian subtropical westerly jet. This baroclinic zone is a preferred band of longitudes for ozone transport and is a fundamental cause of the ozone croissant. Intensification of tropical convection strengthens the Australian High, which interacts with a traveling extratropical stratospheric ridge that is rich in ozone. As the Australian High amplifies, the ridge stalls and merges with it; this is a second dynamical cause of the croissant. Variability of the Australian High explains approximately 40% of variability of the strength of the ozone croissant. New results also indicate a distinct ENSO signal in polar stratospheric clouds and in the ozone croissant. The goals of this project are to understand better the SH stratosphere, in regard to its mean dynamical structures, the pathways for ozone transport, the relevant wave-mean flow dynamics, and its variability related to ENSO. A hierarchy of simulations with assimilated ozone data sets, conserved tracers, and trajectories will be carried out. Global analyses will be dynamically filtered and applied as initial conditions in the University of Wisconsin -Madison non-hydrostatic modeling system (UWNMS), in a domain covering the Southern Hemisphere. Baroclinic life cycle experiments will be performed with zonally symmetric initial conditions and with a wave 1 Australian jet. The interactions of synoptic and planetary waves in the midlatitude surf zone will be studied and the resulting wave behavior compared to linear theory. Pathways for ozone transport from the subtropical stratosphere into the ozone croissant will be determined. The effects of forcing a subtropical anticyclone on the jet and on traveling planetary waves will be investigated. Further statistical analyses will be performed to establish the mean structure of the austral flow and the relationship between ENSO and the stratosphere, allowing the hypothesis that interannual variability of the Antarctic stratosphere is linked to tropical convection to be explored. Broader impacts of this work are in enhancing our understanding of the climate dynamics underlying the distribution of radiatively important trace constituents, interannual predictability, and global teleconnections. A postdoctoral scholar and a graduate student will be trained, and an undergraduate will participate in the research.