VATIS UPDATE Article

A team of scientists from GEOMAR and Christian-Albrechts Universität zu Kiel, Germany, Geophysical Institute, University of Bergen and Bjerknes Centre for Climate Research, Norway, have found evidence that the strengthened stratospheric westerlies arising from the Antarctic ozone hole-induced cooling cause a polar mesospheric warming and a subsequent cooling in the lower thermosphere. While previous studies focused on the role of nonresolved (gravity) wave drag filtering, here the role of resolved (planetary) wave drag and radiative forcing on the Antarctic mesosphere and lower thermosphere (MLT) was explored in detail.

Using simulations with NCAR’s Community Earth System Model, version 1 (Whole Atmosphere Community Climate Model) [CESM1(WACCM)], scientists found that in late spring and early summer the anomalous polar mesospheric warming induced by easterly nonresolved wave drag is dampened by anomalous dynamical cooling induced by westerly resolved wave drag. This resolved wave drag is attributed to planetary-scale wave (k = 1–3) activity, which is generated in situ as a result of increased instability of the summer mesospheric easterly jet induced by the ozone hole.

On the other hand, the anomalous cooling in the polar lower thermosphere induced by westerly nonresolved wave drag is enhanced by anomalous dynamical cooling due to westerly resolved wave drag. The polar MLT temperature response to the Antarctic ozone hole is, through thermal wind balance, accompanied by the downward migration of anomalous zonal-mean wind from the lower thermosphere to the stratopause. The results highlight that a proper accounting of both dynamical and radiative effects is required in order to correctly attribute the causes of the polar MLT response to the Antarctic ozone hole.