Southern Hemisphere Sea Ice Trends: The Roles of Ozone and Greenhouse Gas Changes

Abstract:

Observations of the sea ice extent in the Southern Hemisphere during the last three decades show a weak but positive trend in almost all months. These changes seem to be attributable to an enhancement of the meridional flow in the areas of the Ross and Weddell Seas and changes in the Ekman transport due to a more positive phase of the Southern Annular Mode (SAM), favored by the depletion of Antarctic stratospheric ozone.

At the same time, climate model studies forced with observed Antarctic ozone changes suggested the ozone hole to be responsible for a sea ice retreat, explained by a warming of the Antarctic near coastal waters as a result of enhanced ocean upwelling due to the same mechanism (e.g. stronger westerlies in the positive SAM phase).

In our study we find however that the Antarctic ozone depletion in spring does not only lead to stronger westerlies, but also to a strengthening of the Amundsen Sea Low, causing locally a stronger southerly flow and resulting in an enhanced export of sea ice in summer and autumn, and subsequently to a positive trend in the sea ice extent, as observed.

For this purpose, two simulations with the coupled atmosphere-ocean chemistry-climate model EMAC-O (ECHAM/MESSy Atmospheric Chemistry with MPIOM ocean) have been analyzed and compared, one representing transient climate and ozone change, and one including only ozone depleting substances as driver.