Anthropogenic and internal drivers of wind changes over the Amundsen Sea, West Antarctica, during the 20th and 21st centuries

Abstract. Ocean-driven ice loss from the West Antarctic Ice Sheet is a
significant contributor to sea-level rise. Recent ocean variability in the
Amundsen Sea is controlled by near-surface winds. We combine palaeoclimate
reconstructions and climate model simulations to understand past and future
influences on Amundsen Sea winds from anthropogenic forcing and internal
climate variability. The reconstructions show strong historical wind trends.
External forcing from greenhouse gases and stratospheric ozone depletion
drove zonally uniform westerly wind trends centred over the deep Southern
Ocean. Internally generated trends resemble a South Pacific Rossby wave
train and were highly influential over the Amundsen Sea continental shelf.
There was strong interannual and interdecadal variability over the Amundsen
Sea, with periods of anticyclonic wind anomalies in the 1940s and 1990s,
when rapid ice-sheet loss was initiated. Similar anticyclonic anomalies
probably occurred prior to the 20th century but without causing the present
ice loss. This suggests that ice loss may have been triggered naturally in
the 1940s but failed to recover subsequently due to the increasing
importance of anthropogenic forcing from greenhouse gases (since the 1960s)
and ozone depletion (since the 1980s). Future projections also feature
strong wind trends. Emissions mitigation influences wind trends over the
deep Southern Ocean but has less influence on winds over the Amundsen Sea
shelf, where internal variability creates a large and irreducible
uncertainty. This suggests that strong emissions mitigation is needed to
minimise ice loss this century but that the uncontrollable future influence
of internal climate variability could be equally important.
