Constrained attribution of changes in winds over the Southern Ocean from 1950 to 2100

Abstract. Strong near-surface westerly winds drive the Southern Ocean circulation and play a key role in setting regional and global climate. In the latter half of the 20th century, depletion of stratospheric ozone over Antarctica has caused these winds to accelerate and move polewards, particularly in austral summer. However, the future evolution of these winds remains uncertain. We use reanalysis data and the UK Earth System Model (UKESM1), with full atmosphere interactive chemistry, to assess the drivers of the winds over the recent past and coming century. We first characterize the wind mean state, distribution, and trends over 1980–2020 in the most commonly used atmospheric reanalyses (ERA5, JRA3Q, MERRA2, and R1) to gain insights into observed wind behaviour in the past. We show that while the representation of the mean wind is similar among reanalyses, MERRA2 and R1 show stronger wind acceleration trends that persists year-round, while JRA3Q and ERA5 show weaker acceleration, primarily in austral summer. Using an observational Southern Annular Mode (SAM) index, we show that the weaker, summer-focused trends of JRA3Q and ERA5 are likely more accurate. UKESM1 represents historical trends in winds accurately compared to ERA5. Targeted model simulations show that ozone depletion is overwhelmingly responsible for the wind acceleration observed in 1980–2020, which occurs primarily in austral summer. The effect of ozone depletion on winds peaks in 1980–2000, when it is roughly double that for the entire 40-year period. Ozone recovery is then associated with a slowdown of winds from 2000 to 2050. Beyond 2050, the ozone effect becomes minimal and winds accelerate primarily due to greenhouse gas induced warming, with this trend more evenly distributed across seasons.