The ‘Roaring Forties’ and the Ozone Hole

Email: N.Byrne@pgr.reading.ac.uk

The ‘roaring forties’, often referred to as the ‘brave west winds’, are strong westerly winds in the Southern Hemisphere located between the latitudes of 40 and 50 degrees. These wild winds are some of the strongest on the planet and can traverse the globe at furious speeds, aided in part by the relative dearth of landmasses to serve as windbreaks. Their close companions, the ‘furious fifties’ and the ‘shrieking sixties’ represent regions of even stronger winds that affect the entire Southern Ocean. These strong and steady winds are the driving source of the primary Southern Ocean current (the Antarctic Circumpolar Current) and make it the largest ocean current on the planet.

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Figure 1: (Sourced from earth.nullschool.net.) Surface wind on 20-05-2017. Lighter colours represent regions of larger wind speeds.

The existence of these winds and ocean currents has long been known to sailors and in past centuries, they propelled ships at breakneck speed across the Pacific. In more recent times, vessels that will also travel this route include the British Antarctic Survey’s RRS Sir David Attenborough and the now infamous Boaty McBoatface! Research vessels such as these help contribute to our understanding of how the mid-latitude westerly winds interact with the Southern Ocean and the Antarctic climate, and whether there are any important feedbacks between these different components of the climate system. They are also an important source of evidence for how the climate is changing in one of the most remote places on Earth.

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Figure 2: (Sourced from BBC News.) Boaty McBoatface.

While the rapid increase in CO2 has received much attention for its role in surface climate change in many parts of the globe, in the Southern Hemisphere middle-high latitudes it is arguably ozone depletion (and the associated ozone hole) that has led to the largest changes in surface climate. This is primarily because of the recent discovery that there are important dynamical effects associated with the Antarctic ozone hole – namely a shift in the location of the ‘roaring forties’! This result was quite unexpected at the time of its discovery as it had previously been assumed that surface impacts associated with the Antarctic ozone hole were primarily radiative in nature. Much work in recent years has gone into improving our understanding of how these dynamical effects are transmitted to the surface and what might be the future implications for Southern Hemisphere climate (see references for more details). In any case, the observed impacts of the ozone hole on the westerly winds offer a sobering reminder of the potentially large (and unexpected!) changes that anthropogenic emissions can induce in our climate.

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Figure 3: (Sourced from Wikipedia.) Image of the largest Antarctic Ozone hole ever recorded over the Southern pole (September 2006).

References

Byrne, N. J., T. G. Shepherd, T. Woollings, and R. A. Plumb, (2017), Non-stationarity in Southern Hemisphere climate variability associated with the seasonal breakdown of the stratospheric polar vortex. J. Clim., in press. doi: 10.1175/jcli-d-17-0097.1.

Thompson, D. W. J., S. Solomon, P. J. Kushner, M. H. England, K. M. Grise, and D. J. Karoly, (2011), Signatures of the Antarctic ozone hole in Southern Hemisphere surface climate change. Nat. Geosci., 4: 741–749. doi:10.1038/ngeo1296.