A truly exceptional weather event is happening right now in the Southern Hemisphere

by Martin Jucker (@DrJucker)

At the time of writing, something truly special is happening in the Southern Hemisphere: The air high above the Southern Ocean and Antarctica, anywhere between 20–40 kilometres (12–25 miles) above the surface, is warming a lot in just a few weeks.

In that region, the sun doesn’t rise all winter, resulting in very cold temperatures accompanied by very strong winds. This is called the polar vortex.

Those strong winds are about to slow down completely, and might even revert direction. We call it a “vortex breakdown” or “Stratospheric Sudden Warning”, and in the Southern Hemisphere it only happens for the second time that we know of, and certainly since the era of satellite measurements began in the late 1970s. The first time was in 2002.

West-to-East wind strength at around 30km (~20 miles) altitude above the Southern Ocean at the time of writing. The grey area is what we usually observe, the black line the 2002 Sudden Warming, and the red/magenta/orange line is 2019. Downloaded from https://ozonewatch.gsfc.nasa.gov/meteorology/wind_2019_MERRA2_SH.html on 2019/09/13.

Stratospheric Sudden Warmings happen about every other year in the Northern Hemisphere, but they are extremely rare in the Southern Hemisphere. The reason is that most of Earth’s landmasses and in particular the Tibetan Plateau and the Rocky Mountains happen to be in the Northern Hemisphere, whereas the South is mostly open ocean (for more details, see these videos).

For one, during Sudden Warmings, as their name suggests, the stratosphere over the pole warms a lot — by about 50 degrees celsius over just a few days. In the Southern Hemisphere, this means that the usually very cold temperatures which are a key reason for ozone destruction and the emergence of the ozone hole are warm enough to stop those chemical reactions, or at least slow them down. This allows the ozone hole to close or at least become smaller.

Ozone hole size. 2019 shows the smallest extent of the ozone hole thanks to the weakening and warming of the stratospheric polar vortex. Image from https://www.cpc.ncep.noaa.gov/products/stratosphere/polar/gif_files/ozone_hole_plot.png, downloaded 2019/09/13.

Second, it turns out that what happens tens of kilometres (or miles) above the surface subsequently influences the surface weather over the mid-latitudes (that is, Europe, Russia, North America in the North and mostly Australia and New Zealand in the South) for long periods of time. This does not happen every time, and we still don’t know what determines whether or not these events have an influence at the surface. In the Northern Hemisphere, this is somewhat predictable. It also happens often enough to be of major interest for meteorological services around the world for their seasonal forecasts.

The evolution of a Sudden Stratospheric Warming. The vertical, labeled “pressure” gives heigth in the atmosphere, and the grey surface marks the division between the troposphere (close to the surface) and the stratosphere (above the troposphere). Red surfaces show strong winds, the blue surfaces indicate the wind reversal during a Sudden Warming. Arrows denote strong atmospheric waves propagating upward from the surface and causing the Sudden Warming. From Jucker (2016).

After the one previous event in the Southern Hemisphere, the entire following summer saw drier and warmer weather than usual in Southeastern Australia. We expect something similar to happen this year. Southeastern Australia is already experiencing a drought, and yet another dry and hot spring and summer could be devastating. So yes, these events matter, even though they are rare.

We have observed two events, separated by 17 years. So does that mean we should expect one event every two decades? Maybe, but it is very difficult to say, as reliable records for this part of the atmosphere only go back to 1979. So we can say that there weren’t any other events in the 23 years prior to the 2002 event, but that is still very close to two decades (a few years more or less do not really matter in a statistical sense). So the only hope we have to answering this question (other than just “wait and see”) is to look at computer models, as they can simulate much longer time periods than our now forty-year satellite record. But that analysis will first have to be done, as the focus so far has largely been on the Northern Hemisphere.

Preliminary results indicate that we should expect about one Sudden Warming in the Southern Hemisphere every 300 years. But we’ve had two in as many decades. Does this mean our models are wrong, and that the real frequency is one in twenty years? On the other hand, binomial statistics tells us that an average probability of one in 300 years means that having two in twenty years has a probability of one in 500, which is a bit smaller but not impossible. So, maybe this simply means that we live in a very special time and we won’t see another Sudden Warming for centuries?

While models are the only way to get to the bottom of this (for now), it is also true that different models or the same model with different setups can produce a large spread of Sudden Warming probability in the Northern Hemisphere. But all agree that they are extremely rare in the Southern Hemisphere. But that’s just statistics, and all it tells us is “maybe”.

But I know one thing for certain: We are at this instant in time observing something which is extraordinary (and less likely than a solar eclipse), so enjoy it while it lasts!

Dr Martin Jucker (@DrJucker) is lecturer in atmospheric physics at the University of New South Wales, and his research specializes in the interactions between the troposphere and the stratosphere. More information at www.martinjucker.com.

Uncover the bulk of research. A scientific publication about all the hidden things which lead to discovery. Edited by @DrJucker

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