The Polar Vortex:

Meshing and Stripping Professor Darryn W. Waugh the Gears of the Atmosphere THE POLAR VORTEX: MESHING AND STRIPPING THE GEARS OF THE ATMOSPHERE

Earth’s atmosphere is composed of multiple interlocked rotating parcels of air. At the poles, rotating vortices form an important part of this system. Climate scientist Professor Darryn Waugh of Johns Hopkins University investigates how polar vortices interact with the rest of the atmosphere, and the ways in which they can impact our weather.

Understanding Polar Vortices feature of the polar atmosphere, forming in late autumn and disappearing in spring. Until recently, most people had never However, the tropospheric polar vortex exists heard of polar vortices. It was only during all year round, and occurs at lower latitudes the extremely cold winter of 2014 that the in addition to being lower in the Earth’s term became popular in the United States atmosphere. media. Now when you hear it mentioned in the news, the polar vortex is ofen shorthand So what causes them, and what keeps them for ‘bad winter weather’, which is quite going once they have started? Polar vortices misleading. One researcher seeking to form when a strong temperature gradient better understand polar vortices and to forms between low and high latitudes. This reduce misconceptions surrounding them is happens particularly in winter when the Professor Darryn Waugh, of Johns Hopkins pole gets much less sunlight than areas University in Maryland. closer to the equator. The temperature gradient and the Coriolis Force, caused by The term polar vortex is commonly used to the Earth’s rotation, produces the prevailing describe the circumpolar vortices that cause ‘westerlies’ in the mid-latitudes, which are severe or unusual weather at high latitudes. stronger in winter because of the stronger However, circumpolar vortices can have temperature gradients. At the North Pole, many diferent characteristics and influences greater variability in land and sea surface CREDIT: NASA-GMAO on the weather. Also, far from being unusual temperatures and elevations result in more or rare, they are actually a fairly common waves propagating into the stratosphere, and feature of the Earth’s atmosphere, and occur a weaker and less stable stratospheric polar on other planets. When extreme cold events vortex than at the South Pole. occur, this is not because of the existence of a polar vortex (which was always there) but Vortex Stripping rather because the edge of the vortex has shifed or a piece has been stripped of and In characterising polar vortices, the concept moved south. of ‘vorticity’ is important. This is a measure of the rate of rotation of air parcels. Another Polar vortices occur in two layers of the important term is stratification, which occurs atmosphere: the troposphere (the lowest when layers of stable air form, mixing less layer, up to 5 miles high at the poles) and the than in unstratified air. Both of these terms stratosphere (up to 30 miles high). In both are used in the determination of Potential cases, they refer to the flow of air from west Vorticity – a similar concept to angular to east, circling the pole. These two types momentum which can be used to describe Professor Waugh and his colleagues of polar vortex occur separately from one how a polar vortex behaves. A polar vortex is examined the ‘surf zone’ at the edge of another, and occupy diferent latitudes as usually defined as a region of high Potential stratospheric polar vortices, where a steep well as height ranges. Vorticity (PV), where the air is rotating as a gradient exists in the PV and acts as a barrier parcel and is relatively highly stratified. to mixing. Using a simulation that considers The stratospheric polar vortex is a seasonal the atmosphere to be a shallow layer of fluid

WWW.SCIENTIA.GLOBAL CREDIT: NASA OZONEWATCH

on the surface of a sphere, they were able to bromine from CFCs and other manmade features of the Arctic weather is the sudden describe the dynamics of this system. This compounds. Polar ozone depletion is linked warming or cooling that can take place. enabled them to investigate the efects of to chemical reactions taking place within These sudden changes in temperature are surface elevation (mountain chains and high- polar stratospheric clouds, which occur strongly related to the Arctic polar vortex altitude zones such as Tibet), solar radiation in the stratosphere when the temperature structure. Recent work by the team has and the Earth’s rotation on polar vortices. drops below a threshold value. This efect identified links between upper-atmosphere is particularly strong in the southern polar vortices and lower-atmosphere activity In particular, Professor Waugh’s simulations hemisphere, where the polar vortex is colder such as this sudden warming and cooling, allowed him to examine the characteristics and more stable than northern counterpart. and also to atmospheric activity further from of Rossby waves – north-south meanders The lower temperatures lead to more the poles. in the polar vortex caused by variations chemical destruction while a stable polar in topography, oceanic heating and other vortex means that ozone-depleted air is These links have a range of time scales, from factors. When these Rossby waves ‘break’, not dispersed into middle latitudes. ‘The weeks to years, and the processes behind they can result in vortex stripping, where a formation of the ozone hole leads to an them are still relatively unknown. Professor long filament of air separates from the main even colder, stronger polar vortex,’ Professor Waugh is continuing to work on a number body of the vortex, moves south and mixes Waugh explains. This happens because of problems relating to stratosphere- with the warmer air. ozone absorbs ultraviolet radiation and troposphere coupling, including the efects warms the stratosphere, so less ozone means of polar vortices on surface weather and The team showed a strong agreement lower temperatures, strengthening the vortex ocean circulation. His team is also continuing between their observations and simulations and further driving the creation of polar to make progress on understanding the of vortex stripping, and published their stratospheric clouds. behaviour and movement of polar vortices, findings in the Journal of Geophysical As modelling of climate change predicts and the complex relationships that exist Research in 1994. In this work, the warming in the lower atmosphere but between vortices and stratospheric ozone. simulations were matched with NASA high- cooling in the stratosphere, there is a further altitude observation flights, where stripped risk of more polar stratospheric clouds This research into large-scale atmospheric vortex filaments were found. These filaments forming and a resulting increase in ozone dynamics provides an important connection were detected using equipment that depletion – just as our reduced use of ozone- between greenhouse gas warming of the measured temperature variations and the depleting chemicals seems to be recovering Earth’s atmosphere and the efects of this concentrations of multiple chemical ‘tracers’ stratospheric ozone levels. warming on the weather at a specific place occurring naturally in the atmosphere. These and time. While climate scientists are always tracer compounds are present in diferent In work published in 2011 in the Journal cautious in stating that climate change is concentrations inside and outside polar of Climate, Professor Waugh and his the cause behind any one extreme weather vortex air, and provide a signature of vortex colleagues demonstrated that the Antarctic event, research like Professor Waugh’s is filaments. ozone hole has a much stronger influence giving us a clearer idea of what weather on tropospheric (lower-atmosphere) events are driven by human-induced climate Polar Vortices and the Climate circulation in the southern hemisphere than change. greenhouse gases. This highlights a strong The stratospheric polar vortex exerts link between the southern polar vortex and Martian Polar Vortices: Undeniable an important influence on the Earth’s weather patterns throughout the southern Diferences, Significant Similarities climate. One of the most obvious efects hemisphere. is ozone depletion. This is caused by Polar vortices also form in other planetary chemical destruction by chlorine and In the north, one of the most noticeable atmospheres, through similar processes as

WWW.SCIENTIA.GLOBAL CREDIT: NASA Goddard Space Flight Center

those on Earth. Professor Waugh and his atmosphere freezes out during the winter, colleagues have studied their formation in this is suficient to weaken the potential the winter hemisphere of Mars, where instead vorticity directly over the pole, resulting of an increase in the potential vorticity (PV) in maximum PV values in an annulus towards the pole as is seen on Earth, the surrounding the pole but not directly over it. maximum PV values are seen in a ring around the pole. In 2016, the team published an Even with the presence of a CO2 polar icecap article in the Journal of Geophysical Research to explain the formation of an annular polar detailing similarities between the Martian vortex, this feature should not persist due polar vortex and the Earth’s tropospheric to natural instabilities. However, Professor polar vortex, which forms an approximate Waugh’s team has shown that a combination annulus shape. of the relatively flat topography and the rapidly-cooling Martian atmosphere explains Professor Waugh and his team set out to the relative stability of the Martian polar thoroughly investigate the reasons behind vortex. This work is to be published in the this diferently-shaped polar vortex on Journal of Atmospheric Science in 2017. Mars. Using a simulation of the Martian atmosphere, they showed a connection Looking Deeper into the Vortex with the existence of the Martian CO2 polar vortices in Titan’s stratosphere, and there icecaps. These icecaps form when the The team’s future plans include investigating are questions – similar to those for Earth and temperature drops suficiently for carbon the efects of polar vortices on the Earth’s Mars – regarding the structure and seasonal dioxide to condense out of the air and weather systems and on extreme weather evolution of these vortices and their impact form a solid. When a substance freezes, it events. He also intends to investigate Martian on trace gases,’ Professor Waugh explains. releases what’s known as the latent energy polar vortices and the ways in which trace The formation, structure and behaviour of condensation. This energy is given out atmospheric components such as dust and of Titan’s vortices under conditions very because the molecules have transformed water vapour are transported into and out of diferent to those on Earth may reveal new into a more stable configuration, thus that planet’s distinctive polar vortex system. insights into fundamental vortex processes. releasing much of their kinetic energy. In the case of Mars’ atmosphere, where They also plan to look at polar vortices on approximately one-quarter of the Titan, one of Saturn’s moons. ‘There are polar

WWW.SCIENTIA.GLOBAL Meet the researcher Professor Darryn W. Waugh Johns Hopkins University Baltimore, Maryland USA

Afer obtaining a BSc in Mathematics at the University of Waikato in REFERENCES New Zealand in 1986, Professor Darryn Waugh went on to receive his MSc at Waikato in Mathematics in 1988. He was then awarded his PhD DW Waugh, RA Plumb, et al., Transport of material out of the at the University of Cambridge in the UK in 1992, afer which he pursued stratospheric Arctic vortex by Rossby wave breaking, Journal of postdoctoral work at MIT in the USA and Monash University in Australia. Geophysical Research, 1994, 99, 1071–1088. Since 1998, he has worked at Johns Hopkins University, where he became Professor at the Department of Earth and Planetary Sciences in LM Polvani, DW Waugh, GJP Correa, SW Son, Stratospheric ozone 2004. Professor Waugh has received three Group Achievement Awards depletion: the main driver of 20th Century atmospheric circulation from NASA for projects he has been involved in. He also received the changes in the Southern Hemisphere?, Journal of Climate, 2011, 24, Francois N Frenkiel award for Fluid Mechanics for a 1993 paper and the 795–812. Stanley Jackson Award from the South African Society of Atmospheric Sciences in 2012. DW Waugh, AD Toigo, SD Guzewich, SJ Greybush, RJ Wilson, L Montabone, Martian Polar Vortices: Comparison of Reanalyses, Journal CONTACT of Geophysical Research Planets, 2016, 121, 1770–1785.

T: (+1) 410 516 8344 WJM Seviour, DW Waugh, RK Scott, The Stability of Mars’ Annular Polar E: [email protected] Vortex, Journal of Atmospheric Science, 2017, in press. W: http://sites.krieger.jhu.edu/waugh/

KEY COLLABORATORS

Lorenzo Polvani, Columbia University Scott Guzewich, NASA GSFC Anthony Toigo, JHU APL

FUNDING

NASA NSF NOAA

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