Stratospheric Sudden Warmings
Sudden Stratospheric Warming are defined as dynamical events in the stratosphere identified by a rapid rise in temperature at the rate of 30-40K in a few days along with the reversal or weakening of stratospheric zonal winds around the polar vortex during winter at 10hPa and 60° and thus a consequential reversal of the meridional temperature gradient.These events are observed at a greater frequency in the Arctic region in comparison to the Antarctic region.The relevance of the study of SSW in the poles underlies in the fact that such events are directly correlated with the amount of ozone loss in the stratosphere for that particular year.The ozone loss is comparatively reduced in the years of major and minor SSW.The impact of SSW is not only limited to the troposphere but plays an important role in associated phenomenons in the mesosphere and the ionosphere.
The objective of my project is to analyse the amount of Antarctic ozone loss in the year 2019 due to the associated SSW.In the initial part of the project various ozone matrices are compared to analyse the extent of impact of the SSW in the year 2019. In the second part of the project the ozone loss is computed and thus in the last part a comparison of the losses with the previous years is done.My work would help in understanding the positive correlation of SSW and reduced ozone loss.The decreasing nature of ozone loss is also indicative of the better implementation of the Montreal Protocol.
Sudden Stratospheric Warming
Picture adapted from (Waugh, Sobel, Polvani BAMS 2017)
What is polar vortex?
The polar vortex is a large-scale region of air that is contained by a strong west-to-east jet stream that circles the polar region. Although the polar vortices are sometimes described as extending from the middle troposphere to the upper stratosphere there are actually two quite different polar vortices in Earth’s atmosphere, a tropospheric vortex and a stratospheric vortex.
The stratospheric polar vortex is the region of high atmospheric vorticity that forms with the establishment of the winter stratospheric polar jet. The polar jet begins just above the tropopause in both the Northern and Southern hemispheres reaching maximum wind speeds near the stratopause (∼50 km). The polar jet arises from the strong temperature contrast between the warm tropical stratosphere and the cold polar stratosphere. The tropical stratosphere is heated by the ozone ultraviolet absorption. The polar winter stratosphere is unheated during polar night and cools through infrared emission to space principally by the gases carbon dioxide, ozone, and water. The equator to pole temperature difference creates a strong pressure gradient and, as air moves northward, the Coriolis force deflects this air eastward, creating a strong eastward-flowing jet. Above the stratopause, the temperature gradient between the tropics and the polar region reverses, and polar night jet speed decreases with altitude into the mesosphere.
Scientific interest in the stratospheric polar vortices increased dramatically in the mid- 1980s because of their importance for stratospheric ozone depletion. The low temperatures within the vortices and reduced mixing of polar and midlatitude air across the vortex edge are crucial ingredients for the formation of Antarctic ozone holes.
The edge of tropospheric vortex vortex is often defined by specified geopotential contours, on the 300 or 500 hPa pressure levels, that typically lie within the core of the westerlies. The values of the contours chosen vary, but the tropospheric vortex edge generally lies between 40-50°N.
However, in contrast to the stratospheric vortex, baroclinic instability (and the resulting waves) plays a key role in the variability and long-term maintenance of large-scale tropospheric jet stream (Robinson 2006). Baroclinic instability is the process by which most extratropical tropospheric weather systems extract energy from the basic pole-to-equator temperature gradient, but these weather systems are largely confined to the troposphere.
It should also be clear that the vortex in the troposphere is much larger than the vortex in the stratosphere, and that the two are not directly connected. Furthermore, we wish to highlight another fundamental difference between these two vortices: their seasonal evolution. While the tropospheric vortex exists all year, the stratospheric polar vortex exists only from fall to spring.
What is Sudden Stratospheric Warming(SSW) ?
Richard Scherhag first observed “explosive warmings in the stratosphere” (which he referred to as the “Berlin phenomenon”) in radiosonde measurements in Berlin, Germany, in January/February 1952 (Scherhag 1952, p. 53).
The term sudden stratospheric warming refers to what is observed in the stratosphere:- a rapid warming (up to about 50 °C in just a couple of days), between 10 km and 50 km above the earth’s surface.
Thus the earliest SSW definitions adopted by the World Meteorological Organization (WMO) focused on temperature gradient and zonal wind reversals at the 10-hPa pressure level (~30 km), and poleward of 60° latitude (WMO/IQSY 1964; Quiroz et al. 1975; WMO CAS 1978, p. 36, item 9.4.4; McInturff 1978; Labitzke 1981).
The change in zonal flow is accompanied by deformation in the shape of the polar vortex. The SSW type can be distinguished depending on the vortex shape. More than half of SSW events are categorized as displacement type because the center of polar vortex shifts toward lower latitudes. The other SSW event type is known as split type because the polar vortex splits into two vortices of similar size and strength. This displacement and splitting of the polar vortex are regarded as wave-1 and wave-2 types, respectively.
Detailed account of the same is seen in Butler et al., 2015.(http://dx.doi.org /10.1175/B A M S - D -13 - 0 0173. 2; see Table ES1).
Evolution of polar vortex in a major warm winter (e.g. 2002 is demonstrated here) and the temporal evolution of the zonal wind and temperature at 60°S. The black horizontal line shows the zonal wind speed equal to zero. The vertical black line corresponds to the day of zonal wind reversal (i.e. central date or major warming date).
What is the difference between major and minor SSW?
A major SSW in which the stratospheric winds reverse from climatological-mean westerly flow to easterly flow, waves can no longer propagate upward above the level of the reversal and so subsequently break at lower and lower levels in the stratosphere, reversing the wind downward from the upper stratosphere to the lower stratosphere. The reversal of the zonal circulation is thus a fundamental characteristic of major SSWs and their associated dynamics.
The frequency of major midwinter SSWs is an important metric of polar stratospheric wintertime variability
“Minor'' SSWs (during which the 10hPa circulation remains westerly at 60°N) are frequent but often have little impact on lower stratospheric polar processing. If the reversal of temperature gradient does not follow the zonal-mean wind reversal, then it is a minor SSW
What causes SSW?
Matsuno (1971) suggested that SSWs might be the consequence of a resonant excitation of a free Rossby wave mode of the atmosphere by topographic forcing.
The evolution of the sudden warming can be summarized as follows: Under normal conditions, planetary waves propagate from the troposphere toward the tropical stratosphere. As planetary waves amplify, the direction of propagation switches toward the pole. A rapid increase in the magnitude of the EP flux divergence ensues, which decelerates the zonal-mean zonal wind while producing rapid adiabatic warming at high latitudes via the wave-driven circulation. Incidentally, this circulation also lowers tropical temperatures (due to upwelling, and hence expansional cooling). However, the warming at high latitudes is much more intense and gives the phenomenon its name.
Charney and Drazin (1961) first derived a dispersion relation for vertically propagating Rossby waves and showed that only the planetary-scale waves (λ > 6000 km) could penetrate the strong westerly polar vortex winds during winter.
An upward-propagating large-scale planetary wave increases in amplitude (as measured by the geopotential anomaly, for example) with increasing altitude as the atmospheric density decreases. Eventually, the flow becomes so highly distorted in the upper atmosphere that the wave can no longer propagate and the wave energy and momentum are ‘dumped’ into the flow as a wave-breaking event. The strong nonlinear wave–mean flow interaction takes place at the ‘critical level’ for the stationary planetary wave or the zero zonal wind speed contour.
Schematic diagram of SSW dynamics
Why is SSW more prevalent in the Northern hemisphere than the South?
The dynamical activity in recent winters reveals that the frequency of MWs in the Arctic is increasing (e.g.Charlton-Perez et al.,2008). Studies showed that there were 5 MWs in6 winters over 1967/68–1972/73 (e.g.Bancal ́a et al.,2012;Cohen and Jones,2011;Labitzke and Naujokat,2000;An-drews et al.,1987). Similarly, there were 5 MWs in 6 win-ters from 1983/84 to 1988/89 (e.g.Butler and Polvani,2011;Harada et al.,2010;Manney et al.,2008). On average, during 1957/58–1990/91, MWs occurred only once every twoArctic winters (e.gBancal ́a et al.,2012;Cohen and Jones,2011;Andrews et al.,1987). Conversely, no MW occurred in9 consecutive winters from 1989/90 to 1997/98, except a minor warming in early February 1990 (Manney et al.,2005).However, there were 7 MWs in 5 out of the 6 winters from 1998/99 to 2003/04 (e.g.Kuttippurath et al.,2011;Kleinb ̈ohlet al.,2005;Manney et al.,2005;Liu et al.,2009;Naujokat et al.,2002).
The planetary waves are generated by flow over mountains and continental land-sea temperature contrasts. They can also be generated by year-to-year changes in large scale weather patterns such as El Nino. With large land masses in the Northern Hemisphere compared to the Southern this leads to more Rossby waves and so SSW events are largely a Northern Hemisphere focused phenomenon.
Hemispheric differences in the polar vortices cause differences in wave generation and propagation. The Antarctic vortex is larger with stronger westerlies, and has a longer 6lifespan than its Arctic counterpart. The Antarctic vortex is also much colder with a narrow range of seasonal variability. The larger topography and land-sea contrasts in the northern hemisphere excite more planetary-scale Rossby waves that disturb the polar vortex more than in the southern hemisphere(Waugh and Polvani 2010). As a result, SSW events are commonly observed in the northern hemisphere and very few are observed in the southern hemisphere.
What are the surface impacts of SSW?
On average, SSWs are observed to have substantial, long-lasting effects on surface weather and climate, especially on sea-level pressure (SLP) and the NAM, with associated shifts in the jet streams, storm tracks, and precipitation (e.g., Baldwin & Dunkerton, 2001).
In particular, SSW events tend to be followed by a negative signature of the NAM in the NH (Baldwin & Dunkerton, 1999, 2001) and the SAM in the SH. In the NH, the strongest response to SSW events is observed in the North Atlantic Basin , where the response to SSW events often projects onto the negative phase of the NAO (Charlton-Perez et al., 2018; Domeisen, 2019). The negative phase of the NAO is associated with cold air outbreaks over Northern Eurasia and the eastern United States (Kolstad et al., 2010; King et al., 2019; Lehtonen & Karpechko, 2016) as well as over the Barents and Norwegian Seas (Afargan-Gerstman et al., 2020), and wet anomalies over Southern Europe (Ayarzagüena, Barriopedro, et al., 2018) due to the southward shift and persistence of the North Atlantic eddy-driven jet (Maycock et al., 2020) and the storm track (Afargan-Gerstman & Domeisen, 2020). Further anomalies include positive temperature anomalies over Greenland and eastern Canada, and subtropical Africa and the Middle East. Anomalous tropospheric blocking is often observed after SSW events (Labitzke, 1965; Vial et al., 2013).
Anomalous weakening of the SH polar vortex, tied to shifts in the seasonal evolution of the vortex, are associated with a negative SAM pattern and significant surface impacts over Antarctica, Australia, New Zealand, and South America.
SSW events affect surface weather through the amplification of the jet stream over the midlatitudes. The amplification of the jet stream is caused by slower Rossby wave propagation, making the jet stream appear “wavier” increasing the meridional wave amplitude of the jet stream. As a result of this amplification, cold air anomalies are observed over eastern North America and Europe,and warm air anomalies are observed over western North America
Transportation was disrupted by heavy snowfall, and agricultural crops were destroyed by freezing temperatures that were observed as far south as Florida(Screen et al. 2015).
Due to the meridional amplification of the jet stream in the midlatitudes, cyclogenesis can be affected through slower storm propagation and altered storm tracks (McGuirk and Douglas 1988). Storm tracks are altered following a more southerly track and having a sharper eastward curve inland once off the eastern coast of the United States.
What are the impacts of SSW on the upper atmosphere
The effects of SSW events are now recognized to extend well above the stratosphere and can significantly alter the chemistry and dynamics of the mesosphere, thermosphere, and ionosphere.
SSW events lead to dramatic changes in the mesosphere and lower thermosphere. This includes high-latitude cooling, as well as a reversal of the zonal mean zonal winds from easterly to westerly (the opposite as in the stratosphere) (Hoffmann et al., 2007; Labitzke, 1982; Limpasuvan et al., 2016; Liu & Roble, 2002; Siskind et al., 2010).
The enhanced eastward forcing leads to the reversal of the mesosphere-lower thermosphere winds and also changes the high-latitude residual circulation from downward to upward, resulting in adiabatic cooling of the mesosphere (Limpasuvan et al., 2016; Liu & Roble, 2002; Siskind et al., 2010). The altered stratosphere-mesosphere residual circulation during SSW events may also lead to a warming of the summer hemisphere mesosphere and a decrease in the occurrence of polar mesospheric clouds (Karlsson et al., 2007, 2009; Körnich & Becker, 2010).
Changes in chemistry are particularly notable following elevated stratopause events, when there is significantly enhanced downward transport in the lower mesosphere and upper stratosphere (e.g., Siskind et al., 2015). The enhanced downward transport leads to enhancements in NOx and CO in the stratosphere.
The mesospheric wind changes are related to the ways that winds in the stratosphere influence the filtering of atmospheric gravity waves. The mesospheric anomalies often, although not always, initially appear a week or more prior to the peak stratospheric disturbances
Schematic of the coupling processes and atmospheric variability that occur during sudden stratospheric warming events. Red and blue circles denote regions of warming and cooling, respectively.(taken from EOS. Science news by AGU)
What is the impact of SSW in the ozone depletion cycle?
The dramatic dynamical perturbations during SSWs are associated with anomalies in the transport circulation and thus lead to anomalies in the distributions of constituents such as ozone.the influence of SSWs was recognized and could be shown from observations as early as in the late 1950s, with Dütsch (1963) revealing a close spatial correlation between total column ozone and temperatures in the 50- to 10-hPa layer during the 1957–1958 SSW over Europe. Based on averaged total column ozone observations over all available stations north of 40 N, Züllig (1973) further developed the findings by Dütsch to show that the seasonal evolution of ozone was exhibiting a much more abrupt initial increase during years with SSW events (1962–1963 and 1967–1968) than during a year without an SSW event (1966–1967).
This event was the first to be observed in the SH and as mentioned above led to an impressive split of the 2002 Antarctic ozone hole (Varotsos, 2002; von Savigny et al., 2005), at least partially cutting short ozone depletion during that year (Weber et al., 2003).
As shown by Kiesewetter et al. (2010) and de la Cámara, Abalos, Hitchcock, et al. (2018), after onset of an SSW, ozone anomalies become positive above 500 K and negative below. The positive anomalies then slowly descend to lower altitudes, with the middle stratosphere relaxing back to normal trace gas concentrations the fastest. The enhanced poleward and downward transport during an SSW increases in transport of other species such as carbon monoxide as well, with the breakdown of the polar vortex enhancing mixing between mid and high latitudes and flattening of the tracer gradients (Manney, Schwartz et al., 2009). This will lead to cutting short ozone depletion by halogens in the Arctic polar stratosphere during spring
It is found that changes in the amount of column ozone are positively correlated with polar lower stratospheric temperature with colder (warmer) temperature correlating with less (high) amount column ozone. But in the middle latitude region we observed negative correlations between ozone concentration and stratospheric temperature. In almost all cases there is sudden increase of ozone concentration over the pole and after few days the value is reduced when the warming effect is weak.
How has the frequency of SSW changed in both the poles across the years?
Sudden stratospheric warmings are most common in mid- to late winter, but they have been observed as early as December in the Northern Hemisphere. From the mid-1950s to 1991 major warmings occurred about every other year, but between 1992 and 1998 there were no major warmings. This lull was broken by a spectacular December 1998 major warming.
The dynamical activity in recent winters reveals that the frequency of MWs in the Arctic is increasing (e.g.Charlton-Perez et al.,2008). Studies showed that there were 5 MWs in6 winters over 1967/68–1972/73 (e.g.Bancal ́a et al.,2012;Cohen and Jones,2011; Labitzke and Naujokat,2000 ; Andrews et al.,1987). Similarly, there were 5 MWs in 6 winters from 1983/84 to 1988/89 (e.g.Butler and Polvani,2011; Harada et al.,2010; Manney et al.,2008). On average, during 1957/58–1990/91, MWs occurred only once every two Arctic winters (e.g Bancal ́a et al.,2012;Cohen and Jones,2011; Andrews et al.,1987). Conversely, no MW occurred in 9 consecutive winters from 1989/90 to 1997/98, except a minor warming in early February 1990 (Manney et al.,2005). However, there were 7 MWs in 5 out of the 6 winters from 1998/99 to 2003/04 (e.g. Kuttippurath et al.,2011; Kleinb ̈ohlet al.,2005; Manney et al.,2005; Liu et al.,2009; Naujokat et al.,2002). The winter 1999/00 was unusually cold but each other winter was prone to MWs. Furthermore, two MWs were observed in 1998/99 and 2001/02 (e.g.Charltonand Polvani,2007). This warming sequence continued and there were 5 MWs in 5 winters again in 2005/06–2009/10.
There is an increase in the occurrence of MWs in recent years, their long-term average is likely to stay around the historical value (∼0.7 MWs/winter)
However, the unusual frequency of MWs in recent years has not translated into early final warmings in most cases.
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