WHAT IS THE POLAR VORTEX?
We constantly hear the term Polar Vortex but many of us may be unsure of what it all about. What causes them? Why do we have them?
There are two polar vortices in the atmosphere of planet Earth, one around the North Pole, and one around the South Pole. Each polar vortex is a persistent, large scale cyclone, circling the planet’s geographical pole. The bases of the two polar vortices are located in the middle and upper troposphere and extend into the stratosphere. They surround the polar high and lie in the wake of the polar front. These cold-core low-pressure areas strengthen in the winter and weaken in the summer due to their dependence upon the temperature differential between the equator and the poles. They usually span less than 1,000 kilometers (620 miles) in diameter within which the air circulates in a counter-clockwise fashion in the Northern Hemisphere, and in a clockwise fashion in the Southern Hemisphere. As with other cyclones, their rotation is caused by the Coriolis effect.
The Northern Hemisphere vortex often contains two low pressure centers, one near Baffin Island, Canada and the other over northeast Siberia. Within the Antarctic vortex in the Southern Hemisphere a single low pressure zone tends to be located near the edge of the Ross Ice Shelf near 160 west longitude. When the polar vortex is strong, the Westerlies increase in strength. When the polar cyclone is weak, the general flow pattern across mid-latitudes buckles and significant cold outbreaks occur. Ozone depletion occurs within the polar vortex, particularly over the Southern Hemisphere, and reaches a maximum in the spring.
Polar cyclones are climatological features that hover near the poles year-round. The stratospheric polar vortex develops pole-ward and above the subtropical jet stream. Since polar vortices exist from the stratosphere downward into the mid-troposphere, a variety of heights/pressure levels within the atmosphere can be checked for its existence. Within the stratosphere, strategies such as the use of the 4 mb pressure surface, which correlates to the 1200K isentropic surface, located midway up the stratosphere, is used to create climatologies of the feature. Due to model data unreliability, other techniques use the 50 mb pressure surface to identify its stratospheric location. At the level of the tropopause, the extent of closed contours of potential temperature can be used to determine its strength. Horizontally, most polar vortices have a radius of less than 1,000 kilometers. Others have used levels down to the 500 hPa pressure level (about 5,460 meters (17,910 ft) above sea level during the winter) to identify the polar vortex.
Polar vortices are weaker during summer and strongest during winter. Individual vortices can persist for more than a month. Extra-trophical cyclones that occlude and migrate into higher latitudes create cold-core lows within the polar vortex. Volcanic eruptions in the tropics lead to a stronger polar vortex during the winter for as long as two years afterwards. The strength and position of the cyclone shapes the flow pattern across the hemisphere of its influence. An index which is used in the northern hemisphere to gauge its magnitude is the Artic oscillation.
The Arctic vortex is elongated in shape, with two centers, one normally located over Baffin Island in Canada and the other over northeast Siberia. Around the North Pole, the Arctic vortex spins counterclockwise with wind speeds of 80 mph, stronger than the jet stream’s normal 70 mph winds. In rare events, when the general flow pattern is amplified, the vortex can push farther south as a result of axis interruption. The Antarctic polar vortex is more pronounced and persistent than the Artic one; this is because the distribution of land masses at high latitudes in the Northern Hemisphere gives rise to Rossby waves which contribute to the breakdown of the vortex, whereas in the Southern Hemisphere the vortex remains less disturbed. The breakdown of the polar vortex is an extreme event known as a sudden stratospheric sudden stratospheric warming, here the vortex completely breaks down and an associated warming of 30–50 °C (54–90 °F) over a few days can occur.
The formation of the polar vortex is primarily influenced by the movement of wind and transfer of heat in the polar region. In the autumn, the circumpolar winds increase in speed, causing the polar vortex to spin up further into the stratosphere and the values of potential vorticity to heighten, forming a coherent air mass: the polar vortex. As the winter comes, the winds around the poles decrease, and the air in the vortex core cools. The movement of the air becomes slow, and the vortex stops growing. Once late winter and early spring approach, heat and wind circulation return, causing the vortex to shrink. During the final warming, or the late winter, large fragments of the vortex air are drawn out into narrow pieces into lower latitudes. In the bottom level of the stratosphere, strong potential vorticity gradients remain, and the majority of air molecules remain confined into December in the Southern Hemisphere and April in the Northern Hemisphere, well after the breakup of the vortex in the mid-stratosphere.
The breakup of the polar vortex occurs between middle March to middle May, the average date being April 10. This event signifies the transition from winter to spring, and has impacts on the hydrological cycle, growing seasons of vegetation, and overall ecosystem productivity. The timing of the transition also influences differences in sea ice, ozone, air temperature, and cloudiness. Early and late polar breakup episodes have occurred, due to variations in the stratospheric flow structure and upward spreading of planetary waves from the troposphere. As a result of increased waves into the vortex, the vortex experiences higher amounts of heat sooner than the normal warming period, resulting in a faster season transition from winter to summer. As for late breakups, the waves dismantle the vortex later than normal, causing a delay in the season transition. The early breakup years are also characterized with persistence of remnants of the vortex, while the late breaking years have a quick disappearance of these remnants. In the early breakup phases, only one warming period occurs from late February to middle March, contrasting to the two warming periods that the late breakup phases have in January and March. Zonal mean temperature, wind, and geopotential height exert varying deviations from their normal values before and after early breakups, while the deviations remain constant before and after late breakups. Scientists are connecting a delay in the Arctic vortex breakup with a reduction of planetary wave activities, few stratospheric sudden warming events, and depletion of ozone.
Sudden stratospheric warming events, when temperatures within the stratosphere warm dramatically over a short time, are associated with weaker polar vortices. This warming of stratospheric air can cause the direction of circulation in the Arctic Polar Vortex to go from counter-clockwise to clockwise. These changes aloft force changes below in the troposphere. An example of an effect on the troposphere is the change in speed of the Atlantic Ocean circulation pattern. A soft spot just south of Greenland is where the initial step of down-welling occurs, nicknamed the “Achilles Heel of the North Atlantic”. Small amounts of heating or cooling traveling from the polar vortex can trigger or delay down-welling, causing circulation of heat through the Atlantic Ocean currents to be stopped or sped up. Since all other oceans depend on the Atlantic Ocean for the transmission of heat and energy, climates across the planet can change dramatically. The weakening or strengthening of the polar vortex can alter the sea circulation more than one mile below the waves. Strengthening storm systems within the troposphere can act to intensify the polar vortex by significantly cooling the poles. La Nina – related climate anomalies tend to favor significant strengthening of the polar vortex. Intensification of the polar vortex is also associated with changes in relative humidity as downward intrusions of dry, stratospheric air enter into the vortex core. With a strengthening of the vortex comes a longwave cooling due to a decrease in water vapor concentration near the vortex. The decreased water content is a result of a lower tropo-pause within the inside of the vortex, which places dry stratospheric air above moist tropospheric air. Instability is caused when the vortex tube, the line of concentrated vorticity, is displaced. When this occurs, the vortex rings become more unstable and prone to shifting by planetary waves. The planetary wave activity in both hemispheres varies year-to-year, producing a corresponding response in the strength and temperature of the polar vortex. The number of waves around the perimeter of the vortex are related to the core size; as the vortex core decreases, the number of waves increase.
The degree of the mixing of polar and mid-latitude air depends on the evolution and position of the polar night jet. In general, the combination of these two remains small inside the vortex compared to the outside. Mixing occurs with unstable planetary waves that are characteristic of the middle and upper stratosphere in winter. Prior to vortex breakdown, there is little transport of air out of the Arctic Polar Vortex due to strong barriers exist above 420 km (261 miles). Below this barrier exists the polar night jet, which is weak in the early winter, so any descending polar air mixes with the mid-latitudes. In the late winter, air parcels do not descend as much, causing mixing to be less frequent. After the vortex is broken up, the ex-vortex air is dispersed into the middle latitudes within a month.
Sometimes, a piece of the polar vortex can be broken off before the end of the final warming period. If large enough, the piece can plunge over Canada and the Midwestern, Central, Southern, and Northeastern United States. This diversion of the polar vortex can occur due to the displacement of the polar jet stream, such as the significant northwestern push of the polar jet stream over the western part of the United States in the winter of 2013–2014. Occasionally, the high-pressure Greenland Block can cause the low pressure polar vortex to divert to the south instead of sweeping across the North Atlantic.
A recent study found that stratospheric circulation can have anomalous effects on weather regimes. In the same year researchers found a statistical correlation between weak polar vortex and outbreaks of severe cold in the Northern Hemisphere. In more recent years scientists identified interactions with Artic sea ice decline, reduced snow cover, evapotranspiration patterns, NAO anomalies or weather anomalies which are linked to the polar vortex and jet stream configuration. However, because the specific observations are considered short-term observations there is considerable uncertainty in the conclusions. Climatology observations require several decades to definitively distinguish natural variability from climate trends.
The general assumption is that reduced snow cover and sea ice reflect less sunlight and therefore evaporation and transpiration increases, which in turn alters the pressure and temperature gradient of the polar vortex, causing it to weaken or collapse. This becomes apparent when the jet stream amplitude increases over the northern hemisphere, causing Rossby waves to propagate farther to the south or north, which in turn transports warmer air to the north pole and polar air into lower latitudes. The jet stream amplitude increases with a weaker polar vortex, hence increases the chance for weather systems to become blocked. A recent blocking event emerged when a high-pressure over Greenland steered Hurricane Sandy into the northern Mid-Atlantic States.
The chemistry of the Antarctic polar vortex has created severe ozone depletion. The nitric acid in polar stratospheric clouds reacts with chlorofluorocarbons to form chlorine, which catalyzes the photochemical destruction of ozone. Chlorine concentrations build up during the polar winter, and the consequent ozone destruction is greatest when the sunlight returns in spring. These louds can only form at temperatures below about −80 °C (−112 °F). Since there is greater air exchange between the Arctic and the mid-latitudes, ozone depletion at the North Pole is much less severe than at the south. Accordingly, the seasonal reduction of ozone levels over the Arctic is usually characterized as an “ozone dent”, whereas the more severe ozone depletion over the Antarctic is considered an “ozone hole”. This said, chemical ozone destruction in the 2011 Arctic polar vortex attained, for the first time, a level clearly identifiable as an Arctic “ozone hole.”
WINTER – SEASON
Winter is the coldest season of the year in temperate climates, between autumn and spring. It is caused by the axis of the Earth in the respective hemisphere being oriented away from the Sun. Different cultures define different dates as the start of winter, and some use a definition based on weather, but when it is winter in the Northern Hemisphere it is summer in the Southern Hemisphere, and vice versa. In many regions, winter is associated with snow and freezing temperatures. At the winter solstice, the days are shortest and the nights are longest, with days lengthening as the season progresses after the solstice.
The tilt of the Earth’s axis relative to its orbital plane plays a big role in the weather. The Earth is tilted at an angle of 23.44° to the plane of its orbit, and this causes different latitudes on the Earth to directly face the Sun as the Earth moves through its orbit. It is this variation that primarily brings about the seasons. When it is winter in the Northern Hemisphere, the Southern Hemisphere faces the Sun more directly and thus experiences warmer temperatures than the Northern Hemisphere. Conversely, winter in the Southern Hemisphere occurs when the Northern hemisphere is tilted more toward the Sun. From the perspective of an observer on the Earth, the winter Sun has a lower maximum altitude in the sky than the summer Sun.
During winter in either hemisphere, the lower altitude of the Sun causes the sunlight to hit that hemisphere at an oblique angle. In regions experiencing winter, the same amount of solar radiation is spread out over a larger area. This effect is compounded by the larger distance that the light must travel through the atmosphere, allowing the atmosphere to dissipate more heat. Compared with these effects, the changes in the distance of the earth from the sun are negligible.
Meteorological winter is the method of measuring the winter season used by meteorologists based on “sensible weather patterns” for record keeping purposes, so the start of meteorological winter can change depending on how far north one lives. Winter is often defined by meteorologists to be the three calendar months with the lowest average temperatures. This corresponds to the months of December, January and February in the Northern Hemisphere, and June, July and August in the Southern Hemisphere. The coldest average temperatures of the season are typically experienced in January in the Northern hemisphere and in June or July in the Southern hemisphere. Nighttime predominates the winter season, and in some regions it has the highest rate of precipitation as well as prolonged dampness because of permanent snow cover or high precipitation rates coupled with low temperatures, precluding evaporation. Blizzards often develop and cause many delays. Accumulations of snow and ice are commonly associated with winter in the Northern Hemisphere, due to the large land masses there. In the Southern Hemisphere, the more maritime climate and the relative lack of land south of 40°S makes the winters milder; thus, snow and ice are less common in inhabited regions of the Southern Hemisphere. In this region, snow occurs every year in elevated regions such as the Andes, the Great Dividing Range in Australia, and the mountains of New Zealand, and also occurs in the southerly Patagonia region of South America. It snows year-round in Antarctica.
Some authorities define the period of winter based on astronomical fixed points regardless of weather conditions. One version of this definition, winter begins at the winter solstice and ends at the vernal equinox. Astronomically, the winter solstice, being the day of the year which has fewest hours of daylight, ought to be the middle of the season. In the USA and Canada (sometimes in Britain) the season is regarded as beginning at the solstice and ending on the following equinox. The system of seasons is based on the length of days exclusively. The three-month period of the shortest days and weakest solar radiation occurs during November, December, and January in the Northern Hemisphere and May through July in the Southern Hemisphere. Yet there also many mainland European countries tend to recognize Martinmas (St. Martin’s Day, November 11th), as the first calendar day of winter. Valentine’s Day (February 14th) is recognized by some countries as heralding the first rites of spring.
The three-month period associated with the coldest average temperatures typically begins somewhere in late November or early December in the Northern Hemisphere and lasts through late February or early March. This “thermological winter” is earlier than the solstice delimited definition, but later than the daylight (Celtic) definition. Cultural influences may have led to the winter season being perceived as beginning earlier in recent years, although high latitude countries like Canada are usually well into their real winters before the December solstice.
Many annual plants require winter cold to complete their life cycle, this is called vernalization. Many small perennials profit from the insulating effects of snow by being buried in it. Larger plants, in particular deciduous trees let their upper part go dormant, but their roots are still protected by the snow layer. Few plants bloom in the winter, one exception being the flowering plum, which flowers in time for Chinese New Year.
Even the animals have developed adaptations for winter. Many birds migrate to warmer climates. There are animals that hibernate during the winter and only when it is warmer, just as there are those that store away food for the winter and survive on that instead of hibernating.
During the winter, many people enjoy outdoor winter sports such as skiing, ice-skating, ice hockey (on frozen lakes and ponds),sledding, and tobogganing. When going outside though in the winter, it is imperative to wear hats, gloves/mittens, scarfs, warm winter jackets and more are the standard. Otherwise you run the risk of frostbite, hypothermia or worse. This time of year is the only season that people wear more clothing than in any other time of year. You can remain fashionable even bundled up.
I remember my brother and I ice skating on the lakes in Brightwaters when we were growing up and later on he and his friends played ice hockey on the same lakes. And there was plenty of time on weekends in the winter for sleigh riding and tobogganing. Plus we all had to help in shoveling out the driveway to the house as well as paths to the house so that we could get in and out. Good exercise and fresh air. It was also fun making snowmen and snow women, igloos and doing angels in the snow.
AUTUMN (A/K/A FALL)
Autumn or fall is one of the four temperate seasons. Autumn marks the transition from summer into winter, in September (Northern Hemisphere) or March (Southern Hemisphere) when the arrival of night becomes noticeably earlier.
The equinoxes might be expected to be in the middle of their respective seasons, but temperature lag (caused by the thermal latency of the ground and sea) means that seasons appear later than dates calculated from a purely astronomical perspective. The actual lag varies with region. Some cultures regard the autumnal equinox as “mid-autumn”; others with a longer lag treat it as the start of autumn. Meteorologists (most of the temperate countries in the southern hemisphere) use a definition based on months, with autumn being September, October and November in the northern hemisphere, and March, April and May in the southern hemisphere.
In North America, autumn is usually considered to start with the September equinox. In traditional East Asian solar term, autumn starts on or around August 8th and ends on about November 7th. In Ireland, the autumn months according to the national meteorological service are September, October and November. However, according to the Irish Calendar which is based on ancient Gaelic traditions, autumn lasts throughout the months of August, September, and October, or possibly a few days later, depending on tradition. In Australia, autumn officially begins on March 1st and ends May 31st. According to United States tradition, autumn runs from the day after Labor Day (the Tuesday following the first Monday of September) through Thanksgiving (the fourth Thursday in November), after which the holiday season that demarcates the unofficial beginning of winter begins.
The alternative word fall for the season traces its origins to old Germanic languages. The exact derivation is unclear, with the Old English fiæll or feallan as well as and the Old Norse fall all being possible candidates. However, these words all have the meaning “to fall from a height” and are clearly derived either from a common root or from each other. The term came to denote the season in 16th century England, a contraction of Middle English expressions like “fall of the leaf” and “fall of the year”.
Association with the transition from warm to cold weather, and its related status as the season of the primary harvest, has dominated its themes and popular images. In Western cultures, personifications of autumn are usually pretty, well-fed females adorned with fruits, vegetables and grains that ripen at this time. Many cultures feature autumnal harvest festivals, often the most important on their calendars. Still extant echoes of these celebrations are found in the mid-autumn Thanksgiving holiday of the United States and Canada, and the Jewish Sukkot holiday with its roots as a full-moon harvest festival of “tabernacles” (living in outdoor huts around the time of harvest). There are also the many North American Indian festivals tied to harvest of autumnally ripe foods gathered in the wild, the Chinese Mid-Autumn or Moon festival, and many others. The predominant mood of these autumnal celebrations is gladness for the fruits of the earth mixed with a certain melancholy linked to the imminent arrival of harsh weather. While most foods are harvested during the autumn, foods particularly associated with the season include pumpkins (integral parts and apples, which are used to make the seasonal beverage apple cider. I also go apple picking in the fall at several local farms and get apples to make homemade apple sauce and pies.
Autumn in poetry has often been associated with melancholy. The possibilities of summer are gone, and the chill of winter is on the horizon. Skies turn grey, and many people turn inward, both physically and mentally.
In the Anglo sphere, most notably in Anglo-America, autumn is also associated with the Halloween season (which in turn was influenced by Samhain, a Celtic autumn festival), and with it a widespread marketing campaign that promotes it, in the U.S.A. The television, film, book, costume, home decoration, and confectionery industries use this time of year to promote products closely associated with such a holiday, with promotions going from early September to October 31st, since their themes rapidly lose strength once the holiday ends, and advertising starts concentrating on the Christmas season.
Autumn has a strong association with American football, as the regular season begins during September and ends with playoff competition in December or January, in the winter season. Canadian football, on the other hand, begins in the summer, but extends its season through the autumn season and into November. A normal activity for high schools in the US is attending Friday night football games in Autumn, while Sunday afternoons are reserved for the professional game, particularly the National Football League, and Saturdays are traditionally used for college football. The sport is generally geared around fall weather and playing in cold elements. Autumn also has strong ties to post-season baseball, with the autumnal equinox occurring with about a week left in the regular season, depending on scheduling. Autumn baseball oftentimes signifies excitement in the air for fans who root for teams on the cusp of making the post-season, as well as those that made it. The World Series, baseball’s championship series which determines the champion of Major League Baseball for that season is held in mid-to-late October (sometimes spilling over into November to accommodate longer series) and is nicknamed the “Fall Classic”.
Autumn, particularly in most parts of the United States, also has a strong association with the start of a new school year, particularly for children in primary and secondary education. “Back to school” advertising and preparations usually occurs in the weeks leading to the start of the fall season. Since 1997, Autumn has been one of the top 100 names for girls in the United States. In Indian mythology, autumn is considered to be the preferred season for the goddess of learning Saraswati, who is also known by the name of “goddess of autumn”.
Although colour changes in leaves occur wherever deciduous trees are found, coloured autumn foliage is noted in various regions of the world: most of Anglo-America, Eastern Asia, Europe, parts of Australia and New Zealand’s South Island. Eastern Canada and New England are famous for their autumnal foliage, and this attracts major tourism (worth billions of U.S. dollars) for the regions. I remember when we (my brother and I) were growing up always being fascinated by the changing colours of the leaves. We always raked them up and put them in piles for the town to collect. We also used old clothing and other things we found around to stuff with the leaves we had raked up. Our family also took weekend outings to different areas to see the leaves in all their glorious colors. In school we sometimes brought the best colored leaves in to use in class projects or in art class.