JET STREAMS, WEATHER AND CLIMATE
Circumnavigating the Earth at tremendous speeds of up to two hundred and seventy five miles per hour, jet streams play a major role in the daily transportation of weather systems across the globe. Located in the upper levels of the atmosphere at an altitude of between seven and twelve kilometers above the surface, these narrow bands of fast moving winds significantly affect the weather at the Earth’s surface. Their high speed and position determine not only the nature of the weather at the surface, but also the duration of any given weather system. As we experience somewhat unusual weather patterns, how much can we attribute to jet stream changes not previously seen, while taking into account both natural and seasonal changes?
The movement of air, which we feel as wind, is created by temperature differences between air masses and is formed when air moves quickly from high pressure to low pressure areas. The temperature of an air mass determines the air’s density (and therefore the air pressure). The greater the temperature (and pressure) difference, the faster the wind, as the air flows more quickly to equalize this large difference.
The Earth is not heated evenly by the sun’s rays, with polar regions receiving much less sunlight year round than equatorial and mid-latitude regions. As a result, very fast flowing winds (i.e jet streams) are formed at these boundaries of the contrasting air masses between the warmer mid latitudes and the frigid polar latitudes. It follows therefore, that a greater temperature contrast between the two air masses, leads to a stronger polar jet stream. It is strongest in the northern hemisphere’s winter months, when the temperature contrast between the cold, sunless Arctic and the tropical and midlatitudes is usually at its greatest. The rotation of the Earth directs these winds (which can be several thousand miles long) to travel on average from west to east, although they also meander north to south in a wavelike pattern.
Both the northern and southern hemispheres of the Earth have their own jet streams, both polar and subtropical. Note also that a jet stream can occur anywhere where the air at upper levels is traveling at speeds greater than fifty eight miles per hour.
The polar jet streams are usually found at latitudes of between 50 and 60 degrees both north and south of the Equator, where polar and midlatitude air meet. The subtropical jet streams are usually located at around 20 to 30 degrees (north and south of the Equator) occur where tropical and midlatitude air meet. The northern hemisphere polar jet stream is more widely known due to its importance in weather forecasting for the northern latitudes.
As these jets of upper level air race around the world, surface weather systems are pushed along with them. This important temperature difference between the colder polar air and warmer tropical or mid-latitude air, therefore acts as the main driver of most of the Northern Hemisphere's weather all year round.
Although the prevailing wind direction of the northern hemisphere polar jet stream is from west to east, the polar jet stream undulates north and south in a large wavelike pattern around the circumference of the globe. These waves can be huge, any one of them can half-cover the Atlantic ocean, or a large part of the northern US. Within the wave dips (known as troughs) cold air from the polar regions is able to move further south towards the Equator. Any regions on the Earth’s surface located within the trough will experience these colder temperatures. Conversely, in the wave peaks (known as ridges) warmer air from the equatorial regions is able to move towards the poles, and regions located within the ridges will experience warmer temperatures.
The position and curvature of these polar jet stream waves therefore is a great indicator of prevailing weather experienced at the Earth’s surface. A strong jet stream is able to circle the globe with enough power to effectively smash through any atmospheric ‘obstacles’. Examples of such obstacles include high pressure air mass circulations which move slowly and can tend to cause ‘blocking’ of air masses. During a period with a strong jet stream, weather conditions at the surface tend not to stay in position for long periods of time since the jet stream moves them away quickly.
When the jet stream is weak however, these waves become more amplified (the waves are larger) and move more slowly. Weather patterns at the surface may remain in the same position for longer periods of time and can remain stuck for many days, or even weeks. This is particularly significant during more extreme weather events. Long periods of heavy rain lead to flooding, or conversely long term dry conditions lead to drought.
Variations in the temperature gradient that fuel the jet stream must also affect the surface weather. Natural variations include seasonal changes as the position of the sun in the sky changes throughout the year. In particular, the northern polar jet stream is strongest in the northern hemisphere winter, due to the strong temperature gradient between the polar and equatorial regions. The North Pole receives no sunlight throughout the winter months, leading to this large temperature contrast with the equatorial regions.
Other natural variations that affect the jet streams, include the so-called ENSO (El-Nino/Southern Oscillation) cycle. This scientific term describes the known fluctuations in temperature between the ocean and atmosphere in the east-central Equatorial Pacific. These fluctuations affect both the sea surface temperatures and prevailing winds, which in turn affect global weather, as a result of the positional shift in the subtropical and polar jet streams. While their periodicity can be quite irregular, El Niño (and it’s counterpart known as La Niña) events occur on average every three to five years, and can last for months.
Typical effects of an El Nino event include cold and wet weather in areas that are usually dry, such as California and southeastern US states, resulting from the subtropical jet stream (and therefore the position of storm tracks) shifting in a poleward direction.
The effect of the ENSO cycle on the northern polar jet stream is also well known. For example during a strong El Nino event, the northern polar jet stream (and therefore the storm tracks in the northern hemisphere) shifts eastward. As a result, northeastern US regions usually experience milder and wetter winters during an El Nino winter.
There are other naturally occurring climate variations with implications for the jet streams which occur irregularly over longer time-scales than seasonal and ENSO cycles. One of these includes the phenomenon known as the Arctic Oscillation. This term is used to describe changing air pressure and temperature between the Arctic and the middle latitudes, which ‘oscillate’ between above and below average values. These oscillations affect the upper-level Arctic wind circulations and surface weather around the globe in mid-latitudes, in a similar way to the jet stream. The term ‘polar vortex’ is used to define the system of upper-level winds that circle around the North Pole (note that the South Pole also has a polar vortex). These high-velocity winds usually keep the cold polar air locked within this circulating system.
Although these circumpolar wind systems exist most of the time at both poles, they can change in strength depending on season and are typically strongest during that hemisphere’s winter months (as a result of temperature differences between the air masses, much like the jet stream). Although weather forecasts often refer to the polar vortex as the cold polar air mass itself, note that the vortex is simply a term used to describe the circulating polar winds, and is not referring to the air mass itself.
When this Arctic polar vortex is strong the winds circulate at high velocity and are effective at trapping the coldest air masses within the polar regions. Northern mid-latitudes therefore experience milder winter temperature. When the winds of the polar vortex weaken however, or indeed interact with high-amplitude wave patterns in a weakened polar jet stream, the shape of the polar vortex may become distorted. The circulation pattern around the pole becomes increasingly asymmetrical, elongated and, in more extreme cases, may even split into two or more patterns. As a result, large masses of frigid arctic air may flow southward following the waves of the jet stream into mid-latitudes, causing periods of colder than normal winter temperatures in mid-latitudes such as those experienced in recent winters. Simultaneously, warmer air is allowed into the northern polar latitudes in regions not previously used to such mild temperatures.
There is current ongoing research to detect possible changes in the known patterns of the upper level fast-moving air currents, such as the northern polar jet stream and northern polar vortex. Are natural variations in the climate system sufficient to explain the frequency of prolonged periods of unusually cold, hot, dry and wet spells in recent years in different areas of the globe? Research suggests that the frequency of unusual weather phenomena may be influenced by climate change due to the ongoing decreases in Arctic sea ice and the faster rate of temperature increase in the Arctic. This leads to a lower temperature difference between normally cold polar air masses and warmer mid-latitude air.
Arctic amplification is the scientific term used to describe the accelerated warming that is observed to be occurring in the Arctic in comparison to the rest of the globe. One reason this occurs is that as more of the reflective snow and ice melt in the Arctic, darker land and ocean are exposed, further absorbing more solar energy. This leads to a so-called feedback effect of faster warming in that region, with far-reaching effects. It is counterintuitive that a warming Arctic region may lead to periods of unusually cold winter temperatures in populated northern mid-latitudes, while less densely populated Arctic locations experience warmer winter temperatures. Data for the Arctic amplification however is available for the last two decades only, since the phenomenon was first observed with accurate satellite measurements. An accurate trend therefore is hard to detect until more data is available. Nonetheless this is an area of intense current research and considerable concern.
There are many factors to take into account when measuring the effects of natural and man-made changes to upper air circulations and climate. Long term forecasting of natural climate variations such as ENSO or the Arctic Oscillation is difficult. Several decades’ worth of available satellite data is not long enough to detect any long term trends with accuracy. Scientists are able however to make short-term accurate weather and climate forecasts for the regions of the globe directly affected once such an event has started.
Seemingly unrelated events such as changing sea surface temperatures in equatorial Pacific regions, or altered snow/ice cover in the polar regions can significantly affect weather in many other regions around the world. To predict future effects on climate and weather, all possible factors including natural variations and man-made warming must be incorporated into climate models, since they interact with each other in ways that cannot be anticipated by considering them solely in isolation.
Climate models are becoming increasingly sophisticated and capable of modeling these complex interactions. Predicting future changes to climate and subsequent effects on localized weather by modeling these interactions accurately is one of the major challenges of climate research today.