Shifting Jet Stream Patterns Are Impacting Aviation

Aviation Week | Patrick Veillette, Ph.D. | 23 October 2019

Unseen to the naked eye and often undetectable electronically, clear air turbulence (CAT) often strikes without warning and can toss unbuckled passengers, cabin crew and objects around, sometimes violently. In the U.S. alone, the financial tally for such upsets runs $200+ million annually, and doesn't account for the pain and loss of productivity that results, nor the time lost to aircraft inspection and maintenance.

According to NTSB Senior Meteorologist Donald Eick, turbulence has caused more serious injuries to passengers than any other class of accident. Indeed, 71% of FAR Part 121 air carrier weather-related accidents between 2000 and 2011 were due in part to turbulence, and a quarter of such accidents were tied directly to CAT. In an average year, U.S. airlines experience "significant turbulence" incidents and/or accidents resulting in 14 serious injuries and 69 minor injuries.

An important source of CAT is strong wind shear, which is prevalent especially within the atmospheric jet streams. In the Northern Hemisphere, there are two jet streams, the polar and the subtropical.

The former is the dividing line between colder, polar air and warmer, tropical air. During the summer, the polar jet moves well north, often north of 45 deg. latitude. In the winter it can dip as far south as 25 deg. latitude with outbreaks of polar and even arctic air. Its location and strength are highly variable due to the influence of many different factors from the ocean to the stratosphere. The layout of continents, mountain chains and ocean surface temperatures means that the North Atlantic jet stream is distinct from those in the North Pacific and much of the Southern Hemisphere. The actions of the Pacific and Atlantic jet streams will depend to some extent on the differing responses of their respective ocean basins to climatic changes. Thus, the forecasting of jet stream location, direction and strength is not as precise as needed, especially when planning long-range flights.

The height of the Northern Hemisphere's polar jet stream averages around 30,000 ft., but the core sometimes drops to 25,000 ft. or even lower. The polar jet stream in each hemisphere is created and sustained by the temperature difference between the cold poles and the warm tropics. Since the temperature gradient between the air over the polar region versus the mid-latitudes is greater in the winter, that's when the speed of the polar jet is stronger.

Textbooks and other sources often depict a jet stream as a solid line, giving pilots the impression that its winds are continuous. In fact, the speeds vary considerably along the stream's axis and are neither constant nor always flowing in a west-to-east direction. Areas of stronger winds, or "jet streaks," are found where temperature differences at high altitudes are the strongest and are often referred to as "upper level fronts." These jet streaks have the highest potential for wind shear and maximum turbulence. Jet streak regions move along the jet stream's axis, although at speeds much slower than the winds themselves.

The speed of the jet stream over North America and Europe can reach 200 kt. in the winter but can reach 300 kt., particularly over Southeast Asia.

Typical behavior for the jet stream in the middle latitudes is a mild bending from north to south and then back to the north, not unlike a sine wave. These bends are called "planetary" or Rosby waves and typically progress across the U.S. in three to five days. However, there has been a sizable increase in the amplitude of troughs and ridges in the polar jet since 2000. In addition, these waves are moving slower across the earth, sometimes stopping in place for weeks and bringing long periods of uncommon weather.

Also, these jet streams in both hemispheres are forecast to strengthen at aircraft cruising altitudes. According to the ICAO report "Clear-Air Turbulence in a Changing Climate," the troposphere over the tropics is projected to warm more than the surface in part because a warmer atmosphere has a higher water vapor concentration. In polar regions, the tropospheric impact is to be less pronounced because there is less water vapor present, but changes in atmospheric heat transport and significant climatic feedbacks associated with changes to sea ice and clouds do result in a strong surface warming. The increased heat stored in the Arctic Ocean owing to sea-ice loss is released into the atmosphere in early winter. The troposphere and stratosphere warm unevenly in response to climatic alteration. The stratosphere, by contrast, cools in response to the increased greenhouse gases.

Despite any skepticism, these atmospheric modifications are already being observed and are affecting aviation because prevailing jet stream wind velocity and patterns are altering optimal flight routes, increasing flight times and overall fuel consumption, and creating more CAT.

To better understand these effects, consider the example of making a roundtrip across the North Atlantic from New York's JFK International Airport (KJFK) to London's Heathrow (EGLL). The North Atlantic flight corridor between Europe and North America is one of the world's busiest, with approximately 600 crossings each day, which means there is plenty of historical data to compare recent activity.

While a great circle route minimizes the distance between the two airports, it's more economical to minimize the flight time, which means taking advantage of tailwinds for eastbound flights and minimizing headwinds when going west. For a long-haul flight such as this, a Wind Optimal Route (WOR) trajectory depends on the prevailing jet stream position and strength.

There is a weather phenomenon over the North Atlantic that greatly influences the upper atmosphere and thus the WOR calculations. The North Atlantic Oscillation (NAO) is one of the most prominent climate anomalies composed of a north-south dipole pattern of pressure variances, especially in winter. In the positive phase of the NAO, stronger pressure gradients between the persistent subtropical high and Icelandic low lead to a higher-latitude position of the jet stream. The dominant jet stream shifts northward directly to northwestern Europe (See Figure 1). Weaker pressure gradients in the negative phase of the NAO shift the stream farther south and closer to southern Europe. This interannual variability of the persistent high- and low-pressure systems in the North Atlantic creates different wind-optimized trajectories.

A team led by Dr. Jung Hoon Kim of the Cooperative Institute for Research in the Atmosphere at Colorado State University investigated how upper-level jet stream characteristics associated with the NOA would impact transatlantic WORs. The researchers studied the differences when a positive NAO phase dominated (December 2004 through February 2005) versus a negative NAO phase (December 2009 through February 2010.)

In making a roundtrip between America and Europe during those periods, an aircraft would have experienced a greater total travel time between eastbound and westbound routes in the +NAO phase than in the -NAO phase because the prevailing westerly jet stream along the great circle route is stronger in the positive phase. However, the shorter eastbound and longer westbound times do not cancel out, resulting in an increase of roundtrip time. Eastbound routes in the positive phase are faster because the distances are shorter and tailwinds stronger. Conversely, westbound routes in a negative NAO phase are faster because the distances are shorter and headwinds weaker.  (See Figure 1 opposite page.)

In general, CAT is strongest on the cold/low pressure side of the jet stream (the north side in the Northern Hemisphere), next to and just underneath the axis of the jet stream. Aircraft utilizing jet streams often experience light to moderate turbulence for much of the flight. On the cyclonic side of the jet axis you will find a lower tropopause with sinking dry air that could be of stratospheric origin. The border of the high cirrus clouds in the warm airmass and the lower clouds in the cold airmass are a good indicator for the jet axis.

On Feb. 18-19, 2019, a Virgin Atlantic Boeing 787-9 flying from Los Angeles International Airport (KLAX) to Heathrow reached a record-setting ground speed of 801 mph while at FL 350 over Pennsylvania, thanks to the 240-mph push from the winter jet stream. The flight arrived 48 min. early.

According to Dr. Paul Williams, professor of meteorology at the U.K.'s University of Reading, early arrivals of eastbound flights due to brisk jet stream pushes will become more common. He applied scientifically accepted models on warming to transatlantic flight data between New York and London and calculated that the average tailwind component at cruising altitudes would increase by 14.8%, from 47.9 mph to 55 mph. Although the great circle journey time in still air is 6 hr., 9 min., the calculations for eastbound flights cut transit time by half an hour, to 5 hr., 38 min. That is relatively good news.

However, the average westbound journey time is predicted to be half an hour longer at 6 hr., 40 min. Another important finding from Williams' study is that westbound journey times exhibit significantly more day-to-day variability than eastbound journey times. The probability of a westbound crossing taking over 7 hr. nearly doubles from 8.6% to 15.3%, suggesting that delayed arrivals in North America will become increasingly common.

The changes in the velocity and pattern of the mid-latitude jet stream are also predicted to increase CAT. Climate modeling studies indicate the volume of airspace containing moderate-or-greater turbulence on winter transatlantic routes will increase by 40% to 170%. If proven, this will have serious consequences for aviation.

While that range is admittedly rather wide, the variables that factor into the computations are complex and numerous. Everything in nature, whether it be metal strength or the temperature of the atmosphere, displays variations in its behavior. The complex equations attempt to model as many of the known important factors as possible. This is "state of the art" science thoroughly scrutinized by the leading scientists in these academic fields and accepted for publication in peer-reviewed journals.

Kim points out that eastbound optimized routes are faster but have higher probabilities of encountering CAT because they are closer to the jet streams. This is especially the case near the cyclonic shear side of the streams in the northern part of the great circle route in the positive NAO period, and in the southern part of the great circle route during a negative NAO cycle.

Eastbound optimized routes during a negative NAO phase have the highest chance of encountering CAT because most trajectories tend to pass directly to the cyclonic shear side of the southerly shifted jet stream. This information can be used in aviation to understand how the predicted upper-level teleconnection weather patterns can be translated to make for safe and efficient transatlantic flight. (See illustration above.)

A study titled "Global Response of Clear Air Turbulence to Climate Change," published in the October 2017 issue of Geophysical Research Letters, found that the busiest flight routes, such as those in the Northern Hemisphere, would see the largest increase in turbulence and that severe turbulence will become as frequent as moderate turbulence historically. The study also found moderate turbulence will become as frequent in the summer as it has done historically in winter. This is significant because although CAT is more likely in winter, the study maintains it's likely to become much more of a year-round phenomenon.

This predicted increase in CAT highlights the importance for improving turbulence forecasting and helping crews avoid threatening areas or at least ensuring all aboard are belted before the encounter. As Williams points out, these changes to the upper atmosphere won't necessarily cause more inflight injuries because recent advances in atmospheric modeling and inflight data collection have improved forecast accuracy, improving the odds of avoidance. One of these tools is the Turbulence Auto-PIREP System (TAPS) being developed under the NASA Turbulence Prediction and Warning System (TPAWS) program, which generates real-time, automatic reports of hazardous turbulence.

FAA Advisory Circular 120-88A, "Preventing Injuries Caused by Turbulence," strongly endorses the use of seat belts during a turbulence encounter. The fact is that from 1980-2003, only four people received serious injuries during turbulence when seated with seat belts fastened.

Despite advances by the meteorological science community to better forecast turbulence associated with the jet stream, recent CAT incidents underscore the significance of the threat posed. Specifically, a Turkish Airlines flight to New York on March 9, 2019, sent 30 injured persons to the hospital as a result of severe turbulence encounter. Four months later, an Air Canada flight from Vancouver to Sydney had to divert to Honolulu when 30 people were injured in turbulence.

Given the suddenness of onset and our current inability to accurately detect CAT with sufficient warning, it just makes good sense to buckle up and stay that way.

For Further Information:

Paul D. Williams, Department of Meteorology, University of Reading, U.K. "Increased Light, Moderate and Severe Clear Air Turbulence in Response to Climate Change." Advances in Atmospheric Sciences, Vol. 34, May 2017, pp. 576-586.

Williams, "Transatlantic Flight Times and Climate Change." Environmental Research Letters 11, 2016, 024008.

Jung Hoon Kim et al. "Impact of the North Atlantic Oscillation on Transatlantic Flight Routes and Clear-Air Turbulence." Journal of Applied Meteorology and Climatology, March 2016.

Melissa Gervais, Jeffrey Shaman, Yochanan Kushnir. "Impacts of the North Atlantic Warming Hole in Future Climate Projections: Mean Atmospheric Circulation and the North Atlantic Jet." Journal of Climate, 2019; DOI: 10.1175/JCLI-D-18-0647.1.

Luke N. Storer, Paul D. Williams, Manoj M. Joshi. "Global Response of Clear Air Turbulence to Climate Change." Geophysical Research Letters, Vol. 44, Issue 19, October 2017.

FAA Advisory Circular 120-88A, "Preventing Injuries Caused by Turbulence," Jan. 19, 2006.

Bureau d’Enquêtes et d’Analyses pour la Sécurité de l’Aviation Civile. "Wind Gradients and Turbulence." Incidents in Air Transport, No. 5, December 2006.