Weather of the north Atlantic

Here, ocean currents and large-scale weather patterns often collide.

By Karsten Shein
Comm-Inst. Climate Scientist

Enroute to LCY (Intl, London City, London, UK), the pilots had just finished an HF position check and were enjoying the smooth, crisp air and strong tailwind at FL330.

Neither liked these overnight flights, but they had to get their company’s sales team across the Atlantic in time for a late morning client meeting. Suddenly, their left engine began to surge. As the crew tried to determine the cause, the right engine followed suit and, seconds later, both engines flamed out.

Broadcasting to Reykjavik Radio, one pilot was declaring an emergency and trying to figure out if their glider could make it north to KEF (Intl, Keflavík, Iceland), while the other set up their best glide speed and was hard at work attempting a restart.

Descending through 18,000 ft, they restarted the right engine and, although the left engine could not be coaxed back to life, they had enough power to make it safely to Iceland. Unbeknown to either them or forecasters, ash from an ongoing volcanic eruption had made it to their altitude and they had no way of noticing it in the dark.

Atmosphere and ocean patterns

The north Atlantic stretches from the equator to above the Arctic circle. The weather it produces is governed by a large-scale clockwise atmospheric and oceanic circulation pattern. A semi-permanent high-pressure cell covers the subtropics around 20°–30° N latitude, while corresponding low pressures exist along the equator and near Iceland.

Near the equator, pilots can expect to encounter towering cumuli reaching beyond FL500, particularly during the northern hemisphere summer, when the Intertropical Convergence Zone (ITCZ) ventures northward. Between the equator and the subtropical high, easterly trade winds at the surface flow beneath the subtropical jet stream. Disturbances in this jet can touch off thunderstorms in the western Sahara and Canary Islands, which may organize into strong tropical cyclones that migrate westward before curving northward along the western north Atlantic.

North of the subtropical high, winds at the surface and aloft are westerly, and while the generally colder lower atmosphere means convection cells tend to top out at lower flight levels, the added dynamics of the mid-latitude troposphere can produce storms that are very powerful.

In these latitudes, pilots flying at lower altitudes, such as during takeoffs or landings, should also remember that cyclones crossing the north Atlantic will frequently include distinct and strong cold and warm fronts, which may produce squall lines, blizzards, freezing rain, low level wind shear and other common aviation weather hazards.

The clockwise atmospheric circulation over the north Atlantic also generates an underlying oceanic circulation. Along the US east coast, southerly winds drive surface ocean waters northward in the Gulf Stream current. This current moves energy-rich warm water into higher latitudes. In turn, evaporation of this warm surface water fuels convection in the middle latitude north Atlantic, often invigorating decaying cyclone systems as they cross the Atlantic toward Europe.

The Gulf Stream becomes the north Atlantic current at around 40° N, to the southeast of Newfoundland Canada, and continues its march toward Ireland and the UK, with the Norwegian Current pumping heat into Scandinavia. It is these currents that maintain relatively moderate temperatures in places like London, even though they are at much higher latitudes than other cities that experience similar climates.

Polar jet stream

Interactive tools such as the NWS Aviation Weather Center’s Wide Area Forecast System (WAFS) viewer give pilots a good overview of potential weather hazards along their transatlantic route. This view shows winds and turbulence at FL340 – close to jet stream altitudes.

One of the most important atmospheric features of the north Atlantic for pilots is the polar jet stream. This thin band of high speed air circling the planet at between 40° and 60° N latitude is produced by a strong temperature discontinuity near the top of the troposphere. The rapid temperature change over a short distance produces a correspondingly sharp pressure gradient that generates the high speed of the jet stream wind.

The polar jet delivers wind speeds above 100 kts routinely, making it a favorite for saving time and fuel on eastbound north Atlantic routes. Earlier this year, jet stream winds estimated to have exceeded 230 kts produced commercial aircraft ground speeds of more than 700 kts. It is no wonder that jet stream position and strength is used to determine the daily flight tracks across the north Atlantic. On westbound flights, however, most pilots and dispatchers will opt for non-jet stream altitudes where headwinds are likely to be far lower.

Variations in the temperature discontinuity and differences in pressure around and beneath the jet also create atmospheric waves that, in turn, destabilize the air beneath it, developing or enhancing convective systems that travel with it. Fortunately, most flights across the north Atlantic cruise well above these storm systems. However, the jet and associated storm systems can produce severe to extreme turbulence.

While strong turbulence aloft should be expected around any storm system, the jet stream itself can produce shear turbulence, even when no clouds are present. Clear air turbulence (CAT) can occur anywhere there is a significant change in wind speed or direction over a short distance. This makes the edges and waves of the jet ideal places to encounter CAT.

Jet stream winds will normally accelerate into the base of a wave trough and decelerate out of it. These jet streak areas frequently generate extreme turbulence that has led to injury of unsecured crew and passengers. Although methods for predicting and alerting for CAT are being developed, pilots should not let down their guard anywhere CAT has been forecast.

Volcanic ash

Another challenge for pilots flying the north Atlantic is volcanic ash. Crustal subduction in the Caribbean and sea floor spreading around Iceland have produced several aviation-disrupting eruptions in recent years. Larger eruptions elsewhere can loft ash into the stratosphere to circle the planet. Eruptions are tracked by the Volcanic Ash Advisory Centers worldwide, and sigmets are issued for areas and altitudes where ash is expected.

However, the models used to forecast ash dispersion aren’t perfect, and pilots are encouraged to avoid ash areas by larger margins of safety. Even during the day, the small concentrations of ash capable of abrading windscreens and clogging engine inlet ports may not be visible to the naked eye.

Satellites and weather reports from the few airports across the north Atlantic help forecasters to model and predict the weather, but their information also relies heavily on ship and aircraft weather reports. While your aircraft is likely automatically transmitting standard weather data, pireps reporting unusual weather phenomena can be invaluable to making better forecasts.

Karsten Shein is cofounder of 2DegreesC.org. He was director of the Midwestern Regional Climate Center at the University of Illinois, and a NOAA and NASA climatologist. Shein holds a comm-inst pilot license.