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North Atlantic weather


Often a chaotic mix of weather patterns, the North Atlantic demands careful weather planning.

Gulf Stream and N Atlantic currents are part of a larger ocean circulation that cycles warm surface water poleward and cold deep Arctic water back to the tropics.

Crossing the southern tip of Greenland at FL370, the pilots could see nothing but an endless white blanket of cloud beneath them. The outside air temperature was -56° C and their tailwind was a bit stronger than forecast.

In the distance off to the south, they could make out the line of cumuli marking an extratropical cyclone that had begun as late fall tropical storm before heading northeast into the North Atlantic.

Their northern track was selected partly to keep them out of that area of potential storms and icing, as well as to take advantage of some strong tailwinds from the polar jet.

Although the significant weather charts had suggested they could experience moderate turbulence, the flight had been smooth so far. But the aircraft lurched downward without warning.

After several more jolts, the air smoothed out again, but they had to request a deviation to KEF (Keflavík, Iceland) when the cabin attendant informed them that the company’s CEO had been thrown across the cabin and was unconscious.

Since sailing ships first voyaged across the North Atlantic Ocean, it has been respected as a place where both ocean and the weather conditions can change quickly, and can become dangerous frequently. The main reason for this is the ocean’s position in the middle latitudes.

These latitudes are the battleground where warm subtropical air clashes with cold arctic air masses. Perhaps more importantly, the strong and semi-permanent subtropical high pressure cell that resides in the vicinity of the Azores islands produces a clockwise circulation in the lower atmosphere that drives the ocean surface circulation of the North Atlantic.

Sea surface temperature across the North Atlantic. Surface currents move large quantities of heat from the Caribbean to northern Europe.

The Gulf Stream

The ocean circulation includes the Florida current and Gulf Stream that pull massive quantities of heat into the higher latitudes, and even across to northern Europe.

Although Benjamin Franklin first mapped the Gulf Stream in the 1700s, it took longer to recognize that this current was part of a much larger oceanic circulation that not only regulated Earth’s atmospheric temperature, but also influenced the weather in Europe and beyond.

For example, years with extreme summer and winter temperatures across Europe are highly correlated with warmer-than-normal sea surface temperature of the central North Atlantic.

The heat brought north by the Gulf Stream also enhances the temperature differences across fronts that pass through the area, and as the heat and associated moisture are transferred to the atmosphere, the energy can restrengthen decaying cyclones that have moved into the region either from eastern North America, or northward from the tropics. In fact, many former hurricanes have regained enough strength to survive a journey across the Atlantic, hitting Ireland, the UK, and Portugal with hurricane-force winds.

Although it can seem daunting, particularly because of the apparent lack of weather information over the unpopulated expanse, the weather across the North Atlantic is no different than the midlatitude weather pilots may experience over North America, Europe, or Asia, and generally behaves as predictably – perhaps even more so – because the influence of irregular topography is absent.

Most of the North Atlantic’s adverse weather comes in the form of extratropical cyclones that form as low pressures over North America and migrate either eastward, or, in the case of Nor’Easters, up the Atlantic seaboard.

High-level significant weather (SigWx) chart for the North Atlantic. These prognostic charts identify areas of likely meteorological hazard at transatlantic cruise altitudes. Note the two tropical storms and associated cumulonimbus activity.

The polar jet

With few exceptions, most North Atlantic aviation operations are transatlantic, and thus operate high in the flight levels over the region, usually along prescribed tracks laid out to manage airspace arrivals into North America and Europe. Flights from North America to Europe tend to fly at altitudes that give them the best chance of encountering the strong tailwinds associated with the polar jet stream.

Although the existence of the polar jet was known before then, it was first used to great effect by aircraft ferry flights during the Second World War, when aircraft with service ceilings exceeding 30,000 ft were first manufactured and flew at high altitudes to avoid being spotted by the German navy.

Those pilots discovered they made far better time and used less fuel than anticipated, but that westward flights had to be done at lower altitude to avoid the equally strong headwinds.

Although westerly winds tend to strengthen with increased altitude throughout the midlatitude troposphere, there is a narrow region near the top of the troposphere (around 30,000 ft in winter to 40,000 ft in summer) where the temperature discontinuity between cold Arctic air and warmer subtropical air is exceptionally large over a very short distance.

This translates into an equally strong pressure gradient that drives the geostrophic wind aloft. Because the temperature and pressure difference occurs over a very short horizontal and vertical distance, the zone of highest winds is isolated to that narrow ribbon.

Naturally, wind shear exists wherever air is moving faster than the air surrounding it, such as around the jet boundary and near the core of the jet. In regions of wind shear, turbulent eddies are often shed into the calmer air. These eddies can be horizontal or vertical, but at the high altitude of the jet stream, there are often no clouds or other indicators of these eddies. Thus, they are known as clear air turbulence (CAT).

Although CAT can occur wherever there is a sudden change in wind speed or direction, it is most likely to be found near and within the polar jet. The most likely places are where the jet is changing direction, such as in a trough or ridge.

The more amplified the meander, the more likely CAT will be present. The same holds for trough bases, where a jet streak may exist. The combination of sudden acceleration or deceleration of air and shift in direction makes strong-to-extreme CAT a possibility.

An examination of winds aloft tables, high altitude weather maps, and prognostic charts for the North Atlantic – and even satellite imagery  – will provide a good indication of the jet’s position relative to a flight routing, and pilots can adapt their altitude and routing to maximize tailwinds while avoiding areas where CAT is likely.

In addition, the high-altitude significant weather map for the North Atlantic will also show areas where moderate or greater turbulence is likely, as will model forecasts of the Ellrod Index. This index is used to forecast CAT, where values over 8 indicate moderate CAT, while those over 12 suggest severe CAT.

The stratosphere

Forecast Ellrod Index at around FL340 over the North Atlantic. The Ellrod Index is a tool to forecast the location and potential strength of clear air turbulence.

While eastward flights will generally want to stay in the upper reaches of the troposphere, today’s transcontinental business jets can reach heights the bombers of the 1940s could only dream of. Westward flights can avoid most adverse weather and strong headwinds by cruising above the jet level, in the region of the atmosphere known as the stratosphere.

The lowest level of the stratosphere is a region where temperature doesn’t change much with height, and is often called the tropopause.

Because temperature no longer decreases with height, convection is inhibited and the pressure gradient is weak, meaning there is little cloud formation and winds are much lighter than those just a few thousand feet lower.

However, the stratosphere may not always be within reach. A good rule of thumb is that the midlatitude troposphere over the North Atlantic tops at around 35,000 ft in the winter and 45,000 ft in the summer. Jet stream ridges mean higher troposphere heights, while troughs have lower heights. On a similar note, because the flight levels are pressure altitude surfaces, when flying from a ridge to a trough region, an aircraft at a given flight level will lose absolute altitude.

Sometimes, particularly when the jet is exhibiting strong north-south (meridional) flow and good tailwinds are difficult to come by, eastward flights may opt to fly at stratospheric flight levels. To locate the top of the troposphere, forecasts of winds aloft will show relatively constant temperatures or a sudden decrease in wind speed above a certain level – usually in the FL300–FL400 region.

This change identifies the start of the stratosphere. 300 mb, 250 mb, and 200 mb upper air maps will similarly show the transition (roughly 19% of the atmosphere’s mass is above the troposphere). Similarly, high altitude significant weather maps may indicate the top of the troposphere, and some forecast products will map the transition height. In the absence of this information, anvil tops on towering cumuli are good guides.

Other adverse weather

Because most flights operating in the North Atlantic region are transcontinental, they fly at altitudes which avoid most other adverse weather conditions that may be experienced by aircraft at lower altitudes. However, volcanic ash is a danger over the North Atlantic.

Not only are there active volcanoes in Iceland that have caused major headaches for aviation over the past 20 years, but when larger eruptions occur anywhere in the Northern Hemisphere, ash can be launched all the way into the stratosphere.

The same polar jet that gives us good tailwinds also serves as a means for carrying ash from volcanoes in the Pacific region quickly across to the North Atlantic. Thus, it is always important to check for any volcanic ash advisories and sigmets, even if no eruptions are occurring nearby.

The North Atlantic is also peppered with airports, many of which are business or commercial destinations, such as KEF, or are designated as engine out/emergency strips. The weather in these mid-to-high-latitude maritime locations can be highly variable, particularly in the fall to spring time frame, and adverse conditions such as thick fog, heavy icing, and blizzards are common.

Midlatitude cyclones provide much of this weather, and factors such as fog are often advective and may be present for several days. So, a close look at the satellite picture and forecast charts is important, as is a discussion with a briefer or weather provider who knows and forecasts those local conditions, particularly because an alternate airport may be thousands of miles away.

Given the limited number of weather observation stations across the North Atlantic, it is also critical that pilots contribute pireps whenever possible. Those reports will help meteorologists and fellow pilots get a much clearer picture of the weather in this ever changing region.

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