Surf Science: What Exactly is a Jet Stream and Why is it Important?

From time to time you might hear MSW referencing things like “The jet stream is strong and very southerly orientated” or “There is a large meander in the jet stream”. But what exactly is the jet stream, and why is it so important?

The official definition of a jet stream, from the World Meteorological Organization is as follows: ‘A flat tubular current of air, quasi-horizontal, whose axis is along a line of maximum speed and which is characterised not only by great speeds but also by strong transverse (horizontal and vertical) gradients of speed’

In other words, a kind of isolated flow of air that is faster than the air surrounding it. The speed of the jet stream is typically 200 to 400 km/h – much faster than typical sea-level windspeeds.

In fact there are several different jet streams in the atmosphere, but the one that interests us – and the one that people normally mean when they refer to ‘the jet stream’ – circulates the globe from west to east at around 30 degrees to 60 degrees latitude, and at around 5-to-ten kilometres above sea level.

The jet stream doesn’t just flow straight around the planet from west to east. It has meanders in it – called Rossby waves after the famous meteorologist Carl-Gustaf Rossby (1898-1957) – and it goes through phases where these meanders grow and shrink. Also, the strength of the jet is not constant in position or time: it goes through phases of being stronger or weaker at different points along its trajectory, at different times. These variations are important because they have a major influence on the way the upper airstream affects the surface flow, which is vitally important for low pressures and the generation of surf. The jet stream is an atmospheric super-highway which helps to steer the trajectory of surface low pressure systems

The jet stream is an atmospheric super-highway which helps to steer the trajectory of surface low pressure systems. But it also influences the size and intensity of those systems, because the strength and orientation of the jet controls the amount of energy transferred from the upper atmosphere to the surface. The deepest low pressures tend to develop in places where the jet is strong and straight, or on the poleward arm of a meander.

Because the air in the upper atmosphere is not directly influenced by things going on at the surface, such as temperature changes over the land or sea, it changes less quickly than the surface flows. This is why you often get a series of low pressures following the same trajectory, one after the other. The jet stream – which is influencing the trajectory of those lows – might take a particularly southerly or northerly path, flow straight or contain a meander, and could stay like that for several days.

One factor that was identified as being highly influential for the extreme North Atlantic winter of 2013-2014 was the behaviour of the jet stream. During that winter the jet was up to 30 percent stronger than average, and for over a month it flowed across the North Atlantic in an almost a straight line from Newfoundland to somewhere around Britain and Ireland.