The polar jet stream is a narrow band of high-speed winds at tropopause altitude that move in a wave pattern from west to east above the boundary of the polar front, between the Arctic and the midlatitudes.
The position of the peaks and troughs of the wave determine how far southward cold polar air reaches in a region at any given time.
To a great extent, the jet stream steers the daily weather that we experience. In recent years, there has been a noticeable uptick in the amount of extreme weather in the northern hemisphere, in part due to the interaction between climate change and the jet stream.
Explaining the Polar Jet Stream
Since cold air is more dense than warm air, it takes up less space (i.e. is less thick) and pressure decreases more with elevation gain than warm air – think of cold air as being packed more tightly at the surface of the earth.
As a result, although the cold Arctic air has higher pressure than the warmer midlatitude air at earth’s surface, higher up in the atmosphere the reverse is true – the pressure is higher in the midlatitudes.
Since air wants to flow from higher to lower pressure, air begins to flow northward but is then deflected to the right by the Coriolis force to flow from west to east, parallel to the polar front. The greater the difference in temperatures between the Arctic and midlatitudes, the greater the pressure difference and resulting westerly winds in the jet stream.
How Global Warming Affects the Polar Jet Stream
In recent decades, the Arctic has warmed substantially faster than the rest of the northern hemisphere due to global warming. This effect is known as Arctic Amplification and is mainly due to the melting of sea ice.
As the Arctic gets warmer, the contrast in temperatures that drives the polar jet stream is lessened, and the westerly wind component of jet stream slows. As it weakens, the jet stream begins to meander more, and wave amplitudes become larger as the wave peaks extend further northward along a surface of constant pressure.
The Consequence: Wacky Weather
It’s already widely known that global warming is expected to create more incidences of severe weather because of excess heat and water vapor (which act as fuel for thunderstorms) in the atmosphere. Another way in which global warming contributes to extreme weather is through the effect that it has on the jet stream.
The combination of slower westerly winds and greater wave amplitude has led to a slowdown in the progression of polar jet stream waves eastward. This creates more extreme weather events since weather systems remain in place for longer periods of time.
The longer a weather system lingers, the greater the impact – making events such as droughts, flooding, heat waves, and prolonged Arctic blasts more common. The study, Evidence linking Arctic amplification to extreme weather in mid-latitudes by Professor Jennifer Francis and Stephen Vavrus, cites the 2010 European and Russian heat waves, the 1993 Mississippi River floods, and freezing conditions in Florida during winter 2010–11 as examples of such events induced by an unusually slow moving jet stream. Observations show that the progression of the jet stream is especially sluggish in autumn and winter, when sea-ice loss in the Arctic is greatest.
What about the Meridional Wind in the Jet stream?
This got me wondering, if zonal (east/west) flow is slowed down in the jet stream, what about meridional (north/south) flow? Zonal flow matters because it affects the duration of weather systems, but meridional flow also plays an important role in influencing our weather through heat transport and air mass exchange as well as storm paths and storm development in the midlatitudes. So I asked Jennifer Francis about what is happening to meridional flow.
This seems like a simple question, but it’s really not. While the zonal winds clearly decrease in regions where poleward gradients weaken, changes in meridional winds are not as straightforward. Several possible factors are in play.
The meridional winds are likely to decrease as the contrast in temperature between air masses on either side of a cold front (generally aligned north-south) weakens as Arctic air masses warm faster, but coincidentally, changes in continental versus oceanic temperatures will differ owing to the different heat capacities of the two surfaces. In future summers, the land will heat faster than the ocean, which should strengthen the east-west temperature gradient and increase the meridional winds, but in winter the gradient may weaken as continents don’t get as cold as they used to.
Another factor may be the expected shift to a wavier jet stream, which would align the total wind vectors more north/south and perhaps increase the north/south flow. I’d say the jury is out on the overall meridional wind changes.
What’s in Store for the Future?
As Arctic Amplification continues with the melting of more sea-ice in the future, we can expect zonal winds in the jet stream to weaken even further and that we’ll experience even more wacky weather along with these greater changes in the jet stream. It remains an open question at this point as to whether meridional flow will pick up the pace or weaken along with the westerly wind; only time will tell.