Just a week after record breaking heat affected Central and Western Europe, another exceptional heatwave is getting underway – in an unexpected location… Alaska.
As of writing this on July 4th, it’s already breaking records, with more likely to fall in the coming days.
A bit of light research has identified two key driving forces behind this event. One’s in the surface layers of the Pacific Ocean and the other high up in its atmosphere.
Surface Layers: The Pacific Decadal Oscillation is Cooking
In the extratropical Pacific, sea surface temperature (SST) anomalies* are known to oscillate between two different patterns. They’re known as positive and negative stages of the Pacific decadal oscillation (PDO).
In recent weeks, after months of neutrality, the PDO has become positive. This is characterised by warmer than usual SSTs in a horseshoe shape extending from Hawaii to the Bering Strait. For years with a positive PDO, July’s mean temperatures have tended to be warmer than usual in Alaska, especially on the northwest (see Figure 1).
Figure 1: Charts showing (left) the latest weekly SST anomalies courtesy of NOAA/ESRL/PSD and (right) the mean surface air temperature anomalies for July in years with a PDO index of 1.0 or greater.
However, a positive PDO alone isn’t likely to explain such widespread and strong anomalous warmth as we’re seeing across Alaska in early July 2019.
‘A positive PDO alone isn’t likely to explain such widespread & strong anomalous warmth as we’re seeing across Alaska…’
Getting Higher: The Arctic Atmosphere is Blocking
Much like the preceding month, June 2019 has featured exceptionally high geopotential heights (GPH) in the upper troposphere (the layer of atmosphere we mostly spend our lives in) and lower stratosphere (the layer above the troposphere). Figure 2 provides a visualisation of this.
Many of you will be wondering what GPH is exactly. Well… it’s a scientific measure that’s related to both air pressure and temperature, but also takes the effect of gravity into account. So, it’s very complicated to explain in full! Delve in here, if you’re up for the challenge.
Thankfully, for the purposes of climate science, it’s usually sufficient to know simply that higher geopotential height correlates strongly with higher air pressure, and weakly with air temperature.
Figure 2: Mean geopotential height difference to the long-term average for June 2019 at (left) 100 mb – the lower stratosphere – and (right) 500 mb – the upper troposphere.
These departures from the average are both connected to an anomalous amount of heat and descending air (i.e. air pushing downward, increasing the pressure on what’s below) in the upper atmosphere.
Temperature Contrasts Move the Atmosphere
This heat impacts the bands of upper-atmospheric winds that generally run from west to east. They’re ‘thermal’ winds, driven by a difference between the colder Arctic and the south’s warmer lower-stratospheric temperatures.
You might have heard of this as… the jet stream.
As illustrated in Figure 3, when the Arctic is much warmer than usual, the reduced temperature difference causes the jet stream to slow down. With less eastward inertia, it’s liable to meander. Much like how a slower vehicle corners more readily than a faster one.
‘With less eastward inertia, the jet stream is liable to meander. Much like how a slower vehicle corners more readily than a faster one.’
Figure 3: Diagram showing the effect on the jet stream of the lower stratosphere on the poleward side
becoming warmer relative to that on the equatorward side (i.e. south side in N. Hemisphere).
Today (July 4th 2019), a weaker than usual jet stream is arcing a long way north from the Pacific Ocean, such that it’s tracking over northern Alaska (see Figure 4). The area contained within these northward arcs is supportive of surface high pressure formation (denoted by a blue H below the surface level pressure in mb).
Figure 4: Wind speed (shaded) and direction (arrows) at the 250 mb air pressure level of the atmosphere. The jet stream exists at or near this level. The large northward arc across Alaska is circled for emphasis. Chart source: tropicaltidbits.com.
In the coming days, the surface high beside Alaska will move directly overhead. Within it, there’s a lot of sunny weather to be had. Meanwhile, the northward portion of the jet stream to the southwest of Alaska has transported a lot of warm air north from the subtropics. This combination is an ideal recipe for high temperatures to affect much of the state, baking Alaska.
Running hot and cold…
With the jet stream so weak and meandering, it’ll take a great many days for the surface high to be ‘pushed’ away or broken down.
So it looks like Alaska will spend the near future feeling more like the Mid-Atlantic states.
‘Let’s keep an eye on where these hot and cold anomalies lead…’
On the flip-side, the southward portion of the big jet stream arc will drive Arctic air equatorward to produce some pleasantly cool (for the time of year) weather across much of the western half of the US.
For every up, there’s a down – and there’s evidence evolutions in climate are leading to bigger ups and downs, happening more often than was typical of, for example, the 20th Century. Let’s keep an eye on where these hot and cold anomalies lead…
James Peacock MSc
Head Meteorologist at MetSwift