“We’ve never seen anything like this before,” said Dustin Guy, a meteorologist with NWS’s office in Seattle. “We’ve only had three days of 100 or more degrees in 126 years, and it looks like we’re ready to get three of them in a row now. There’s really nothing to compare it to. We’ve never in anybody’s lifetimes seen anything quite like this before in Seattle.”
The sweltering weather, expected to cool slightly tomorrow, appears to be part of a broader climate change trend. The Pacific Northwest’s average temperature has warmed more than 2 F compared with a century ago, with most of that change in the last 40 years. In addition, the number of extreme heat days has doubled in less than a century, and it likely won’t stop at that, said Larry O’Neill, Oregon’s state climatologist.
In 1940, he said, Portland had only about 10 days per year when the daily high temperature topped 90 F.
“By 2020, that number is at about 20 days per year,” O’Neill said. In Multnomah County, where Portland sits, “the projected change, by the middle of the century … is an additional 20 days (per year) with temperatures above 90 degrees,” he said.
One of the mechanisms for the formation of a high-pressure system is tropical cyclone activity in the western Pacific Ocean, he said. Those are the West Coast equivalent of hurricanes. And like hurricanes, they are strengthened by warmer ocean temperatures.
High-pressure systems like the one driving the Pacific Northwest heat wave is “something like three times more likely to occur when we have a tropical cyclone out in the Pacific,” he said. “So climate change is impacting tropical cyclone activity through modulation of sea surface temperatures, and also things like wind shear.”
Climate change is making the heat wave more severe.
Baseline temperatures were already higher than in the past, due to Earth’s changing climate. Globally, Earth’s average temperatures are increasing, with 2016 and 2020 tied for the hottest years on record (SN: 1/14/21).
Those changes are reflected in what’s now officially considered “normal.” In May, for example, the U.S. National Oceanographic and Atmospheric Administration reported that the country’s new baseline reference temperature, or “climate normal,” will be the period from 1991 to 2020 — also now the hottest 30-year period on record for the country (SN: 5/26/21).
That changing reference makes it tough to place such an unprecedented heat wave in any kind of historical context. “We have a historical data record that’s 100 years long,” O’Neill says. Saying that the heat wave is a once-in-a-millennium event means that “you would expect that, at random chance, this would occur once every 1,000 years. But we’ve never observed this. We have no basis to say this,” he adds. “This is a climate that we’re not accustomed to.”
Kristina Dahl, a senior climate scientist at the Union of Concerned Scientists, says the heat wave is “unprecedented.”
“We saw heat records over the weekend only to be broken again the next day,” Dahl told CNN, “particularly for a part of the country where this type of heat does not happen very often.”
Michael E. Mann, a climate scientist at Pennsylvania State University, put it very simply: climate change is making heat waves more frequent and more intense. “You warm up the planet, you’re going to see an increased incidence of heat extremes,” Mann told CNN.
Experts like Dahl and Mann say climate change is reshaping the planet’s weather patterns. As humans emit more planet-warming greenhouse gas into the atmosphere, more energy is added to the climate system. The excess energy, according to Kristie Ebi, a climate and health researcher at the University of Washington, appears by way of extreme weather events.
“Heat waves have always occurred and will always occur, but we’ve got a very different pattern of heat waves now than we did a couple of decades ago,” Ebi told CNN. “And it’s not just the intensity, it’s also the geographic extent.”
Some experts said that as the Earth heats up, climate change could have a more dramatic impact on summer heat, posing risks to humans in different ways.ADVERTISEMENT
“All the indicators are very clearly that we are entering continually hotter, dryer, riskier summers,” Sarah Myhre, a climate scientist and executive director of the Rowan Institute, told The Hill.
“From a climate change context, the idea that heat extremes are going to become more extreme and more intense” has been known to scientists for decades, she added.
Jane Baldwin, a postdoctoral research scientist at Columbia University’s Lamont-Doherty Earth Observatory, added that climate change will affect not only temperatures across the board but also the nature of atypically hot periods.
“We have very clear evidence that as greenhouse gases in the atmosphere increase, the overall temperature warms, but also because of that, heat waves increase in intensity and duration,” Baldwin said.
“Until the event has passed, it’s hard to do the attribution of the event in terms of how much of this event can be attributed to climate change, but I do think it is a harbinger of what is projected to come with global warming,” she added.
A number of factors came together simultaneously to produce the extreme high temperatures over the Northwest observed last week.
The key factor in this and previous regional heatwaves is the development of an unusually strong and persistent area of high pressure over the Northwest. Such high-pressure areas are also called ridges and often extend vertically through great depth (10-30 thousand feet).
The figure below illustrates what the ridge looked like at 11 AM last Sunday around 18,000 feet above the surface (500 hPa pressure). At this level, the ridge was the most intense ever observed over the region (will prove that later).
High-pressure areas/ridges are associated with warm temperatures during the summer. First, high-pressure areas possess strong sinking, and sinking causes powerful warming as air is compressed as it descends to the higher pressure that exists at low levels (pressure decreases with height). Your air pump, very warm after inflating a tire, is a good illustration of this mechanism.
Ridges also possess southerly (from the south) flow on their western sides (apparent above), bringing subtropical warmth northwards.
But there is more!
The sinking air in high-pressure areas prevents clouds, thus allowing maximum solar heating (and the sun is near maximum strength now). And on the southern side of upper-level ridges there is often easterly (from the east) wind, which can move down the western slopes of terrain barriers, producing even MORE compressional heating as the air descends the slopes.
It is no accident that every major summer heatwave in our region is associated with a ridge of high pressure.
Ridges are veritable heating machines during summer and sometimes are colloquially referred to as heat domes. The media loves this term.
The origin of the intense ridge of high pressure of last week is fascinating.
Our ridge appears to have originated in the far western Pacific, where a tropical disturbance rammed into the Pacific jet stream, causing high-amplitude waviness in the jet stream thousands of miles downstream to the east. The result was a strong ridge over the Northwest, with the waviness also producing a deep trough over the central Pacific (see upper-level map on Wednesday, June 23rd, 500 hPa pressure–about 18,000 ft).
Blue and purple indicate much lower than typical pressures (troughs or lows), red indicates above-average values (ridges or highs). You can see the waviness of the atmosphere over the north Pacific from Asia to North America.
The Atmospheric Heat Supercharger
In our heatwave, there was a feature that supercharged the warming west of the Cascade crest and the coastal mountains (e.g., the Olympics): an approaching upper-atmospheric low-pressure area (or trough) that was west of northern California in Map A above.
Chromed supercharger air intake, carburetor and manifold on a hot rod 123rf.com
Our high-pressure ridge (“heat dome”) had a potent supercharger. Picture courtesy of Nick Ares. Between the offshore trough and the ridge over the Pacific Northwest, there were strong southeasterly (from the southeast) winds that pulled up air from the warm desert Southwest. This air subsequently descended the western slopes of the Cascades, where the air was further compressed and warmed.
You can see this “supercharger” in action on Monday afternoon in a forecast map valid for around 5000 ft (850 hPa pressure level). The colors indicate temperatures (darker red is warmer) and winds are also shown. You can think of the solid lines as representing the pressure at that level. Note the low offshore and the high to the northeast. The white arrow shows the warm southeasterly flow that descended the Cascade’s western slopes.
And the impact of the supercharger is seen in the super-warm air (brown colors, 104F, and more) found downstream of the Cascades at 5 PM on Monday, June 28.
Everything had to come together just “right” to give us this extreme event.
Record amplitude of a ridge/high pressure over our region, forced by a tropical disturbance in the western Pacific, that produced a downstream “wave train”. An environment that allowed the resulting wave to amplify. The ridge had to be in exactly the right position relative to our terrain. An upper-level trough had to develop in just the right location offshore and move in the optimal direction to cause strong southeasterly flow, fostering the supercharger noted above. We needed a period when the sun was very strong. And a summer stretch without smoke, which has a profound cooling effect.
The meteorological dice had to come up all sixes. And they did.
This concurrence of a number of factors coming together at one place and time was why the extreme heat occurred, with a very small assist from global warming, which added a few degrees to an already extreme event. It is important to note that the atmosphere comes up sixes regularly, but not necessarily in the same place. The atmosphere is churning with all kinds of variability inherent in the physics of the atmosphere (also called natural variability).
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