Scientific prophecy in CA: A new kind of heat wave

California stands at “ground zero” in the world of climate change, as the MacArthur grantee/genius Peter Gleick likes to say. This is not good news for forty million or so of us who live here, but for far-sighted scientists, it’s an opportunity. Gleick himself became known when as a graduate student decades ago he wrote a thesis arguing that climate change would threaten the Sierra snowpack, on which California depends for a steady supply of water in the summer.

But Gleick isn’t the only scientific prophet in California today: another is the Russian-born scientist Alexander “Sasha” Gershunov, a researcher at Scripps in La Jolla. In a couple of papers published about a decade ago, Gershunov argued that California was seeing a new kind of heat wave, a “Type II” heat wave. As he wrote in 2012 for the Geophysical Research Letters journal:

“California heat wave activity falls into two distinct types: (1) typically dry daytime heat waves and (2) humid nighttime-accentuated events (Type I and Type II, respectively). The four GCMs [Note: general circulation models of the atmosphere] considered project Type II heat waves to intensify more with climate change than the historically characteristic Type I events, although both types are projected to increase. This trend is already clearly observed and simulated to various degrees over all sub-regions of California.”

To translate a little: in the past, heat in California tended to be dry, even along the coast, and at night, temperatures typically waned, allowing residents to cool down safely. Today we see a new kind of heat wave, the so-called “Type II” heat wave, which more closely resembles the humid heat waves of the East, which don’t cool off quickly at night.

The charts are a little hard to read, but note that the Relative Humidity Index in the future will decline in the deserts, but rise quite sharply along both the southern and northern coasts of California, which historically have cooled off at night.

Now take a look at this LA Times story from a couple of weeks ago, on the new sort of heat wave hitting California.

“When a major heat wave hits Southern California, it begins with a jab — a ridge of high pressure builds over Nevada or Mexico and sweeps into the region, bringing scorching temperatures along with it.

Then comes the right hook: A mass of humid air created by unusually warm ocean water just off the northern coast of Baja California moves in from the southeast. Combined, they deliver a deadly blow, wreaking havoc on heavily populated regions such as Los Angeles County.

“We understand pretty well how and why they form,” said Glynn Hulley, a climate scientist at NASA’s Jet Propulsion Laboratory who has documented a shift toward hotter, more humid heat waves in urban areas of Southern California since 2000. “It’s almost like the heat waves have changed their personality, shifting to warmer and more humid nighttime events.'”

Here is a drawing of the Type II heating phenomenon:

from graphic reporter Paul Duginski of the LA Times

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