Heat Waves Coming

As I've pointed out in previous posts, here and here, we may be in for some hot days.  And it appears right now.

There is “a vast and growing body of research,” as Climate Central explained in February. “Humanity is about to experience a historically unprecedented spike in temperatures.” 

This may come about from a confluence of climate events.  The warming of the Arctic weakened the "Polar Vortex" bringing severe cold to the Northeast U.S. last winter.  But for the summer it is affecting the jet stream, a consequence of which is likely to be a hot summer(pay wall).

warming map
SCORCHING SUMMER  The waning of summer storms due to Arctic warming can exacerbate summertime heat waves across the Northern Hemisphere, such as the record-setting summer 2003 season chronicled above in Europe, new research suggests. Red regions experienced hotter July temperatures than those measured in 2001.

Sweltering summertime heat waves are on the rise across the Northern Hemisphere because of atmospheric changes brought on by Arctic warming, new research shows. After examining 35 years of weather data, researchers spotted a decline in the strength of summer storms that carry cool, moist air across the northern continents. The sagging of these storms is the result of wind pattern changes induced by the rapidly warming Arctic, the researchers report online March 13 in Science.

Another factor is the "PDO", the Pacific Decadal Oscillation,

Trenberth explained that it’s significant the Pacific Decadal Oscillation (PDO) “seems to have gone strongly positive” because that is “perhaps the best single indicator to me that a jump is imminent.” During a PDO, he explains, “the distribution of heat in the oceans changes along with some ocean currents.”
The PDO is a “pattern of Pacific climate variability similar to ENSO [El Niño Southern Oscillation] in character, but which varies over a much longer time scale.” While El Niños and La Niñas tend to last only 6 to 18 months, the PDO can remain primarily in one phase for a decade or even longer, as this figure from NOAA’s March “Global Ocean Monitoring” report shows:

Pacific Decadal Oscillation
“The positive phase of PDO [Pacific Decadal Oscillation] index has persisted 8 months since Jul 2014 with PDO index = + 1.6 in Feb 2015.” Via NOAA.
The hottest year so far was 1998, corresponding to the peak in the PDO, and as can be seen, we're heading from another peak in the PDO.

It is important to understand that these factors, the weakening of the jet stream from warming of the Arctic, and the PDO, are separate but influenced by global warming.  Global warming caused by the concentration of greenhouse gases in the atmosphere is a "secular" trend, i.e., an underlying trend that is moving consistently with that concentration.  The variations around that trend are due to La Nina's that bring the global temperature down, El Nino's that bring it up, and PDO's that at different times lower and raise it.  These processes are affected by global warming over larger scales of time.  El Nino's for example are getting stronger.  But over short periods of time, a decade, say, are independent of global warming.  Thus we may be getting a long, hot, summer this year, but to some extent it's the ancillary forces, not the secular warming trend due to greenhouse gas concentration in the atmosphere.

The thing is, though, these ancillary forces are themselves affected by global warming in the long term.  So this summer is taste of what's to come.

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