Rising Heights

A couple of quick notes today.  First, I highly recommend Rick Thoman's latest blog post over at Substack.  In classic Rick fashion, he creates an index to quantify the combined frequency of uncomfortable heat and smoke conditions in Fairbanks; the results are worth seeing.

https://alaskaclimate.substack.com/p/interior-alaska-summer-discomfort

On another note, a few weeks ago I speculated that the very high 500mb height of nearly 6000 meters just south of the Aleutians might be an all-time record.  (The 500mb height is a measure of pressure in the mid-atmosphere; high heights correspond to a ridge with warm, subsiding air aloft.)  It turns out that it wasn't quite a record, but it was very close.

Here's a map of the record maximum 500mb height since 1940, based on the ERA5 reanalysis (click to enlarge):

Interestingly, the global record occurred over the North Pacific to the south of Alaska in July 2006.  The mid-latitude North Pacific and North Atlantic are the global hot spots for very high mid-atmosphere heights - higher than in the tropics, because the pressure is higher to begin with.

The red "blob" of record high heights over Bristol Bay reflects a record-breaking ridge in August 2019:

The days leading up to this event brought remarkable warmth to southern Alaska; Kodiak tied its all-time record high temperature of 86°F, and Anchorage tied its warmest overnight low temperature (63°F).

Summer 2019 was the last time the PDO phase was strongly positive, with very warm SSTs surrounding Alaska; the PDO has been more or less negative ever since.

Here's a chart showing the global area fraction with 500mb height exceeding 6000 meters each year:

The rapidly rising trend reflects the recent surge in global mean temperatures, because 500mb heights rise as the lower atmosphere becomes warmer and therefore less dense (and the surface pressure doesn't change).