Cause Of Unusually Intense Pacific Storms of The Past Week

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While “atmospheric river” and “bomb cyclone” are new meteorological expressions which were used to describe the potent West Coast storms over-the-weekend there are some climate diagnostics completely unique which helped two 943-947 MB low-pressure systems to form…one just southwest of Alaska last Thursday night and another due west of Washington Sunday morning.

Fig. 1: The North Pacific basin daily SSTA analysis valid Oct. 25, 2021.

Discussion: While “atmospheric river” and “bomb cyclone” are new meteorological expressions which were used to describe the potent West Coast storms over-the-weekend there are some climate diagnostics completely unique which helped two 943-947 MB low-pressure systems to form…one just southwest of Alaska last Thursday night and another due west of Washington Sunday morning.

Since 2013 a semi-permanent region of shallow and deep-water anomalous warmth has occupied the northeast Pacific Ocean. The “warm blob” is well-correlated to anomalous high-pressure usually in the vicinity of the U.S. West Coast and responsible for long-term drought in California while sometimes reaching cross-polar (“ridge bridge”) engaging Siberian arctic air into North America. The “warm blob” is a significant contributor to North America climate of the past decade.

As 2021 comes to a close, the “warm blob” remains strong but has shifted toward the Dateline and located mostly northwest of Hawaii (Fig. 1). Meanwhile, the Gulf of Alaska has cooled sharply in recent months for the first time in nearly a decade. The SSTA across the new cool zone in the northeast Pacific is -0.46C and likely close to -1.00C in the Gulf of Alaska. Interestingly, the cool zone in the northeast Pacific has cooled by nearly 1C during the past 30 days.

The “warm blob” is very strong and within the Pacific decadal oscillation (PDO) monitoring area known as the “Mantua Box” the SSTA pattern is a robust warm +1.26C. The Mantua Box has warmed by +0.60C the past 30 days. The SSTA contrast between the Gulf of Alaska cool pool and the “warm blob” northwest of Hawaii is likely the strongest SSTA thermal gradient on record.

So…what does that mean to the weather pattern? Normally, cool pools of ocean cooling cause the atmosphere to (also) cool causing a low-pressure area to form. Within this low-pressure area storms are generated. Across the warm SSTA region the warmer atmosphere spawns a persistent high-pressure ridge. An extreme example of this pattern has occurred the past 10 days in the eastern Pacific Ocean (Fig. 2).

The thermal contrast of the ocean surface from the Gulf of Alaska to northwest of Hawaii is possibly the most extreme on record leading to super amplified trough and ridge areas in very close proximity. The thermal gradient in the atmosphere (and ocean) produced by these two pressure regimes caused the extreme low pressure of this past weekend storm intensity.

The pattern described is likely to last at least 1-3 months. Implied is more unusually intense storms into the West Coast of North America, a semi-permanent upper trough over new snow areas of Western Canada and a tendency for milder air farther downstream over southern and eastern continent.

Fig. 2: The 500 MB heights for Oct. 15-24, 2021 identifying a deep upper trough over cool SSTA in the Gulf of Alaska and an amplified upper ridge pattern stretched across the “warm blob” SSTA region northwest of Hawaii.