Rainy scenes like the one above were prevalent during the summer of 2017. Overlooking the Juneau Harbor in August 2017.
Scenes like the one above became familiar sights across Juneau, Alaska this past summer with local headlines reading “June Finishing Half an Inch above Average” and “July’s Weather was a Busted Summer”. August 2017 finished as the 12th wettest on record and rained for 18 consecutive days. Meanwhile, Seattle went 55 consecutive days without rainfall, beating a previous record set in 1951. While it is not unusual for the Pacific Northwest to be “drier” during the summer, August 2017 was the 2nd warmest on record with 17 days reaching 80 degrees or above.
Volcanoes are thought to contribute to long-term global warming and short-term global cooling. Bogoslof Volcano, a small stratovolcano nestled in the Aleutian Islands, had been in the midst of an active eruption pattern since December 2016, erupting on average once or twice a week. Output from the Community Earth System Model (CESM), a coupled ocean-atmosphere model, shows some cooling due to volcanic aerosols in some regions (specifically the north-central United States, tropical oceans, and over China). Precipitation is likely to be reduced, as cooler sea surface temperatures decrease evaporation into the atmosphere and, therefore, decrease global precipitation. Juneau experienced below normal temperatures and above normal precipitation for June and July, while August was warm and wet. Ketchikan experienced a similar pattern, but this is not a pattern expected of high-latitude volcanic activity.
Beginning in spring 2013, toxic algae blooms spread, fin whales appeared for the first time near Kodiak Island, and massive numbers of sea otters were dying along the shore. A patch of warm water had developed in the Gulf of Alaska, infamously named “the blob”. Just as blowing on hot coffee cools the surface, westerlies along North America’s West Coast had weakened enough to stop churning the sea surface and cool it. A stubborn high-pressure system known as the “Ridiculously Resilient Ridge” (RRR) was keeping storms at bay along the Washington/Oregon coast and diverting precipitation towards Alaska. But, does the “blob” cause the RRR or does the RRR cause the blob? No one really knows for sure, but it is likely that this type of pattern is attributed to normal fluctuations in the Pacific Decadal Oscillation (PDO). Generally, this type of pattern is not abnormal for Alaska during the summer, however, the persistence is certainly anomalous.
Will this summer be similar? Winter 2017-2018 was considered a borderline, moderate La Niña event, after following ENSO neutral conditions from January 2017 onward. PDO remained strongly positive (warm phase) through June 2017, then declining towards more neutral conditions. In general, cold ENSO-neutral and La Niña events typically trend towards warmer and drier conditions, although week-to-week patterns will be highly variable.
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©2018 Meteorologist Sharon Sullivan