Highlight: California’s unique climate pattern.
Fig. 1: Quarterly temperature/precipitation rankings given the 1895-2021 climatology for the state of California for 2016-2021 (so far).
Discussion: California is engulfed in a record hot summer. Dryness has been dominant in 2021 acting as a catalyst to the record heat. Is there anything about the 5-year trend that helps to explain the record heat of summer 2021? Note that the quarterly temperature ranking for California during the past 5 years has generally been in the top 20% warmest years in the 1985-2021 climatology (Fig. 1). Early 2019 was cool and wet and in general 2019 was temperate. Otherwise, anomalous warmth has dominated. The quarterly precipitation pattern across California during the past 5 years has varied widely. There have been 6 dry spikes and 4 wet spikes. Not surprising is the wet climate accompanying the cool Q1 of 2019 climate. However, the 2nd wettest Q1 on record, which occurred in 2017 was accompanied by anomalous warmth which is exceptionally unusual. The wet Q2 of 2020 was also surprisingly warm.
Fig. 2: The upper air pattern as defined by 500 MB height anomalies for 2016-2020. California has been exposed to anomalous warm high-pressure.
Fig. 3: Born in late 2013/early 2014 is a large mass of surface and subsurface anomalous warm water in the northeast Pacific Ocean which in-turn has warmed the atmosphere above causing persistent high-pressure ridging to develop.
The upper air pattern during 2016-2020 has featured an unusually strong high-pressure system over the Gulf of Alaska and vicinity (Fig. 2). Dry and warm climate is usually associated with these systems. Residual high-pressure has extended to the east across the Southern U.S. including California helping to explain the persistent anomalous warm climate. The upper-level high-pressure ridge is associated with a large area of anomalous warm ocean water in the northeast Pacific Ocean which developed in late 2013/early 2014 and has persisted into 2021 (Fig. 3).
The upper ridge is preventing winter-time heavy precipitation from occurring when California gains most of their water. Q1 of 2017 and 2019 were wet otherwise precipitation during the wet season has not occurred. Consequently, California is in a long-term drought capable of intensifying if hot weather associated with the semi-permanent upper ridge pattern described comes along.
Conclusion: From a climate perspective, the accelerant of anomalous heat which is currently at record levels for California for summer 2021 so far is a semi-permanent upper-level high-pressure ridge present much of the time since 2013 and associated with reginal warming of the northeast Pacific Ocean. The semi-permanent ridge has allowed only 2 quarters since 2015 to feature the excessive precipitation needed to prevent drought. Consequently, soil moisture is depleted. Below normal soil moisture over a large region is the catalyst (and predictor) of anomalous warmth. A very dry scenario, such as the 2021 California drought is the trigger for the super heat this state has observed since summer began.