The "Ridiculously Resilient Ridge" leads to warmer, stagnant surface ocean water
The Pacific warm blob likely formed in response to the "ridiculously resilient ridge" (RRR), a phenomenon better known for its role in the California drought. The RRR is a static high pressure region in the atmosphere that—in addition to bringing record-breaking warmth and dryness to California—slows down ocean heat loss to the atmosphere and surface ocean water circulation, causing the surface ocean to become stagnant. The lack of air movement also impacts the wind-forced currents and the wind-generated stirring of surface waters.
California drought study identifies upward trend in anomalous northeastern Pacific ridging
An April 2016 study investigating changes during California's October to May "rainy season” identifies a significant increase in the occurrence of atmospheric patterns associated with certain precipitation and temperature extremes over the 67-year period. The study identifies in particular that both thermal expansion and sea level pressure trends contribute to a notable increase in anomalous northeastern Pacific ridging patterns similar to that observed during the 2012–2015 California drought.