Large‐Scale Conditions for the Record‐Setting Southern California Marine Heatwave of August 2018.

In early August 2018, a record‐setting marine heatwave (MHW) occurred along the coast of Southern and Baja California. Water temperature at Scripps Pier rose to 26.4°C, the highest in 102 years of measurements. This paper investigates the large‐scale ocean‐atmospheric conditions for this Southern California MHW event. This intense and sustained event is the result of the superposition of "weather" timescale warming by a coastal wind relaxation and intraseasonal warming by coastally trapped waves. For weather‐scale warming, climatological upwelling was weakened by a wind relaxation, due to anomalous offshore low pressure associated with passing tropical cyclones John and Kristy. For intraseasonal warming, the upwelling was inhibited and the thermocline was deepened by poleward‐propagating coastally trapped waves, which provided a slow warming background. The mixed layer heat budget analysis indicates that the reduced upwelling and deepened thermocline are the major cause of the MHW. Plain Language Summary: The Southern California marine heatwave in August 2018, which set a record for warmest sea surface temperature ever recorded at San Diego, encompasses two timescales. "Slow" warming began due to an earlier coastal wind relaxation, which reduced the upwelling of cool water; and the propagation of coastally trapped waves. "Fast" warming occurred in August due to a wind relaxation associated with an offshore tropical cyclone. Key Points: A record‐setting marine heatwave (MHW) occurred along the coast of Southern and Baja California in early August 2018This MHW is the result of a coastal wind relaxation on the weather timescale and intraseasonal coastally trapped wavesThe ocean upwelling and vertical mixing are the primary cause of the MHW event [ABSTRACT FROM AUTHOR]