Contextualizing marine heatwaves in the Southern California Bight under anthropogenic climate change

In early August 2018, record-high sea surface temperatures were recorded in the 102 year old Scripps Institution of Oceanography daily temperature time series (SIOT) at La Jolla, CA, USA. The previous record of 25.8 degrees C, set in July 1931, was broken four times over 9 days, peaking at 26.4 degrees C on 9 August 2018. Optimum Interpolation Sea Surface Temperature data suggest that the marine heatwave (MHW) manifested in the northern half of the Baja California Peninsula, tapering off into the Southern California Bight. Here we use the Optimum Interpolation Sea Surface Temperature data to categorize this MHW as severe with relatively high maximum intensity (3.9 degrees C) and long duration (44 days) when compared to other events in the time series. Glider profiles show that the thermal anomaly was largest near the surface and extended to at least 100 m depth. By detrending the SIOT to remove the long-term anthropogenic climate signal and comparing the resulting data set to the unadjusted, we contextualize this MHW with respect to the entire time series and demonstrate that the long-term trend played a crucial role in amplifying the intensity and duration of the MHW. In this case, the anthropogenic warming signal in the SIOT accounts for an additional 19 MHW days (from 24 to 43) and an increase in cumulative intensity of 56.8 degrees C (from 83.1 to 139.9). Plain Language Summary Marine heatwaves, like their counterparts in the atmosphere, are becoming more frequent and intense relative to historical norms owing to the long-term increase of ocean temperature. The century-long, continuous ocean surface temperature measurements at La Jolla, CA, USA, are used to put the August 2018 record-breaking temperature into context. The duration and intensity of the event are altered significantly by the removal of the long-term ocean warming trend, indicating that the trend itself is largely responsible for the increase in MHW conditions. Key Points The August 2018 Southern California Bight marine heatwave (MHW) contained the highest ocean temperature recorded in La Jolla, CA, USA, since daily manual measurements began in 1916 The event was a relatively long duration, high-intensity MHW that broke records mostly due to the anthropogenic warming trend and the season of occurrence while being driven by solar heating of a thin mixed layer By comparing detrended and unadjusted (nondetrended) time series of daily manual measurements since 1916, we isolate the anthropogenic warming signal present in the data, in this case accounting for an increase of 19 MHW days (from 24 to 43) and an increase in cumulative intensity of 56.8 degrees C (from 83.1 to 139.9)