Processes That Influence Bottom Temperatures in the California Current System

Abstract Bottom water temperature (BWT) strongly influences marine organisms in coastal waters. Although subsurface observations are limited, the recent development of high‐resolution reanalyses enables a detailed three‐dimensional view of the ocean, including on the continental shelf. Here, we use the GLORYS 1/12° (∼9 km) reanalysis during 1993–2019 to examine processes that influence BWT anomalies on the shelf (≤400 m bottom depth) off the West Coast of Baja California and the contiguous US. We examine the relationship between BWT anomalies and other ocean variables, including mixed layer depth (MLD), thermocline depth (TD), thermocline and bottom temperature gradients, and bottom currents. The strongest and most spatially coherent connections with BWT anomalies occur for MLD during winter and TD in summer. BWT anomalies are also correlated with the local sea surface height (SSH) anomalies with maxima at 2–5 days lag. On subseasonal timescales, the lag of the maximum coherence of SSH at the southern tip of Baja California with both SSH and BWT increases northwards, consistent with propagating coastally trapped waves (CTWs), although the BWT anomalies decrease with latitude. Like SST and SSH, BWT anomalies are coherent along the entire West Coast on interannual timescales, reflecting ENSO's influence on the northeast Pacific. In contrast to CTWs, wind‐driven upwelling's impact on BWT anomalies increases with latitude. Regional ocean model experiments confirm the GLORYS analysis and highlight the importance of remote wind‐driven effects, in addition to local winds, on BWT in the northern part of the domain.