Processes That Influence Bottom Temperatures in the California Current System

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 degrees (similar to 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. Plain Language Summary Temperature strongly influences marine organisms through their metabolism, growth and behavior, including species that live on or near the bottom, such as shellfish, crabs, and flounder. Although there have been many studies documenting sea surface temperature variability, much less is known about bottom water temperatures due to a lack of observations. Here, we use a recently developed reanalysis, which combines a wide array of observations with a computer model to obtain a fine-scale view of the coastal ocean. We examine processes that influence bottom water temperature along the West Coast of the contiguous United States and Baja California, where the ocean depth is less than 400 m. The temperature near the bottom and at the surface often vary together, especially during winter where the ocean is shallow when storms vertically mix the upper ocean. Deeper in the ocean, vertical movement of the thermocline, where the temperature decreases rapidly with depth, can also generate large bottom water temperature anomalies. Bottom water temperatures are more strongly affected by winds off the Pacific Northwest US coast and ocean processes initiated in the tropics further south. ENSO events have a strong influence on bottom water temperatures in addition to those at the surface.