Interannual Variations of Submesoscale Circulations in the Subtropical Northeastern Pacific

Impacts of oceanic submesoscale circulations on oceanic circulation as well as marine ecosystem and air‐sea heat flux have been reported by recent studies. Therefore, their regionality and variations on various timescales are important to understand their impacts. The submesoscale circulations are active in winter, which is mostly enhanced by mixed layer instability (MLI). However, their variations on timescales longer than interannual are not well understood. This study shows the interannual variations of active submesoscale circulations in winter and their mechanism in the subtropical northeastern Pacific using the output of a submesoscale permitting simulation. The submesoscale kinetic energy varies synchronously with the available potential energy release in a process of MLI. El Niño events appear to modulate the interannual variations. In addition, the similarities and differences between the variations in the eastern and western subtropics are highlighted. Plain Language Summary Oceanic submesoscale circulations at scales from one to several tens of kilometers are ubiquitous in the world ocean. Recent studies have reported their significant roles in not only ocean circulation but also marine ecosystem and air‐sea coupling via enhanced vertical motions. Therefore, revealing their variations and regionality are crucial to further understanding their roles. Previous studies reported that submesoscale circulations are enhanced within the surface mixed layer in winter by baroclinic instability that extracts energy from sloping isopycnals. However, submesoscale variations on timescales longer than interannual have not been well understood. This study explores the interannual variations in the subtropical northeastern Pacific based on a submesoscale permitting hindcast simulation. The winter submesoscale circulations are more active in the El Niño years than other years. The enhanced submesoscale circulations, as well as their seasonal enhancement in winter, are caused by strengthened baroclinic instability. Key Points Interannual variations of winter submesoscale circulations in the subtropical northeastern Pacific are examined using a hindcast simulation The winter submesoscale kinetic energy in the eastern subtropics was high in the years when El Niño events occurred The similarities and differences of their variations between the eastern and western subtropics are highlighted