Weaker Seasonal Variation in Potential Energy Anomaly in the Upper Beaufort Gyre Favors the Upward Release of Subsurface Heat

Abstract The stability of the upper ocean is crucial for the exchange of momentum, heat, and salt between sea ice and subsurface warm water in the Arctic Ocean’s Beaufort Gyre (BG) region. Here, based on multiple in situ observations, the shifting phases of the BG during 2003–23 are objectively defined. We find that the potential energy anomaly (PEA) in the upper 55 m decreased from 130.9 ± 2.3 J m −3 during 2006–12 with BG intensification to 90.3 ± 2.0 J m −3 during 2013–19 with BG relaxation. Further, the mixed layer became saltier and deeper across all seasons. Decreasing PEA indicates an overall weaker stratification in the upper water column which promotes stronger vertical entrainment. We also find that the mixed layer heat content increased across nearly all seasons, except during July–September (summer). Our analysis using a Price–Weller–Pinkel model suggests that the cause of this warming was not atmospheric heat fluxes from above, but rather subsurface heat entrainment upward. The key mechanism is that the seasonal amplitude of PEA is smaller during 2013–19 when the BG relaxes, thereby allowing mixing to greater depths under the same surface salt flux as in 2006–12. This is important for the future evolution of the sea ice melting and oceanic vertical mixing if the BG relaxes further. Significance Statement Our study demonstrates that the relaxation of the Beaufort Gyre during 2013–19 resulted in a saltier and deeper mixed layer across all seasons, accompanied by a reduction in potential energy anomaly (i.e., weakening stratification) and also its seasonal variation in the upper 55 m. This enhances subsurface heat entrainment from both the near-surface temperature maximum and the Pacific Summer Water layers during fall and winter, which will likely impact future sea ice melting.