The Mixed Layer Salinity Balance in the Western Arctic Ocean

In this study, we explore the mixed layer salinity (MLS) balance in the western Arctic Ocean based on the Arctic Subpolar gyre sTate Estimate (ASTE) results. The key components of the MLS budgets and their variabilities in response to the Beaufort Gyre (BG) spin‐up are identified. Seasonally, the surface forcing (brine rejection plus freshwater input) is the most important dominant contributor to the MLS balance. On the other hand, the entrainment dominates the interannual variability of MLS tendency inside the BG, while the advection dominates that in the Beaufort Sea. The sensitivity test of increased river discharge revealed a greater role of the advection term, along with weakened contributions from the surface forcing and entrainment, in determining the interannual variability of MLS balance. In contrast, the seasonal variabilities remained largely unchanged. The Lagrangian particle tracking reveals that the majority of BG freshwater within the mixed layer exits through the Canadian Archipelago prior to the BG spin‐up (2002–2006) and during its relaxation (2012–2017). We found a reduction in mixed layer freshwater sources from the external BG that could feed the gyre during its spin‐up (2007–2011), with the major contributions coming from the Beaufort Sea and the BG region itself through Ekman convergence. The mixed layer freshwater pathways are similar in the two versions of ASTE, but with noticeable proportion changes with the increasing river discharge. Plain Language Summary The mixed layer salinity (MLS) is crucial for both ocean dynamics and biogeochemistry, as well as ecosystems. However, the basin‐scale variability of MLS budgets in the western Arctic Ocean is difficult to obtain from observation. Here, using a dynamic consistent model that is validated against multiple observations, we revealed how the MLS balance and the key factors in regulating its spatial and temporal variability. Different MLS balance is identified inside versus outside the large‐scale Beaufort Gyre (BG) circulation. Oceanic advection plays a greater role in determining the MLS balance with an increased river discharge. We tracked the variability of mixed layer freshwater pathways in response to different BG phases, and found a reduction in freshwater sources from the external BG that could feed the gyre during its strongest phase. The interannual proportion of different freshwater sources within the mixed layer that flows into the BG is determined, as well as their sens