In the West Antarctic Peninsula (WAP), complex interactions between the cryosphere, ocean and atmosphere produce an environment with large geographical, seasonal and interannual variability which is highly vulnerable to climate change. The seasonal sea ice cycle and its interactions with upper-ocean mixing play an important role in structuring this environment. Here we show that the relationship between sea ice and mixed layer depth (MLD) varies regionally between the WAP shelf and off-shelf regions. Using an MITgcm regional model of the WAP and Bellingshausen Sea for 1989-2018, we find that on the WAP shelf, high winter sea ice coverage is related to shallow spring mixed layers, whereas in a region offshore of the shelf, high winter sea ice coverage is related to deep spring mixed layers. The exact boundary between positive and negative correlations between winter sea ice concentration (SIC) and spring MLD varies decadally. Our results can be explained by a nonlinear relationship between SIC and momentum flux into the ocean, with a minor additional role for the timing of seasonal processes. Transport of sea ice across the model domain dampens this mechanism except in regions of very large sea ice export such as polynyas. With sea ice conditions projected to undergo large changes over the course of the century, understanding the relationship between sea ice and upper-ocean mixing in this unique and vulnerable location is crucial for understanding the wider impacts of climate change on biological productivity in the polar oceans.

The Rockall Trough is one of the main conduits for warm Atlantic Water to the Nordic Seas. Ocean heat anomalies, originating from the eastern subpolar gyre, are known to influence Arctic sea ice extent, marine ecosystems, and continental climate. Knowledge of the transport through this basin has previously been limited to estimates from hydrographic sections which cannot characterise the intra‐annual and multi‐annual variability. As part of the Overturning in the Subpolar North Atlantic Programme (OSNAP), a mooring array was deployed in the Rockall Trough in order to obtain the first continuous measurements of transport. Here, we define the methodology and the errors associated with estimating these transports. Results show a 4‐year mean northward transport of 6.6 Sv (1 Sv = 106 m3/s) by the North Atlantic Current (NAC) in the east and interior of the Rockall Trough (2014‐2018). A mean transport of ‐2.0 Sv (southward) is observed in the west of the basin, which could be part of a recirculation around the Rockall Plateau. The 90‐day low‐pass filtered transport shows large sub‐annual and inter‐annual variability (‐1.6 Sv to 9.1 Sv), mostly resulting from changes in the mid‐basin geostrophic transport. Satellite altimetry reveals the periods of low and high transport are associated with significant changes in the Rockall Trough circulation. There is a detectable seasonal signal, with the greatest transport in spring and autumn.