• Canada Basin;
  • Pacific Summer Water;
  • climate change;
  • near-surface temperature maximum;
  • remnant winter mixed layer;
  • water masses

[1] Increased sea ice melt and decreased surface albedo have changed the near-surface water mass structure of the Canada Basin. From 1993–2009, the near-surface temperature maximum (NSTM) and remnant of the previous winter's mixed layer (rWML) warmed by about 1.5°C and 0.5°C and freshened by about 4 and 2 practical salinity units, respectively. Results from a 1-D model suggest rWML warming can be explained by heat diffusion from both the NSTM and Pacific Summer Water (PSW). The same model predicts salinization of the rWML, whereas freshening was observed. This suggests that changes to the rWML are from both diffusion and the accumulation of freshwater. The rWML's salinity was associated with distance from the center of the Beaufort Gyre; the rWML at stations inside the gyre was on average 1.9 salinity units fresher than at stations outside. In addition, the salinity of PSW in the Canada Basin - defined by its local temperature maximum - freshened from about 30–32 in 1993 to 28–32 in 2008. Order of magnitude calculations suggest that neither changes in PSW source waters nor changes in advection pathways of PSW explain this freshening. Our model suggests that salt diffused from PSW to the freshening rWML; this diffusion increased (and freshened the PSW salinity range) as the rWML freshened. These results show that surface effects through warming and ice melt are felt to at least the depth of PSW. Observations from 2009 show the appearance of a third temperature maximum from an as yet unknown source.