Freshwater Input and Vertical Mixing in the Canada Basin’s Seasonal Halocline: 1975 versus 2006–12

Abstract

The Arctic seasonal halocline impacts the exchange of heat, energy, and nutrients between the surface and the deeper ocean, and it is changing in response to Arctic sea ice melt over the past several decades. Here, we assess seasonal halocline formation in 1975 and 2006-2012 by comparing daily, May to September, salinity profiles collected in the Canada Basin under sea ice. We evaluate differences between the two time periods using a one-dimensional (1D) bulk model to quantify differences in freshwater input and vertical mixing. The 1D metrics indicate that two separate factors contribute similarly to stronger stratification in 2006-2012 relative to 1975: (1) larger surface freshwater input and (2) less vertical mixing of that freshwater. The larger freshwater input is mainly important in August-September, consistent with a longer melt season in recent years. The reduced vertical mixing is mainly important from June until mid-August, when similar levels of freshwater input in 1975 and 2006-2012 are mixed over a different depth range, resulting in different stratification. These results imply that decadal changes to ice-ocean dynamics, in addition to freshwater input, significantly contribute to the stronger seasonal stratification in 2006-2012 relative to 1975. These findings highlight the need for near-surface process studies to elucidate the impact of lateral processes and ice-ocean momentum exchange on vertical mixing. Moreover, the results may provide insight for improving the representation of decadal changes to Arctic upper-ocean stratification in climate models that do not capture decadal changes to vertical mixing.

Surface Salinity Under Transitioning Ice Cover in the Canada Basin: Climate Model Biases Linked to Vertical Distribution of Fresh Water

“…The data in this study are identical to those employed by Rosenblum et al. (2021), who showed that June‐September surface changes between the ITP and AIDJEX datasets are consistent with 30‐year mixed‐layer trends reported by Peralta‐Ferriz and Woodgate (2015) using data mainly associated with low sea ice concentration in the same region. They used only quality‐controlled data (level 3) in the ITP archive, screened profiles to select those that include samples shallower than 10 m depth (as in Jackson et al., 2010), and that were collected during the period May 1 ‐ December 31, which is common to both datasets.…”