Climate Change Drives Evolution of Thermohaline Staircases in the Arctic Ocean
Abstract
A thermohaline staircase detection algorithm, applied to mooring and ice-tethered profiler data, systematically assessed the variability of fine-scale, diffusive-convective staircase abundance in the Arctic Ocean thermoclines in 2004–2023. Over that period, staircase occurrence statistically decreased in both the Amerasian and Eurasian basins, with thinner, shallower staircase layers preferentially decreasing over the Eurasian Basin's slope. In stark contrast to the Amerasian Basin, seasonality of detected staircase occurrence was pronounced in the Eurasian Basin and appeared to be increasing. Interannual and long-term variability of detectable staircase abundance and background thermocline density stratification were correlated, negatively so in the Amerasian Basin and positively in the Eurasian Basin, indicating reversed sensitivities of staircase constructive and destructive processes to stratification. Seasonal and long-term staircase variabilities in both basins were consistent with known environmental contrasts and tendencies, including upper freshening of the stronger, thicker Amerasian Basin halocline, the shift toward deeper winter ventilation of the weaker Eurasian Basin halocline, and more near-surface velocity shear over the Eurasian Basin's slope. There is no reason to believe that climate change will stop anytime soon, and we have good cause to believe that the observed tendencies in staircase structure will persist.
Key Points
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The number of fine-scale diffusive-convective staircases have decreased in much of the upper Arctic Ocean over the 2004 to 2023 period
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Seasonality of detected staircase occurrence was pronounced in the Eurasian Basin and was likely increasing
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The relative sensitivity of staircase constructive and destructive processes to density stratification differed between major Arctic basins
Plain Language Summary
We found that staircase structures, expressed as steps in vertical temperature and salinity profiles, decreased in occurrence within the Arctic Ocean thermocline over 2004–2023. In the Eurasian Basin, this tendency accompanied increasing seasonal variation and a greater loss of shallower staircase steps over the slope. These developments accorded with the environmental changes occurring in these waters, which included surface freshening in the Amerasian Basin, weakening background density stratification and stronger winter vertical mixing in the Eurasian Basin, and increasing near-surface velocity shear over the latter's boundary. While detected staircase occurrence responded to background density stratification, this response was dependent on the relative sensitivities of staircase constructive and destructive processes to that stratification.
Conflict of Interest
The authors declare no conflicts of interest relevant to this study.
Open Research
Data Availability Statement
The data used in this study is available at https://uaf-iarc.org/nabos/data/ (NABOS project) and https://www2.whoi.edu/site/itp/data/ (ITP program). MATLAB R2023a was used to implement the method. Thermodynamic properties were calculated using the Gibbs Seawater (GSW) Oceanographic Toolbox, version 3.0 for MATLAB (McDougall & Barker, 2011). The M_Map package was used to create the map in Figure 1 (Pawlowicz, 2020). Detailed algorithmic information for the entire NDM, including pseudocode and its MATLAB implementation (Lundberg, 2024), is at https://doi.org/10.18739/A26M33558 (NDM_algorithm.pdf).