Shear dispersion and delayed propagation of temperature anomalies along the Norwegian Atlantic Slope Current

Abstract

Using satellite altimetric sea surface height (ADT) data, we search for propagation of hydrographic anomalies along the Norwegian Atlantic Slope Current (NwASC) from the Svinøy section in the south to the Fram Strait in the north. Our analyses indicate that ADT anomalies, related to low-frequency temperature variations, propagate downstream with speeds of about 2 cm s${}^{-1}$. Notably, this speed is nearly an order of magnitude slower than the speed of the NwASC, which in agreement with previously estimated propagation speeds of hydrographic anomalies along the flow. A conceptual tracer advection model, consisting of a thin current core interacting with an adjacent slow moving reservoir, is introduced to examine temperature anomaly propagation along the NwASC. It is shown that shear dispersion effects, resulting from cross-stream eddy mixing and velocity shear, can qualitatively explain the observed delayed propagation of hydrographic anomalies: low-frequency temperature anomalies move downstream with an effective velocity that corresponds to a mean velocity across the entire Atlantic Water layer, rather than the speed of Norwegian Atlantic Slope Current.

Keywords:

• Nordic Seas
• propagation ocean heat anomalies towards the Arctic Ocean
• satellite altimetry
• shear dispersion and delayed propagation

Acknowledgements

This work was supported by a grant from the Swedish National Space Board. We thank Léon Chafik and Liam Brannigan for valuable comments and interesting discussions.

Notes

No potential conflict of interest was reported by the authors.

1 A few studies have suggested asymmetries in propagation speeds of temperature and salinity anomalies (Yashayaev and Seidov, 2017) and of warm and cold temperature anomalies (Sundby and Drinkwater, 2007). The inferred speeds are still ’slow’ but the robustness of these results is not fully clear.

2 Such ‘passive’ anomalies can have a steric-height signature at the sea surface and no bottom pressure anomaly; see LaCasce (2018).