Thermohaline Staircases in the Amundsen Basin: possible disruption by shear and mixing.

As part of the 2013 and 2014 Field Seasons of the North Pole Environmental Observatory in the Amundsen Basin of the Arctic Ocean, two temperature microstructure experiments were performed. A Rockland Scientific Microrider with FP07 fast response thermistors was attached to a Conductivity-Temperature-Depth unit (SBE 19+ in 2013, SBE 911 in 2014). From a heated hut, the instrument package was lowered through a 30” hole in the sea ice at a speed of 25 cm s^-1 with a motorized winch. The 2013 data set was characterized by an extensive thermohaline staircase, indicative of the diffusive convective type of double diffusion (DDC), from 150-250 m depths. The staircase was absent in the upper part of that depth range in 2014, even though an analysis of density ratio, R_ρ, still shows high susceptibility to DDC. In the depth range of interest, survey averaged R_ρ = 3 in 2013 and R_ρ = 3.3 in 2014, indicating that hydrographic differences might not be the cause of staircase disappearance. We propose that increased upper ocean shear and turbulent mixing, possibly associated with strong wind events, caused disruption of the staircase in 2014. Average thermal diffusivity, K_T, between 70 – 120 m is elevated in 2014, 7 x 10^-6 m²s^-1, compared to 2013, 2 x 10^-6 m²s^-1. Also, finescale shear variance between 70-200 m calculated from eXpendable Current Profilers in 2014 is nearly 3 times as high when compared to 2013. Despite increased turbulence over the staircase region in 2014, heat fluxes are remarkably consistent between the surveys. Even though they have been previously reported and the basin has favorable stratification, DDC staircases are less prevalent in the Amundsen Basin when compared to the Canada Basin. Possible reasons behind the lack of an omnipresent thermohaline staircase in the Amundsen Basin will be discussed.