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Thermohaline staircases in the A mundsen B asin: Possible disruption by shear and mixing
Author(s) -
Guthrie John D.,
Fer Ilker,
Morison James H.
Publication year - 2017
Publication title -
journal of geophysical research: oceans
Language(s) - English
Resource type - Journals
eISSN - 2169-9291
pISSN - 2169-9275
DOI - 10.1002/2017jc012993
Subject(s) - thermohaline circulation , pycnocline , temperature salinity diagrams , stratification (seeds) , turbulence , thermal diffusivity , mixing (physics) , the arctic , convective mixing , geology , convection , salinity , atmospheric sciences , physics , oceanography , mechanics , thermodynamics , biology , seed dormancy , botany , germination , dormancy , quantum mechanics
As part of the 2013 and 2014 North Pole Environmental Observatories (NPEO) in the Amundsen Basin of the Arctic Ocean, two similar temperature microstructure experiments were performed with different results. In 2013, vertical fluxes were through a thermohaline staircase, and in 2014, the thermohaline staircase was largely absent. Here we investigate the reasons for this difference. The 2013 data set was characterized by an extensive thermohaline staircase, indicative of the diffusive convective type of double diffusion (DC), from 120 to 250 m depths. The staircase was absent above 200 m in 2014, even though analysis of density ratio, R ρ , still shows high susceptibility to DDC. In the depth range of interest, survey‐averaged R ρ = 3.8 in 2013 and R ρ = 3.6 in 2014, indicating that the temperature‐salinity structure in the pycnocline was not the cause of the lack of a staircase in 2014. We propose that exceptionally weak turbulent mixing, even for the typically quiescent Arctic Ocean, allowed formation of the staircase in 2013. Average thermal diffusivity, K T , between 50 and 120 m is elevated in 2014, 2 × 10 −5 m 2 s −1 , compared to 2013, 1 × 10 −6 m 2 s −1 . However, vertical Atlantic Water (AW) DC heat fluxes in 2013 are remarkably consistent with turbulent heat fluxes in 2014. Similar data sets collected in 2007 and 2008 both resemble 2014, showing consistently higher mixing values compared to 2013. The suppression of turbulence during NPEO 2013 resulted from increased near‐surface stratification, possibly caused by a different large‐scale circulation pattern that year.