Turbulent Upper-Ocean Mixing Affected by Meltwater Layers during Arctic Summer

Randelhoff, Achim; Fer, Ilker; Sundfjord, Arild

doi:10.1175/jpo-d-16-0200.1

Cited by 38 publications

(55 citation statements)

References 46 publications

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“…During this event, significant turbulent mixing was observed in the ocean surface layer in response to the strong winds, but this mixing did not break through the seasonal pycnocline at ~20 m even during the strongest winds on 18–19 September (Figure b). This is consistent with previous studies by Randelhoff et al (, ), who reported a rapid attenuation in the turbulent mixing at the seasonal pycnocline. On the other hand, strong mixing was observed down to depths of 20–30 m below the seasonal pycnocline on 11–12 September, despite the wind being calm.…”

Section: Discussionsupporting

confidence: 94%

Do Strong Winds Impact Water Mass, Nutrient, and Phytoplankton Distributions in the Ice‐Free Canada Basin in the Fall?

et al. 2020

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In general, strong wind events can enhance ocean turbulent mixing, followed by episodic nutrient supply to the euphotic zone and phytoplankton blooms. However, it is unclear whether such responses to strong winds occur in the ice-free Canada Basin, where the seasonal pycnocline is strong and the nutricline is deep. In the present study, we monitored a fixed-point observation (FPO) station in the Canada Basin for about 3 weeks in the fall of 2014 to examine the oceanic and biological responses to strong winds. At the FPO site, oceanic microstructure measurements, hydrographic surveys, and water sampling were performed with high temporal resolution, recording internal wave propagation, eddy passage, and water mass changes. Strong winds and internal wave propagation significantly enhanced the mixing above and at the seasonal pycnocline, but their effects were diminished at the nutricline, which was much deeper than the seasonal pycnocline. Therefore, wind-induced mixing did not increase the upward nutrient supply from the nutricline and did not impact phytoplankton (chlorophyll a) distribution in the surface layer of the FPO site. The temporal evolution of the chlorophyll a concentration was most closely related to water mass changes. We also observed prominent subsurface chlorophyll a maxima with abundant large-sized phytoplankton that were likely carried by warm-core eddies to the FPO site. Phytoplankton biomass may have been sustained by the high concentration of ammonium within the eddy and ammonium regeneration at the seasonal pycnocline, where particulate organic matter likely accumulated.Plain Language Summary The Arctic basins, once covered with multiyear ice, are gaining ice-free areas in summer and fall as an effect of global warming. An ice-free upper ocean is more susceptible to wind forcing than ice-covered waters. Here, we studied the oceanic and biological responses to strong winds in the ice-free Canada Basin, where a density gap (pycnocline) was seasonally formed at a depth of about 20 m between the sea surface layer containing sea ice meltwater and the layer below. Although significant wind-induced mixing was observed above and at this seasonal pycnocline, mixing was attenuated at nutricline depths (~60 m), where nutrient concentrations increased strongly with depth. Thus, nutrients were not supplied from the nutricline to the sea surface layer through wind-induced mixing, and phytoplankton biomass did not increase in response to strong winds. However, phytoplankton biomass sometimes increased around the seasonal pycnocline, where ammonium was available for use in photosynthesis. This ammonium was likely supplied by warm-core eddies transporting shelf water into the Canada Basin. Another conceivable source of ammonium was particulate organic matter suspended at the seasonal pycnocline that decomposed, producing ammonium.

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Section: Discussionsupporting

confidence: 94%

Do Strong Winds Impact Water Mass, Nutrient, and Phytoplankton Distributions in the Ice‐Free Canada Basin in the Fall?

et al. 2020

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“…Overall, there was an 80% increase in internal wave amplitude between full ice cover (amplitudes of 0.7 m at C2–C4 in March to 26 April) and open water (1.3 m for Seagliders in August–September), greater than the seasonal cycle documented in Dosser and Rainville () as that measure includes a variety of ice concentrations in the summer season. The open water conditions observed by the Seagliders had amplitudes 20% larger than the low ice concentration conditions observed at C2, consistent with past studies showing less dramatic contrasts between low ice concentrations and open water conditions (Randelhoff et al, ). The specific mechanism that results in increased energy between low ice concentrations and open water is unclear; decreased dissipation at the surface (due to absence of rough ice or a lack of any ice) or increased local generation is plausible.…”

Section: Discussionsupporting

confidence: 89%

“…From our observations, Richardson number provides only some constraints on the possible variations in vertical mixing. The 4‐m Richardson number was almost always larger than 1/4 during the MIZ field program, consistent with the paradigm of weak turbulent diapycnal diffusivities in the Arctic (e.g., Fer, ; Guthrie et al, ; Lincoln et al, ; Randelhoff et al, ). Lower values were observed for rougher ice and lower ice concentrations, suggesting that vertical mixing may be sensitive to changes in the ice cover.…”

Section: Discussionsupporting

confidence: 76%

Internal Waves in the Arctic: Influence of Ice Concentration, Ice Roughness, and Surface Layer Stratification

et al. 2018

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The Arctic ice cover influences the generation, propagation, and dissipation of internal waves, which in turn may affect vertical mixing in the ocean interior. The Arctic internal wavefield and its relationship to the ice cover is investigated using observations from Ice‐Tethered Profilers with Velocity and Seaglider sampling during the 2014 Marginal Ice Zone experiment in the Canada Basin. Ice roughness, ice concentration, and wind forcing all influenced the daily to seasonal changes in the internal wavefield. Three different ice concentration thresholds appeared to determine the evolution of internal wave spectral energy levels: (1) the initial decrease from 100% ice concentration after which dissipation during the surface reflection was inferred to increase, (2) the transition to 70–80% ice concentration when the local generation of internal waves increased, and (3) the transition to open water that was associated with larger‐amplitude internal waves. Ice roughness influenced internal wave properties for ice concentrations greater than approximately 70–80%: smoother ice was associated with reduced local internal wave generation. Richardson numbers were rarely supercritical, consistent with weak vertical mixing under all ice concentrations. On decadal timescales, smoother ice may counteract the effects of lower ice concentration on the internal wavefield complicating future predictions of internal wave activity and vertical mixing.

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“…First, the convection driven entrainment is presumably even smaller than that given by Randelhoff and Guthrie []. Second, upper halocline waters have to be renewed either through a convective‐advective mechanism [ Rudels et al ., ] to achieve an interannual steady state, or by upward diffusion of deeper halocline waters during summer when surface meltwater restricts vertical mixing to above upper halocline waters [ Randelhoff et al , ].…”

Section: Discussionmentioning

confidence: 99%

One‐dimensional evolution of the upper water column in the Atlantic sector of the Arctic Ocean in winter

et al. 2017

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A one‐dimensional model is employed to reproduce the observed time evolution of hydrographic properties in the upper water column during winter, between 26 January and 11 March 2015, in a region north of Svalbard in the Nansen Basin of the Arctic Ocean. From an observed initial state, vertical diffusion equations for temperature and salinity give the hydrographic conditions at a later stage. Observations of microstructure are used to synthesize profiles of vertical diffusivity, K, representative of varying wind forcing conditions. The ice‐ocean heat and salt fluxes at the ice‐ocean interface are implemented as external source terms, estimated from the salt and enthalpy budgets, using friction velocity from the Rossby similarity drag relation, and the ice core temperature profiles. We are able to reproduce the temporal evolution of hydrography satisfactorily for two pairs of measured profiles, suggesting that the vertical processes dominated the observed changes. Sensitivity tests reveal a significant dependence on K. Variation in other variables, such as the temperature gradient of the sea ice, the fraction of heat going to ice melt, and the turbulent exchange coefficient for heat, are relatively less important. The increase in salinity as a result of freezing and brine release is approximately 10%, significantly less than that due to entrainment (90%) from beneath the mixed layer. Entrainment was elevated during episodic storm events, leading to melting. The results highlight the contribution of storms to mixing in the upper Arctic Ocean and its impact on ice melt and mixed‐layer salt and nutrient budgets.

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Turbulent Upper-Ocean Mixing Affected by Meltwater Layers during Arctic Summer

Cited by 38 publications

References 46 publications

Do Strong Winds Impact Water Mass, Nutrient, and Phytoplankton Distributions in the Ice‐Free Canada Basin in the Fall?

Do Strong Winds Impact Water Mass, Nutrient, and Phytoplankton Distributions in the Ice‐Free Canada Basin in the Fall?

Internal Waves in the Arctic: Influence of Ice Concentration, Ice Roughness, and Surface Layer Stratification

One‐dimensional evolution of the upper water column in the Atlantic sector of the Arctic Ocean in winter

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