Temporal Variability of Internal Wave‐Driven Mixing in Two Distinct Regions of the Arctic Ocean

Abstract

This work investigates how internal wave-driven turbulence varies in time, from hourly to yearly timescales, and in space, across two distinct regions of the Arctic Ocean. We apply a shear-based fine-scale parameterization to mooring records in Nares Strait and on the Beaufort Sea shelf-slope that sampled the upper stratified water column every 30-45 min and span 2003-2006 and 2003-2004, respectively. In doing so, we generate over 600,000 estimates of the internal wave-driven dissipation rate. These estimates exhibit large temporal variability in both regions, spanning over 3 orders of magnitude. Despite these wide ranges, we find distinct distributions at each site. In Nares Strait, the time series of dissipation shows systematic variation at tidal frequencies, and tidal forcing appears to influence dissipation more strongly than winds, sea ice, and stratification on daily timescales. On longer timescales, dissipation exhibits a weak seasonal cycle, being elevated when the stratification is high and during the ice melt season. In the Beaufort Sea, we detect no dominant timescales or significant relationships with forcing metrics, but note that the dissipation rate is typically 2 orders of magnitude lower than that in Nares Strait. This region is characterized as being in a turbulent mixing regime for only 2% of the record, compared to 73% of the Nares Strait record, implying that turbulence here is rarely energetic enough relative to the stratification to drive a turbulent heat flux. Inferred Beaufort Sea heat fluxes are an order of magnitude lower than the O(1) W m −2 average value found in Nares Strait. Plain Language Summary Arctic Ocean mixing rates and how they vary in space and time have important consequences for the transport of heat, salt, and nutrients. However, the tasks of understanding how mixing changes water properties and of predicting future changes to Arctic Ocean mixing rates remain challenging, particularly given that most observations used to study mixing in the Arctic Ocean to date are geographically and temporally limited. In this study, we infer mixing rates by using an estimation technique that relates ocean turbulence to properties of internal waves in the ocean interior. We apply this technique to highly resolved, multiyear measurements obtained from two distinct regions of the Arctic Ocean. Using these estimates, we characterize the statistical distributions of mixing metrics and find strong regional differences between the two study sites. We find that one region is highly energetic and displays systematic patterns in mixing intensity at tidal and seasonal timescales. Turbulent energy at the other site is 2 orders of magnitude weaker, and active turbulent mixing occurs extremely infrequently. These results help to put previous mixing observations into a more informed context and provide insight into the underlying causes of Arctic Ocean mixing patterns.

Diapycnal diffusivities in Kelvin–Helmholtz engendered turbulent mixing: the diffusive–convection regime in the Arctic Ocean

“…In the recent work on direct or indirect measurements in the Arctic Ocean (for example, Chanona, Waterman & Gratton 2018; Scheifele et al. 2018, 2021; Chanona & Waterman 2020; Dosser et al. 2021), a critical level of or is usually chosen to differentiate the turbulent regime from the molecular regime.…”

“…An algorithm for the determination of diapycnal diffusivities in the stratified turbulence In the practical measurement of turbulence and mixing in the Arctic Ocean, there are generally two most critical physical quantities that are especially important to understand: the diapycnal diffusivities for density K ρ and the vertical heat flux F H . In the recent work on direct or indirect measurements in the Arctic Ocean (for example, Chanona, Waterman & Gratton 2018;Scheifele et al 2018Scheifele et al , 2021Chanona & Waterman 2020;Dosser et al 2021), a critical level of Re cr b = 10 or Re cr b = 20 is usually chosen to differentiate the turbulent regime from the molecular regime. In the molecular regime the difference between the molecular diffusion for temperature and salinity is identified so that…”

Validating Finescale Parameterizations for the Eastern Arctic Ocean Internal Wave Field

“…(2021) investigated temporal variability of internal wave‐driven turbulence and related parameters using 8 days of moored observations in the Beaufort Gyre in an environment with highly variable shear‐to‐strain ratio, averaging 17 on a range between 3 and 50. Using moored observations and a fixed R ω of 7, Chanona and Waterman (2020) investigated temporal variability of internal wave driven mixing at two Arctic locations: Nares Strait and in the Beaufort Sea. Dosser et al.…”

Impact of Vertical Mixing on Summertime Net Community Production in Canadian Arctic and Subarctic Waters: Insights From In Situ Measurements and Numerical Simulations

“…Weak mixing rates are prevalent in the central Arctic basins (e.g., Fer, 2009; Rainville & Winsor, 2008), while the continental margins appear to be sites of enhanced mixing (e.g., Fer et al., 2010; Rippeth et al., 2015; Sundfjord et al., 2007). In Canadian Arctic shelf‐slope waters, mixing modulated by tides, topography, and stratification exhibits multiple order‐of‐magnitude variability, and localized episodic events may generate large, albeit rare, mixing events (Chanona et al., 2018; Chanona & Waterman, 2020; Scheifele et al., 2021).…”