Tidal Conversion and Mixing Poleward of the Critical Latitude (an Arctic Case Study)

Abstract

The tides are a major source of the kinetic energy supporting turbulent mixing in the global oceans. The prime mechanism for the transfer of tidal energy to turbulent mixing results from the interaction between topography and stratified tidal flow, leading to the generation of freely propagating internal waves at the period of the forcing tide. However, poleward of the critical latitude (where the period of the principal tidal constituent exceeds the local inertial period), the action of the Coriolis force precludes the development of freely propagating linear internal tides. Here we focus on a region of sloping topography, poleward of the critical latitude, where there is significant conversion of tidal energy and the flow is supercritical (Froude number, Fr > 1). A high-resolution nonlinear modeling study demonstrates the key role of tidally generated lee waves and supercritical flow in the transfer of energy from the barotropic tide to internal waves in these high-latitude regions. Time series of flow and water column structure from the region of interest show internal waves with characteristics consistent with those predicted by the model, and concurrent microstructure dissipation measurements show significant levels of mixing associated with these internal waves. The results suggest that tidally generated lee waves are a key mechanism for the transfer of energy from the tide to turbulence poleward of the critical latitude. Plain Language Summary The decline in aerial extent of sea ice covering the Arctic Ocean in the recent years is perhaps one of the leading indications of climate change. Warm water enters the Arctic Ocean at depths of 100-200 m; however, it is isolated from melting the ice by the lack of mixing in the Arctic Ocean. This lack of mixing has been attributed to the ocean being isolated from the wind by ice, and the fact that much of the Arctic Ocean is north of the critical latitude, beyond which the type of internal tide that is believed to drive mixing across other major oceans on the planet cannot occur. However, new evidence has been found that suggests that the tide might be important in driving mixing in certain areas of the Arctic Ocean. Here we combine state-of-the-art numerical modeling with new turbulence measurements to identify the mechanism by which the tide can drive mixing at these high latitudes.

Topographic Shelf Waves Control Seasonal Melting Near Antarctic Ice Shelf Grounding Lines

“…Unlike freely propagating, semidiurnal modes, the diurnal topographic waves associated with the barotropic tides over the Gunnerus Bank are trapped, that is, they cannot propagate, because they are poleward of the critical latitude, and their conversion into internal waves may be important to facilitate the resonance of these modes inside the ice shelf cavity. Nonlinear and linear examples of propagating baroclinic waves at above‐critical latitudes exist (Hughes & Klymak, ; Rippeth et al, ) and comparable seasonal resonances of baroclininc waves with diurnal tides have been observed at similar configurations along the Antarctic continental slope (Jensen et al, ; Semper & Darelius, ). Idealized models that have been used to quantify the internal wave energy rely on constant geometry assumptions, which is challenging for the complex and partially unknown bathymetry around the Riiser‐Larsen Peninsula and beneath the RBIS.…”

Understanding Arctic Ocean Circulation: A Review of Ocean Dynamics in a Changing Climate

“…There is relatively weak tidal forcing in the Arctic, and most of the region is above the critical latitude north of which the semidiurnal lunar tide can propagate freely (Kowalik & Proshutinsky, ). Topographic waves generated over bathymetric slopes and rough topography, forced by the tides, are the main source of energy for higher tidal dissipation observed over topography (Holloway & Proshutinsky, ; Kowalik & Proshutinsky, ; Luneva et al, ; Padman et al, ; Rippeth et al, ). Sea‐ice cover is present for most of the year and acts as a buffer to wind‐driven momentum input to the upper ocean; further, internal wave energy can be dissipated under sea ice (Morison et al, ; Pinkel, ).…”

Variability of Internal Wave‐Driven Mixing and Stratification in Canadian Arctic Shelf and Shelf‐Slope Waters

“…Rippeth et al () also detected elevated turbulence across rough Arctic continental shelf breaks that were linked to the tide. Unique environmental factors in high‐latitude oceans, including the suppression of freely propagating linear internal tides poleward of the critical latitude, the importance of tidal energy conversion to nonlinear waves (Rippeth et al, ), and the insulating effects of sea ice cover, are further expected to impose regional and seasonal limitations on internal wave generation, evolution and, ultimately, wave‐driven mixing. Previous studies, including a mixture of both direct turbulence measurements and finescale estimates of wave‐driven turbulence, have found that the turbulent kinetic energy dissipation rate, ε , and associated eddy diffusivity, κ , can each span 2 to 4 orders of magnitude in localized Arctic regions (Table ).…”

“…Rippeth et al (2015) also detected elevated turbulence across rough Arctic continental shelf breaks that were linked to the tide. Unique environmental factors in high-latitude oceans, including the suppression of freely propagating linear internal tides poleward of the critical latitude, the importance of tidal energy conversion to nonlinear waves (Rippeth et al, 2017), and the insulating effects of sea ice cover, are further expected to Table 1 Summary of Dissipation Rate, , Mixing Rate, , and Heat Flux, F H , Results From Previous Turbulent Mixing Studies Wijesekera et al (1993) Yermak Plateau 10 −10 -10 −8 Fine Sundfjord et al (2007) Barents Sea 10 −8 -10 −6 10 −5 -10 −3 0.3-50 Micro Fer and Sundfjord (2007) Barents Sea 10 −8 -10 −6 10 −4 -10 −3 10-20 Micro Lenn et al (2009) East Siberian slope 10 −10 -10 −9 10 −6 -10 −4 0.01-5 Micro Fer (2009) Amundsen Basin 10 −9 -10 −7 10 −6 -10 −4 0-0.5 Micro Fer et al (2010) Yermak Plateau 10 −9 -10 −7 10 −5 -10 −4 −15-15 Micro/fine Bourgault et al (2011) Amundsen Gulf 10 −9 -10 −7 10 −6 -10 −3 Micro Lenn et al (2011) Laptev Sea 10 −6 -10 −3 10 −6 -10 −3 Micro Guthrie et al (2013) Central Arctic 10 −6 -10 −4 −2-2 Micro/fine /Canada Basin Rippeth et al (2015) Circumpolar shelves 10 −9 -10 −7 0.05-50 Micro /Canada Basin Kawaguchi et al (2016) Chukchi Plateau 10 −10 -10 −8 O(1) Micro/fine…”