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Hayley V. Dosser and Luc Rainville

ABSTRACT

The dynamics of the wind-generated near-inertial internal wave field in the Canada Basin of the Arctic Ocean are investigated using the drifting Ice-Tethered Profiler dataset for the years 2005 to 2014, during a decade when sea ice extent and thickness decreased dramatically. This time series, with nearly 10 years of measurements and broad spatial coverage, is used to quantify a seasonal cycle and interannual trend for internal waves in the Arctic, using estimates of the amplitude of near-inertial waves derived from isopycnal displacements. The internal wave field is found to be most energetic in summer when sea ice is at a minimum, with a second maximum in early winter during the period of maximum wind speed. Amplitude distributions for the near-inertial waves are quantifiably different during summer and winter, due primarily to seasonal changes in sea ice properties that affect how the ice responds to the wind, which can be expressed through the “wind factor”—the ratio of sea ice drift speed to wind speed. A small positive interannual trend in near-inertial wave energy is linked to pronounced sea ice decline during the last decade. Overall variability in the internal wave field increases significantly over the second half of the record, with an increased probability of larger-than-average waves in both summer and winter. This change is linked to an overall increase in variability in the wind factor and sea ice drift speeds, and reflects a shift in year-round sea ice characteristics in the Arctic, with potential implications for dissipation and mixing associated with internal waves.

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Hayley V. Dosser and Bruce R. Sutherland

Abstract

As upward-propagating anelastic internal gravity wave packets grow in amplitude, nonlinear effects develop as a result of interactions with the horizontal mean flow that they induce. This qualitatively alters the structure of the wave packet. The weakly nonlinear dynamics are well captured by the nonlinear Schrödinger equation, which is derived here for anelastic waves. In particular, this predicts that strongly nonhydrostatic waves are modulationally unstable and so the wave packet narrows and grows more rapidly in amplitude than the exponential anelastic growth rate. More hydrostatic waves are modulationally stable and so their amplitude grows less rapidly. The marginal case between stability and instability occurs for waves propagating at the fastest vertical group velocity. Extrapolating these results to waves propagating to higher altitudes (hence attaining larger amplitudes), it is anticipated that modulationally unstable waves should break at lower altitudes and modulationally stable waves should break at higher altitudes than predicted by linear theory. This prediction is borne out by fully nonlinear numerical simulations of the anelastic equations. A range of simulations is performed to quantify where overturning actually occurs.

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Hayley V. Dosser and Mary-Louise Timmermans

Abstract

The deep waters in the Canada Basin display a complex temperature and salinity structure, the evolution of which is poorly understood. The fundamental physical processes driving changes in these deep water masses are investigated using an inverse method based on tracer conservation combined with empirical orthogonal function analysis of repeat hydrographic measurements between 2003 and 2015. Changes in tracer fields in the deep Canada Basin are found to be dominated by along-isopycnal diffusion of water properties from the margins into the central basin, with advection by the large-scale Beaufort Gyre circulation as well as localized, vertical mixing playing important secondary roles. In the Barents Sea branch of the Atlantic Water layer, centered around 1200-m depth, diffusion is shown to be nearly twice as important as advection to lateral transport. Along-isopycnal diffusivity is estimated to be ~300–600 m2 s−1. Large-scale circulation patterns and lateral advective velocities associated with the anticyclonic Beaufort Gyre are inferred, with an average speed of 0.6 cm s−1. Below about 1500 m, along-isopycnal diffusivity is estimated to be ~200–400 m2 s−1.

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Hayley V. Dosser, Luc Rainville, and John M. Toole

Abstract

Salinity and temperature profiles from drifting ice-tethered profilers in the Beaufort gyre region of the Canada Basin are used to characterize and quantify the regional near-inertial internal wave field over one year. Vertical displacements of potential density surfaces from the surface to 750-m depth are tracked from fall 2006 to fall 2007. Because of the time resolution and irregular sampling of the ice-tethered profilers, near-inertial frequency signals are marginally resolved. Complex demodulation is used to determine variations with a time scale of several days in the amplitude and phase of waves at a specified near-inertial frequency. Characteristics and variability of the wave field over the course of the year are investigated quantitatively and related to changes in surface wind forcing and sea ice cover.

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