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Jesse M. Cusack, Alberto C. Naveira Garabato, David A. Smeed, and James B. Girton

1. Introduction Lee waves can be generally defined as internal gravity waves generated by the interaction of a quasi-steady stratified flow with topography. Observations of such phenomena in the ocean are rare, with notable examples including high-frequency, tidally forced waves in the lee of ridges (e.g., Pinkel et al. 2012 ; Alford et al. 2014 ). Propagating waves must have a frequency between the local inertial frequency f and buoyancy frequency N , which precludes their generation in

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Dhruv Balwada, Joseph H. LaCasce, Kevin G. Speer, and Raffaele Ferrari

submesoscale flows and eddies in interior ocean, without any surface association, can also potentially result from interaction between internal waves and balanced flows ( Thomas and Yamada 2019 ), or result due to breaking waves creating mixed patches that then coalesce into pancake vortices due to an inverse cascade ( Sundermeyer et al. 2005 ; Polzin and Ferrari 2004 ), or be generated by flow interacting with topography and spinning off eddies ( Srinivasan et al. 2019 ; Vic et al. 2018 ; Bracco et al

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J. Alexander Brearley, Katy L. Sheen, Alberto C. Naveira Garabato, David A. Smeed, and Stephanie Waterman

little understood. Ferrari and Wunsch (2009) review several candidate mechanisms including frictional bottom drag, loss of balance in the ocean interior, interactions with the internal wave field, continental margin absorption, and suppression by wind work. While friction in the bottom boundary layer is important, Wunsch and Ferrari (2004) argued that it is too weak to be the dominant sink. In addition, Molemaker et al. (2010) suggest that loss of balance is unlikely to be a large term in the

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J. R. Ledwell, L. C. St. Laurent, J. B. Girton, and J. M. Toole

remain sparse and mostly limited to mid- and low latitudes ( Gregg et al. 1973 ; Toole et al. 1994 ; Gregg et al. 2003 ; Klymak et al. 2006 ). Measurements of vertical shear and strain at scales of tens of meters, from which mixing estimates can be inferred, are more widespread, but formulations relating these internal wave characteristics to dissipation rates and diapycnal diffusivity are subject to a number of added approximations ( Gregg 1989 ; Kunze et al. 2006 ). The Southern Ocean is a

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Byron F. Kilbourne and James B. Girton

1. Introduction The study of near-inertial oscillations and internal waves began with the advent of moored, self-recording current meter measurements in the 1960s. These instruments revealed considerable variance near the local inertial frequency ( Webster 1968 ) and motivated a series of efforts to better understand near-inertial variability, its predominant generation mechanisms, and its role in other ocean processes such as diapycnal mixing and energy transport: Pollard and Millard (1970

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F. Sévellec, A. C. Naveira Garabato, J. A. Brearley, and K. L. Sheen

1. Introduction Oceanic vertical flow is an important element of the ocean circulation, playing a central role in the redistribution of water and tracers between (and within) the upper-ocean mixed layer and the ocean interior. In the interior, the occurrence of vertical motion is associated with a wide range of processes characterized by distinct dynamics: from relatively high-frequency and small-scale geostrophically unbalanced flows (such as internal waves and three-dimensional turbulent

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L. St. Laurent, A. C. Naveira Garabato, J. R. Ledwell, A. M. Thurnherr, J. M. Toole, and A. J. Watson

. 1997 ; Ledwell et al. 2000 ). Typically located in the middle of oceanic basins away from enhanced boundary currents, the tides are often the largest source of energy at midocean ridge sites. Tidal flow incident on bathymetry leads to an internal wave response, principally at tidal frequencies ( St. Laurent and Garrett 2002 ), that can radiate energy into the deep interior of the ocean. As internal waves radiate, they lose energy through a variety of instability and scattering mechanisms ( Munk

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Sophia T. Merrifield, Louis St. Laurent, Breck Owens, Andreas M. Thurnherr, and John M. Toole

1. Introduction A number of processes in the Southern Ocean are thought to support high levels of mixing relative to other regions of the global ocean. At the surface, strong winds and storms force the ocean at near-inertial frequencies, generating internal waves that can propagate downward ( Price 1981 ). Upper-ocean and middepth values of diapycnal diffusivity are believed to be set in part by the breaking of these near-inertial waves (e.g., Wu et al. 2011 ). Deep-reaching currents

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Michael Bates, Ross Tulloch, John Marshall, and Raffaele Ferrari

a zonal two-level quasigeostrophic channel flow. 2) Stochastic mixing theory Ocean eddies are nonlinear, and it is not clear how to interpret the quantities that appear in Eq. (3) . In particular, eddies are not continuously growing at some rate ω i , rather they rapidly grow and slowly decay, reaching a statistical equilibrium. Ferrari and Nikurashin (2010) showed that an equation similar to Eq. (4) can be derived by considering the mixing induced by a random superposition of Rossby waves

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Ru Chen, Sarah T. Gille, Julie L. McClean, Glenn R. Flierl, and Alexa Griesel

.g., Ledwell et al. 1998 ). The breakdown of the F–N theory in some ocean scenarios is unsurprising, considering that it is built on a number of assumptions that are violated in the ocean, including a flat bottom, a spatially and temporally constant mean flow, and scale separation between the mean flow and eddies (e.g., Ferrari and Nikurashin 2010 ). One assumption upon which these theories, including the F–N theory, are based is that eddies only contain a single wave corresponding to the most unstable

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