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Mei Han
,
Robert M. Rauber
,
Mohan K. Ramamurthy
,
Brian F. Jewett
, and
Joseph A. Grim

filter small-scale motions but retain the information required to diagnose the frontal circulations. The ageostrophic secondary circulation in the trowal and warm-frontal regions of the two cyclones, a dynamic response to frontogenesis/frontolysis, was investigated by solving the Sawyer–Eliassen (SE) equation ( Sawyer 1956 ; Eliassen 1962 ) under the geostrophic momentum (GM) approximation. The SE equation describes how a two-dimensional cross-frontal circulation is forced by various mechanisms

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Michael S. Dinniman
and
Michele M. Rienecker

primary dynamical mechanism acting in the OFZs. Roden and Paskausky (1978) were able to estimate patterns of frontogenesis and frontolysis in the STFZ in winter using a simple model based on Ekman dynamics with one degree horizontal resolution but only over timescales of a week or less. Camerlengo (1982) used a four-layer hydrodynamical model of the SAFZ to show that the effect of a negative wind stress curl is to produce convergence at the front, strongly favoring frontogenesis. However, many

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Yutaka Yoshikawa
,
Craig M. Lee
, and
Leif N. Thomas

1. Introduction The transfer of mixed layer water into the pycnocline, referred to as subduction, sets the flux of heat and tracers from the surface mixed layer into the stratified ocean interior. Previous numerical studies ( Samelson and Chapman 1995 ; Spall 1995 ; Wang 1993 ) find that frontogenesis at a meandering density front can induce a three-dimensional (3D) ageostrophic cross-front circulation that subducts surface water into or below the pycnocline, which can result in the formation

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Jonathan Gula
,
M. Jeroen Molemaker
, and
James C. McWilliams

indicate a submesoscale strain-induced frontogenesis acting on the sharp front of the Gulf Stream, which is confirmed in the frontal sharpness and frontogenetic tendency discussed in section 5a . Fig . 10. Instantaneous horizontal patterns for the frontal eddy at z = −10 m for the frontal eddy of Fig. 8 : (a) relative vorticity ζ = υ x − u y , normalized by f ; (b) Okubo–Weiss parameter St 2 − ζ 2 , normalized by f 2 ; (c) frontal sharpness 0.5||∇ b || 2 (i.e., variance of the horizontal

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Thomas J. Galarneau Jr.
and
Xubin Zeng

frontogenesis The National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) analysis pressure-level data, available at 6-h intervals at 0.25° × 0.25° latitude–longitude grid spacing, was used to determine the synoptic-scale environment and physical mechanisms that produced the Harvey rainstorm. Accumulated rainfall analyses in the southern Great Plains and southeast United States were derived from the NCEP Stage-IV rainfall dataset available at 4-km horizontal grid spacing ( Lin 2011

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Jen-Ping Peng
,
Peter Holtermann
, and
Lars Umlauf

instability of the weakly stratified SBL ( Fox-Kemper et al. 2008 ), or by other processes related to, e.g., upwelling, differential mixing, and river plumes. The classical view is that submesoscale fronts and filaments are quickly intensified by frontogenesis induced by a quasigeostrophic mesoscale strain field ( Hoskins 1982 ; Capet et al. 2008 ). There is, however, increasing evidence from recent numerical studies that late-stage submesoscale frontogenesis is affected, or even dominated, by an

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Jonathan Gula
,
M. Jeroen Molemaker
, and
James C. McWilliams

) , which shows much stronger secondary circulation and intensification rate for cold filaments compared to warm ones. The cause is a horizontal deformation flow that acts on an isolated, favorably aligned filament, causing rapid narrowing and a two-celled secondary circulation with even stronger surface convergence and downwelling at its center than in frontogenesis for a monotonic density gradient (i.e., a conventional front). The Gulf Stream is full of fronts, filaments, and eddies at meso- and

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Leif N. Thomas
and
Callum J. Shakespeare

( Urakawa and Hasumi 2012 ). Urakawa and Hasumi noted that the water mass transformation ascribable to cabbeling in their simulations was strongly influenced by numerical diffusion. This implies that processes that cascade temperature and salinity variance from the mesoscale to the grid scale of their model are essential to the mechanism. One process of particular relevance is frontogenesis, that is, the intensification of horizontal tracer gradients by a strain field. In the next section, we describe

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Dhruv Balwada
,
Qiyu Xiao
,
Shafer Smith
,
Ryan Abernathey
, and
Alison R. Gray

) surface strain, and (d) the vorticity–strain JPDF in a region with a strong front. A depth-across front section of the (e) temperature, (f) vertical velocity, and tracer on days (g) 8 and (h) 10 after the tracer forcing is turned on. The black contours in (a), (e), (f), (g), and (h) are some chosen temperature contours to highlight the front. The yellow contours in (e) show the meridional velocity, which is northward, decaying away from the front. During the process of frontogenesis, when a background

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Sebastian Essink
,
Verena Hormann
,
Luca R. Centurioni
, and
Amala Mahadevan

1. Introduction Measuring kinematic properties is of particular interest at submesoscales (0.1–10 km length scales), where lateral buoyancy gradients are intensified by surface forcing, topographic interaction, frontogenesis, baroclinic instability, and turbulent thermal wind. Large local Rossby number Ro = ζ / f ∼ O ( 1 ) can be generated, where ζ = υ x − u y is the relative vorticity and f is the Coriolis frequency (e.g., Thomas et al. 2008 ; McWilliams 2016 ), along with

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