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relative to the geostrophic jet. We define θ w = 0 to correspond to a wind-forcing ellipse with one principal axis oriented parallel to the mean flow and with initial winds pointing in the direction of the jet (positive x ). The parameters are defined more generally via the parametric formulas for the wind stress as a function of time t , forcing frequency ω w , and wind orientation angle θ w as follows: where τ a and τ b are the amplitudes of the wind stress in the direction of the
relative to the geostrophic jet. We define θ w = 0 to correspond to a wind-forcing ellipse with one principal axis oriented parallel to the mean flow and with initial winds pointing in the direction of the jet (positive x ). The parameters are defined more generally via the parametric formulas for the wind stress as a function of time t , forcing frequency ω w , and wind orientation angle θ w as follows: where τ a and τ b are the amplitudes of the wind stress in the direction of the
modes of KE and APE suggests our vortices undergo a mixed barotropic–baroclinic instability. Relating our results to classic instability problems, we might therefore expect the barotropic instability growth time to scale with the horizontal shear; for example, where C is a scale factor typically much larger than 1. Helfrich and Send (1988) used such scaling to examine the instability of quasigeostrophic hetons. In their case, the horizontal shear was defined in terms of the Bickley jet
modes of KE and APE suggests our vortices undergo a mixed barotropic–baroclinic instability. Relating our results to classic instability problems, we might therefore expect the barotropic instability growth time to scale with the horizontal shear; for example, where C is a scale factor typically much larger than 1. Helfrich and Send (1988) used such scaling to examine the instability of quasigeostrophic hetons. In their case, the horizontal shear was defined in terms of the Bickley jet
polar vortices as barriers to mixing tracers have been studied through both observations ( Nakamura and Ma 1997 ; Haynes and Shuckburgh 2000a , b ) and modeled flows ( Shuckburgh and Haynes 2003 ). It has also been used for oceanic jets along the Antarctic Circumpolar Current that are shown to act as barriers to mixing ( Marshall et al. 2006 ; Shuckburgh et al. 2009 ). Under the approximation that the evolution of a certain tracer concentration class C takes place along isopycnals in the
polar vortices as barriers to mixing tracers have been studied through both observations ( Nakamura and Ma 1997 ; Haynes and Shuckburgh 2000a , b ) and modeled flows ( Shuckburgh and Haynes 2003 ). It has also been used for oceanic jets along the Antarctic Circumpolar Current that are shown to act as barriers to mixing ( Marshall et al. 2006 ; Shuckburgh et al. 2009 ). Under the approximation that the evolution of a certain tracer concentration class C takes place along isopycnals in the
observed fluid is not exactly in a Lagrangian frame of reference at all depths and cross-stream locations observed. A typical cross-front section from the Atlantis ( Fig. 2 ) exhibits a strong surface-intensified jet with velocities exceeding 2 m s −1 in the streamwise direction x , order-one vorticity Rossby numbers (Ro = ζ / f ≈ −∂ u /∂ y / f , where f is the Coriolis frequency, u is the along-stream velocity, and y points in the cross-stream direction), isopycnal slopes s b of order 0
observed fluid is not exactly in a Lagrangian frame of reference at all depths and cross-stream locations observed. A typical cross-front section from the Atlantis ( Fig. 2 ) exhibits a strong surface-intensified jet with velocities exceeding 2 m s −1 in the streamwise direction x , order-one vorticity Rossby numbers (Ro = ζ / f ≈ −∂ u /∂ y / f , where f is the Coriolis frequency, u is the along-stream velocity, and y points in the cross-stream direction), isopycnal slopes s b of order 0
water density class (0.13–0.14 kg m −3 ) between 0- and 25-m depth. The newly densified mode water parcels then sink at the front in a vertical jet (as seen in Fig. 3f ). These parcel paths are at odds with the linear model of Thomas and Shakespeare (2015) , which proposed dense water formation by essentially separate horizontal mixing of water masses at each depth, after which the density of parcels reaches ρ 100 . Instead, here we have upward advection of water to near the surface, where the
water density class (0.13–0.14 kg m −3 ) between 0- and 25-m depth. The newly densified mode water parcels then sink at the front in a vertical jet (as seen in Fig. 3f ). These parcel paths are at odds with the linear model of Thomas and Shakespeare (2015) , which proposed dense water formation by essentially separate horizontal mixing of water masses at each depth, after which the density of parcels reaches ρ 100 . Instead, here we have upward advection of water to near the surface, where the
diameters). During the moderate-straining case study, a rich structure of submesoscale strands became evident in survey transects, most noticeable in the salinity field ( Fig. 9b, II ). To what extent these features represented spatial variability in the front versus temporal evolution is not known. Sea surface temperature (SST) imagery ( Fig. 12b ) suggests fragmentation and warming of the cold filament at the surface. However, the subsurface northward jet advecting the dye and drifters remained
diameters). During the moderate-straining case study, a rich structure of submesoscale strands became evident in survey transects, most noticeable in the salinity field ( Fig. 9b, II ). To what extent these features represented spatial variability in the front versus temporal evolution is not known. Sea surface temperature (SST) imagery ( Fig. 12b ) suggests fragmentation and warming of the cold filament at the surface. However, the subsurface northward jet advecting the dye and drifters remained
presence of a large, barotropic dipole. This suggests that mesoscale vortices would not impose a strong limit to upscale energy transfer. However, stronger mesoscale vortices could still destroy smaller vortices. Other sources of shears and strains include fronts and currents. Brunner-Suzuki et al. (2012) found that a jet (for a small range of jet velocities) can, in fact, strain and shear a single S vortex enough to inhibit dipole splitting. This in turn may also inhibit upscale energy transfer
presence of a large, barotropic dipole. This suggests that mesoscale vortices would not impose a strong limit to upscale energy transfer. However, stronger mesoscale vortices could still destroy smaller vortices. Other sources of shears and strains include fronts and currents. Brunner-Suzuki et al. (2012) found that a jet (for a small range of jet velocities) can, in fact, strain and shear a single S vortex enough to inhibit dipole splitting. This in turn may also inhibit upscale energy transfer
surfaces (though this need not be the case in the presence of isopycnal salinity gradients subject to internal-wave straining, as we shall see). Site 2 was sampled during 10–19 June 2011. Confluences based on a 1–10-km drifter array were ~3 × 10 −5 s −1 ~0.38 f on 13 June, reducing to ~10 −6 s −1 ~0.01 f during 14–18 June (D. A. Birch et al. 2014, unpublished manuscript), which sharpened a meridional water-mass front ( Figs. 1 – 3 ) and accelerated a north-northwestward jet that subsequently
surfaces (though this need not be the case in the presence of isopycnal salinity gradients subject to internal-wave straining, as we shall see). Site 2 was sampled during 10–19 June 2011. Confluences based on a 1–10-km drifter array were ~3 × 10 −5 s −1 ~0.38 f on 13 June, reducing to ~10 −6 s −1 ~0.01 f during 14–18 June (D. A. Birch et al. 2014, unpublished manuscript), which sharpened a meridional water-mass front ( Figs. 1 – 3 ) and accelerated a north-northwestward jet that subsequently
response to a hurricane . J. Phys. Oceanogr. , 11 , 153 – 175 , doi: 10.1175/1520-0485(1981)011<0153:UORTAH>2.0.CO;2 . Ragone , F. , and G. Badin , 2016 : A study of surface semi-geostrophic turbulence: Freely decaying dynamics . J. Fluid Mech. , 792 , 740 – 774 , doi: 10.1017/jfm.2016.116 . Rypina , I. I. , M. G. Brown , F. J. Beron-Vera , H. Koçak , M. J. Olascoaga , and I. A. Udovydchenkov , 2007 : On the Lagrangian dynamics of atmospheric zonal jets and the
response to a hurricane . J. Phys. Oceanogr. , 11 , 153 – 175 , doi: 10.1175/1520-0485(1981)011<0153:UORTAH>2.0.CO;2 . Ragone , F. , and G. Badin , 2016 : A study of surface semi-geostrophic turbulence: Freely decaying dynamics . J. Fluid Mech. , 792 , 740 – 774 , doi: 10.1017/jfm.2016.116 . Rypina , I. I. , M. G. Brown , F. J. Beron-Vera , H. Koçak , M. J. Olascoaga , and I. A. Udovydchenkov , 2007 : On the Lagrangian dynamics of atmospheric zonal jets and the
