The distribution of eddy kinetic and potential energies in the global ocean

Understanding of the major sources, sinks, and reservoirs of energy in the ocean is briefly updated in a diagram. The nature of the dominant kinetic energy reservoir, that of the balanced variablity, is then found to be indistinguishable in the observations from a sum of barotropic and first baroclinic ordinary quasi-geostrophic modes. Little supporting evidence is available to partition the spectra among forced motions and turbulent cascades, along with significant energy more consistent with weakly non-linear wave dynamics. Linear-response wind-forced motions appear to dominate the high frequency (but subinertial) mooring frequency spectra. Turbulent cascades appear to fill the high wavenumber spectra in altimetric data and numerical simulations. Progress on these issues is hindered by the difficulty in connecting the comparatively easily available frequency spectra with the variety of theoretically predicted wavenumber spectra.National Oceanographic Partnership Program (U.S.)United States. National Aeronautics and Space AdministrationNational Science Foundation (U.S.) (Award OCE-0849233

The Role of Closed Gyres in Setting the Zonal Transport of the Antarctic Circumpolar Current

Eddy-permitting simulations are used to show that basinlike gyres can be observed in the large-scale barotropic flow of a wind-driven channel with a meridional topographic ridge. This is confirmed using both two-layer quasigeostrophic and 25-level primitive equation models at high horizontal resolution. Comparing results from simulations with and without the topographic ridge, it is shown that the zonal baroclinic transport in the channel increases with increasing wind stress when the bottom topography is flat but not when there is a meridional ridge. The saturation of transport for increasing wind occurs in conjunction with the development of recirculating gyres in the large-scale barotropic streamfunction. This suggests that the total circulation can be thought of as a superposition of a gyre mode (which has zero circumpolar transport) and a free circumpolar mode (which contains all of the transport). Basinlike gyres arise in the channel because the topography steers the barotropic streamlines and supports a frictional boundary layer similar to the more familiar ones observed along western boundaries. The gyre mode is thus closely linked with the bottom form stress exerted by the along-ridge flow and provides the sink for the wind momentum input. In this framework, any increase in wind forcing spins a stronger gyre as opposed to feeding the circumpolar transport. This hypothesis is supported with a suite of experiments where key parameters are carried over a wide range: wind stress, wind stress curl, ridge height, channel length, and bottom friction.Mathematics and Climate Research Network (Fellowship)National Science Foundation (U.S.) (Award OCE-1233832

Shutdown of turbulent convection as a new criterion for the onset of spring phytoplankton blooms

The onset of phytoplankton blooms in late winter, early spring has been traditionally associated with the shoaling of the mixed layer above a critical depth. Here we show that the onset of a bloom can also be triggered by a reduction in air–sea fluxes at the end of winter. When net cooling subsides at the end of winter, turbulent mixing becomes weak, thereby increasing the residence time of phytoplankton cells in the euphotic layer and allowing a bloom to develop. The necessary change in the air–sea flux generally precedes mixed-layer shoaling, and may provide a better indicator for the onset of the spring bloom than the mixed-layer depth alone. Our hypothesis is supported by numerical simulations and remote sensing data.United States. Office of Naval Research (Award N00014-08-1-1060

Diagnosing the vertical structure of the eddy diffusivity in real and idealized atmospheres

The Earth's extratropical troposphere is equilibrated by turbulent eddy fluxes of potential temperature and momentum. The equilibrated state has the remarkable characteristic that isentropic slopes leaving the surface in the subtropics reach the tropopause near the Poles. It has been speculated that turbulent eddy fluxes maintain this state for a wide range of radiative forcing and planetary parameters. In a previous study, the authors showed that this state needs to be associated with an eddy diffusivity of Ertel potential vorticity that is largest at the surface and decays through the troposphere to approximately zero at the tropopause. This result is confirmed in this study using atmospheric reanalysis and idealized numerical simulations. However, it is also shown that the vertical profile of the eddy diffusivity can change, resulting in different isentropic slopes and climates. This is illustrated with a series of idealized numerical simulations with varying planetary scales and rotation rates.National Science Foundation (U.S.) (Award OCE-0849233

On the development of thermohaline correlations as a result of nonlinear diffusive parameterizations

Some oceanographic mixing parameterizations assume that transports depend nonlinearly on the buoyancy gradient; e.g., diffusivities are proportional to some power of the buoyancy gradient. In this paper we examine the consequences of these nonlinear-diffusion parameterizations by solving an initial value problem in which the t = 0 thermohaline fields are prepared as random and uncorrelated distributions of temperature and salinity. Solutions of the nonlinear diffusion equation as a ‘rundown’ problem show that correlations develop between the temperature and salinity. These correlations are such that the evolving thermohaline gradients tend to be strongly compensating in their joint effect on buoyancy

Transition from geostrophic turbulence to inertia–gravity waves in the atmospheric energy spectrum

Midlatitude fluctuations of the atmospheric winds on scales of thousands of kilometers, the most energetic of such fluctuations, are strongly constrained by the Earth’s rotation and the atmosphere’s stratification. As a result of these constraints, the flow is quasi-2D and energy is trapped at large scales—nonlinear turbulent interactions transfer energy to larger scales, but not to smaller scales. Aircraft observations of wind and temperature near the tropopause indicate that fluctuations at horizontal scales smaller than about 500 km are more energetic than expected from these quasi-2D dynamics. We present an analysis of the observations that indicates that these smaller-scale motions are due to approximately linear inertia–gravity waves, contrary to recent claims that these scales are strongly turbulent. Specifically, the aircraft velocity and temperature measurements are separated into two components: one due to the quasi-2D dynamics and one due to linear inertia–gravity waves. Quasi-2D dynamics dominate at scales larger than 500 km; inertia–gravity waves dominate at scales smaller than 500 km.United States. Office of Naval Research (Grant ONR-N-00014-09-1-0458)National Science Foundation (U.S.) (Grant NSF-CMG-1024198

Wave–vortex decomposition of one-dimensional ship-track data

We present a simple two-step method by which one-dimensional spectra of horizontal velocity and buoyancy measured along a ship track can be decomposed into a wave component consisting of inertia–gravity waves and a vortex component consisting of a horizontal flow in geostrophic balance. The method requires certain assumptions for the data regarding stationarity, homogeneity, and horizontal isotropy. In the first step an exact Helmholtz decomposition of the horizontal velocity spectra into rotational and divergent components is performed and in the second step an energy equipartition property of hydrostatic inertia–gravity waves is exploited that allows a diagnosis of the wave energy spectrum solely from the observed horizontal velocities. The observed buoyancy spectrum can then be used to compute the residual vortex energy spectrum. Further wave–vortex decompositions of the observed fields are possible if additional information about the frequency content of the waves is available. We illustrate the method on two recent oceanic data sets from the North Pacific and the Gulf Stream. Notably, both steps in our new method might be of broader use in the theoretical and observational study of atmosphere and ocean fluid dynamics.United States. Office of Naval Research (Grant N-00014-09-1-0458)Grant GMG-102419