Dynamics of large-amplitude geostrophic flows over bottom topography

International audienceWe examine the interaction of near-surface and near- bottom flows over bottom topography. A set of asymptotic equations for geostrophic currents in a three-layer fluid is derived. The depths of the active (top/bottom) layers are assumed small, the slope of the bottom is weak, the interfacial displacement is comparable to the depths of the thinner layers. Using the equations derived, we examine the stability of parallel flows and circular eddies. It is demonstrated that eddies with non-zero near-surface component are always unstable; eddies localized in the near-bottom layer may be stable subject to additional restrictions imposed on their horizontal profiles and bottom topography

Seasonal-to-decadal predictions with the ensemble Kalman filter and the Norwegian Earth System Model: a twin experiment

Here, we firstly demonstrate the potential of an advanced flow dependent data assimilation method for performing seasonal-to-decadal prediction and secondly, reassess the use of sea surface temperature (SST) for initialisation of these forecasts. We use the Norwegian Climate Prediction Model (NorCPM), which is based on the Norwegian Earth System Model (NorESM) and uses the deterministic ensemble Kalman filter to assimilate observations. NorESM is a fully coupled system based on the Community Earth System Model version 1, which includes an ocean, an atmosphere, a sea ice and a land model. A numerically efficient coarse resolution version of NorESM is used. We employ a twin experiment methodology to provide an upper estimate of predictability in our model framework (i.e. without considering model bias) of NorCPM that assimilates synthetic monthly SST data (EnKF-SST). The accuracy of EnKF-SST is compared to an unconstrained ensemble run (FREE) and ensemble predictions made with near perfect (i.e. microscopic SST perturbation) initial conditions (PERFECT). We perform 10 cycles, each consisting of a 10-yr assimilation phase, followed by a 10-yr prediction. The results indicate that EnKF-SST improves sea level, ice concentration, 2 m atmospheric temperature, precipitation and 3-D hydrography compared to FREE. Improvements for the hydrography are largest near the surface and are retained for longer periods at depth. Benefits in salinity are retained for longer periods compared to temperature. Near-surface improvements are largest in the tropics, while improvements at intermediate depths are found in regions of large-scale currents, regions of deep convection, and at the Mediterranean Sea outflow. However, the benefits are often small compared to PERFECT, in particular, at depth suggesting that more observations should be assimilated in addition to SST. The EnKF-SST system is also tested for standard ocean circulation indices and demonstrates decadal predictability for Atlantic overturning and sub-polar gyre circulations, and heat content in the Nordic Seas. The system beats persistence forecast and shows skill for heat content in the Nordic Seas that is close to PERFECT

Three-dimensional acoustic scattering by vortical flows. I. General theory

When an acoustic wave is incident on a three-dimensional vortical structure, with length scale small compared with the acoustic wavelength, what is the scattered sound field that results? A frequently used approach is to solve a forced wave equation for the acoustic pressure, with nonlinear terms on the right-hand side approximated by the bilinear product of the incident wave and the undisturbed vortex: we refer to this as the “acoustic analogy” approximation. In this paper, we show using matched asymptotic expansions that the acoustic analogy approximation always predicts the leading-order scattered sound field correctly, provided the Mach number of the vortex is small, and the acoustic wavelength is a factor of order M−1 larger than the scale of the vortex. The leading-order scattered field depends only on the vortex dipole moment. Our analysis is valid for acoustic frequencies of the same order or smaller than the vorticity of the vortex. Over long times, the vortex may become significantly disturbed by the incident acoustic wave. Additional conditions are derived to maintain validity of the acoustic analogy approximation over times of order M−1, long enough for motion of the vortex to be significant on the length scale of the acoustic waves

Effect of soil treatment and fertilizers on changes in agrophysical properties of soil and soybean yield in the link of cereal crop rotation

The research was conducted in the Nizhny Novgorod region in 2015-2016 on light gray forest soil in the link of crop rotation “winter wheat-soybean”. The scheme of field experiment included 5 soil treatment systems: 1. Traditional moldboard tilling based on autumn plowing with PN-3-35 at 20-22 cm (control); 2. Nonmoldboard “deep” cultivation - autumn plowing with PN-3-35 (without moldboard) at 20-22 cm; 3. Nonmoldboard “shallow” tilling with the stubble cultivator Pöttinger at a depth of 14-16 cm; 4. The minimum machining with the disc harrow XM 44660 NOTHAD at a depth of 10-12 cm; 5. Zero tilling (No-till) with the Sunflower 9421-20 drill. For each soil treatment system, the effectiveness of the action of mineral fertilizers (N60P60K60) and straw in combination with the bio-preparation Stimix®Niva (21/ha) and mineral nitrogen (10 kg a.i. per 1 ton of straw) was studied. By the time of soybean sowing, only mechanical treatment provided favorable conditions for germination of seeds in the 0-10 cm soil layer (volumetric soil weight 1.25-1.30 g / cm3). The technology of direct seeding led to an increased compaction of the soil already in the initial period of vegetation (1.43 g / cm3). The variants of treatment systems didn’t have a significant effect on the soil moisture. By the time of soybean sowing the moisture content in 0-20 cm soil layer was 14.7-15.5%, and in 0-50 cm layer -16.4-17%. According to the soil treatment systems studied, the highest yield of soybean Svetlaya was obtained on the background of mineral fertilizers (N60P60K60) and straw as well as in variants with bio-preparation Stimix®Niva or mineral nitrogen for plant matter decay. Soybean cultivation by mechanical treatment systems provided grain yields of 1.64-1.761 / ha, and no-till technology reduced it to 0.61 t/ha