**Date:** October 25, 2017

**Time:** 14h

**Location:** E314

**By:** Cesar Rocha (Scripps Institution of Oceanography)

**Title: **Extraction of energy from balanced flow by near-inertial waves

**Abstract:**

Primitive-equation numerical simulations and analysis of reduced models suggest that stimulated generation—the transfer of energy from balanced flows to existing internal waves—is a leading contender for an ocean mesoscale energy sink. Here we study stimulated generation using an asymptotic model that couples barotropic quasi-geostrophic flow and near-inertial waves with the exp(imz) structure on the f-plane. A detailed description of the conservation laws of this vertical plane-wave model illuminates the mechanism of stimulated generation associated with vertical vorticity and lateral strain. In particular, there are two sources of wave potential energy, and corresponding sinks of balanced kinetic energy: (1) the refractive convergence of the wave action density into anticyclones (and divergence from cyclones) and (2) enhancement of wave-field gradients by geostrophic straining.

We quantify the energy conversion and describe the phenomenology of stimulated generation using numerical solutions of decaying ocean macroturbulence modified by near-inertial waves. The initial conditions are a uniform inertial oscillation and a two-dimensional turbulent field emergent from random initial conditions. In all solutions, stimulated generation co-exists with a transfer of balanced kinetic energy to large scales, which is associated with vortex merger. And geostrophic straining accounts for most of the generation of wave potential energy, which represents a sink of 10-20% of the initial balanced kinetic energy. But refraction is fundamental because it creates the initial eddy- scale lateral gradients in the near-inertial field that are then enhanced by advection. In these quasi-inviscid solutions, wave dispersion is the only mechanism that upsets stimulated generation: with barotropic balanced flow, lateral straining enhances the wave group velocity; the waves accelerate and thus rapidly escape from the straining regions. Because of this wave escape, the wave field does not suffer a direct cascade to dissipative scales.

**Date: **October 27, 2017

**Time: **14h

**Location: ***L378*

**By:** Sebastian Schemm (ETH Zurich)

**Title: **

**Abstract:
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**Date: **November 10, 2017

**Time: **11h

**Location: **E314

**By:**

**Title: **

**Abstract:**

**Date: **November 21, 2017

**Time: **14h

**Location: **E314

**By:** P. Yiou (LSCE)

**Title: **Analogues of circulation and stochastic weather generators

**Abstract: **

Analogues of circulation have found many innovative uses in the past few years, including climate reconstructions, attribution of event and data assimilation. After presenting the gist of the methodology of analogues and its link with recurrences in dynamical systems, I will show how this methodology can be used to design stochastic weather generators with coherent spatial features. I will also argue that such tools can also be used to perform ensemble predictions of climate variables such as temperature. I will give several examples of such applications.

**Date: **December 14, 2017

**Time: **14h

**Location: **E314

**By:** E. Horne (Ladhyx)

**Title: **Energetics aspects and irreversible mixing in stratified turbulence: numerical study

**Abstract: **

The local mixing produced by turbulence in the ocean interior plays a crucial role in its global energy budget. This mixing partially drives large scale dynamics, as evidence in the meridional overturning circulation (MOC). The circulation is produced thanks to the downward transport of energy from the surface to the deep bottom of the ocean, possible thanks to vertical mixing. Many processes produce mixing in the ocean, mostly forced by interior tides and winds. In addition, fine measurements of the density in the ocean show that the stratification can vary quite abruptly at small scales. Nevertheless, the proportion of energy transferred from turbulent structures to effective mixing is very difficult to measure in the ocean, and the details of the distribution of the injected energy is yet not fully understood. In order to answer these questions, a set of 3D Direct Numerical Simulations (DNS) of a turbulent stratified flow are performed by solving the Navier-Stokes equations under Boussinesq approximation. A classical Fourier pseudo-spectral method is used with 1024^3 grid points. A porous penalization region is introduced to take into account non-flux conditions at the bottom and at the top of the box. A turbulent velocity field is introduced at t=0 and perturbs the initially linear buoyancy profile which is then free to evolve in time.

The instantaneous irreversible mixing is compute by comparing the potential energy and the background potential energy of the system. The mixing efficiency, which is a form to measure how the injected energy is partitioned between available potential energy and kinetic energy, is computed for a broad range of degrees of stratification (characterized by the non-dimensional Richardson number, Ri). Our results indicate that the mixing efficiency presents a non trivial, yet simple, dependency with respect to the Richardson number. In addition, these results are captured by a statistical model developed by Venaille 2016. These results allow to improve notoriously the historical approach to model the mixing efficiency in oceanic conditions, which is often taken to be constant.

**Date:** October 3, 2017

**Time:** 14h

**Location:** E314

**By:** Ali R. Mohebalhojeh, Institute of Geophysics, University of Tehran

**Title:** On the quantification of imbalance and inertia-gravity waves generated in numerical simulations of moist baroclinic waves

**Abstract:**

Quantification of inertia–gravity waves (IGWs) generated by upper-level jet-surface front systems and their parametrization in global models of the atmosphere relies on suitable methods to estimate the strength of IGWs. A harmonic divergence analysis (HDA) that has been previously employed for quantification of IGWs combines wave properties from linear dynamics with a sophisticated statistical analysis to provide such estimates. A question of fundamental importance that arises is how the measures of IGW activity provided by the HDA are related to the measures coming from the wave–vortex decomposition (WVD) methods. The question is addressed by employing the nonlinear balance relations of the first- order delta-gamma , the Bolin–Charney, and the first- to third-order Rossby-number expansion to carry out WVD. The global kinetic energy of IGWs given by the HDA and WVD are compared in numerical simulations of moist baroclinic waves by the Weather Research and Forecasting (WRF) model in a channel on the f plane. The estimates of the HDA are found to be two to three times smaller than those of the optimal WVD. This is in part due to the absence of a well-defined scale separation between the waves and vertical flows, the IGW estimates by the HDA capturing only the dominant wave packets and with limited scales. It is also shown that the difference between the HDA and WVD estimates is related to the width of the IGW spectrum.

**Date: **October 19, 2017

**Time: **14h

**Location: **E314

**By:** Nili Harnik (Tel Aviv Univ.)

**Title: **

**Abstract:
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**Date:** October 19, 2017

**Time:** 15h

**Location:** E314

**By:** Pablo Zurita Gotor

**Title: **

**Abstract:**