Seminaires

Upcoming seminars

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: Understanding ENSO’s influence on extratropical cyclones and cyclogenesis over the North Atlantic.

 

Abstract:

El Niño-Southern Oscillation (ENSO) is arguably one of the strongest internal drivers of global weather variability.
While the North Atlantic was traditionally assumed to be one of the regions which are only marginally affected by ENSO, there is a renewed interested in how ENSO affects the North Atlantic circulation on the scale of individual weather systems. I present an overview of our recent research on variability in extratropical cyclogenesis downstream of the Rocky Mountains, over the Gulf Stream and at Greenland and how ENSO-related large-scale circulation anomalies affect cyclone formation in these regions. Surprisingly, there is a significant relationship between extratropical cyclogenesis over the Gulf Stream and leeward of the Rocky Mountains. During La Niña winters, the more active polar jet creates conditions amenable for cyclogenesis leeward of the Rocky Mountains chain. The more active polar jet is a consequence of stationary wave propagation from tropical sources and the stronger La Niña ridge in the northeast Pacific. The downstream propagation of equatorward oriented upper-level eddies, which are reinforced by the polar jet, cause cyclogenesis-friendly conditions at Greenland by pushing the jet poleward. The eddies cause an overall positive NAO response to La Niña as described in the previous literature. During El Niño winters, cyclogenesis occurs more frequently over the Gulf Stream, as a consequence of a more extended Pacific jet. There is an interesting difference between central Pacific and eastern Pacific El Niño-variantes. During the former, Gulf Stream cyclogenesis occurs below the jet exit region, during the latter it preferentially occurs below the jet’s entrance region. Accordingly, during central Pacific El Niño winters, the storm track anchor over the Gulf Stream shifts poleward.

Date: November 10, 2017

Time: 11h

Location: E314

By: Xiyue (Sally) Zhang (Environmental Science and Engineering, California Institute of Technology, Pasadena, CA)

Title: Sensitivities of Arctic clouds to climate change

 

Abstract:

The Arctic has been experiencing rapid changes in recent decades. In addition to the prominent sea ice loss, Arctic amplification is a robust feature in climate models under greenhouse warming and has also been observed over the past decades. Cloud feedbacks and the trapping of heat under the stable inversion are thought to contribute to this Arctic amplification of global warming. However, the sign of any cloud feedback in high latitudes is uncertain. Understanding and constraining Arctic cloud feedback is difficult because of the ubiquitous temperature inversion in high latitudes and the presence of mixed-phase clouds, both of which are challenging to capture in global climate models (GCMs). Here we use high-resolution large-eddy simulations (LES) with a one-moment mixed-phase microphysics scheme to investigate how Arctic clouds respond to climate changes. To represent changing large-scale conditions such as meridional moisture advection in a realizable way, we drive the LES with output from a GCM: temperature and moisture tendencies from a grid cell at 82N are used as forcing terms in the LES. We find that low-cloud fraction decreases with increasing temperature across a wide range of climates. The cloud fraction changes are associated with a change in cloud regimes from stratocumulus in the coldest climate, to cumulus in the warmest climate. This can be understood from changes in the saturation deficit, the difference between specific humidity and saturation specific humidity, which increases with temperature because relative humidity changes in the boundary layer are modest. We also investigate the relationship between lower-tropospheric stability and cloud fraction. A sensitivity experiment that uniformly increases temperature in an atmospheric columns shows that decreased cloud fraction can be associated with increased lower tropospheric stability, contrary to common beliefs. Implications for cloud feedbacks in the Arctic are discussed.

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.

Past seminars

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: Gravity waves in a moist atmosphere: A mechanistic picture

 

Abstract:

Motivated by the need to interpret the influence of convection schemes on tropical variability, the interaction between gravity-driven waves and moisture in a shallow water model is analyzed with an emphasis on physical interpretation. A Betts-Miller type convective parameterization is used, and analytical solutions for the influence of moisture on wave speed and stability are obtained, both at the limit of a vanishing convective relaxation timescale (or “strict quasi-equilibrium” (SQE)) and for finite relaxation timescales. We show that the divergence and moisture fields are exactly out of phase only when the system is at the SQE limit. A relaxation timescale dependent “gross moist stability” and equivalent depth are derived for both one-dimensional gravity waves and Kelvin waves.
The wavenumber dependence of the effect of moisture is also analyzed, and it is seen that for any given value of the convective relaxation time, the larger scale waves are always closer to SQE than the smaller scale waves, as a natural consequence of the equivalence between SQE and the moisture-divergence phasing. The phasing between the height, divergence and moisture fields is calculated, and the behavior of moist gravity and Kelvin waves for finite relaxation timescales is explained using the phase differences between the various fields. Using this analysis, physically based explanations are provided for the results of prior GCM-based studies.

Date: October 19, 2017

Time: 15h

Location: E314

By: Pablo Zurita Gotor  (Univ. Madrid)

Title: Superrotation and the spinup of equatorial eddy-driven jets

 

Abstract:

This talk will discuss the spinup of equatorial westerly jets by tropical eddies, relevant for the problem of planetary superrotation. Motivated by the well-established phenomenology of extratropical eddy-driven jets, most superrotation models have focussed on the latitudinal propagation of the eddies and hence on meridional eddy momentum fluxes. However, it will be argued in this talk that because tropical vorticity dynamics makes meridional momentum advection inefficient accelerating the mean flow in the tropics, equatorial superrotation will be driven by vertical momentum advection in many cases of interest. We first consider the limit of small absolute vorticity in the tropics, relevant for the equilibration of Kelvin-Rossby instability. In this limit, vorticity fluxes are small and meridional momentum advection weak. More generally, it will be argued that meridional advection can only decelerate the mean flow with weak vorticity forcing. This is shown to be the case in the Earth’s deep tropics with the exception of the Northern monsoon season.