Coupling coefficients and kinetic equation for Rossby waves in multi-layer ocean

International audienceThe kinetic description of baroclinic Rossby waves in multi-layer model ocean is analysed. Explicit analytical expressions for the coupling coefficients describing energy exchange intensity between different modes are obtained and their main properties are established for the three-layer model. It is demonstrated that several types of interactions vanish in the case of simple vertical structures of the ocean, e.g. when all layers have equal depth. These cases correspond to a zero component of the eigenvectors of the potential vorticity equations. The kinetic equation always possesses a fully barotropic solution. If energy is concentrated in the baroclinic modes, the barotropic mode will necessarily be generated. Motion systems consisting of a superposition of the barotropic and a baroclinic mode always transfer energy to other baroclinic modes

Shifts in early spring wind regime in North-East Europe (1955?2007)

International audienceChanges of the winter-to-spring switch-time of the upper air flow regime at 850 and 500 hPa levels over the north-eastern Baltic Sea are analyzed based on a data set extending until 2007. The long-term variation of the air flow in early spring (March) exhibits multiple regime shifts. The shifts are extracted by means of a vector analysis of the monthly mean air flow as well as the statistical shift detection technology. In the middle of the 1960s the average air flow turned from NW (WNW) to W (WSW) at the 500 (850) hPa level. The original regime was restored in the mid-1990s. The regime shifts in the average air flow in March can be interpreted as changes in the transition time from winter to summer circulation type

Spectral evolution of two-layer weak geostrophic turbulence. Part I: Typical scenarios

International audienceLong-time evolution of large-scale geophysical flows is considered in a ?-plane approximation. Motions in an infinite 2-layer model ocean are treated as a system of weakly nonlinear Rossby waves (weak geostrophic turbulence). The evolution of the energy spectrum of the barotropic and the baroclinic modes is investigated on the basis of numerical experiments with the kinetic equation for baroclinic Rossby waves. The basic features of free (nonforced inviscid) spectral evolution of baroclinic flows are similar to those of the barotropic motions. A portion of the energy is transferred to a sharp spectral peak while the rest of it is isotropically distributed. The peak corresponds to an intensive nearly zonal barotropic flow. Typically, this well-defined barotropic zonal anisotropy inhibits the reinforcement of its baroclinic analogy. For a certain set of initial conditions (in particular, if the barotropic zonal flow is not present initially), a zonal anisotropy of both modes is generated. The interplay between the multimodal nearly zonal flow components leads to the excitation of large-scale (several times exceeding the scale of the initial state), mostly meridional, baroclinic motions at the expense of the barotropic nearly zonal flow. The underlying mechanism is explained on the level of elementary mixed-triad interaction. The whole wave field retains its essentially baroclinic as well as spectrally broad nature. It evidently tends towards a thermodynamically equilibrated final state, consisting of the superposition of a (usually barotropic, but occasionally multimodal) zonal flow and a wave system with a Raleigh-Jeans spectrum. This evolution takes place as a multi-staged process, with fast convergence of the modal spectra to a local equilibrium followed by a more gradual adjustment of the energy balance between the modes

Wave conditions in the Baltic Proper and in the Gulf of Finland during windstorm Gudrun

International audienceWave conditions in the northern Baltic Proper during windstorm Erwin/Gudrun (January 2005) are analysed based on in situ measurements in three locations and output of operational wave models from the German Weather Forecast Service, the Danish Meteorological Institute and the Finnish Institute of Marine Research. The measured significant wave height reached 7.2 m in the northern Baltic Proper and 4.5 m in the Gulf of Finland. The roughest wave conditions, estimated from the comparison of the forecast and measured data, occurred remote from the sensors, off the coasts of Saaremaa and Latvia where the significant wave height was about 9.5 m. Peak periods exceeded 12 s in a large part of the northern Baltic Proper and in the central part of the Gulf of Finland

Wave climate in the Arkona Basin, the Baltic Sea

The basic features of the wave climate in the Southwestern Baltic Sea (such as the average and typical wave conditions, frequency of occurrence of different wave parameters, variations in wave heights from weekly to decadal scales) are established based on waverider measurements at the Darss Sill in 1991–2010. The measured climate is compared with two numerical simulations with the WAM wave model driven by downscaled reanalysis of wind fields for 1958–2002 and by adjusted geostrophic winds for 1970–2007. The wave climate in this region is typical for semi-enclosed basins of the Baltic Sea. The maximum wave heights are about half of those in the Baltic Proper. The maximum recorded significant wave height <i>H</i><sub>S</sub> =4.46 m occurred on 3 November 1995. The wave height exhibits no long-term trend but reveals modest interannual (about 12 % of the long-term mean of 0.76 m) and substantial seasonal variation. The wave periods are mostly concentrated in a narrow range of 2.6–4 s. Their distribution is almost constant over decades. The role of remote swell is very small

Depression Waves Generated by Large Ships in the Venice Lagoon

At present cruise ships use the Lido and Giudecca channels to berth at the Port of Venice, and cargo vessels use the Malamocco Marghera channel to berth at the Marghera industrial area. Following a government decree to restrict large ship access to the Port of Venice through the Lido and Giudecca channels, the Port authority has been investigating the construction of a new channel between Marghera and the cruise terminal, to re-route all large vessels through the Malamocco lagoon entrance. Ships in the Venice Lagoon travel at low speeds with associated low depth-based Fronde numbers (<0.5), and classical Kelvin wave effects are minimal. However, under certain channel geometries, depression waves (also known as Bernoulli wake) can be substantial with significant far-field effects. In the Malamocco Marghera Channel, depression waves up to 2.5 m, with high (similar to 1.5 m/s) near bed water speeds, transform into asymmetric Riemann waves, able to propagate large distances from the navigation channel into the shallow lagoon. By contrast, depression waves associated with large ships in the Lido and Giudecca channels are much smaller (similar to 0.3-0.4 m). Rerouting large cruise ships entering the Port of Venice through the Malamocco Marghera channel and a newly constructed channel will have adverse effects on the Venice lagoon with increased water velocities and sediment resuspension associated with the nonlinear Riemann waves. It is therefore essential that the potential effects be thoroughly evaluated using an appropriate methodology before the development of a new lagoon channel

The Cerenkov effect revisited: from swimming ducks to zero modes in gravitational analogs

We present an interdisciplinary review of the generalized Cerenkov emission of radiation from uniformly moving sources in the different contexts of classical electromagnetism, superfluid hydrodynamics, and classical hydrodynamics. The details of each specific physical systems enter our theory via the dispersion law of the excitations. A geometrical recipe to obtain the emission patterns in both real and wavevector space from the geometrical shape of the dispersion law is discussed and applied to a number of cases of current experimental interest. Some consequences of these emission processes onto the stability of condensed-matter analogs of gravitational systems are finally illustrated.Comment: Lecture Notes at the IX SIGRAV School on "Analogue Gravity" in Como, Italy from May 16th-21th, 201

Complex metacommunity structure for benthic invertebrates in a low-diversity coastal system

The majority of studies in metacommunity ecology have focused on systems other than marine benthic ecosystems, thereby providing an impetus to broaden the focus of metacommunity research to comprise marine systems. These systems are more open than many other systems and may thus exhibit relatively less discrete patterns in community structure across space. Metacommunity structure of soft-sediment benthic invertebrates was examined using a fine-grained (285 sites) data set collected during one summer across a large spatial extent (1700km(2)). We applied the elements of metacommunity structure (EMS) approach, allowing multiple hypothesis of variation in community structure to be tested. We demonstrated several patterns associated with environmental variation and associated processes that could simultaneously assemble species to occur at the sites. A quasi-Clementsian pattern was observed frequently, suggesting interdependent ecological relationships among species or similar response to an underlying environmental gradient across sites. A quasi-nested clumped species loss pattern was also observed, which suggests nested habitat specialization. Species richness declined with depth (from 0.5 to 44.8m). We argue that sensitive species may survive in shallower water, which are more stable with regard to oxygen conditions and present greater habitat complexity, in contrast to deeper waters, which may experience periodic disturbance due to hypoxia. Future studies should better integrate disturbance in terms of temporal dynamics and dispersal rates in the EMS approach. We highlight that shallow water sites may act as sources of recruitment to deeper water sites that are relatively more prone to periodic disturbances due to hypoxia. However, these shallow sites are not currently monitored and should be better prioritized in future conservation strategies in marine systems.Peer reviewe