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Understanding and forecasting the evolution of geophysical fluids is a major scientific and societal challenge. Forecasting algorithms should take into account all the available information on the considered dynamic system. The variational data assimilation (VDA) technique combines all these informations in an optimality system (O.S.) in a consistent way to reconstruct the model inputs. VDA is currently...
Recent decadal salinity changes in the Greenland‐Scotland overflow‐derived deep waters are quantified using CTD data from repeated hydrographic sections in the Irminger Sea. The Denmark Strait Overflow Water salinity record shows the absence of any net change over the 1980s–2000s; changes in the Iceland–Scotland Overflow Water (ISOW) and in the deep water column (σ0 > 27.82), enclosing both overflows,...
The generation of internal waves from barotropic tides can be quantified in terms of the conversion rates. These have often been obtained by applying the WKB approximation, which yields an expression for the conversion rates which is proportional to the seabed buoyancy frequency NB. For small values of NB or strong variations of the buoyancy profile N(z), this gives unreliable results. Using homogenization...
The performance of an atmospherically forced ocean general circulation model (OGCM) in simulating daily and monthly sea surface temperature (SST) is examined during the historical El Niño Southern Oscillation (ENSO) events during the time period 1993–2003. For this purpose, we use the HYbrid Coordinate Ocean Model (HYCOM) configured for the North Pacific north of 20°S at a resolution of ≈9 km. There...
Zonally propagating wave solutions of the linearized shallow water equations (LSWE) in a zonal channel on the rotating spherical earth are constructed from numerical solutions of eigenvalue equations that yield the meridional variation of the waves’ amplitudes and the phase speeds of these waves. An approximate Schrödinger equation, whose potential depends on one parameter only, is derived, and this...
Recent loss of summer sea ice in the Arctic is directly connected to shifts in northern wind patterns in the following autumn, which has the potential of altering the heat budget at the cold end of the global heat engine. With continuing loss of summer sea ice to less than 20% of its climatological mean over the next decades, we anticipate increased modification of atmospheric circulation patterns...
This study revisits the problem of assimilation of asynchronous observations, or four‐dimensional data assimilation, with the ensemble Kalman filter (EnKF). We show that for a system with perfect model and linear dynamics the ensemble Kalman smoother (EnKS) provides a simple and efficient solution for the problem: one just needs to use the ensemble observations (that is, the forecast observations...
We study the evolution of finite perturbations in the Lorenz ‘96 model, a meteorological toy model of the atmosphere. The initial perturbations are chosen to be aligned along different dynamic vectors: bred, Lyapunov, and singular vectors. Using a particular vector determines not only the amplification rate of the perturbation but also the spatial structure of the perturbation and its stability under...
Marine cold air outbreaks (MCAOs) are events where cold air flows over a relatively warm sea surface. Such outbreaks are associated with severe mesoscale weather systems that are not generally resolved in global climate models, such as polar lows and boundary‐layer fronts. Here, an analysis of winter climatology and variability of MCAOs in the Southern Hemisphere (SH) is presented. Near the sea ice...
The dynamics of a salinity‐dominated thermohaline circulation in a sill basin is examined using a two‐layer model. A prescribed freshwater supply acts to establish a stable stratification, working against a prescribed destabilizing temperature difference. The upper‐layer outflow is in geostrophic balance and the upwelling is driven by a fixed energy supply to small‐scale vertical mixing. The salinity‐dominated...
An experimental study of horizontal convection with a free surface has been conducted. Fresh water was heated from above by an infrared lamp placed at one end of a tank, and cooled by evaporation as the water moved away from the heat source. The heat radiated from the lamp was absorbed in a thin (less than 1 mm) layer next to the surface, and then advected and diffused away from the lamp region. Latent...
An important experimental result, as yet poorly understood, is that mechanical stirring can significantly enhance the strength of horizontal convection. A contentious issue is whether this necessarily implies that the mechanical stirring replaces the buoyancy forcing as the main source of energy driving the observed overturning circulation, as has been suggested for the Atlantic meridional overturning...
Extensive changes have been reported from the Arctic Mediterranean. The ice cover is retreating, the temperature in the Atlantic layer has been increasing, the salinity in the upper layers shows large variations and deep waters in the Greenland Sea have become warmer and more saline. These changes all appear externally forced; by the radiation balance, by the atmosphere, and by ocean advection. The...
An original one‐dimensional (1‐D) retrieval followed by a three‐dimensional variational (1D+3DVar) assimilation technique is being developed to assimilate volumes of radar reflectivity data in the high‐resolution numerical weather prediction Arome model.
The good performance of the 1‐D retrieval is shown for an isolated storm over southwestern France through an observing system simulation experiment...
An upper bound is derived for the total dissipation rate in an ocean forced exclusively by surface fluxes of heat and freshwater, assuming a non‐linear equation of state. This generalizes the upper bound found by Paparella and Young, which is valid for a flow forced by an imposed temperature distribution at the surface and a linear equation of state. Like this previous result, the present one shows...
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...
High‐latitude sea breezes and the related daytime calm zones were studied through fine‐scale two‐dimensional idealized simulations by varying the direction and speed of the ambient large‐scale geostrophic flow in small steps. Strongest coastal afternoon breezes were obtained for moderate large‐scale flows 45°–90° left from the pure offshore direction (as seen from the sea), while calm zones appeared...
A statistical–dynamical regionalization approach is developed to assess possible changes in wind storm impacts. The method is applied to North Rhine‐Westphalia (Western Germany) using the FOOT3DK mesoscale model for dynamical downscaling and ECHAM5/OM1 global circulation model climate projections. The method first classifies typical weather developments within the reanalysis period using K‐means cluster...
The influence of sea surface temperature (SST) on the locations of the genesis and of landfall of tropical cyclones in the North Atlantic is analyzed. For that purpose, these locations are calculated from HURDAT and split into two disjoint subsets according to whether SST in the North Atlantic was above or below average in the year the corresponding storms occurred. Landfalls are investigated separately...
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