Implementation of a Coupled Atmosphere-Wave-Ocean Model in the Mediterranean Sea: Sensitivity of the Short Time Scale Evolution to the Air-Sea Coupling Mechanisms

In this study a coupled regional atmosphere-wave-ocean model has been implemented in the Mediterranean Sea and applied to the simulation of the atmospheric circulation and of the upper ocean structure on the short time scale range typical of regional meteorological predictions. The coupling accounts for the feedback of the upper ocean on the atmospheric circulation, that is for the variation of the SST (Sea Surface Temperature) during the development of the cyclones and for the dependence of the SSR (Sea Surface Roughness) on the wind-wave spectrum. The model can operate also in uncoupled mode, with prescribed SST and SSR computed using the Charnock formula (depending on wind speed only). Six case-studies, characterized by strong cyclones and intense air-sea interaction, have been analyzed. A sensitivity analysis is carried out by comparing uncoupled and coupled simulations, carried out including the SST and SSR-feedbacks both separately and simultaneously. The feedbacks have systematic effects on the atmospheric precipitation, on heat and momentum fluxes, and, consequently, on MLD (Mixed Layer Depth), SST, and SWH (Significant Wave Height). For all these quantities, except for SWH, the two feedbacks act in opposite directions. The effect of the SST feedback is larger on latent heat flux, precipitation, sea surface cooling, and, marginally, on mixed layer growth and it determines their reduction when the two feedbacks act simultaneously. The SSR and SST-feedbacks have comparable effect on the SWH field and they both contribute to the SWH reduction. Though the SLP (Sea Level Pressure) and Geopotential minima of single "fall" case-studies can be appreciably modified by the inclusion of the feedbacks, no systematic effect has been identified in these fields, presumably because of the intrinsic unpredictability of the atmospheric circulation.     

Mediterranean wave climate variability and its links with NAO and Indian Monsoon

This study examines the variability of the monthly average significant wave height (SWH) field in the Mediterranean Sea, in the period 1958–2001. The analysed data are provided by simulations carried out using the WAM model (WAMDI group, 1988) forced by the wind fields of the ERA-40 (ECMWF Re-Analysis). Comparison with buoy observations, satellite data, and simulations forced by higher resolution wind fields shows that, though results underestimate the actual SWH, they provide a reliable representation of its real space and time variability. Principal component analysis (PCA) shows that the annual cycle is characterised by two main empirical orthogonal functions (EOF) patterns. Most inter-monthly variability is associated with the first EOF, whose positive/negative phase is due to the action of Mistral/Etesian wind regimes. The second EOF is related to the action of southerly winds (Libeccio and Sirocco). The annual cycle presents two main seasons, winter and summer characterised, the first, by the prevalence of eastwards and southeastwards propagating waves all over the basin, and the second, by high southwards propagating waves in the Aegean Sea and Levantin Basin. Spring and fall are transitional seasons, characterised by northwards and northeastwards propagating waves, associated to an intense meridional atmospheric circulation, and by attenuation and amplification, respectively, of the action of Mistral. These wave field variability patterns are associated with consistent sea level pressure (SLP) and surface wind field structures. The intensity of the SWH field shows large inter-annual and inter-decadal variability and a statistically significant decreasing trend of mean winter values. The winter average SWH is anti-correlated with the winter NAO (North Atlantic Oscillation) index, which shows a correspondingly increasing trend. During summer, a minor component of the wave field inter-annual variability (associated to the second EOF) presents a statistically significant correlation with the Indian Monsoon reflecting its influence on the meridional Mediterranean circulation. However, the SLP patterns associated with the SWH inter-annual variability reveal structures different from NAO and Monsoon circulation. In fact, wave field variability is conditioned by regional storminess in combination with the effect of fetch. The latter is likely to be the most important. Therefore, the inter-annual variability of the mean SWH is associated to SLP patterns, which present their most intense features above or close to Mediterranean region, where they are most effective for wave generation.

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Mapping Solar Wind Streams from the Sun to 1 AU: A Comparison of Techniques

A variety of techniques exist for mapping solar wind plasma and magnetic field measurements from one location to another in the heliosphere. Such methods are either applied to extrapolate solar data or coronal model results from near the Sun to 1 AU (or elsewhere), or to map in-situ observations back to the Sun. In this study, we estimate the sensitivity of four models for evolving solar wind streams from the Sun to 1 AU. In order of increasing complexity, these are: i) ballistic extrapolation; ii) ad hoc kinematic mapping; iii) 1-D upwinding propagation; and iv) global heliospheric MHD modeling. We also consider the effects of the interplanetary magnetic field on the evolution of the stream structure. The upwinding technique is a new, simplified method that bridges the extremes of ballistic extrapolation and global heliospheric MHD modeling. It can match the dynamical evolution captured by global models, but is almost as simple to implement and as fast to run as the ballistic approximation.     

KINK MODES AND CURRENT SHEETS IN CORONAL LOOPS

We present simulations of the non-linear evolution of the m=1 kink mode in line-tied coronal loops. We focus on the structure of the current concentrations which develop as a consequence of the instability in two different types of magnetic field configuration, one containing a net axial current and the other with a vanishing total axial current. In the first case, current sheets develop one third of the way from footpoint to loop apex (where the non-linear kink mode folds on itself) within the body of the current channel, while in the second case the current sheet develops at the loop apex at the interface between the current containing channel and the outer axial potential field. In both cases line-tying, while playing a stabilizing role in the linear theory, acts as a destabilizing agent for the non-linear resistive evolution. The unwrapping of magnetic field lines in the vanishing axial current model appears to be consistent with the geometry of compact recurrent loop flares.     

Future changes in cyclone climatology over Europe as inferred from a regional climate simulation

This study analyzes the cyclone climatology in regional climate model simulations of present day (1961–1990) and future (2071–2100, A2 and B2 emission scenarios) european climate conditions. The model domain covers the area from Scandinavia to Northern Africa and from the Eastern Atlantic to Russia at a horizontal grid spacing of 50 km. Compared to present day, in the A2 and B2 scenario conditions the annual average storm track intensity increases over the North-East Atlantic and decreases over Russia and the Eastern Mediterranean region. This overall change pattern is larger in the A2 than in the B2 simulations. However, the cyclone climatology change signal shows a large intermonthly variability and important differences across European regions. The largest changes are found over the North-East Atlantic, where the storm track intensity increases in winter and decreases in summer. A significant reduction of storm track intensity is found during late summer and autumn over the Mediterranean region, and from October to January over Russia. The number of cyclones decreases in future conditions throughout Europe, except over the Central Europe and Mediterranean regions in summer (where it increases). The frequency of intense cyclones and the depth of extreme cyclones increase over the North-East Atlantic, decrease over Russia and show an irregular response over the rest of the domain.

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Global MHD Modeling of the Solar Corona and Inner Heliosphere for the Whole Heliosphere Interval

In an effort to understand the three-dimensional structure of the solar corona and inner heliosphere during the Whole Heliosphere Interval (WHI), we have developed a global magnetohydrodynamics (MHD) solution for Carrington rotation (CR) 2068. Our model, which includes energy-transport processes, such as coronal heating, conduction of heat parallel to the magnetic field, radiative losses, and the effects of Alfvén waves, is capable of producing significantly better estimates of the plasma temperature and density in the corona than have been possible in the past. With such a model, we can compute emission in extreme ultraviolet (EUV) and X-ray wavelengths, as well as scattering in polarized white light. Additionally, from our heliospheric solutions, we can deduce magnetic-field and plasma parameters along specific spacecraft trajectories. In this paper, we present a general analysis of the large-scale structure of the solar corona and inner heliosphere during WHI, focusing, in particular, on i) helmet-streamer structure; ii) the location of the heliospheric current sheet; and iii) the geometry of corotating interaction regions. We also compare model results with i) EUV observations from the EIT instrument onboard SOHO; and ii) in-situ measurements made by the STEREO-A and B spacecraft. Finally, we contrast the global structure of the corona and inner heliosphere during WHI with its structure during the Whole Sun Month (WSM) interval. Overall, our model reproduces the essential features of the observations; however, many discrepancies are present. We discuss several likely causes for them and suggest how model predictions may be improved in the future.