European ammonoid diversity questions the spreading of anoxia as primary cause for the Cenomanian/Turonian (Late Cretaceous) mass extinction

Ammonoid diversity patterns show that the spreading of oceanic anoxia is not the initial and major kill mechanism for the Cenomanian/Turonian mass extinction as usually suggested. In the Anglo-Paris Basin and the Vocontian Basin, the drop of ammonoid species richness starts around the middle/late Cenomanian boundary, i.e. 0.75 myr before the occurrence of anoxic deepwater sediments. The stepwise extinction of first heteromorphs and then acanthoceratids is incompatible with the rise of the oxygen minimum zone. Moreover, shelf environments of these basins remained well oxygenated during the Cenomanian/Turonian boundary interval. Thus, we stress that other causative mechanisms initiated the ammonoid extinction even if anoxia subsequently participated in the demise of marine ecosystems. Editorial handling: M.J. Benton & J.-P. Billon-Bruyat     

Water tank and numerical model studies of flow over steep smooth two-dimensional hills

The present work investigates the role of different treatments of the lower boundary condition on the numerical prediction of flows over two-dimensional, smooth, steep hills. Four different law of the wall formulations are tested when a large recirculating region is formed on the lee side of the hill. Numerical implementation of the near-wall functions was made through a finite elements code. The standard κ–ε model was used to close the averaged Navier–Stokes equations. Results are validated through original data obtained in a water tank. Measurements resorted to laser Doppler anemometry. The experiment provide detailed data for the characterization of the reverse flow in the region between the separation and the reattachment points, with emphasis on the near wall region. The experimental wall shear stress distribution is compared with the results provided by the different laws of the wall showing good agreement. The numerical predictions are shown to vary markedly between different formulations.     

The Influence of the North Atlantic Oscillation on Rainfall Triggering of Landslides near Lisbon

The majority of landsliding episodes in the area north of Lisbon are associated with rainfall events of short (less than 5 days) medium (5–20 days) or long duration (more than 20 days). The precipitation regime in Portugal is highly irregular, with large differences between wet and dry years. We have assessed the impact of the North Atlantic Oscillation (NAO) on both the winter precipitation and the timing and magnitude of associated landslide events. Results show that the large inter-annual variability of winter precipitation is largely modulated by the NAO mode. The precipitation composite corresponding to high NAO index presents a considerable lower median value (47 mm/month) than the corresponding low NAO index class (134 mm/month). The entire precipitation distribution associated with the low NAO index composite encompasses a wider range of values than the corresponding high NAO index composite. This non-linear behavior is reflected in the probability of occurrence of a very wet month (precipitation above the 90% percentile) that is just 1% for the positive NAO class and 23% for low NAO index months. Results for the low NAO class are crucial because these months are more likely associated with long-lasting rainfall episodes responsible for large landslide events. This is confirmed by the application of a 3-month moving average to both NAO index and precipitation time series. This procedure allowed the identification of many months with landslide activity as being characterized by negative average values of the NAO index and high values of average precipitation (above 100 mm/month). Finally, using daily data we have computed the return periods associated with the entire set of landslide episodes and, based on these results, obtained a strong linear relationship between critical cumulative rainfall and the corresponding critical rainfall event duration.     

Diffuse Component Spectra of Solar Active Regions at Submillimeter Wavelengths

Solar maps at 212 and 405 GHz obtained by the Solar Submillimetric Telescope (SST) show regions of enhanced brightness temperature, which coincide with the location of active regions. A statistical study of the radio emission from these active regions was performed for the first time at such high frequencies during 23 days on June and July 2002, when the atmospheric opacity was low. The brightest regions on the maps were chosen for this study, where the brightness excess observed varies from 3 to 20% above quiet Sun levels (i.e., 200–1000 K) at both wavelengths. Sizes of the regions of enhanced emission calculated at half the maximum value were estimated to be between 2′ and 7′. These sizes agree with observed sizes of active regions at other wavelengths such as Hα and ultraviolet. An important result is that the flux density spectra of all sources increase toward submillimeter frequencies, yielding flux density spectral index with an average value of 2.0. The flux density of the active region sources were complemented with that from maps at 17 and 34 GHz from the Nobeyama Radio Heliograph. The resulting spectra at all four frequencies were fit considering the flux density to be due to thermal bremsstrahlung from the active region. In the calculations, the source radius was assumed to be the mean of the measured values at 212 and 405 K. The effective temperatures of the radio emitting source, assumed homogeneous, obtained from this fit were 0.6–2.9 × 10⁴ K, for source diameters of 2′–7′.     

Multi-model calibration and combination of tropical seasonal sea surface temperature forecasts

Different combination methods based on multiple linear regression are explored to identify the conditions that lead to an improvement of seasonal forecast quality when individual operational dynamical systems and a statistical–empirical system are combined. A calibration of the post-processed output is included. The combination methods have been used to merge the ECMWF System 4, the NCEP CFSv2, the Météo-France System 3, and a simple statistical model based on SST lagged regression. The forecast quality was assessed from a deterministic and probabilistic point of view. SSTs averaged over three different tropical regions have been considered: the Niño3.4, the Subtropical Northern Atlantic and Western Tropical Indian SST indices. The forecast quality of these combinations is compared to the forecast quality of a simple multi-model (SMM) where all single models are equally weighted. The results show a large range of behaviours depending on the start date, target month and the index considered. Outperforming the SMM predictions is a difficult task for linear combination methods with the samples currently available in an operational context. The difficulty in the robust estimation of the weights due to the small samples available is one of the reasons that limit the potential benefit of the combination methods that assign unequal weights. However, these combination methods showed the capability to improve the forecast reliability and accuracy in a large proportion of cases. For example, the Forecast Assimilation method proved to be competitive against the SMM while the other combination methods outperformed the SMM when only a small number of forecast systems have skill. Therefore, the weighting does not outperform the SMM when the SMM is very skilful, but it reduces the risk of low skill situations that are found when several single forecast systems have a low skill.     

Influence of Antarctic ice sheet lowering on the Southern Hemisphere climate: modeling experiments mimicking the mid-Miocene

A coupled global atmosphere-ocean model is used to study the influence of the Antarctica ice sheet in a configuration that mimics that of the early Miocene on the atmospheric and oceanic circulations. Based on different climate simulations of the present day (CTR) and conducted with distinct Antarctic ice sheet topography (AIS-EXP), it is found that the reduction of the Antarctic ice sheet topography (AIS) induces warming of the Southern Hemisphere and reduces the meridional thermal gradient. Consequently, the atmospheric transient low level eddy heat flux $[(\overline{v^{\prime}T^{\prime}})]$ and the eddy momentum flux $[(\overline{u^{\prime}v^{\prime}})]$ are reduced causing the reduced transport of heat from the mid-latitudes to the pole. The stationary flow and transient wave anomalies generate changes in the SSTs which modify the rate of deep water formation, strengthening the formation of the Antarctic Bottom Water. Substantial changes are predicted to occur in the atmospheric and oceanic heat transport and a comparison between the total heat transport of the atmosphere-ocean system, as simulated by the AIS-EXP and the CTR runs, shows that the reduction of the AIS height leads to reduced Southern Hemisphere poleward and increased equatorward heat transport. These results are in agreement with reduced storm track activities and baroclinicity.