Crisan Demetrescu

Toward a better representation of the secular variation. Case study: The European network of geomagnetic observatories

In the present paper we discuss a few issues regarding the secular variation (SV) and secular acceleration (SA) of the geomagnetic field that have consequences on mapping them at regional scales. Data from the European network of geomagnetic observatories have been analyzed from the perspective offered by existing long time series of annual means. The existence of high-frequency ingredients in the temporal change of the main field has been taken into account too. The importance of eliminating, from observatory and main field model data, prior to any discussion on secular variation, the signal related to external variations is demonstrated. Its consequences for SV analysis and/or mapping, including the jerk concept, are shown. Also, the importance of the geographical scale at which the SV is represented is discussed. To that aim, we used gufm1, IGRF and CM4 models for the main field from which the residual external signature was eliminated. The contribution of high-frequency ingredients to the map pattern is revealed. The results of the paper set additional observational constraints to the main field and geodynamo modeling.

Long-term evolution of the Lower Danube discharge and corresponding climate variations: solar signature imprint

The possible changes in temperature and precipitation regime are expected to lead to changes in the water regime of rivers. In this study, we investigate the long-term evolution of Lower Danube discharge in connection to variations in the precipitation in the Upper-Middle and Lower Danube Basins. The analysis is given by using annual means data from four gauges along the river, on the Romanian territory, namely, Orsova, Ceatal, Sulina, and Sf. Gheorghe, and from 27 weather stations in the Danube Basin. The comparison of the average precipitation in the Upper and Middle Danube Basin, as calculated from the records of 17 weather stations, with the discharge at Orsova, at the entry in the Lower Danube segment, shows correlated interannual and multi-decadal variations. The variations in precipitation in the Lower Danube Basin, recorded at ten weather stations, show up to a certain degree in variations of the tributary rivers discharge and in the discharge difference between the upstream station Orsova and the downstream station Ceatal. The precipitation and discharge data from the two sub-basins have been examined from the viewpoint of multi-decadal variability associated with Atlantic variability and with solar variability at decadal and multi-decadal timescales. Significant variations at the two timescales have been found.