Ileana Mares

NAO impact on the summer moisture variability across Europe

Abstract The Palmer Drought Severity Index (PDSI) for the summer time over the 1891–1991 interval as it was calculated by Briffa et al. [ Int. J. Climatol 14 (1994) 475–506] is considered. PDSI is analysed in this paper for six regions. As we expected the NAO signal is evident in wintertime, but its influence is not simultaneous in the same month––but with some lags. For instance, the best significant NAO signal has been obtained by considering the NAO in January as predictor and the drought index in Romania in January–March as predictand. For this pair the highest signal-to-noise ratio has been obtained. The following result is related to the influence of NAO in January on the precipitation behaviour in February–April. The other results emphasised PDSI responses at the NAO signal in the cold half of the year with several lag months delay.

A Hidden Markov Model Applied to the Daily Spring Precipitation over the Danube Basin

The main goal of this study is to obtain an improvement of the spring precipitation estimation at local scale, taking into account the atmospheric circulation on the Atlantic-European region, by a statistical downscaling procedure. First we have fitted the precipitation amounts from the 19 stations with a HMM with 7 states. The stations are situated in localities crossed by the Danube or situated on the principal tributaries. The number of hidden states has been determined by means of BIC values. A NHMM has been applied then to precipitation occurrence associated with the information about atmospheric circulation over Atlantic-European region. The atmospheric circulation is quantified by the first 10 components of the decomposition in the EOFs or MEOFs. The predictors taking into account CWTs for SLP and the first summary variable from a SVD have also been tested. The atmospheric predictors are derived from SLP, geopotential, temperature, and specific and relative humidity at 850 hPa. As a result of analyzing the multitude of the predictors, a statistical method of selection based on the informational content has been achieved. The test of the NHMM performances has revealed that SLP and geopotential at 850 hPa are the best predictors for precipitation.

Improvement of long-range forecasting by EEOF extrapolation using an AR-MEM model

Abstract This paper presents an optimum combination of two robust statistical techniques that can be used to improve the skill of long-range weather forecasts. The first method uses decomposition and analysis based on extended empirical orthogonal functions (EEOFs), with a 3-month data window, for temperature and precipitation fields in Romania. Using rule N to select the significant components led to three modes for temperature and to nine modes for precipitation. An autoregressive (AR) model is used to produce forecasts of the time series of the EEOF components. The parameters of this model are determined by a method consistent with the maximum entropy method, which is why this model is named AR-MEM. In order to select model order, seven criteria are tested, some of which are efficient, while the others are consistent. The skill of these methods is tested using simulated time series. Model parameters are determined from observational data over the period 1950–90. The Heidke skill score is computed using...

Testing of NAO and ENSO signals in the precipitation field in Europe

The summer Palmer Drought Severity Index (PDSI) for the 1891–1991 period is considered in five from nine homogeneous regions, as they were defined by Briffa et al. (1994), and in a region representative for the Romania zone (ROMZ).

Discharge variability in Romania using Palmer indices and a simple atmospheric index of large-scale circulation

ABSTRACT The aim of this study is to analyse the seasonal characteristics of four Palmer indices calculated on the basis of data from 27 meteorological stations in Romania, and the impact of these indices on river discharges in the period 1931–1998. Our research also tests the influence of large-scale atmospheric circulation on these indices and on discharge. For each season, developments in the empirical orthogonal functions (EOF) and multivariate EOF (MEOF) are achieved. The MEOF representation highlights the overall characteristics of the four Palmer indices. It maximizes specific information for each season compared with individual information of each Palmer index. We then identify geographical areas with homogeneous distribution, taking into account both the discharge distribution and the rotated EOF components of each Palmer index. Finally, we analysed the impact of large-scale atmospheric circulation on hydro-climatic events in Romania by means of the Greenland-Balkan Oscillation Index (GBOI), which is shown to have a greater influence on southeastern Europe than the North Atlantic Oscillation Index (NAOI).

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.