Seasonal modes of dryness and wetness variability over Europe and their connections with large scale atmospheric circulation and global sea surface temperature

The relationship between the seasonal modes of interannual variability of a multiscalar drought index over Europe and the large-scale atmospheric circulation and sea surface temperature (SST) anomaly fields is investigated through statistical analysis of observed and reanalysis data. It is shown that the seasonal modes of dryness and wetness variability over Europe and their relationship with the large-scale atmospheric circulation and global SST anomaly fields differ from one season to another. During winter, the dominant modes of dryness and wetness variability are influenced by the Arctic Oscillation (AO)/North Atlantic Oscillation (NAO), the Scandinavian pattern, the East Atlantic pattern and the East Atlantic/Western Russia pattern. The spring dryness/wetness modes are influenced mainly by the AO, Polar/Eurasian patterns and the Atlantic Multidecadal Oscillation conditions. The phases (positive or negative) and the superposition of these large scale variability modes play a significant role in modulating the drought conditions over Europe. During summer, the atmospheric blocking is one of the main drivers of dryness and wetness conditions, while during autumn dryness/wetness conditions variability can be related to the NAO or with a wave train like pattern in the geopotential height at 850mb, which develops over the Atlantic Ocean and extends up to Siberia. It is also found that the response of the dryness and wetness conditions to global SST is more regional in summer, compared to the other seasons, when local processes may play a more important role

Hydrology needed to manage droughts: the 2015 European case

Hydrology needed to manage droughts: the 2015 European cas

Assessment of droughts in Romania using the Standardized Precipitation Index

This paper analyses the temporal and spatial variability of droughts in Romania, over the last five decades, based on a high-resolution data set developed at country level, namely ROCADA. Droughts are analyzed by means of the Standardized Precipitation Index (SPI) for 3-, 6- and 12-month time scales. The time period 1979–1995 was identified as the period with the highest number of months affected by moderate, severe as well as extreme drought conditions. The 2000–2001 episode was identified as the major drought event, concerning the severity and the spatial extent, with an area of 60 % of the country affected by extreme drought for more than 10 consecutive months. The results of the trend analysis emphasize an inhomogeneous spatial aspect of the dryness/wetness trends. Statistically significant positive trends (wetter conditions) over small areas distributed inhomogeneous around the country like the southernmost corner as well as the northeastern part and some small areas in the western part of the country have been identified. Statistically significant negative (drier conditions) trends have been obtained over the southwestern part of the country and over the eastern part. In general, the SPI trends follow the observed trends in the monthly precipitation totals, at country level. The results indicate that there is no spatial consistency in the occurrence of droughts at country level and the SPI at different time scales may vary in its usefulness in drought monitoring, due to the fact that in the case of shorter time scales the SPI values have the tendency to fluctuate frequently above and below the zero line, while for longer time scales there are well-defined dry and wet cycles

Spatio-Temporal Variability of Seasonal Drought over the Dobrogea Region

In this study we have examined the spatial and temporal variability of seasonal short-term drought over Dobrogea region over the period 1965 -2005. The dominant mode of spatial variability captures an in-phase relationship of drought conditions over the entire analyzed region, for all the seasons. We show that the Arctic/North Atlantic Oscillation patterns control a significant part of the interannual drought variability over the Dobrogea region in all seasons. Dry (wet) periods in Dobrogea region are associated with geopotential height anomalies at 850mb that project onto the negative (positive) phase of Arctic/North Atlantic Oscillation. Moreover, the SST anomalies from the Atlantic Ocean realm and potential evapotranspiration anomalies over the south eastern part of Romania play also a significant role on the variability of drought conditions over Dobrogea region

Identification of Dry Periods in the Dobrogea Region

The main objective of this study is to identify different types of drought (moderate, severe and extreme) in the Dobrogea region based on three indicators: the Standardized Precipitation Index (SPI), the Standardized Precipitation and Evapotranspiration Index (SPEI) and the Standardized Flow Index (SFI). The dry periods, from a meteorological point of view, were identified based on a drought index that takes into account only precipitation (SPI) and another one that takes into account both precipitation as well as mean air temperature (SPEI). To highlight the dry periods from a hydrological point of view we applied the procedure for calculating the SPI to monthly discharge time series, through the Standardized Flow Index (SFI)

Interannual variability of Rhine river streamflow and its relationship with large-scale anomaly patterns in spring and autumn

Interannual to decadal variability of Rhine river streamflow and their relationship with large-scale climate anomaly patterns for spring (MAM) and autumn (SON) are investigated through statistical analysis of observed streamflow data and global climate anomaly fields. A wavelet analysis reveals that spring streamflow variability is non-stationary with enhanced variability in the 8–16 years band from 1860 to 1900 and in the 2–8 years and 16–30 years after 1960. A composite analysis reveals that streamflow anomalies during spring are related with a sea surface temperature (SST) pattern which resembles the corresponding El Nino-Southern Oscillation (ENSO) SST pattern. The corresponding atmospheric circulation pattern favors enhanced moisture advection over Rhine catchment area during positive streamflow anomalies. During autumn the streamflow variability follows a distribution similar to spring streamflow but with a strong peak in the 30–60 year band. Autumn streamflow anomalies are significantly related only with the North Atlantic SST anomalies. The atmospheric circulation pattern associated to high streamflow during autumn, which is more regional than the corresponding spring pattern, shows a deep low-pressure system over the British Isles and the north-western part of Europe and a shift southward of the Atlantic jet axis. The orientation of the axis of the Atlantic and African Jet, as well as the advection of the moist air from the ocean, plays a crucial role in the variability of Rhine streamflow both in spring and autumn

Spatial and temporal variability of winter streamflow over Romania and its relationship to large-scale atmospheric circulation

In this study we have examined the spatial and temporal variability of winter (DJF) streamflow over Romania as recorded at 46 hydrological stations over the period 1935 -2010. An empirical orthogonal function analysis (EOFs) was employed to characterize the spatial variability of winter streamflow. The dominant mode captures in-phase variability of river flow anomalies over the entire country. The second mode is characterized by a north-south dipole, emphasizing the influence of topography over the streamflow variability. Both modes are related with large-scale atmospheric circulation and sea surface temperature patterns. We show that the Arctic/North Atlantic Oscillation, East Atlantic, East Atlantic/Western Russia and Scandinavian patterns control a significant part of the interannual winter streamflow variability as captured by these two modes. Moreover, we show that the winter streamflow is very sensitive to the influence of winter temperatures. Positive streamflow anomalies are recorded during warm winters, which are favorable to precipitation fallen as rain, while cold winters can favor snowy winters and frozen ground and hence reduced winter discharges

Interannual to decadal summer drought variability over Europe and its relationship to global sea surface temperature

Interannual to decadal variability of European summer drought and its relationship with global sea surface temperature (SST) is investigated using the newly developed self calibrated Palmer drought severity index (scPDSI) and global sea surface temperature (SST) field for the period 1901–2002. A European drought severity index defined as the average of scPDSI over entire Europe shows quasiperiodic variations in the 2.5–5 year band as well as at 12–13 years suggesting a possible potential predictability of averaged drought conditions over Europe. A Canonical Correlation Analysis between summer scPDSI anomalies over Europe and global SST anomalies reveals the existence of three modes of coupled summer drought scPDSI patterns and winter global SST anomalies. The first scPDSI-SST coupled mode represents the long-term trends in the data which manifest in SST as warming over all oceans. The associated long-term trend in scPDSI suggests increasing drought conditions over the central part of Europe. The second mode is related to the inter-annual ENSO and decadal PDO influence on the European climate and the third one captures mainly the drought pattern associated to Atlantic Multidecadal Oscillation. The lag relationships between winter SST and summer drought conditions established in this study can provide a valuable skill for the prediction of drought conditions over Europe on interannual to decadal time scales

Multidecadal variability of summer temperature over Romania and its relation with Atlantic Multidecadal Oscillation

We investigate the multidecadal variability of summer temperature over Romania as measured at 14 meteorological stations with long term observational records. The dominant pattern of summer temperature variability has a monopolar structure and shows pronounced multidecadal variations. A correlation analysis reveals that these multidecadal variations are related with multidecadal variations in the frequency of four daily atmospheric circulation patterns from the North Atlantic region. It is found that, on multidecadal time scales, negative summer mean temperature (TT) anomalies are associated with positive sea level pressure (SLP) anomalies centered over the northern part of the Atlantic Ocean and Scandinavia and negative SLP anomalies centered over the northern part of Africa. It is speculated that a possible cause of multidecadal fluctuations in the frequency of these four patterns are the sea surface temperature anomalies associated to the Atlantic Multidecadal Oscillation. These results have implications for predicting the evolution of summer temperature over Romania on multidecadal time scales