ENSO-related quadriennial variations in European rainfall (original) (raw)
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El Ni�o-southern oscillation events and associated European winter precipitation anomalies
International Journal of Climatology, 2005
The winter precipitation anomalies in the European area have been analysed over the period 1900-98 based on the El Niño-Southern oscillation (ENSO) state. A set of winter and autumn ENSO events is first selected using the Sea-Surface temperature (SST) data of the Niño 3 region, with the constraint that the ENSO event is well developed during the winter and autumn of study, and that it is an extreme event. Cold and warm ENSO events and periods that can be regarded as normal are selected. For the selected winter ENSO events and for the winter following the selected autumn ENSO events, composites of European winter precipitation anomalies have been obtained and compared with each other. A study of the consistency among events of the relationship between ENSO and precipitation anomalies was also carried out. The analysis of the winter precipitation anomalies based on the selected winter ENSO events shows the existence, for the European area and during La Niña events, of a statistically significant precipitation anomaly pattern with positive precipitation anomalies north of the British Isles and in the Scandinavian area and negative anomalies in southern Europe, resembling the precipitation pattern associated with the positive phase of the North Atlantic oscillation (NAO). Particularly, for the southwestern area of the Iberian Peninsula, the negative anomaly reaches 20% of the winter average precipitation. The consistency analysis shows that this precipitation pattern is not the result of a few major events, but rather that it is stable and qualitatively similar to that found during the positive phase of the NAO. A non-linear response to ENSO is found in the eastern Mediterranean area: negative precipitation anomalies are found, having similar amplitude anomalies, both during El Niño and La Niña events. The analysis of the precipitation anomalies for the winter following the selected autumn ENSO events shows very similar results to those found for the previous analysis, thus suggesting the existence of a potential source of seasonal forecasting of European precipitation. An analysis of the sensitivity of the precipitation anomalies to the strength of the ENSO events shows that, when the strength of the ENSO increases, the magnitude of the rainfall anomalies does not change, but the area influenced and the coherence between events do increase slightly.
Euro-Mediterranean rainfall and ENSO- A seasonally varying relationship
Geophysical Research Letters, 2002
1] Using observational datasets and atmospheric reanalyses, we show that interannual variability of rainfall in the Euro-Mediterranean sector is significantly influenced by ENSO in a way that is seasonally varying. Spatially coherent correlation patterns are found in central and eastern Europe during winter and spring, and in western Europe and the Mediterranean region during autumn and spring. A composite analysis of ENSO events indicates that during an El Nino western Mediterranean rainfall has a 10% increase (decrease) in the autumn preceeding (spring after) the mature phase of an event, corresponding to a rainy season arriving (retreating) earlier compared to the climatology. The atmospheric reanalyses show that an anomalous atmospheric circulation and moisture transport extending from the Atlantic Ocean into the Euro-Mediterranean region accompanies the observed rainfall anomalies. Multidecadal variations characterize the ENSO Euro-Mediterranean relationship during the 20th century.
International Journal of Climatology, 2003
The link between El Niño-southern oscillation (ENSO) variability in boreal winter (represented by the NIÑO3 index, i.e. East Pacific sea-surface temperature anomalies) and the large-scale circulation and weather conditions over Europe-northwest Africa in spring is explored, considering station reports of precipitation, sea-level pressure (SLP) anomalies and two North Atlantic oscillation (NAO) indices. It is found that these relations have undergone consistent and simultaneous changes in the 20th century. Three characteristic periods can be identified. During 1900-25 and 1962-87, positive NIÑO3 index values are associated with enhanced precipitation over central Europe and reduced rainfall in southern Europe and northern Africa. The ENSO influence on precipitation over Scotland and Norway is small. The rainfall anomalies can be explained from the advective and dynamical implications of a north-south dipole in SLP correlations (warm ENSO events followed by low pressure in northern Europe and high pressure over the Mediterranean Sea-North Africa). This dipole hardly projects on the commonly used NAO centres (Iceland and Azores/Gibraltar) and thus ENSO-NAO correlations are insignificant. During 1931-56 the NIÑO3 index reveals little influence on precipitation over the Iberian Peninsula and Morocco, but there are large negative correlations with precipitation over Scotland and Norway. This is related to an alteration of the NIÑO3-SLP correlation pattern, which implies high pressure over northern Europe and low pressure over central Europe after warm events, and thus a virtually inverted dipole with respect to the other two periods. The large westward extension of the dipole leads to a significant NAO-NIÑO3 correlation of r = −0.5. These alterations were accompanied by substantial large-scale circulation changes during the period 1931-56, as revealed by anomalously high pressure and dry conditions over central-western Europe, a change in precipitation-producing SLP patterns for Morocco and an anomalously low number of positive NAO and NIÑO3 index values. It is left for discussion as to whether the decadal variations described are due to a change in the physics of the teleconnection or to stochastic fluctuations.
Interannual oscillations in winter rainfall over Europe. Iberia study case
Finisterra, 2012
Low -frequency variability is a good starting point for modelling the climate system and understanding mechanisms that can give us some indication of future climate evolution. this work aims to better understand the degree of association between the atmospheric circulation and precipitation over europe and the iberian Peninsula. to achieve this goal, the standard precipitation anomalies in europe and the anomalies of sea level pressure (sLP) in the north atlantic region during the twentieth century were subjected to Multi -Channel singular spectrum analysis (Mssa). this procedure allows us to measure the degree of association between the sLP and precipitation fields. the identification of common oscillations between the two climatic fields led not only to the confirmation of the physical meaning of these oscillations, but also to the identification of the dynamical evolution of the ocean -atmosphere system in the north atlantic. two oscillations, statistically significant, common to...
Spatial-temporal structures of quasi-periodic oscillations in precipitation over Europe
International Journal of Climatology, 2000
Based on a 5°×5° gridded monthly precipitation data set from 1911 to 1990, quasi-periodic oscillations are identified by performing a multichannel singular spectrum analysis (MSSA) on the first ten principal components (PCs; explaining 53.1% of total variance) of normalized monthly precipitation fields over Europe. Two robust quasi-periodic oscillations are isolated with periods of 23.8 months (2.0 years) and 43.5 months (3.6 years), respectively. Applying a reconstruction technique and composite analysis, spatial structures and temporal propagation of the quasi-periodic oscillations are revealed. The two oscillations behave differently: the 43.5-month oscillation clearly propagates northward; however, the 23.8-month oscillation seems quasi-stationary, although it propagates slowly southward. This may imply that the causes of these two oscillations are different. Oscillatory behaviour in the time series of a North Atlantic Oscillation (NAO) index has also been analysed by performing singular spectrum analysis (SSA) for the same period used for the analysis of precipitation. However, in the time series of the NAO index, the dominant oscillation is a 27.7-month (2.3-year) period, with no robust oscillation with a period of 3.6 years being detected. Furthermore, the two oscillations cannot be identified by MSSA from the precipitation fields regressed upon the NAO index. Therefore, it appears that the two quasi-periodic oscillations are not well related to the NAO, which may not be the most important atmospheric pattern associated with non-seasonal precipitation variability over Europe.
Space-time structure of extreme precipitation in Europe over the last century
International Journal of Climatology, 2014
We investigate the space-time structure of extreme precipitation in Europe over the last century, using daily rainfall data from the European Climate Assessment & Dataset (ECA&D) archive. The database includes 267 stations with records longer than 100 years. In the winter season (October to March), for each station, two classes of daily rainfall amount values are selected that, respectively, exceed the 90th and 95th percentile of daily rainfall amount over all the 100 years. For each class, and at each location, an annual time series of the frequency of exceedance and of the total precipitation, defined respectively as the number of days the rainfall threshold (90th and 95th percentiles) is exceeded and total precipitation on days when the percentile is exceeded, are developed. Space-time structure of the frequency and total precipitation time series at the different locations are then pursued using multivariate time and frequency domain methods. The identified key trends and organized spectral modes are linked to well-known climate indices, as North Atlantic Oscillation (NAO) and El Nino Southern Oscillation (ENSO). The spectra of the leading principal component of frequency of exceedance and of total precipitation have a peak with a 5-year period that is significant at the 5% level. These are also significantly correlated with ENSO series with this period. The spectrum of total rainfall is significant at the 10% level with a period of ∼8 years. This appears to be significantly correlated to the NAO index at this period. Thus, a decomposition of both secular trends and quasi-periodic behaviour in extreme daily rainfall is provided.
International Journal of Climatology, 2004
December-February (DJF) extreme rainfall was analysed at 347 European stations for the period 1958-2000. Two indices of extreme rainfall were examined: the maximum number of consecutive dry days (CDD); and the number of days above the 1961-90 90th percentile of wet-day amounts (R90N). A principal component analysis of CDD found six components that accounted for 52.4% of the total variance. Six components of DJF R90N were also retained that accounted for 39.1% of the total variance. The second component of R90N has a very significant trend and the factor loadings closely resemble the observed linear trend in this index, suggesting that the analysis has isolated the mode of variability causing the trend as a separate component. The principal components of the indices were correlated with surface and upper-air observations over the North Atlantic. The best correlations were generally found to be with sea-level pressure (SLP) observations. A separate canonical correlation analysis of each of the two indices with SLP revealed several coupled modes of variability. The North Atlantic oscillation (NAO) was isolated as the first canonical pattern for R90N. For CDD the first two canonical coefficients of CDD were significantly correlated with the NAO index. Generally, the canonical coefficients with the highest correlations with the NAO had the most significant trends, suggesting that the observed trend in the NAO has strongly contributed to the observed trends in the indices. Two other important canonical patterns were isolated: a pattern of anomalous mean SLP (MSLP) centred over the North Sea, which seems to be related to local sea-surface temperature over this region; and a dipole-like pattern of MSLP with poles over the eastern Mediterranean and the central North Atlantic. Repeating the canonical correlation analysis with two other indices of extreme rainfall, the 90th percentile of wet day amounts and the maximum 10 day rainfall total, gives very similar coupled patterns. Copyright
Probabilistic Precipitation Anomalies Associated with ENSO
Bulletin of The American Meteorological Society, 2001
Extreme phases of the El Niño-Southern Oscillation (ENSO) phenomenon have been blamed for precipitation anomalies in many areas of the world. In some areas the probability of above-normal precipitation may be increased during warm or cold events, while in others below-normal precipitation may be more likely. The percentages of times that seasonal precipitation over land areas was above, near, and below normal during the eight strongest El Niño and La Niña episodes are tabulated, and the significance levels of the posterior probabilities are calculated using the hypergeometric distribution. These frequencies may provide a useful starting point for probabilistic climate forecasts during strong ENSO events. Areas with significantly high or low frequencies or above- or below-normal precipitation are highlighted, and attempts are made to estimate the proportion of land areas with significant ENSO-related precipitation signals. There is a danger of overstating the global impact of ENSO events because only about 20%-30% of land areas experience significantly increased probabilities of above- or below-normal seasonal precipitation during at least some part of the year. Since different areas are affected at different times of the year, the fraction of global land affected in any particular season is only about 15%-25%. The danger of focusing on the impact of only warm-phase events is emphasized also: the global impact of La Niña seems to be at least as widespread as that of El Niño. Furthermore, there are a number of notable asymmetries in precipitation responses to El Niño and La Niña events. For many areas it should not be assumed that the typical climate anomaly of one ENSO extreme is likely to be the opposite of the other extreme. A high frequency of above-normal precipitation during strong El Niño conditions, for example, does not guarantee a high frequency of below-normal precipitation during La Niña events, or vice versa. On a global basis El Niño events are predominantly associated with below-normal seasonal precipitation over land, whereas La Niña events result in a wider extent of above-normal precipitation.
A growing interest in extreme precipitation has spread through the scientific community due to the effects of global climate change on the hydrological cycle, and their threat to natural systems' higher than average climatic values. Understanding the variability of precipitation indices and their association to atmospheric processes could help to project the frequency and severity of extremes. This paper evaluates the trend of three precipitation extremes: the number of consecutive dry/wet days (CDD/CWD) and the quotient of the precipitation in days where daily precipitation exceeds the 95th percentile of the reference period and the total amount of precipitation (or contribution of very wet days, R95pTOT). The aim of this study is twofold. First, extreme indicators are compared against accumulated precipitation (RR) over Europe in terms of trends using non-parametric approaches. Second, we analyse the geographically opposite trends found over different parts of Europe by considering their relationships with large-scale processes, using different teleconnection patterns. The study is accomplished for the four seasons using the gridded E-OBS data set developed within the EU ENSEMBLES project.
Multidecadal variability of the continental precipitation annual amplitude driven by AMO and ENSO
As the water vapor content in the atmosphere scales with temperature, a warmer world is expected to feature an intensification of the hydrological cycle. Work to date has mainly focused on mean precipitation changes, whose connection to climatic modes is elusive at a global scale. Here we show that continental precipitation annual amplitude, which represents the annual range between minimum and maximum (monthly) rainfall, covaries with a linear combination of the Atlantic Multidecadal Oscillation and low-frequency variations in the El Niño–Southern Oscillation on a decadal to multidecadal scale with a correlation coefficient of 0.92 (P < 0.01). The teleconnection is a result of changes in moisture transport in key regions. Reported trends in the annual amplitude of global precipitation in recent decades need to be assessed in light of this substantial low-frequency variability, which could mask or enhance an anthropogenic signal in hydrological cycle changes.