The North Atlantic Oscillation (NAO) is a major mode of large-scale climate variability which contains a broad spectrum of variations. There are substantial contributions from short-term 2 -5 year variations, which have clearly marked teleconnections. Decadal trends are also apparent in the historical record of the NAO and may be due to either stochastic or deterministic processes. Evidence is presented that suggests the NAO exhibits 'long-range' dependence having winter values residually correlated over many years. Several simple stochastic models have been used to fit the NAO SLP (sea-level pressure) wintertime index over the period 1864 -1998, and their performance at predicting the following year has been assessed. Long-range fractionally integrated noise provides a better fit than does either stationary red noise or a non-stationary random walk.

The North Atlantic Oscillation (NAO) is a major mode of large‐scale climate variability which contains a broad spectrum of variations. There are substantial contributions from short‐term 2–5 year variations, which have clearly marked teleconnections. Decadal trends are also apparent in the historical record of the NAO and may be due to either stochastic or deterministic processes. Evidence is presented that suggests the NAO exhibits ‘long‐range’ dependence having winter values residually correlated over many years. Several simple stochastic models have been used to fit the NAO SLP (sea‐level pressure) wintertime index over the period 1864–1998, and their performance at predicting the following year has been assessed. Long‐range fractionally integrated noise provides a better fit than does either stationary red noise or a non‐stationary random walk. Copyright © 2000 Royal Meteorological Society

[1] In this article authors explore the relation in the observed data between the Eurasian snow cover anomalies and the NAO variability on interannual to decadal time scales. Results reached in this study suggest that in winter and early spring, NAO type atmospheric circulation influences the extent of snow cover and the snow cover affects the atmosphere in the late spring, summer and early autumn leading to a mechanism that seems to be responsible for the multiannual NAO persistence in the last half century.

Box 11.1 (continued) Box 11.1, Figure 2 | A schematic illustrating the progression from an initial-value based prediction at short time scales to the forced boundary-value problem of climate projection at long time scales. Decadal prediction occupies the middle ground between the two. (Based on Meehl et al., 2009b.)

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A climatology and trend of hail events in Romania are presented using hail data spanning the years 1961–2014. Hail observations from weather stations and model reanalysis data were used to document the spatial and temporal distributions, variabilities, and environments of hail events. The results show that hail occurs most frequently in mountainous areas, while the smallest average number of hail days per year is found in the southeast. Herein, the convective season was defined as April–September, given that 94.2% of all mean monthly hail days were identified in this period. During the convective season the hail events prevail with most of these occurring in the afternoon and evening hours between 1000 and 1800 UTC. The severe hail events occur, overall, between 1400 and 1600 UTC, while in the southwest severe hail occurs later between 1600 and 1900 UTC. The spatial distribution of the convective parameters is consistent with the spatial distribution of hail days, revealing that hail is not favored in southeastern Romania, but in the rest of the country. The trend analysis of mean hail days per year disclose that 55.2% of all stations show a statistically significant upward trend, 3.8% show a statistically significant downward trend, while 40.9% show no statistically significant trend. A correlation between the variability of hail days per year and the variability in the occurrence of low pressure systems of Atlantic origin exists, the latter generating low pressure systems over the Mediterranean Sea that supply eastern Europe with the moist air needed for convection.

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