Comparing historic records of storm frequency and the North Atlantic Oscillation (NAO) chronology for the Azores region

The storminess of the Azores region was investigated using newspaper records from AD 1836 onwards. The information obtained was rank-ordered for intensity and the time series of storm frequency analysed for interannual- to century-scale variability. The documentary data set was validated by comparison with objective cyclones intensity for the period AD 1958-2000. Results indicate that four periods of contrasting storm frequency are present ( AD 1836-1870, 1870-1920, 1920-1940 and 1940-1998). The average storm lasts for 2.3 days and the average secular storm frequency is 3.1 storms/yr. Low intensity events occur four times every five years whereas an extreme storm occurs on average once every seven years. The documentary index of storminess is highly variable at different timescales, which is consistent with other studies of storminess in the North Atlantic. Nevertheless, an objective comparison between late nineteenth- and late twentieth-century storm frequency does not reveal a significant difference. Between AD 1865 and the late twentieth century the winter NAO and storminess indices show a statistically significant anti-correlation pattern at the monthly and seasonal scales. In the late nineteenth century and between AD 1950 and 1970 the NAO index was low and the storminess index high, whilst the opposite occurred from the early twentieth century until the middle 1950s; since AD 1970 both indexes reveal positive trends and are predominantly positive. The NAO mode of circulation is partially responsible for the storminess spatial pattern and temporal distribution over the Azores region since AD 1865 and for about a century, however this relation appears to have weakened since the 1960s.