Anomalies in the seasonal cycle of sea level pressure in Iceland and the North Atlantic Oscillation

This letter presents an analysis of a new homogenized time series of daily mean sea level pressure (SLP) from Reykjavik and Stykkisholmur, Iceland fro m 1823-1999. Time series statistical techniques including harmonic analysis are used to identify the seasonal march of pressure and its variability th rough the record. The results are assessed regarding the North Atlantic Osc illation (NAO) index. In addition to the annual cycle, the seasonal march o f SLP has an appreciable semi-annual cycle, as well as anomalies (abrupt ri ses and falls) occurring at remarkably distinct times in spring and winter. Multidecadal variability is apparent in the seasonal cycle and the anomali es. A shift in intraseasonal variability is observed in recent decades, inc luding an extension of the winter Icelandic Low enhancement period into Mar ch. Increasing failure or delay of the "usual" abrupt pressure rise in Icel and in late February has contributed to the unusually positive winter NAO i ndex values since the 1960s.