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

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This letter presents an analysis of a new homogenized time series of daily mean sea level pressure (SLP) from Reykjavík and Stykkishólmur, Iceland from 1823–1999. Time series statistical techniques including harmonic analysis are used to identify the seasonal march of pressure and its variability through the record. The results are assessed regarding the North Atlantic Oscillation (NAO) index. In addition to the annual cycle, the seasonal march of SLP has an appreciable semi‐annual cycle, as well as anomalies (abrupt rises and falls) occurring at remarkably distinct times in spring and winter. Multidecadal variability is apparent in the seasonal cycle and the anomalies. A shift in intraseasonal variability is observed in recent decades, including an extension of the winter Icelandic Low enhancement period into March. Increasing failure or delay of the “usual” abrupt pressure rise in Iceland in late February has contributed to the unusually positive winter NAO index values since the 1960s.