Asymmetries in Circulation Anomalies Related to the Phases of the North Atlantic Oscillation on Synoptic Time Scales.

The North Atlantic Oscillation (NAO) index is often characterized by independent positive and negative NAO events with a characteristic spatial pattern and a typical lifetime of around 1 week. These events are separated by periods of near‐neutral NAO conditions. Here, we challenge this view by showing in reanalysis and observed data that the strength and spatial shape of NAO events depends on the NAO index prior to the window of 1 week and this dependency is most pronounced for negative NAO events. The influence is seen in the mean sea level pressure, and in other important features, including blocking frequency and jet stream characteristics, and also in air surface temperature and precipitation in parts of Europe. This new appreciation is important for efforts to improve methods for subseasonal‐to‐seasonal predictions of NAO. Plain Language Summary: Northern European winters differ. Some are mild and moist with prevailing westerly winds and some are cold and dry, dominated by the Siberian air masses from East. When the Northern European winters are mild, the winter in Greenland is cold, and vice versa. This is the North Atlantic Oscillation (NAO) and its companion, the Europe–Greenland temperature seesaw. They were described in qualitative terms already in the late eighteenth century. Nowadays, the NAO is often described as a series of independent positive or negative events with a duration of around 1 week with intermittent periods of near‐neutral conditions and variable duration. The net result of these events determines the NAO strength of the winter. We show that this picture is too simplified because the NAO outside the time horizon of 1 week influences the strength of the individual NAO events. This is most pronounced for the negative events where this influence can both distort and amplify or diminish the pattern of the event. Also the typical temperatures and precipitation patterns associated with NAO are influenced significantly by this effect. One important consequence for our discovery is the improved prospects of doing subseasonal‐to‐seasonal predictions. Key Points: The daily North Atlantic Oscillation (NAO) index consists of positive and negative events of duration around 1 week separated by periods of near‐neutral conditionsThe strength and spatial shape of negative NAO events depends on NAO‐index values prior to the eventBlocking frequencies, jet strength and position, and surface temperature and precipitation have similar dependencies [ABSTRACT FROM AUTHOR]