Arctic Oscillation anomaly in winter 2009/2010 and its impacts on weather and climate

Using the Arctic Oscillation (AO) index, the exceptional winter (DJF) of 2009 has been analyzed. The middle-to-high latitudes of the Northern Hemisphere suffered from a nearly zonally symmetric anomaly of temperature and pressure. This situation revealed that two negative AO events occurred in the winter of 2009/2010, with unprecedented low values in January 2009 and February 2010. The negative AO event in January 2009 can be further divided into two stages: the first stage was mainly driven by enhanced upward-propagating planetary waves, which led to a weak stratospheric polar vortex associated with a downward-propagating negative AO signal; the second stage was caused by a lower tropospheric positive temperature anomaly in the high latitudes, which maintained the positive geopotential height anomaly of the first stage. The two successively occurring stages interacted and caused the lower troposphere to experience a strong and lengthy persistence of the negative AO event. We consider that the second event of negative AO in February 2010 is related to the downward-propagating negative AO after sudden stratospheric warming. Eleven long-persistence negative AO events were analyzed using reanalysis data. The results suggest that the negative AO in the troposphere might have been caused by stratospheric sudden warming, a downward-propagating weak stratospheric circulation anomaly or dynamic processes in the troposphere. Further study shows that the negative phase of the AO in the winter of 2009/2010 corresponded to a wide range of temperature and precipitation anomalies in the Northern Hemisphere. Therefore, to improve the accuracy of weather forecasting and climate prediction, more attention should be paid to the AO anomaly and its impact.