Downward Arctic Oscillation signal associated with moderate weak stratospheric polar vortex and the cold December 2009

The climatic anomalies and related mechanism for the cold December 2009 in Northern Hemisphere are analyzed preliminarily. It is revealed that both anomalously low monthly mean temperature and more extreme cold days occurred over many areas of mid‐latitude Northern Hemisphere in this December. This cold condition can be accounted for by the extreme negative Arctic Oscillation (AO) at surface level, which is the lowest one for the past 31 years. It is shown that the negative surface AO in December is closely associated with the downward propagating anomalies from the stratosphere, implying the involvement of stratospheric processes in this cold month. Since the preceding stratospheric signal is not as strong as those reported in previous studies, it suggests that some relatively weaker stratospheric anomalies can also propagate downward and influence the troposphere.