摘要
Winter North Atlantic Oscillation(NAO)indexes from observations based on various winter durations are compared.Results show that there are significant differences in the interannual and decadal variations of these NAO indexes.For the same data source,a different definition of winter duration can lead to different signs of NAO index in some years,which mainly appear to be in the period of decadal phase transition.The different winter durations induce different cycles of the observation-based NAO.The longer the winter duration,the stronger the decadal variation.The NAO defined by different winter durations also can generate different descriptions of the NAO action centers,including the position and movement.The longer the winter duration,the more southerly action centers appear to be.The movement of the action centers affects not only site-based NAO indexes but also those defined by other methods,such as empirical orthogonal function(EOF)analysis.The length of time used in EOF analysis has a great influence on the spatial pattern of the NAO mode,which results in a considerable difference between the corresponding NAO indexes.Regardless of which NAO index is used,the NAO-related SST anomalies show the same tripole pattern.The longer the winter duration,the more significant the relationship between the NAO and SST affected by the timescale of sea-air interaction.
对于冬季NAO指数的定义有很多,但尚不清楚这些不同的定义尤其是定义冬季NAO指数时所用的冬季时间定义不同是否会对NAO的研究带来影响。我们主要运用相关性和功率谱分析研究了基于不同定义方法和不同数据来源得到的冬季NAO指数的特征差异,分析了这种差异可能对研究NAO与北大西洋海表温度(SST)关系的影响。结果显示:不同的数据来源对NAO的定义影响不大,不同的定义方法有一定的影响;不同冬季时间定义得到的NAO对NAO活动中心位置和变化规律的描述不同;不同定义方法得到的NAO与SST的关系比较一致,但冬季定义持续时间越长,NAO与海温的关系越显著。
基金
supported jointly by the National Key Research and Development Program of China [grant number 2016YFB02008001]
the National Natural Science Foundation of China [grant number 41530426]
