In this paper, we generalize the concept of “external forcing” to include any mechanism that modulates the long-term evolution of a meteorological variable but is not directly related to the internal variability of the climate system. Applying this concept, the corresponding ‘external forcings’ are removed from several long record datasets of oceanic and atmospheric variables at the surface in the North Atlantic. We perform a multivariate analysis in the frequency domain over both the original data fields and the new ‘internal variability’ fields. This multivariate analysis is based on a MultiTaper Method-Singular Value Decomposition (MTM-SVD). It is noteworthy that, after the removal of the external forcings, there is an almost perfect alignment of the main multidecadal oscillatory band (f = 0.21 cycles/decade) with all the spectra of the analysed fields. This alignment was not observed before the external forcings were removed. Particularly striking is the case of the sea level pressure (SLP), which shows a notable variation in its oscillation period despite the fact that this variable has traditionally been considered to be unaffected, at global scale, by any external forcing. The external forcing in the SLP records is very probably caused by the scarcity of the observed data during the first hundred years of the record (most evident, during the earliest decades), by the spatial distribution of those observations and, possibly, by the assimilation model employed to build those long record datasets. When we analysed the relationship between the ocean and the atmosphere using this approach, we found strong evidence of a cyclic behaviour in which oceanic conditions modulate the atmospheric variability, with a lead time of up to about 10 years.

在本文中，我们概括了“外部强迫”的概念，以包括任何可调节气象变量长期演变但与气候系统内部可变性没有直接关系的机制。应用这一概念，就从北大西洋地表海洋和大气变量的几个长期记录数据集中删除了相应的“外部强迫”。我们在频域上对原始数据字段和新的“内部可变性”字段执行多元分析。该多变量分析基于MultiTaper方法-奇异值分解（MTM-SVD）。值得注意的是，在去除外部力之后，主要的多年代际振荡频带（f = 0）几乎完美对准。21个周期/十年），其中包含所分析场的所有光谱。在去除外部力之前未观察到这种对准。特别令人震惊的是海平面压力（SLP）的情况，尽管在传统意义上该变量在全球范围内不受任何外部强迫影响，但其振荡周期却显示出显着变化。SLP记录中的外部强迫很可能是由于记录的前一百年（最明显的是，最早的几十年）中观测到的数据稀缺，这些观测值的空间分布以及可能是由于同化引起的。用于构建那些长期记录数据集的模型。当我们使用这种方法分析海洋与大气之间的关系时，