Independent Components in the Northern Hemisphere Winter: Is the Arctic Oscillation Independent?

[摘要] This paper examines true oscillations in the Northern Hemisphere winter by using independent component analysis (ICA). ICA can distinguish between true and apparent oscillations under the assumption that the true oscillations are mutually independent. Particular attention is paid to the Arctic Oscillation (AO). For this purpose, the NCEP-NCAR reanalysis data (NCEP-NCAR data) and the data of the present climate experiment of the Meteorological Research Institute (MRI data) are used. There may be certain issues in ICA applied to meteorology: the selection of data periods, treatment of noise, and relationship between the number of dimensions in phase space and the number of independent components. We make several considerations and proposals about these issues. ICA should be performed for periods for which the variance is almost the same. Since independent components after whitening are not necessarily uncorrelated with each other under the existence of noise, and the relation between the dimension of phase space and the number of independent components cannot be predetermined, we propose the method for seeking independent components by kurtosis as the most relevant in meteorology. On the basis of the above considerations and proposals, ICA is performed on the NCEP-NCAR data. Independent components are found for the sea level pressure (SLP) and 500 hPa height (Z500) fields. They are the North Atlantic Oscillation (NAO) and the Pacific-North American Oscillation (PNA). Thus, the AO is an apparent mode derived from them. However, since the period of the data is too short, statistical significance cannot be obtained. Then, ICA is performed on the MRI data. Also in this data, the NAO and the PNA-like oscillation are independent for both the SLP and Z500, where the PNA-like oscillation implies that its pattern is somewhat different from the observed PNA pattern. It can be concluded again that the AO is not independent, but this time with statistical significance.

[学科分类] 大气科学