| description abstract | Based on a wavelet transform, a new method referred to as maximal wavelet filter (MWF) is proposed to extract temporal structure changes of a climatic oscillation, which varies its pattern corresponding to the changes of the oscillation period. The MWF is a bandpass filter having a narrow pass band, the central frequency of which temporally varies according to the periods of maximal wavelet amplitudes for a specific region. MWF is applied to wintertime sea level pressures (SLPs) in the Northern Hemisphere from 1899 to 2000 to extract SLP changes associated with the bidecadal oscillation (BDO), which distributes globally but has the strongest amplitudes in the North Pacific. In the Pacific sector, the BDO center of action captured by the MWF was located over Alaska in the first few decades of the record, and then moved southward to the central North Pacific from 1920 to 1950, with maximal BDO amplitudes in the middle of the century. The southward migration was accompanied by the previously reported increase of the oscillation period from 15 to 20 years. On the other hand, Atlantic SLP variations coherent with the Pacific BDO had large amplitudes in midlatitudes (high latitudes) in the early (late) part of the twentieth century. In association with these spatial structure changes, the pattern of the recent BDO resembles the pattern of the Arctic Oscillation. The analysis of the sea surface temperatures (SSTs) gridded from the Comprehensive Ocean?Atmosphere Data Set (COADS) and the newly digitized Kobe collections suggests that BDO pattern in the SSTs also shifted toward the south between the first and last few decades of the twentieth century. Furthermore, covariability between the land?air temperatures and Aleutian low strength is observed through the twentieth century for Alaska, but only after 1940 for the midlatitudes of western North America and Hawaii, indicating that the BDO influence was limited to the high latitudes in the first few decades of the twentieth century in these regions, consistent with the spatial structure changes in the SLP field over the North Pacific. | |
