Interannual Temperature Events and Shifts in Global Temperature: A “Multiwavelet” Correlation Approach

Abstract

For the purpose of climate signal detection, we introduce a method for identifying significant episodes of large-scale oscillatory variability. The method is based on a multivariate wavelet algorithm that identifies coherent patterns of variation simultaneously within particular ranges of time and periodicity (or frequency) that may vary regionally in the timing and amplitude of the particular temperature oscillation. By using this methodology, an analysis is performed of the instrumental record of global temperatures spanning the past 140 years. The duration of an ?episode? is chosen to correspond to 3?5 cycles at a specified oscillation period, which is useful for detecting signals associated with the global El Niño/Southern Oscillation (ENSO) phenomenon. To confirm the robustness of signals detected in the earliest, sparse data (only 111 5° longitude by 5° latitude grid points are available back to 1854), we performed multiple analyses overlapping in time, using increasingly dense subsets of the full (1570 grid point) temperature data. In every case, significant interannual episodes are centered in the 3?7 year period range corresponding to the conventional band of ENSO-related variance and describe intervals of quasi-oscillatory variability of decadal-scale duration. These episodes consist of a sequence of one or two warm and cold events with sea surface temperature fluctuations in the eastern tropical Pacific of amplitude ±0.6°?1.1°C. Each episode includes one or more historically prominent El Niño events. The signals are characterized as significant, however, by virtue of their global-scale pattern of temperature variations as well as their oscillatory pattern in time. The 1920?1940 interval of increasing global temperatures was bracketed by oscillatory episodes with unusual global patterns of expression relative to the recent ENSO episodes of the 1970s and 1980s. The episodes that preceded the 1920?1940 and 1975?present intervals of rapid warming were associated with globally averaged temperature fluctuations of TGLB > 0.4°C, the largest among those identified. In contrast, the episode that concludes the 1920?1940 temperature rise exhibits a global-mean fluctuation TGLB = 0.05°C, smallest among the observed episodes. These observations motivate speculation about the possible relationship between ENSO variability and global warming, in particular, the relationship between ENSO and the transient storage of heat in the tropical upper ocean layer, and the relationship between secular climate change and the amplitude of interannual ENSO events.