Wavelet-Based Multifractal Analysis on Climatic Regime Shifts

Abstract

A climatic regime shift is characterized by an abrupt transition from one quasi-steady climatic state to another. We attempted to explain the change of multifractal behavior of climate indices when a regime shift occurred. We used the wavelet - transform method to analyze the multifractal behaviors of the El Niño/Southern Oscillation (ENSO) index (Niño3.4 index), Pacific Decadal Oscillation (PDO) index, North Pacific Index (NPI), Pacific/North American pattern (PNA) index, and West Pacific pattern (WP) index. We showed the change of multifractality of these climate indices. When the wavelet coherences between the Niño3.4 index and NPI, NPI and PDO index, and Niño3.4 and PDO indices became strong, changes from multifractal to monofractal behavior were observed at climatic regime shifts. It may be possible to explain the background of the change of fractality by regarding climate change as the consequence of mutual interactions of various climatic elements. A fluctuation increase is observed in a coupled chaotic system just before chaos synchronization, which is when fractality and states change. We expect that a similar mechanism possibly exists for a climatic regime shift. When fluctuations became large and multifractality became strong, a climatic regime shift occurred and a change from multifractal to monofractal behavior was observed. The strong interaction of climatic phenomena, such as the ENSO, PDO, and Aleutian Low, caused a climatic regime shift. The fractality change of the PDO index almost corresponded to the regime shifts. In terms of multifractal analysis, we conclude that a climatic regime shift corresponds to a change from multifractality to monofractality of the PDO index.