Modes of climate variability and their relationships with interhemispheric temperature asymmetry: a Granger causality an
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ORIGINAL PAPER
Modes of climate variability and their relationships with interhemispheric temperature asymmetry: a Granger causality analysis Umberto Triacca1 Received: 27 March 2020 / Accepted: 11 November 2020 © The Author(s) 2020
Abstract The aim of this paper is to investigate the relationships among Interhemispheric Temperature Asymmetry (ITA) and the principal modes of natural variability: the Atlantic Multidecadal Oscillation (AMO), the Southern Oscillation Index (SOI), and the Pacific Decadal Oscillation (PDO). In particular, Granger causality tests are used to capture the linkages among these variables. Our analysis provides strong evidence that AMO causes ITA, the causal role of PDO is weak, and SOI seems to have no causal influence.
1 Introduction The Interhemispheric Temperature Asymmetry (ITA) index is defined as the difference between the hemispheric mean surface air temperatures Northern Hemisphere (NH) minus Southern Hemisphere (SH). Recently, ITA has been proposed as an emerging indicator of climate change (Friedman et al. 2013); thus, it is of extreme interest to investigate the causes of ITA changes. In particular, it is important to establish if this indicator depends or not on internal variability.1 In fact, if a significant effect of the internal variability over ITA was present, this variable may not be a good indicator of climate change. The natural internal variability may mask the climate change signal. This is similar to global temperature, in which anthropogenic aerosols are believed to have masked GHGs over the middle of the twentieth century (Friedman et al. 2013). 1 By natural internal climate variability, we mean variability that occurs in the absence of natural or anthropogenic forcing; that is, variability that occurs solely due to the internal dynamics of the coupled atmosphere-ocean-biosphere-cryosphere system (see DelSole (2011, p.909-910).
Umberto Triacca
[email protected] 1
Department of Computer Engineering, Computer Science and Mathematics, University of L’Aquila, I-67010 Coppito, Italy
The aim of this paper is to investigate the relationships among ITA and the principal modes of natural variability: the Atlantic Multidecadal Oscillation (AMO), the Southern Oscillation Index (SOI), and the Pacific Decadal Oscillation (PDO). In particular, Granger causality tests are used to capture the linkages among these variables. We use these indices (AMO, SOI, and PDO) because they are an appropriate way to represent the natural interannual variability. However, as far as AMO is concerned, it is important to note a growing line of research suggesting that much (if not most) of its variability is forced. See for example Bellucci et al. (2017), Murphy et al. (2017), Bellomo et al. (2018), Haustein et al. (2019), and Mann et al. (2020). The paper is organized as follows: Section 2 describes the methodology used. Section 3 presents the empirical results. Section 4 provides a summary of the key result and provides ideas for future work.
2 Testing for Granger causality Grang
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