Editing IPCC:AR6/WGI/Chapter-2 (section)

=== 2.4.5 Pacific Decadal Variability (PDV) ===

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Pacific Decadal Variability (PDV) refers to the ocean-atmosphere climate variability over the Pacific Ocean at decadal-to-interdecadal time scales and is usually described by the Pacific Decadal Oscillation (PDO) or the Inter-decadal Pacific Oscillation (IPO) indices. The AR5 and SROCC reported a large shift of the PDO in the late 1970s, with a predominantly positive phase until the end of the 1990s, being mainly negative afterwards. There was no significant change assessed in the PDO during the instrumental period as a whole, and no confidence level was assigned. Changes in the pre-instrumental era PDO were not assessed in AR5.

The existence of the PDV in the centuries prior to the instrumental period is evidenced by a variety of proxy records based on tree rings ( [[#Biondi--2001|Biondi et al., 2001]] ; [[#D’Arrigo--2015|D’Arrigo and Ummenhofer, 2015]] ), corals ( [[#Felis--2010|Felis et al., 2010]] ; [[#Deng--2013|Deng et al., 2013]] ; [[#Linsley--2015|Linsley et al., 2015]] ) and sediments ( [[#Lapointe--2017|Lapointe et al., 2017]] ; [[#O’Mara--2019|O’Mara et al., 2019]] ). There is little coherence between the various paleo-proxy indices prior to the instrumental record, and neither these nor the instrumental records provide indications of a clearly defined spectral peak ( [[#Chen--2015|Chen and Wallace, 2015]] ; M. [[#Newman--2016|]] [[#Newman--2016|Newman et al., 2016]] ; [[#Henley--2017|Henley, 2017]] ; L. [[#Zhang--2018|]] [[#Zhang--2018|]] [[#Zhang--2018|]] [[#Zhang--2018|Zhang et al., 2018]] ; [[#Buckley--2019|Buckley et al., 2019]] ). For instance, spectral analysis from millennia length PDV reconstructions shows spectral peaks at multi-decadal, centennial and bi-centennial time scales ( [[#Beaufort--2017|Beaufort and Grelaud, 2017]] ), while only multi-decadal oscillations can be detected in the shorter (less than 400 years into the past) paleoclimate reconstructions. A variety of proxies suggest a shift in the PDV from the early-mid Holocene, which was characterized by a persistently negative phase of the PDO (i.e., weak Aleutian Low), to the late Holocene, and more variable and more positive PDO (i.e., strong Aleutian Low) conditions. This shift at around 4.5 ka is also evident in the PDO periodicities, changing from bidecadal and pentadecadal variability in the early Holocene to only pentadecadal periodicities in the late Holocene ( [[#Hernández--2020|Hernández et al., 2020]] ). Several proxy records indicate that the strengthening in the Aleutian Low inferred since the late 17th century is unprecedented over the last millennium (Z. [[#Liu--2017|]] [[#Liu--2017|]] [[#Liu--2017|Liu et al., 2017]] ; [[#Osterberg--2017|Osterberg et al., 2017]] ; [[#Winski--2017|Winski et al., 2017]] ), in line with an increase in PDV low-frequency variability ( [[#Williams--2017|Williams et al., 2017]] ; [[#Hernández--2020|Hernández et al., 2020]] ).

The PDO and IPO indices are significantly correlated during the instrumental period, showing regime shifts in the 1920s, 1940s, 1970s and around 1999. Positive PDV phases were observed from the 1920s to the mid-1940s and from the late 1970s to the late 1990s, while negative phases occurred from mid-1940s until the late 1970s, and since 1999 (Figure 2.38; [[#Han--2014|Han et al., 2014]] ; [[#Chen--2015|Chen and Wallace, 2015]] ; M. [[#Newman--2016|]] [[#Newman--2016|Newman et al., 2016]] ). The associated spatial patterns are quite similar, but the PDO pattern exhibits stronger SST anomalies in the extra-tropical North Pacific than the IPO ( [[#Chen--2015|Chen and Wallace, 2015]] ). The strength and structure of the SST patterns also differ among the periods (M. [[#Newman--2016|]] [[#Newman--2016|Newman et al., 2016]] ).

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[[File:76139e2272e794efb7b34b688784344c IPCC_AR6_WGI_Figure_2_38.png]]

'''Figure 2.38''' '''|''' '''Indices of multi-decadal climate variability from 1854–2019 based upon several sea surface temperature data products.''' Shown are the indices of the AMV and PDV based on area averages for the regions indicated in Annex IV. Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

Instrumental observations are sparse prior to 1950, and thus the fidelity of any PDV index derived for the second part of the 19th century and early decades of the 20th century is relatively low (Figure 2.38; [[#Deng--2013|Deng et al., 2013]] ; [[#Wen--2014|Wen et al., 2014]] ). This results in ''low agreement'' in the classification of the PDO/IPO phase among several indices, even during recent years with the availability of high-quality data. Nevertheless, the teleconnection patterns are robust regardless of the index used to characterize the PDO ( [[#McAfee--2017|McAfee, 2017]] ). Analysis of time series of PDO and IPO highlights the (multi-) decadal nature of this mode of variability with no significant trends, but highlights a recent switch from a positive to a negative phase since 1999/2000 across all indicators ( [[#England--2014|England et al., 2014]] ; [[#Henley--2017|Henley, 2017]] ).

In summary, the PDV in the instrumental record is dominated by (multi-)decadal-scale shifts between positive and negative phases over the last 150 years with no overall trend ( ''high confidence'' ). There is ''low confidence'' in paleo-PDV reconstructions due to discrepancies among the various available time series in terms of phasing and timing. However, there is ''high confidence'' in the occurrence of a shift from predominantly negative to positive PDO conditions from the middle to the late Holocene.

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