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

==== 2.4.1.2 Southern Annular Mode (SAM) ====

<div id="h3-31-siblings" class="h3-siblings"></div>

The AR5 concluded that it was ''likely'' that the SAM had become more positive since the 1950s and that this increase was unusual in the context of the prior 400 years ( ''medium confidence'' ). Both AR5 and SROCC reported statistically significant trends in the SAM during the instrumental period for the austral summer and autumn.

Several studies have attempted to reconstruct the evolution of the SAM during the Holocene using proxies of the position and strength of the SH zonal winds, although with no clear consensus regarding the timing and phase of the SAM ( [[#Hernández--2020|Hernández et al., 2020]] ). The early Holocene was dominated by SAM positive phases ( [[#Moreno--2018|Moreno et al., 2018]] ; [[#Reynhout--2019|Reynhout et al., 2019]] ), consistent with increasing westerly wind strength ( [[#Lamy--2010|Lamy et al., 2010]] ), with some reconstructions showing significant centennial and millennial variability but no consistent trend after 5 ka ( [[#Hernández--2020|Hernández et al., 2020]] ). For the CE, enhanced westerly winds occurred over 0–1000 CE, as reflected in increased burning activity in Patagonia ( [[#Turney--2016a|Turney et al., 2016a]] ) and tree ring records from southern New Zealand ( [[#Turney--2016b|Turney et al., 2016b]] ) imply a predominantly positive SAM phase. Pollen records and lake sediments from Tasmania, southern mainland Australia, New Zealand and southern South America, inferred the period of 1000 to 1400 CE to be characterized by anomalously dry conditions south of 40°S, implying a positive SAM ( [[#Moreno--2014|Moreno et al., 2014]] ; [[#Fletcher--2018|Fletcher et al., 2018]] ; [[#Evans--2019|Evans et al., 2019]] ; [[#Matley--2020|Matley et al., 2020]] ). Nevertheless, proxy reconstructions of the SAM based on temperature-sensitive records from tree rings, ice cores, lake sediments and corals spanning the mid-to-polar latitudes show alternating positive and negative phases (Figure 2.35).

<div id="_idContainer087" class="Basic-Text-Frame"></div>

[[File:328f0580e5a1117c67759075c3d6a122 IPCC_AR6_WGI_Figure_2_35.png]]

'''Figure 2.35''' '''|''' '''Southern Annular Mode (SAM) reconstruction over the last millennium. (a)''' SAM reconstructions as seven-year moving averages (thin lines) and 70-year LOESS filter (thick lines). '''(b)''' Observed SAM index during 1900–2019. Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

Prolonged periods of negative SAM values were identified during the period 1400–1700 CE in several reconstructions (Figure 2.35, [[#Villalba--2012|Villalba et al., 2012]] ; [[#Abram--2014|Abram et al., 2014]] ; [[#Dätwyler--2018|Dätwyler et al., 2018]] ), with a minimum identified during the 15th century ( [[#Hernández--2020|Hernández et al., 2020]] ), although some disagreements exist between proxy records before 1800 CE ( [[#Hessl--2017|Hessl et al., 2017]] ). [[#Abram--2014|Abram et al. (2014)]] concluded that the mean SAM index during recent decades is at its highest levels for at least the last 1 kyr. Similarly, the summer SAM reconstruction by [[#Dätwyler--2018|Dätwyler et al. (2018)]] indicates a strengthening over the last 60 years that is outside the ''very likely'' range of the last millennium natural variability. The largest 30- and 50-year trends in the annual SAM index occurred at the end of the 20th century (after 1969 and 1950 respectively), indicating that the recent increase in the SAM is unprecedented in the context of at least the past three centuries ( [[#Yang--2018|Yang and Xiao, 2018]] ).

Before the mid-1950s, SAM indices derived from station-based datasets, and centennial reanalyses show pronounced interannual and decadal variability but no significant trends, with low correlation between SAM indices due to the diversity across different datasets and sensitivity to the definition used for the index calculation ( [[#Barrucand--2018|Barrucand et al., 2018]] ; [[#Schneider--2018|Schneider and Fogt, 2018]] ; J. [[#Lee--2019|]] [[#Lee--2019|Lee et al., 2019]] ). Various SAM indices exhibit significant positive trends since the 1950s, particularly during austral summer and autumn ( [[#Barrucand--2018|Barrucand et al., 2018]] ; [[#Schneider--2018|Schneider and Fogt, 2018]] ; J. [[#Lee--2019|]] [[#Lee--2019|Lee et al., 2019]] ), unprecedented for austral summer over the last 150 years (J.M. [[#Jones--2016|]] [[#Jones--2016|Jones et al., 2016]] ; [[#Fogt--2020|Fogt and Marshall, 2020]] ). This indicates a strengthening of the surface westerly winds around Antarctica, related to both the position and intensity of the subpolar jet in the SH ( [[#2.3.1.4.3|Section 2.3.1.4.3]] ; [[#Ivy--2017|Ivy et al., 2017]] ; [[#IPCC--2019|IPCC, 2019]] ). The SAM trends have slightly weakened after about 2000 ( [[#Fogt--2020|Fogt and Marshall, 2020]] ).

In summary, historical station-based reconstructions of the SAM show that there has been a robust positive trend in the SAM index, particularly since 1950 and for the austral summer ( ''high confidence'' ). The recent positive trend in the SAM is ''likely'' unprecedented in at least the past millennium, although ''medium confidence'' arises due to the differences between proxy records before 1800 CE.

<div id="2.4.2" class="h2-container"></div>

<span id="el-niñosouthern-oscillation-enso"></span>