The evolution and volcanic forcing of the southern annular mode during the past 300 years

Abstract

A positive change in the southern annular mode (SAM), which is the primary pattern of climate variability in the Southern Hemisphere, has been induced predominantly by polar stratospheric ozone depletion. However, the lack of long‐term observational records limits our understanding of the long‐term SAM behaviour. In this study, we found that the geochemical record of the LGB69 ice core from the eastern coast of Antarctica was significantly correlated with the winter SAM index (SAMI). In addition, we developed an annual mean SAMI beginning in 1701 based on 15 annually resolved ice cores and relevant proxy–climate relationships. Our reconstruction accounted for 54.8% of the total variance from 1957 to 2000 (the calibration period). We demonstrate that the recent positive phase shift in the annual mean SAMI since the 1970s is unprecedented, with the estimate for the latest regime in the 1990s reaching values 2.5 times the standard deviation above the baseline (1701–2000). This peak value also coincides with the largest 30‐ and 50‐year trends, which have occurred at the end of the 20th century. From the reconstructed SAMI, we also found that the response to large volcanic events was likely positive in the 3 years after the eruption, but this positive response can be masked by internal climate variability when a strong El Niño event occurs in the eruption year.

Multidecadal climate oscillations during the past millennium driven by volcanic forcing

“…We adopt the convention that the t = 0 (zero phase) pattern corresponds to the peak tropical (and global mean) cooling, i.e., the main expected direct response to volcanic forcing. The pattern also shows evidence of extratropical linkages, including a dynamical AO or NAO-like response in the Northern Hemisphere (54) and a warming off Antarctica, consistent with a dynamical coupling with the Southern Annular Mode (55)(56)(57).…”

Southern African temperature responses to major volcanic eruptions since 1883: Simulated by CMIP5 models

“…CMIP5 models tend to show a positive Southern Annular Mode (SAM) following major volcanic eruptions (Barnes et al ., 2016; McGraw et al ., 2016), as was also the case after the Agung and Pinatubo eruptions. A positive SAM generally means strengthened westerlies, which contract towards the poles and result in intensified cyclogenesis (Yang and Xiao, 2017). Mid‐latitude cyclones cause most of the extreme weather encountered over mid‐latitudes of the SH, or southernmost terrestrial regions of South America, southern Africa and Australasia (Pepler et al ., 2016).…”

The Southern Annular Mode: Variability, trends, and climate impacts across the Southern Hemisphere

“…Even in austral summer, natural variability can play an important role on shorter timescales, such as the pre‐ozone hole peaks in the SAM index around 1960 (Figure 1) (J. M. Jones et al, 2009; J. M. Jones & Widmann, 2003, 2004), or the recent negative Antarctic‐wide pressure anomalies since 2000 (Fogt et al, 2017), associated with tropical SSTs during a period of weaker ozone forcing. Volcanic activity may also drive short‐term changes in the SAM index (Fogt et al, 2017; G. J. Marshall, 2003; Yang & Xiao, 2018) through modulations in the extratropical temperature gradient and wave driving of the polar stratosphere, although the response varies between eruptions due to changes in tropical variability and other factors (Robock et al, 2007) (Box 1).…”

“…Other SAM indices (or SH pressure / wind estimates) have been reconstructed over much longer time periods, many spanning back to AD 1000, using midlatitude proxy data such as tree rings and lake sediments (J. M. Jones & Widmann, 2003; Moreno et al, 2018; Villalba et al, 2012), ice core proxy data from Antarctica (I. D. Goodwin et al, 2014; I. D. Goodwin et al, 2004; Yang & Xiao, 2018), or multiproxy reconstructions based on a combination of midlatitude and Antarctic data (Abram et al, 2014; Zhang et al, 2010). Notably, as seen in South American (G. Silvestri & Vera, 2009) and Antarctic (G. J. Marshall et al, 2011, 2013) temperatures, recent work also suggests that many paleo–proxy relationships with the SAM are likely to be nonstationary, although this relationship varies regionally (Dätwyler et al, 2018).…”

“…Volcanic activity may also drive short-term changes in the SAM index (Fogt et al, 2017;G. J. Marshall, 2003;Yang & Xiao, 2018) through modulations in the extratropical temperature gradient and wave driving of the polar stratosphere, although the response varies between eruptions due to changes in tropical variability and other factors (Robock et al, 2007) (Box 1). , long-term historical estimates of the SAM are achieved through reconstructions.…”