Evolution of the Southern Annular Mode during the past millennium

Abstract

The Southern Annular Mode (SAM) is the primary pattern of climate variability in the Southern Hemisphere 1,2 , influencing latitudinal rainfall distribution and temperatures from the subtropics to Antarctica. The positive summer trend in the SAM over recent decades is widely attributed to stratospheric ozone depletion 2 ; however, the brevity of observational records from Antarctica 1 -one of the core zones that defines SAM variability-limits our understanding of long-term SAM behaviour. Here we reconstruct annual mean changes in the SAM since AD 1000 using, for the first time, proxy records that encompass the full mid-latitude to polar domain across the Drake Passage sector. We find that the SAM has undergone a progressive shift towards its positive phase since the fifteenth century, causing cooling of the main Antarctic continent at the same time that the Antarctic Peninsula has warmed. The positive trend in the SAM since ∼AD 1940 is reproduced by multimodel climate simulations forced with rising greenhouse gas levels and later ozone depletion, and the long-term average SAM index is now at its highest level for at least the past 1,000 years. Reconstructed SAM trends before the twentieth century are more prominent than those in radiative-forcing climate experiments and may be associated with a teleconnected response to tropical Pacific climate. Our findings imply that predictions of further greenhouse-driven increases in the SAM over the coming century 3 also need to account for the possibility of opposing e ects from tropical Pacific climate changes.

Regional Antarctic snow accumulation over the past 1000 years

“…In the Pacific sector the calibration period coincides with a shift in the phase of the Interdecadal Pacific Oscillation (IPO), which is known to influence the atmospheric circulation transporting moisture to Law Dome (Vance et al, 2015), from positive at the start of the period to negative from the late 1990s. The Southern Annular Mode (SAM), the dominant mode of atmospheric variability in the Southern Hemisphere, is predominantly in its positive phase during this period, which has been demonstrated to influence precipitation, especially in AP (Thomas et al, 2008;Abram et al, 2014) and WAIS (Fogt et al, 2012;Raphael et al, 2016). Therefore, some care must be taken when extrapolating results beyond the instrumental period.…”

Southern Ocean Overturning Compensation in an Eddy-Resolving Climate Simulation

“…Over the past 50 years, Southern Ocean winds have been increasing at a steady rate and have shifted poleward in response to anthropogenic forcing from the Antarctic ozone hole in the lower stratosphere and global climate change (Thompson et al 2011). Recent work from paleo records suggests that the Southern Ocean winds have been weaker in past climates because of an equatorward shift of the polar Westerlies (Toggweiler 2009) and are currently the strongest they have been in the past 1000 years (Abram et al 2014). Understanding how the strength of the Antarctic Circumpolar Current (ACC) and MOC respond to changing winds is fundamental to understanding global climate change.…”

Dendrohydrology and water resources management in South-Central Chile: Lessons from the Río Imperial streamflow reconstruction

“…Zúñiga et al (2012)), this relationship may well explain the decreasing summer streamflow trend observed for the last decades, as detected in the September-December negative correlation between the AAO in relation to both instrumental records and the tree-ring reconstruction (Table 5). This way, the post-1980 trend in summer streamflow of the Río Imperial may be linked to the aforementioned positive phase of the SAM (Abram et al, 2014). More so, this close relationship 25 could mean an unprecedented decreasing trend in summer streamflow in the Río Imperial for the last millennia.…”

“…This makes rainfall 15 runoff the principal source of water for the river. During the last 75 years or so, the SAM has been shifting to a positive phase, unprecedented in the last 1000 years (Abram et al, 2014). A positive phase means that the pressure gradient between the midlatitudes and the polar latitudes is negative, that is, more positive or less negative in the midlatitudes relative to polar latitudes.…”