Assessment of zonally symmetric and asymmetric components of the Southern Annular Mode using a novel approach

Abstract

The Southern Annular Mode (SAM) is the main mode of variability in the Southern Hemisphere extra-tropical circulation and it is so called because of its zonally symmetric ring-like shape. However, the SAM pattern actually contains noticeable deviations from zonal symmetry. Thus, the purpose of this study is to describe the zonally asymmetric and symmetric components of the SAM variability and their impacts. We regress monthly geopotential height fields at each level onto the asymmetric and symmetric component of the SAM to create two new indices: Asymmetric SAM (A-SAM) and Symmetric SAM (S-SAM). In the troposphere, the A-SAM is associated with a zonal wave 3 which is rotated a quarter wavelength with respect to the climatological zonal wave 3, is much stronger in the Pacific ocean, where it extends vertically to the stratosphere with an equivalent barotropic structure. On the other hand, the S-SAM is associated with negative geopotential height anomalies over Antarctica surrounded by a zonally symmetric ring of positive geopotential height anomalies. The observed relationship between the El Niño Southern Oscillation and the SAM is fully explained by the A-SAM index. The positive trend of the SAM is present only in its symmetric component. Despite this, the SAM is becoming more zonally asymmetric. The regional impacts of the SAM in temperature and precipitation are strongly affected by its asymmetric component. We show that the asymmetric component of the SAM has its own unique variability, trends and impacts, some of these signals are only evident when the two SAM components are separated.

Antarctic sea ice regime shift associated with decreasing zonal symmetry in the Southern Annular Mode

“…2007-2021; Figure 5c) the annular structure becomes more wave-like, as the non-annular components in the midlatitudes contract southwards. The difference between early samples and more recent samples (Figure 5d) essentially reproduces the pattern of the asymmetric SAM component at 700hPa (Campitelli et al, 2022), and zonal SAM anomalies averaged between -70°S and -55°S (Figure 5d) show a consistent shift in structure from annular to increasingly wave-3-like. The deepening and longitudinal contraction of the non-annular component in the Amundsen Sea positions it further away from the Ross Sea, with a clear reduction in cyclonic flow over the west of this region (Figure 5c).…”

“…Surface air temperatures and Southern Ocean SSTs reflected the weakened blocking state (Freitas et al, 2015;O'kane et al, 2013b;Franzke et al, 2015), and the strength of the subtropical jet decreased while upper tropospheric zonal wind strength increased north of the sea ice zone (Frederiksen et al, 2011;Frederiksen and Frederiksen, 2007). However, as SAM has a zonal wave-3 structure embedded within it, the intensifying annular mode has in recent years undergone a shift towards its asymmetric component (Wachter et al, 2020;Campitelli et al, 2022), increasing meridional flow over the sea ice zone and driving spatially heterogeneous anomalies. This may at first appear counterintuitive, as our analysis shows that recent sea ice anomalies are in greater agreement across the regions of highmagnitude change; however, we also show that the structural shift in SAM is co-located with changing Antarctic sea ice anomalies and reversing trends, implying that a sea ice response to north-south wind changes is already underway and likely to continue in line with an intensifying asymmetric flow pattern.…”

“…However, though the structure of SAM shows seasonal and decadal variability (Fogt et al, 2012a), in recent years the structure of the SAM has become increasingly asymmetric (Wachter et al, 2020;Campitelli et al, 2022), indicating more north-south airflow around the continent and, as a result, more spatially heterogeneous sea ice behaviour. In EOF1 of SLP between 1979 and 1993 (Figure 5b), the structure of SAM closely reflects the long-term mean (Figure 5a).…”

Anomalous subtropical zonal winds drive decreases in southern Australian frontal rain

“…Our analysis also indicates no change in the latitude of the subtropical jet (not shown), but a possible weakening in its intensity on front days in eastern Australia, although the average change in zonal wind speed is smaller at 300hPa than it is at 700hPa. The mean state changes in Figures 5 and S1 are also indicative of a modification of the ZW3 in Southern Hemisphere , which has been associated with rainfall variability in parts of southeast Australia but has no clear trend in the cool season (Campitelli et al, 2021).…”

Anomalous subtropical zonal winds drive decreases in southern Australian frontal rain

“…The mean state changes in Figs. 6 and S1 are also indicative of a modification of the ZW3 in the Southern Hemisphere, which has been associated with rainfall variability in parts of southeast Australia but has no clear trend in the cool season (Campitelli et al, 2021).…”