Discussion
The identified importance of SAM and ENSO to Antarctic Ice Sheet spatiotemporal change is robust to variations in our regression methodology. SAMΣ and Niño3.4Σ were derived from anomalies relative to their climatological mean over 1971-2000, a well-observed period just before the GRACE period. This choice of period has little impact on Niño3.4Σ and avoids uncertainties in earlier periods of the SAM record(18 ) but likely under-estimates SAM anomalies and SAMΣ trends in the GRACE period. Shifting the climate-index reference-period to 1961-1990 results in 64% of AIS mass loss rate being attributed to the SAMΣ term due to the increased trend in SAMΣ. However, this change in reference period does not change the SAMΣ regression term. Adopting alternative SAM and ENSO indices (Methods, Fig. S1) results in changes to the regression terms, with 62% of the AIS trend contributed by SAM and ENSO (58% SAM, 2% ENSO; compare Tables S1 and S2). The spatial patterns of the regression coefficients (Fig. 2a-b) are not substantially altered by these index perturbations. Finally, the SAMΣ regression coefficient is not sensitive to the presence or absence of the SAMΣ trend (compare Tables S1 and S3). Overall, these perturbations indicate our estimates are robust and likely conservative in the role of SAM in AIS mass variability.
Repeating the multiple linear regression after first subtracting modelled SMB mass anomalies from the gridded GRACE data shows a substantial reduction in the magnitude of the SAMΣ and Niño3.4Σ coefficients in many regions (compare Figure S10 with Fig. 2), but not to negligible levels. While SMB models show large inter-model differences(36 ), introducing uncertainty in this analysis, a substantial part of both the SAM and ENSO signal in GRACE appears to be SMB signal(15 ) (Figure S11). Analysis of the spatial distribution of the GRACE-minus SMB signal, which could suggest SAM/ENSO-induced dynamic mass change(23 ), is hampered by the 200-300 km spatial resolution of GRACE, and the respective contributions of each of SMB and ocean-driven melt requires further quantification.
The finding that approximately 40 percent contribution of total ice-mass loss over 2002-2021 is likely forced by SAM provides a possible pathway to attributing some Antarctic Ice Sheet change to anthropogenic forcing, something thus far only partially established in a few regional Antarctic studies(37-39 ). The shift toward positive SAM since the 1940s, contributing to upward trends in SAMΣ, has been attributed to a combination of ozone and greenhouse forcing(34 ), and understanding of the potential anthropogenic drivers of this component of ice-sheet change could be advanced with a dedicated attribution study of SAM shifts over the GRACE period. In the future, it is unclear if ENSO will intensify(40 ) or dampen(41 ) but there is broad agreement that SAM will continue to shift toward its positive phase, driven by greenhouse gases(42 ), resulting in ongoing or accelerated trend in SAMΣ. If the relationship between SAM and ice-sheet change remains strong over other periods, as ice cores analysis suggests(19 ), SAM will play a substantial role in ice-sheet mass loss in the future in addition to responses to other forcing. Projections of the ice sheet will, in that case, only be robust if climate models reproduce and project SAM accurately, a source of present inter-model disagreement(43 ). Accurate representation of the future evolution of SAM and ENSO is therefore likely important for projecting future sea-level contribution from Antarctica.
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