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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