4 Discussion and conclusions
Most widely-used sea-ice and ocean models represent AIS meltwater fluxes
by adding spatially uniform freshwater fluxes along the Antarctic coast,
a protocol suggested by OMIP1. However, such uniform freshwater flux
representation contrasts with the spatially varying melting patterns
observed for the AIS [Rignot et al ., 2013; 2019;]. This
misrepresentation creates surface salinity biases that can propagate
into AABW properties [Jongma et al, 2009 ]. In this study,
four different freshwater flux experiments were performed to better
understand the AABW response to the location (uniform and varying) and
to increasing AIS melting. Our results show that AABW salinity is
sensitive to where the meltwater flows around the Antarctic continent
(i.e., zonal distribution of freshwater fluxes). Varying the AIS melting
fluxes zonally according to observations (VARI , Fig 1b,d)
produces a 1.9±0.6×10-3PSU increase in AABW salinity
compared to a uniform meltwater flux. In VARI , freshwater flux is
the largest in the Amundsen sector (Fig 1d) and smaller in the remaining
regions along the Antarctic coast. As a result, surface salinity is
increased across the Weddell and Ross Seas in VARI compared toUNIF (Fig 2a,d,f). The positive salinity anomalies in the Weddell
and Ross Seas propagate to the bottom layer by the shelf overflow. Since
no coastal bottom water formation is simulated in the Amundsen sector
[Danabasoblu et al, 1999], the freshening signal in this sector does
not propagate into the bottom waters (Fig 2f). The higher AABW salinity
found in the experiment with spatially varying freshwater fluxes
(VARI ) has important implications for the representation of AABW
in global ocean models. Specifically, to avoid the low salinity bias in
AABW imposed by uniform AIS melting a better practice is to force ocean
& sea-ice models with zonally varying fluxes that mimic the observed
basal melting and calving distribution as in VARI . This practice
is also consistent with the surface meltwater flux protocol suggested in
OMIP phase 2 [Tsujino et al 2020].
In contrast, the AABW transport in the Southern Ocean is insensitive to
the spatial distribution of AIS melting. Any decrease in the eulerian
component of the overturning in the Southern Ocean in response to the
freshwater fluxes is compensated by an increase in the eddy-induced
component of the overturning (Fig S2). Eddy compensation of the
overturning has been reported in experiments with altered Southern
Hemisphere Westerlies [Bishop et al., 2016]. Here, we report that
eddy compensation can also occur under small surface freshwater
perturbations. Note that the CESM1 representation of the ocean
overturning and the Southern Ocean circulation is similar in 100 km
horizontal resolution (used here) and eddy-rich (10 km) resolution
[Hewitt et al., 2020]. Therefore, eddy compensation to freshwater
fluxes is unlikely an artifact of oceanic eddy parameterizations.
Furthermore, in VARI and UNIF , AABW transport was
evaluated for equilibrium conditions. Consequently, the similar AABW
transport in these simulations can also be caused by salt flux
compensation from sea ice expansion - similar to what happens in the
20% increase experiment.
Finally, observations show that AABW salinity decreased since the 1980s
[Menezes et al ., 2017; Purkey et al ., 2012]. To answer
if AABW freshening in the Southern Ocean could be driven by increased
AIS melting, we explored the sensitivity of AABW transport and salinity
to a 20% increase in AIS melting (415Gt/year). The short-term (30
years) AABW freshening in CESM1 in response to the increased freshwater
flux is similar to observed trends in the Southern Ocean during the past
decades. The average Southern Ocean AABW freshening of
3.0±0.2×10-3 PSU/decade simulated in this study lies
within the range of freshening rates estimated from observations (i.e.,
1.0×10-3~5.0×10-3PSU/decade)
since 1980 [Purkey and Johnson 2012; Anikulmar et al., 2020].
Regional freshening rates also agree with observations. In particular,
observations on the Indian and West Pacific sectors show freshening
rates of 4.0±1.0×10-3PSU/decade [Menezes et al.,
2017] and 3.1×10-3PSU/decade [Shimada et al.,
2012], respectively, while simulated freshening rates for these
sectors are 4.8±0.4×10-3PSU/decade and
3.4±0.5×10-3PSU/decade, respectively. The agreement
between the observed and the simulated AABW freshening rates under a
20% increase in AIS melting suggests that increased AIS melting could
be a fundamental driver of the observed AABW freshening.
Observations also show that sea ice in the Southern Ocean has expanded
since 1974 at a rate of
33×103km2/yr. In our experiments, a
20% increase in freshwater fluxes from AIS caused sea ice to expand by
22×103km2/yr, roughly in agreement
with observations. This similarity in sea-ice expansion rates suggests
that enhanced AIS melting could be at least partially responsible for
the observed sea-ice expansion trend. Although the mechanisms
responsible to expand sea ice in our simulations are not explored in
this study, previous studies have suggested that increases in meltwater
fluxes from AIS can isolate the Southern Ocean surface from warm deep
waters, triggering sea ice expansion [Mackie et al, 2020; Jeong et al,
2020].
It is important to highlight that the simulated impact of increased AIS
melting on AABW salinity, transport, and sea ice is not permanent. After
60 years of enhanced AIS melting, both AABW salinity and GMOC transport
are restored to their initial values (Fig 3a), suggesting that observed
AABW freshening could yet be reversed by enhanced brine rejection from
sea ice expansion. Nevertheless, the agreement of the observed sea ice
and AABW trends with the simulated ones indicates that both of these
trends could be triggered by a 415Gt/yr increase in AIS melting. Indeed,
previous studies report an increase in AIS melting between 180Gt/yr and
480Gt/yr since 1978 [Rignot et al, 2019; Adsumilli et al 2020].
It is important to highlight that these results are constrained by the
specific CESM1 configuration. For example, shelf overflow
parameterizations had a critical role in propagating the surface
salinity anomalies to the bottom layer in CESM1. Models without overflow
parameterizations on the Antarctic coast form AABW by shelf or open
ocean deep convection, which can dampen the surface freshening by mixing
the surface freshwater with high-salinity deep waters. Eddy compensation
in CESM1 also maintained AABW transport stable in the freshwater forcing
experiments. However, the efficiency of eddy compensation can change
under eddy parameterization schemes other than the one used in CESM1.
Finally, ice shelves can reach up to 1 km down the water column, and as
such freshwater fluxes from basal melting are not restricted to the
surface as in our experiments. Ice shelf melting at deeper levels can
cause coastal freshening rates different from those in our surface
freshwater flux experiments [Pauling et al. , 2016]. Further
studies are necessary to assess if specific model settings, such as eddy
and shelf overflow parameterizations can determine AABW sensitivity to
AIS melting.