2 Methods
2.1 The Community Earth System Model Version 1
All simulations were performed using the ocean and sea-ice components of
the Community Earth System Model version 1 (CESM1) [Danabasoglu
et al ., 2012] from the National Center for Atmospheric Research.
Ocean and sea ice were forced with surface fluxes from the European
Center for Medium-Range Weather Forecast ERA5 reanalysis for the
1951-1980 period [Hersbach et al ., 2020]. A horizontal
resolution of ~1o (grid gx1v6) was
used for both ocean and sea-ice components. The ocean is divided into 60
unevenly-spaced levels, with thicknesses ranging from 10 m at the
surface to 250 m in the deep ocean. All simulations were initialized
with temperature and salinity fields from the Polar Hydrographic
Climatology [Steele et al ., 2001]. A spin-up approach fromLee et al., [2017, 2019 ] is followed here where each
year in the model is forced with randomly chosen historic fields from
ERA5.
Previous work showed that most of the widely used Earth System Models
inaccurately represent AABW formation. A possible reason for this
misrepresentation is that most models’ current horizontal and vertical
resolutions are too coarse to reproduce the downslope flow of dense
waters off the Antarctic shelf and its mixing along the way, an
essential process to form AABW. Coarse-resolution models exhibit
excessive mixing of dense shelf waters with lighter surface waters,
decreasing the density of shelf water and inhibiting AABW formation on
the shelf [Heuz é et al , 2013]. These models create
AABW through excessive deep convection in the open ocean, resulting in
bottom waters that are often too fresh [Heuzé 2021]. Nevertheless,
since the addition of shelf overflow parameterizations in the Southern
Ocean [Danabasoglu et al ., 2012], AABW in CESM1 is properly
formed on the continental shelf [Heuz é, 2021], making CESM1
an appropriate model to study AABW sensitivity to coastal freshwater
fluxes.
2.2 Water mass definition
We define AABW as the waters with a neutral density higher than 28.27 kg
m-3 [Orsi et al., 1999 ]. AABW salinity was
calculated as the average salinity of AABW waters south of
60oS. The Southern Ocean was divided into five sectors
(Fig 1a) according to Parkinson and Cavalieri [2012 ]: the
Weddell Sector, Indian Sector, West Pacific, Ross Sector, and Amundsen
Sector. The AABW transport in the Southern Ocean was measured as the
absolute value of the minimum of the streamfunction at
65oS in density coordinates. The confidence interval
for the AABW transport was obtained from the 95% confidence level of
the yearly-averaged AABW transport estimates from the last 100 years of
each simulation. Finally, although basal melting occurs in both ice
shelves and under icebergs, in this study the term basal melting
specifically refers to melting under ice shelves.
2.3 Freshwater distribution simulations
We carried out three model simulations to test to what extent AABW
formation and salinity are affected by the spatio-temporal distribution
of Antarctic meltwater fluxes. The first experiment (UNIF. , as in
uniform) was forced with a freshwater flux of 2075 Gt/yr added uniformly
in the ocean grid points closest to the Antarctic coast (Fig. 1a, Table
S1). The flux magnitude of 2075 Gt/yr is based on total ice mass loss
estimates from Rignot et al, [2019], and a full derivation of the
value can be found in Supplementary material S2. The freshwater flux
field of UNIF represents the surface meltwater fluxes suggested
by OMIP1 and does not account for the spatial variation in freshwater
fluxes from calving or basal melting.
The second simulation (BM ) was forced with zonally varying fluxes
to mimic the spatial variations in AIS basal melting (Fig. 1c). The
third simulation (VARI , Fig 1b) was forced with a freshwater flux
that mimics the spatial variation of both ice sheet basal melting and
calving (Supplementary S1). The spatial distribution of basal melting
takes into account the meltwater production of Antarctic ice shelves
estimated by Rignot et al [2013], while iceberg melting distribution
is based on satellite-tracked iceberg positions from 1979 until 2017
(Supplementary S2). VARI differs from BM in the meridional
distribution of freshwater fluxes, i.e., by having part of its
freshwater fluxes displaced offshore, as seen by the higher melting
fluxes offshore (Fig 1d, northward distances from coast larger than
3o of latitude), and lower melting fluxes along the
coast (Fig 1d, distances lower than 3o) in VARIcompared to BM . The spatially varying freshwater flux fields used
in VARI and BM simulations were produced according to the
method discussed in Hammond and Jones [2016].