1 Introduction
Freshwater originating from the Antarctic Ice Sheet (AIS) enters the ice
shelf cavities and marginal seas, lowering local salinities and
potentially hindering the formation of Antarctic Bottom Water (AABW)
[Silvano et al. , 2018; Wijk and Rintoul , 2014]. AABW
formation is one of the main drivers of the Global Meridional
Overturning Circulation (GMOC) [Gordon 1986], a large-scale
ocean circulation system that connects all ocean basins and controls
global climate stability [Broecker , 1991]. Thus, by changing
the rate of AABW formation, AIS melting can significantly affect the
state of the global climate.
AABW is mainly formed through the mixing of Antarctic ice shelf waters
and Circumpolar Deep Water (CDW), which flows towards the Antarctic
coast and onto the continental shelf where it enters ice shelf cavities
[e.g., Talley 2013 ]. Under the ice shelves, CDW mixes with
Dense Shelf Water, creating dense waters that flow through the
continental shelf, forming AABW through mixing along the shelf overflow
path [Carmack and Foster, 1975; Foster and Carmack, 1976 ].
Ice melting also occurs under ice shelves, releasing freshwater and thus
decreasing salinities along the Antarctic coast. Hence, increasing AIS
melting can increase the buoyancy of shelf waters, hindering the
formation of AABW [Wijk and Rintoul , 2014]. Alternatively,
AABW can be formed offshore by deep convection in open-ocean polynyas
[e.g., Killworth , 1983], a process commonly present in models
[Aguiar et al , 2017; Azaneu et al , 2013], but only
observed a few times since satellites started monitoring sea ice in the
Southern Ocean in late 1972 [Campbell et al , 2019;Gordon 1978].
Two main processes release meltwater from AIS to the Southern Ocean.
First, while circulating within ice-shelf cavities, warm CDW exchanges
heat with the overlying ice and induces melting on the ice shelf base, a
process known as basal melting [Jenkins et al., 2001 ].
Second, ice can detach from the ice shelves creating icebergs - a
process referred to as calving [e.g., Joughin and
MacAyeal, 2005 ]. Detached icebergs are advected by surface winds and
ocean currents [e.g., Wagner et al., 2017 ], releasing
freshwater along their advective path. Besides their potential impact on
AABW formation, both ice shelf basal melting and calving are thought to
increase sea-ice production and thus brine rejection, which can preclude
the direct surface freshening by AIS melting [Bintanja et al .,
2013]. However, the most widely used Earth System Models do not
explicitly simulate these ice melting processes, making it difficult to
understand their impact on AABW formation and sea ice production
[Jongma et al ., 2013].
Coupling ice sheet models to climate models to simulate AIS melting is
technically complex and requires considerable computational effort
[Nowicki and Seroussi, 2018 ]. An alternative approach is to
add virtual freshwater fluxes in the Southern Ocean, a protocol
suggested by the Ocean Model Intercomparison Project Phase 1 (OMIP1)
[Farneti et al, 2015; Griffies et al, 2009 ]. Embedded model
configurations often follow OMIP1 protocol by distributing AIS melting
equally over every surface grid cell of the ocean model along the
Antarctic coast, in a uniform flux configuration (Fig 1a). However, this
uniform flux misrepresents the spatially varying calving and basal
melting of ice shelves [Rignot et al., 2019 ], and therefore
could impose salinity biases under ice shelf cavities, changing the
salinity of AABW and its source waters [Jongma et al, 2009 ].
Since most state-of-the-art climate models misrepresent AABW properties
[Heuzé et al, 2013 ], assessing the role of the spatial
distribution of AIS melting in determining AABW properties is a useful
step to improve the representation of the AABW in model simulations.
Furthermore, a significant freshening of the AABW was reported over the
last three decades [Purkey and Johnson, 2012; Anikulmar et al,
2021 ], with regional increases in AIS melting and calving possibly
triggering this freshening [Fogwill et al, 2015 ]. This
connection between enhanced regional AIS melting and AABW freshening
makes it even more critical to assess how spatial variations in AIS
melting alter AABW properties.
Therefore, this work assesses the impact of the freshwater distribution
(uniform along the Antarctic coast versus spatially varying) on the AABW
properties and transport in a global ocean and sea-ice coupled model,
and further analyzes the potential response of AABW and the Antarctic
sea ice to an enhanced AIS melting. Section 2 describes the methods,
including the model, water mass definitions, and experimental design.
Section 3 compares the surface and bottom responses to diverse
spatio-temporal meltwater flux distributions. The main discussion and
conclusions are provided in section 4.