Tracing the impacts of recent rapid sea ice changes and the A68 megaberg on the surface freshwater balance of the Weddell and Scotia Seas

Abstract

The Southern Ocean upper-layer freshwater balance exerts a global climatic influence by modulating density stratification and biological productivity, and hence the exchange of heat and carbon between the atmosphere and the ocean interior. It is thus important to understand and quantify the time-varying freshwater inputs, which is challenging from measurements of salinity alone. Here we use seawater oxygen isotopes from samples collected between 2016 and 2021 along a transect spanning the Scotia and northern Weddell Seas to separate the freshwater contributions from sea ice and meteoric sources. The unprecedented retreat of sea ice in 2016 is evidenced as a strong increase in sea ice melt across the northern Weddell Sea, with surface values increasing approximately two percentage points between 2016 and 2018 and column inventories increasing approximately 1 to 2 m. Surface meteoric water concentrations exceeded 4% in early 2021 close to South Georgia due to meltwater from the A68 megaberg; smaller icebergs may influence meteoric water at other times also. Both these inputs highlight the importance of a changing cryosphere for upper-ocean freshening; potential future sea ice retreats and increases in iceberg calving would enhance the impacts of these freshwater sources on the ocean and climate. This article is part of a discussion meeting issue ‘Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities’.

Finale: impact of the ORCHESTRA/ENCORE programmes on Southern Ocean heat and carbon understanding

“…Meredith et al . [ 69 ] used repeat δ 18 O data from the A23 section to examine year-on-year changes in sea ice and meteoric water input. They found a particularly strong pulse of sea ice melt injected to the surface of the northern Weddell Sea, following the unprecedented decrease in sea ice extent in 2016.…”

Impact of Giant Iceberg A68A on the Physical Conditions of the Surface South Atlantic, Derived Using Remote Sensing

“…Fortunately, in situ oceanographic measurements were collected by ship‐borne sensors and gliders close to the A68 fragments near South Georgia in early 2021 (Lucas et al., 2022; Meredith et al., 2023). The lowest surface salinity detected by a deployed glider was 25.59 psu, on 4 March 2021, at 56.254°S, 33.243°W, immediately beside A68A (Lucas et al., 2022).…”

Giant icebergs increase mixing and stratification in upper-ocean layers

“…Satellite-derived measurements [25] reveal significant surface salinity and temperature freshening and cooling up to a month before and after A-68As approach to South Georgia. Data collected using a contemporaneous hydrographic section, taken between South Georgia and the South Scotia Ridge, revealed surface meteoric water concentrations exceeding 4% close to South Georgia due to meltwater from A-68A [32].…”