Meeting Documents

Southern Ocean Overturning Response to Antarctic Sea Ice Decline

Abstract

From 1979 to 2016, Antarctic sea ice extent (SIE) experienced a small but steady increase, despite rising global mean surface temperatures. Since 2016, however, Antarctic SIE has undergone multiple dramatic declines, including the lowest SIE on record in February 2023 and the lowest wintertime SIE in September 2023. The seasonal cycle of sea ice growth, redistribution, and melt is the dominant process controlling the strength and structure of the Southern Ocean's overturning circulation through surface water-mass transformation. Changes in SIE will modify both the magnitude of surface buoyancy forcing and the density classes into which the freshwater is transported, thereby impacting the patterns of water-mass transformation.

In this study, we use model output from ECCO Version 4, release 5 (V4r5) to quantify the contributions of heat fluxes, ice and snow freshwater fluxes, and surface buoyancy patterns to water-mass transformation rates from 1992 to 2024. Importantly, V4r5 captures the rapid declines in sea ice area in 2016 and 2023, enabling a comparison of water-mass transformation rates during the increasing SIE period (1992-2015) and the declining SIE period (post-2016). Using the freshwater and heat fluxes from V4r5, we calculate surface buoyancy fluxes in potential density space. In denser classes, positive water-mass transformation occurs as sea ice formation near the Antarctic coast produces denser waters. At lower latitudes, melting sea ice contributes to lighter density classes, resulting in negative water-mass transformation rates. Preliminary results suggest that the reductions in SIE give rise to both thermal and freshwater contributions to water-mass transformation changes. This comprehensive buoyancy budget provides insight into how changes in Antarctic sea ice influence both the Southern Ocean and global overturning circulation.