Figure 3 . Effects of increased AIS melting. In [a] are plotted time series of mean bottom ocean salinity anomaly (blue), mean Sea Ice Concentration anomaly (orange), overturning streamfunction at 65oS (green), calculated as VARI120%-VARI,averaged over the Southern Ocean south of 60oS and smoothed with a 10-year moving average. Dotted red line in [a] is the salt flux from sea ice formation smoothed with a 10-year moving average, while the full red is the salt flux after a low-pass filter with a cutout frequency of 0.1 yrs-1. [b] is the initial 30-years-long (i.e., years 101 - 130) salinity trend at the bottom of the ocean due to a 20% increase in AIS melting. Values indicated in [b] are the 30-year salinity trends averaged over the model bottom cells for each sector (in 10-3 PSU per decade).
3.2 AABW response to increased AIS melting
To investigate the transient response to a 20% increase in AIS melting fluxes, we compare results from VARI and VARI120%simulations. Increasing the AIS melting generates a freshening trend of 3±0.2×10-3PSU/decade in the bottom layer during the first 30 years of the VARI120% simulation (Fig 3a). This freshening trend is the strongest in the Indian (4.8±0.4×10-3PSU/decade) and Weddell (-4.7±0.3×10-3PSU/decade) sectors, followed by the West Pacific, Amundsen, and Ross Sea sectors (Fig 3b). After the peak of bottom layer freshening at the year 26, salinity anomalies start to increase and stabilize by around the year 60. The average salinity anomaly in the bottom layer computed between the years 60 and 100, i.e., after stabilization of the bottom layer salinity, is -2.0×10-3PSU (Fig 3a). The overturning circulation at 65oS first decelerates by ~8.3Sv until approximately year ~30, then accelerates again after year 34, and eventually becomes more stable closer to the initial condition at the year 60. Salinity decreases in AABW are linked to increasing buoyancy of surface waters, which can slow down AABW formation [Stouffer et al. 2007]. Specifically, enhanced freshwater fluxes in VARI120% can increase the buoyancy of surface waters, hindering the AABW formation initially. Decreased AABW formation then hinders momentum transfer to the bottom layers, slowing down the overturning circulation at 65oS. The synchronous decrease in AABW salinity and overturning in the first 30 years ofVARI120% (Fig 3a) suggests that surface freshening is responsible for slowing down the overturning in the Southern Ocean. It is interesting to note that no net changes in AABW salinity occurred in the long-term (timescales longer than 60 years).
Anomalies in sea ice concentration (SIC) also respond to the increase in AIS melting. The anomaly in SIC averaged over the Southern Ocean stays stable in the first 20 years. SIC starts increasing at year 20 and reaches its maximum at year 28 (Fig 3a). SIC anomaly remains high until year 50, then starts to decrease and reaches a new equilibrium at year 60. The sea ice expansion after year 20 drives a stronger salt flux from sea ice into the ocean after year 25, due to brine rejection from the increased sea ice formation (Fig 3a). This enhanced sea ice production and salt flux could counterbalance the AABW freshening triggered by increased freshwater flux [Bintanja et al ., 2013], and thus reverse the overturning circulation increase after year 30 (Fig 3a). Therefore, sea ice expansion seems to have a critical role in stabilizing the AABW formation rate after an increase in AIS melting. The sea ice expansion trend (Fig 3a) amounts to 22×103km2/yr, which is comparable to the observed sea-ice expansion of 33×103km2/yr for the period of 1979 - 2015 [Sun and Eisenman, 2021].