Sustained ocean changes contributed to sudden Antarctic sea ice retreat in late 2016

Abstract

After nearly three decades of observed increasing trends of Antarctic sea ice extent, in September-October-November 2016, there was a dramatic decrease. Here we document factors that contributed to that decrease. An atmosphere-only model with a specified positive convective heating anomaly in the eastern Indian/western Pacific Ocean, representing the record positive precipitation anomalies there in September-October-November 2016, produces an anomalous atmospheric Rossby wave response with mid- and high latitude surface wind anomalies that contribute to the decrease of Antarctic sea ice extent. The sustained decreases of Antarctic sea ice extent after late 2016 are associated with a warmer upper Southern Ocean. This is the culmination of a negative decadal trend of wind stress curl with positive Southern Annular Mode and negative Interdecadal Pacific Oscillation, Ekman suction that results in warmer water being moved upward in the column closer to the surface, a transition to positive Interdecadal Pacific Oscillation around 2014–2016, and negative Southern Annular Mode in late 2016.

Tropical Teleconnections to Antarctic Sea Ice During Austral Spring 2016 in Coupled Pacemaker Experiments

“…Anomalies in SST are also overall well reproduced in pattern, if not in magnitude, in this ensemble set. The similarity in MSLP patterns between observations and the Indian ensemble mean, along with a Rossby wave source analysis as in Wang et al (2019; see Figure S6 in the supporting information and discussed below), provides further support that anomalous convection over the eastern Indian Ocean during spring 2016 was a driving force influencing the early phase of the decline in Antarctic sea ice in this season (Meehl et al, 2019;Wang et al, 2019). In response to the anomalous MSLP, the pattern of SIC anomalies also shows a zonal wave three pattern, similar to observed.…”

“…Nevertheless, there is a relationship between the SAM and ENSO (Fogt & Bromwich, 2006;L'Heureux & Thompson, 2006), due to the SAM modulation of the high-latitude ENSO teleconnection, with ENSO events influencing the asymmetric structure of the high-latitude SAM (Fogt et al, 2011). For example, Meehl et al (2019) find a strong negative SAM to be associated with convective heating over the eastern Indian/western Pacific Oceans in their atmospheric model experiment: whether the difference between their results and ours is due to differences in the atmospheric dynamics between models, or due to different atmosphere-ocean-ice feedbacks in our coupled experiments, remains unknown. For example, Meehl et al (2019) find a strong negative SAM to be associated with convective heating over the eastern Indian/western Pacific Oceans in their atmospheric model experiment: whether the difference between their results and ours is due to differences in the atmospheric dynamics between models, or due to different atmosphere-ocean-ice feedbacks in our coupled experiments, remains unknown.…”

“…Both our Pacific and Ind-Pac ensemble means show a clear and very strong surface warming in this region during summer 2015/2016 (Figures S10c and S10d); however, this warming has almost entirely dissipated in the Pacific ensemble by spring 2016 (Figures 2c, 3c, and S10c), providing only limited evidence for a lingering influence of the 2015/2016 extreme El Niño. Considerable spread across each ensemble was found (Figures S3-S5 and S7-S9), making interpreting the results challenging and contrasting more tightly constrained experiments, such as convective heating atmospheric model experiments, which clearly simulate a teleconnection from the eastern Indian Ocean to southern high latitudes (Meehl et al, 2019). This additional examination of high-latitude SST anomalies in our pacemaker experiments provides further evidence that the Indian Ocean played a more dominant role than the Pacific in contributing to the spring 2016 Antarctic sea ice decline.…”

“…Meehl et al (2019) propose that in the lead up to spring 2016, the Southern Ocean had been "preconditioned" by a period of prolonged positive SAM and negative Interdecadal Pacific Oscillation (∼2000-2014), leading to enhanced westerly winds causing Ekman pumping of warm subsurface water upward in the water column. Meehl et al (2019) propose that in the lead up to spring 2016, the Southern Ocean had been "preconditioned" by a period of prolonged positive SAM and negative Interdecadal Pacific Oscillation (∼2000-2014), leading to enhanced westerly winds causing Ekman pumping of warm subsurface water upward in the water column.…”

“…There is also the possibility that longer-term ocean contributions building up prior to 2014 may have played a role in the observed decline (Meehl et al, 2019); however, these longer timescale oceanic effects are not included in our experimental design. This suggests that intrinsic atmospheric variability is likely also playing a role.…”