Compounding tropical and stratospheric forcing of the record low Antarctic sea-ice in 2016

Abstract

After exhibiting an upward trend since 1979, Antarctic sea ice extent (SIE) declined dramatically during austral spring 2016, reaching a record low by December 2016. Here we show that a combination of atmospheric and oceanic phenomena played primary roles for this decline. The anomalous atmospheric circulation was initially driven by record strength tropical convection over the Indian and western Pacific Oceans, which resulted in a wave-3 circulation pattern around Antarctica that acted to reduce SIE in the Indian Ocean, Ross and Bellingshausen Sea sectors. Subsequently, the polar stratospheric vortex weakened significantly, resulting in record weakening of the circumpolar surface westerlies that acted to decrease SIE in the Indian Ocean and Pacific Ocean sectors. These processes appear to reflect unusual internal atmosphere-ocean variability. However, the warming trend of the tropical Indian Ocean, which may partly stem from anthropogenic forcing, may have contributed to the severity of the 2016 SIE decline.

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

“…Similarly, in our pacemaker experiments we find the observed anticyclonic Rossby wave source off the west coast of Australia ( Figure S6a; dark red patch) to be captured in the Indian ensemble ( Figure S6b), and the cyclonic Rossby wave source off the east coast of Australia ( Figure S6a; dark blue patch) to be due to the combined influences of the Indian ( Figure S6b) and Pacific ( Figure S6c) Oceans. Thus, a Rossby wave source analysis of our pacemaker output complements the observational regression analysis conducted by Wang et al (2019) to disentangle the roles of the Indian and Pacific Oceans in contributing to the high-latitude circulation during September-October 2016 and provides further support for the tropical Indian Ocean driving the zonal wave three pattern at high latitudes.…”

“…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.…”

“…The analysis by Wang et al (2019) found the anticyclonic Rossby wave source off the west coast of Australia to be due to southward divergent flow from enhanced convection in the eastern Indian Ocean (their Figure 3c), while the cyclonic Rossby wave source off the east coast of Australia was due to both tropical Indian and Pacific influences (their Figures 3c and 3e). Similarly, in our pacemaker experiments we find the observed anticyclonic Rossby wave source off the west coast of Australia ( Figure S6a; dark red patch) to be captured in the Indian ensemble ( Figure S6b), and the cyclonic Rossby wave source off the east coast of Australia ( Figure S6a; dark blue patch) to be due to the combined influences of the Indian ( Figure S6b) and Pacific ( Figure S6c) Oceans.…”

“…The main features seem to be an average of the Indian ensemble mean pattern (Figure 2b) and the Pacific ensemble mean pattern ( Figure 2c); for example, there is a low-high-low pattern between 90 • E to 90 • W as seen in the Indian ensemble mean; however, the locations of the high and eastern low anomalies are more consistent with a PSA pattern. It is possible that in the coupled model, forcing in the tropical Pacific Ocean dominates over the tropical Indian Ocean during September-October, leading to a high-latitude MSLP pattern in the Ind-Pac ensemble that more closely resembles that of the Pacific ensemble, whereas in the real world, forcing from the tropical Indian Ocean dominated in September-October 2016 (Wang et al, 2019). It is possible that in the coupled model, forcing in the tropical Pacific Ocean dominates over the tropical Indian Ocean during September-October, leading to a high-latitude MSLP pattern in the Ind-Pac ensemble that more closely resembles that of the Pacific ensemble, whereas in the real world, forcing from the tropical Indian Ocean dominated in September-October 2016 (Wang et al, 2019).…”

Understanding the Seasonal Cycle of Antarctic Sea Ice Extent in the Context of Longer‐Term Variability

“…The decay was particularly strong in November 2016, when Antarctic SIE exhibited a negative anomaly compared to the 1979-2015 average (Schlosser et al, 2018). Several studies have investigated the possible factors driving this recent decline and found it to be linked to a warmer upper Southern Ocean (Meehl et al, 2019), to an anomalous atmospheric circulation with an amplified zonal wave 3 pattern (Schlosser et al, 2018;Wang et al, 2019) and to a significantly weakened polar stratospheric vortex, resulting in record weakening of the circumpolar surface westerlies that acted to decrease SIE (Wang et al, 2019).…”