Recent sudden Antarctic sea ice retreat caused by connections to the tropics and sustained ocean changes around Antarctica

After nearly three decades of observed increasing trends of Antarctic sea ice extent, in September-October-November (SON) 2016, there was a dramatic and subsequently sustained decrease in ice extent. Expansion of sea ice accelerated after the year 2000 and a sudden retreat started in late 2016 through 2017 (see figure below). Two coupled, air-sea processes investigated in this study and previous that could impact this region are:

  1. The Interdecadal Pacific Oscillation (IPO), a coupled air-sea interaction phenomenon in the Pacific Basin which modulates sea surface temperature (SST) in the tropics and extra-tropics, begins in 2000 and ends in 2014 coinciding with the increase and decrease of sea ice extent trends (indicated by vertical, dashed lines in the below figure)
  2. The Southern Annular Mode (SAM), a shifting in atmospheric pressure between the Southern Pole and mid-latitudes over the Southern Ocean, which can impact wind and thus SST in the Southern Hemisphere high latitudes
Differences from the seasonal average of sea ice extent from observations (10<sup>6</sup>km<sup>2</sup>). Vertical dashed lines denote the start (2000) and end (2014) of a negative IPO period
Figure: Differences from the seasonal average of sea ice extent from observations (106km2). Vertical dashed lines denote the start (2000) and end (2014) of a negative IPO period.

Climate models included in the Coupled Model Intercomparison Project indicate a decrease of sea ice extent in the post-1979 period when in reality, an increase occurred; and thus, a number of studies have attempted to attribute factors that could have produced the expansion of Antarctic sea ice in this period. Evidence from observations and model sensitivity experiments have supported the idea that the expansion of Antarctic sea ice extent was associated with atmospheric teleconnection patterns driven from various areas in the tropics and associated surface wind anomalies at high southern latitudes, as well as a freshening of the Southern Ocean.

Previous studies have also addressed the subsequent sudden decrease of Antarctic sea ice extent in SON 2016 and have attributed it mainly to atmospheric processes. The post-2000 period was when the interdecadal Pacific oscillation (IPO) was in its negative phase with below-normal SST anomalies in the tropical Pacific on decadal timescales associated with a slowdown in the rate of global warming. The previous 1979–2000 period saw the IPO in its positive phase with an acceleration of global warming.

In this study, we analyze the observations and a model sensitivity experiment to show that a combination of factors contributed to the sudden and sustained decrease in Antarctic sea ice extent that started in late 2016. We find that the sustained decreases of Antarctic sea ice extent after late 2016 are associated with a warmer upper Southern Ocean (see below figure) due to a culmination of events. First, teleconnections from strong tropical convection in the eastern Indian Ocean produced surface wind anomalies in the vicinity of the Antarctic sea ice that contributed to its decreased extent. Second, decadal-timescale trends of strengthened circum-Antarctic westerly winds associated with the negative phase of the IPO and positive phase of the SAM moved warm subsurface water upward in the water column due to a process called Ekman suction. Third, a negative phase of the SAM in SON 2016 in the context of a transition to a positive phase of the IPO produced warm SSTs that completed a warming of the upper 600 m over much of the circum-polar ocean around Antarctica to sustain the decreases of Antarctic sea ice extent after late 2016.

Difference in zonal mean ocean temperature among the time periods of interest
Figure: Left - Cold Upper southern ocean during sea ice expansion, Middle&Right - Warm upper southern ocean in 2016 sustained through 2017 with sea ice retreat.
Difference in zonal mean ocean temperature among the time periods of interest.

Reference

  • Meehl, G.A., J.M. Arblaster, C.T.Y. Chung, M. M. Holland, A. DuVivier, L. Thompson, D. Yang, and C.M. Bitz, 2019: Recent sudden Antarctic sea ice retreat caused by connections to the tropics and sustained ocean changes around Antarctica, Nature Comms., 10:14, https://doi.org/10.1038/s41467-018-07865-9