In September-October-November (SON) 2016, there was a dramatic decrease of Antarctic sea ice extent. This followed nearly three decades of observed increasing trends of Antarctic sea ice extent since the beginning of the satellite record in 1979 that accelerated after the Interdecadal Pacific Oscillation (IPO) transitioned from positive to negative around 2000. Here we perform a sensitivity experiment using an atmosphere-only model with a specified positive convective heating anomaly in the eastern Indian/western Pacific Ocean to represent the record (for the 2000s) positive precipitation anomaly in SON 2016. We show that an anomalous atmospheric Rossby wave response, associated with that tropical heating, produces a teleconnection pattern around Antarctica with surface wind anomalies that likely contributed to the decrease of Antarctic sea ice extent. However, the sustained decreases of Antarctic sea ice extent after late 2016 suggest a regime-change that was associated with a warming of the upper layers of the ocean at high southern latitudes. This warming was the culmination of a negative trend of wind stress curl in the 2000s, associated Ekman suction that brought warmer water to the surface, and a negative phase of the Southern Annular Mode (SAM) in SON 2016. A likely transition of the IPO to its positive phase around 2014 could sustain the regime of warmer high latitude upper ocean temperatures and associated sea surface temperatures, and a decadal timescale trend of decreasing Antarctic sea ice extent.