The Anomalous 2012–13 Boreal Winter Oceanic Excitation of Earth’s Polar Motion

Recent work has shown that the Madden–Julian Oscillation (MJO) winds around the Maritime Continent can drive a see-saw in oceanic mass between the Indian and Pacific oceans on intraseasonal time scales. During the boreal winter of 2012–13, this see-saw accounted for about two thirds of an unusually large (~ 30 mas, milliarcseconds) fluctuation in the oceanic excitation of Earth’s polar motion about the 90°E meridian. Interestingly, the magnitude of the oceanic influence was nearly at par, but out-of-phase with that of the atmosphere and a factor of ~ 10 larger than effects associated with the hydrological cycle. Here we show that oceanic mass changes and transport anomalies during the 2012–13 boreal winter were indeed most pronounced in the Indo-Pacific basin, and that they possessed a favorable geometry to excite polar motion variations about the 90°E meridian. Phase alignment of the excitation signals from different regions, as well as between mass and motion terms, was a key characteristic of the 2012–13 event, but was far less distinct in other strong see-saw years. Basin-wise, the Indian Ocean acted as a dominant contributor to the 2012–13 polar motion excitation at MJO periods, followed by the Pacific basin. Overall, ocean dynamics in the 10°−65°S latitudinal belt over the Indo-Pacific basin accounted for ~ 93% of the global oceanic excitation function during the 2012–13 winter. The processes figuring most prominently in modulating intraseasonal polar motion were mass rearrangements in the southern Indian Ocean and the south-east Pacific Ocean, the east Australian current, and the Antarctic circumpolar current around the Antarctic landmass. Wind-driven dynamics in the Southern Ocean thus appear to be the main cause for the 2012–13 oceanic excitation signal not attributable to the see-saw.