The Antarctic circumpolar wave and its seasonality: Intrinsic travelling modes and El Niño–Southern Oscillation teleconnections

Interannual variability in the Southern Ocean is investigated via nonlinear Laplacian spectral analysis (NLSA), an objective eigendecomposition technique for nonlinear dynamical systems that can simultaneously recover multiple timescales from data with high skill. Applied to modelled and observed sea surface temperature and sea ice concentration data, NLSA recovers the wavenumber-2 eastwards propagating signal corresponding to the Antarctic circumpolar wave (ACW). During certain phases of its lifecycle, the spatial patterns of this mode display a structure that can explain the statistical origin of the Antarctic dipole pattern. Another group of modes have combination frequencies consistent with the modulation of the annual cycle by the ACW. Further examination of these newly identified modes reveals that they can have either eastwards or westwards propagation, combined with meridional pulsation reminiscent of sea ice reemergence patterns. Moreover, they exhibit smaller-scale spatial structures and explain more Indian Ocean variance than the primary ACW modes. We attribute these modes to teleconnections between ACW and the tropical Indo-Pacific Ocean; in particular, fundamental El Niño–Southern Oscillation (ENSO) modes and their associated combination modes with the annual cycle recovered by NLSA. Another mode extracted from the Antarctic variables displays an eastwards propagating wavenumber-3 structure over the Southern Ocean, but exhibits no significant correlation to interannual Indo-Pacific variability.