Mechanisms of zonal index variability in an aquaplanet GCM

Data from a 2025-day Geophysical Fluid Dynamics Laboratory aquaplanet GCM integration are used to examine the temporal evolution of the zonal index, defined as the principal component of the first EOF of the zonally and vertically integrated absolute angular momentum. This EOF represents meridional displacements of the subtropical jet. Positive (negative) values of the zonal index correspond to poleward (equatorward) displacements of the subtropical jet and are referred to as the high (low) zonal index. Composites of various quantities are used to examine the temporal evolution of both the zonal-mean zonal winds and the eddy fields associated with either index. The high index is initiated by an enhancement in the equatorward wave activity propagation, which causes the subtropical jet to move in a poleward direction. The low index is led by a weakening in the equatorward propagation, which results in an equatorward displacement of the subtropical jet. More importantly, for the high index, the corresponding increase in eddy forcing is confined to a brief period near onset, resulting in rapid growth and slow decay of the zonal wind anomaly. This suggests that the high index state is ''impulsively'' forced by the eddies rather than maintained by eddy-zonal mean feedback. In contrast, for the low index, a reduction in the eddy forcing extends throughout the entire persistent episode, although the weakest eddy forcing occurs during onset. The authors believe that such forcing causes the low index anomaly to persist for a longer period of time than the high index anomaly and also results in a similar growth and decay rate for the low index. Furthermore, case studies show that the onset for both indices is associated with wave breaking. For the high (low) index, this wave breaking takes the form of filamentation (blocking).