A New Zonal Wave-3 Index for the Southern Hemisphere

Abstract

Abstract Zonal wave 3 (ZW3) is an important feature of the Southern Hemisphere extratropical atmospheric circulation and has strong impacts on meridional heat and momentum transport, regional Antarctic sea ice extent, and Southern Hemisphere blocking events. Attempts have been made in the past to define an index that quantifies the variability in the ZW3 pattern; however, existing methods are based on fixed geographical locations and fail to capture certain ZW3 events because of strong variability in phase. In addition, a fixed spatial index poorly characterizes ZW3 in CMIP models, which can exhibit biases in the mean phase of the ZW3 pattern. In this study, we introduce a new way to characterize ZW3 variability by incorporating two indices, one each for magnitude and phase, based on the combination of the first two empirical orthogonal functions (EOFs) of the 500-hPa meridional wind anomalies. We show that the new ZW3 index provides a clear advantage over past indices because it captures a substantially higher proportion of variance (∼40% compared to ∼16%), and it can be used for both reanalysis datasets and coupled climate models regardless of model biases. A composite analysis associated with the new index reveals a strong relationship between the ZW3 defined by our index and sea ice fraction around Antarctica, with significant regional sea ice anomalies during strong ZW3 events with different phases.