Quasi-biennial modulation of the Southern Ocean Coherent Mode

Abstract

Empirical Orthogonal Function (EOF) analysis of two independent Southern Ocean sea-level datasets for the period 1992–2002 shows that the coherent oceanic mode–a zonally symmetric counterpart to the atmospheric Southern Annular Mode (SAM)–consistently emerges as the predominant mode of variability and is revealed to be a reliable proxy for circumpolar transport upon subannual to interannual time-scales. Cross-validation of the coherent mode from each dataset and that of a global barotropic model demonstrates a high degree of correspondence between sea-level observations and model estimations and also indicates that the coherent oceanic mode is largely barotropically driven upon subannual time-scales, whilst baroclinic oceanic effects predominate at longer periods. Both the coherent mode and transport are shown to be modified by the stratospheric Quasi-Biennial Oscillation (QBO) and a mechanism is described by which QBO-dependent planetary wave activity may be guided from the stratosphere towards the surface by high-latitude easterly and westerly jets, inducing wind anomalies that force sea-level variability through equatorward Ekman transport. A longer (18-year) tide-gauge record from the Vernadsky/Faraday station and a 22-year time series of sea-level estimations from an ocean general circulation model are used to show that QBO-induced sea-level and transport variability may be modulated upon longer time-scales by a quasi-decadal cycle of solar activit