Southern Ocean Intrinsic and Forced Modes of Low Frequency

A coupled Ocean-Sea ice General Circulation Model (OGCM) is used to identify a Southern Ocean southeast Pacific intrinsic mode of low frequency variability. This mode is co-located with a major region of Sub-Antarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) formation. Using CORE data a comprehensive suite of experiments were carried out to elucidate excitation and amplification mechanisms of this intrinsic mode by low frequency forcing (ENSO, SAM) and stochastic forcing due to high frequency winds. Subsurface thermocline anomalies were found to teleconnect the Pacific and Atlantic regions of the Antarctic Circumpolar Current (ACC). The Pacific region of the ACC is characterised by intrinsic baroclinic disturbances that respond to both SAM and ENSO, while the Atlantic sector of the ACC is sensitive to higher frequency winds that act to amplify thermocline anomalies propagating downstream from the Pacific resonant with eastward travelling Rossby waves. This simulation study identifies plausible mechanisms that determine the predictability of the Southern Ocean climate on multi-decadal timescales.