Decadal variability in an OGCM Southern Ocean: intrinsic modes, forced modes and metastable states

• Published in: Ocean Modelling
• Main Authors: O'Kane, TJ, Matear, R, Chamberlain, MA, Risby, JS, Sloyan, BM, Horenko, I
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier Sci Ltd 2013
• Subjects: Earth Sciences; Oceanography; Physical Oceanography; Antarctic; Southern Ocean; The Antarctic; Pacific; Antarc*; Sea ice
• Online Access: https://doi.org/10.1016/j.ocemod.2013.04.009; http://ecite.utas.edu.au/119350

An ocean general circulation model (OGCM) is used to identify a Southern Ocean southeast Pacific intrinsicmode of low frequency variability. Using CORE data a comprehensive suite of experiments were carriedout to elucidate excitation and amplification responses of this intrinsic mode to low frequencyforcing (ENSO,SAM) and stochastic forcing due to high frequency winds. Subsurface anomalies werefound to teleconnect the Pacific and Atlantic regions of the Antarctic Circumpolar Current (ACC) thermocline.The Pacific region of the ACC is characterised by intrinsic baroclinic disturbances that respond toboth SAM and ENSO, while the Atlantic sector of the ACC is sensitive to higher frequency winds thatact to amplify thermocline anomalies propagating downstream from the Pacific. Non-stationary clusteranalysis was used to identify the systems dynamical regimes and characterise meta-stability, persistenceand transitions between the respective states. This analysis reveals significant trends, indicating fundamentalchanges to the meta-stability of the ocean dynamics in response to changes in atmospheric forcing.Intrinsic variability in sea-ice concentration was found to be coupled to thermocline processes. Seaicevariability localised in the Atlantic was most closely associated with high frequency weather forcing.The SAM was associated with a circumpolar sea-ice response whereas ENSO was found to be a major driverof sea-ice variability only in the Pacific. This simulation study identifies plausible mechanisms thatdetermine the predictability of the Southern Ocean climate on multi-decadal timescales.