Abstract
A coupled ocean-sea ice general circulation model is used to identify a Southern Ocean southeast Pacific intrinsic mode of low frequency variability and its response to inter-annual atmospheric forcing. Using forcing data from the co-ordinated ocean-ice reference experiment, a comprehensive suite of experiments elucidated excitation and amplification mechanisms of this intrinsic mode by low frequency forcing and stochastic forcing due to high frequency winds. Subsurface thermocline anomalies are found to teleconnect the Pacific and Atlantic regions of the Antarctic circumpolar current (ACC). It is found that the Pacific region of the ACC is characterised by intrinsic baroclinic disturbances that respond to both zonal and latitudinal wind variations, while the Atlantic sector of the ACC is sensitive to higher frequency synoptic 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. References T. Delworth, A. J. Broccoli, A. Rosati, R. J. Stouffer, V. Balaji, J. A. Beesley, W. F. Cooke, and K. W. Dixon. GFDL's CM2 global coupled climate models. Part 1: Formulation and simulation characteristics. J. Climate, 19:643--674, 2006. doi:10.1175/JCLI3629.1 H. Dijkstra and M. Ghil. Low-frequency variability of the largescale ocean circulation: A dynamical systems approach. Rev. Geophys., 43 RG3002, 2005. doi:10.1029/2002RG000122 C. W. Hughes. Rossby waves in the Southern Ocean: A comparison of TOPEX/POSEIDON altimetry with model predictions. J. Geophys. Res., 100:15933--15950, 1995. doi:10.1029/95JC01380 W. G. Large, J. C. McWilliams, and S. C. Doney. Oceanic vertical mixing: A review and a model with a nonlocal boundary layer parameterization. Rev. Geophys., 32:363--403, 1994. doi:10.1029/94RG01872 W. G. Large and S. G. Yeager. Diurnal to decadal global forcing for ocean and sea-ice models: the datasets and flux climatologies. NCAR Technical Note:, NCAR/TN-460+STR, CGD Division of the National Centre for Atmospheric Research, 2004. doi:10.5065/D6KK98Q6 T. J. McDougall and P. C. McIntosh. The temporal-residual-mean velocity. Part I: Derivation and the scalar conservation equations. J. Phys. Oceanogr., 26:2653--2665, 1996. doi:10.1175/1520-0485(1996)026<2653:TTRMVP>2.0.CO;2 R. J. Murray. Explicit generation of orthogonal grids for ocean models. Journal of Computational Physics, 126:251--273, 1996. doi:10.1006/jcph.1996.0136 A. B. Pezza, H. A. Rashid, and I. Simmonds. Climate links and recent extremes in antarctic sea ice, high-latitude cyclones, southern annular mode and ENSO. Climate Dynamics, 38:57--73, 2012. doi:10.1007/s00382-011-1044-y G. Quattrocchi, S. Pierini, and H. A. Dijkstra. Intrinsic low-frequency variability of the gulf stream. Nonlinear Processes in Geophysics, 19:155--164, 2012. doi:10.5194/npg-19-155-2012 A. Timmermann. Changes of ENSO stability due to greenhouse warming. Geophys. Res. Let., 28:2061--2064, 2001. doi:10.1029/2001GL012879
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