Ensemble prediction study of the East Australian Current

Abstract

We examine the role of dynamical instabilities and flow dependent, growing errors in an ensemble prediction study of the East Australian Current. In the region of the Tasman front where the East Australian Current separates from the coast, the circulation is characterised by meandering and significant mesoscale eddy variability, and is dominated by nonlinear dynamics, thereby representing a severe challenge for operational forecasting. We explore the mechanisms for and structures of forecast errors over a month using the Australian operational ocean forecast system. To examine the role of dynamical instabilities, forecast ensemble perturbations are generated as bred vectors allowing the identification of those perturbations to a given initial state that grow most rapidly. We consider a period spanning the Austral autumn/summer corresponding to the seasons of maximum eddy variability. We find that the bred vectors span a low dimensional subspace whose structures correspond to regions of large forecast error. Further, these structures typically occur in regions of instability and in particular in the vicinity of the Tasman front and the East Australian Current extension. Our results suggest that even a very few bred vectors will be useful in adaptive sampling and targeted observations. References G. R. Brassington, P. R. Oke, and T. Pugh, BLUElink Operational ocean prediction in Australia, vol. 5, ch. 11, pp. 87--95, World Scientific Publishing, 2006. M. Corazza, E. Kalnay, D. J. Patil, S.-C Yang, R. Morss, M. Cai, I. Szunyogh, B. R. Hunt, and J. A. Yorke, Use of the breeding technique to estimate the structure of the analysis errors of the day, Nonlin. Proc. in Geophys. 10 (2003), 233--243. http://hal.archives-ouvertes.fr/hal-00302201/ S. M. Griffies, M. J. Harrison, R. C. Pacanowski, and A. Rosati, A technical guide to {MOM4} {GFDL} ocean technical group report no 5, 2003. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.143.8321&rep=rep1&type=pdf H. E. Hurlburt, G. B. Brassington, Y. Drillet, M. Kamachi, M. Benkiran, R. Bourdalle-Badie, E. P. Chassignet, G. A. Jacobbs, O. Le Galloudec, J.-M. Lellouche, E. J. Metzger, P. R. Oke, T. F. Pugh, A. Schiller, O. M. Smedstad, B. Tranchant, H. Tsujino, N. Usui and A. J. Walcraft, High-resolution global and basin-scale ocean analyses and forecasts., Oceanography 22 (2009), no. 3, 80--97. http://handle.dtic.mil/100.2/ADA508067 M. M. Mata, M. Tomczak, S. Wijffels, and J. A. Church, East Australian Current volume transports at 30S: Estimates from the World Ocean Circulation Experiment hydrographic sections PR11/P6 and the PCM3 current meter array., J. Geophys. Res. 105 (2000), 28, 509--28, 526. doi:10.1029/1999JC000121 M. M. Mata, S. E. Wijffels, J. A. Church, and M. Tomczak, Eddy shedding and energy conversion in the East Australian Current, J. Geophys. Res. 111 (2006), C09034. doi:10.1029/2006JC003592 T. J. O'Kane and J. S. Frederiksen, Statistical dynamical subgrid-scale parameterizations for geophysical flows, Physica Scripta T132 (2008), 014033. doi:10.1088/0031-8949/2008/T132/014033 T. J. O'Kane, P. R. Oke and P. A. Sandery, Ocean Modelling, to appear (2011). doi:10.1016/j.ocemod.2011.04.003 P. R. Oke, M. Balmaseda, M. Benkiran, J. A. Cummings, E. Dombrowsky, Y. Fujii, S. Guinehut, G. Larnicol, P.-Y. Le Traon, and M. J. Martin, Observing System Evaluations using GODAE systems, Oceanography (2009), 144--153. http://handle.dtic.mil/100.2/ADA511325. P. R. Oke and D. A. Griffin, The cold-core eddy and strong upwelling off the coast of New South Wales in early 2007, Deep Sea Research 58 (2011), 574--591. doi:10.1016/j.dsr2.2010.06.006 D. Pazo, M. A. Rodr\OT1\i guez, and J. M. Lopez, Spatio-temporal evolution of perturbations in ensembles initialized by bred, Lyapunov and singular vectors, Tellus 62A (2010), 10--23. doi:10.1111/j.1600-0870.2009.00419.x P. A. Sandery, G. B. Brassington, and J. Freeman, Adaptive nonlinear dynamical initialization., J. Geophys. Res. 116 (2011), C01021. doi:10.1029/2010JC006260 A. Schiller, P. R. Oke, G. B. Brassington, M. Entel, R. Fiedler, D. A. Griffin, and J.V. Mansbridge, Eddy-resolving ocean circulation in the Asian-Australian region inferred from an ocean reanalysis effort., Progress in Oceanography 76 (2008), 334--365. doi:10.1016/j.pocean.2008.01.003 Z. Toth and E. Kalnay, Ensemble forecasting at {NMC}, Bull. Amer. Meteor. Soc. 74 (1993), 2317--2330. doi:10.1175/1520-0477(1993)074<2317:EFANTG>2.0.CO;2