Methods of ensemble prediction for seasonal forecasts with a coupled ocean-atmosphere model

Abstract

Computational methods for efficient seasonal ensemble prediction with a coupled ocean-atmosphere model, consisting of a global atmosphere and a Pacific basin ocean, are described. Nonlinearly modified Lyapunov vectors, termed bred modes, and finite time normal modes, termed cyclic modes, that grow fastest over a month are found to be suitable ensemble perturbations. The skill of seasonal ensemble prediction is examined in hindcast simulations for the period 1980 to 2000. In general, ensemble mean forecasts are significantly more skilful than the control forecasts. We find that cyclic mode perturbations are generally more effective than bred vectors in improving ensemble forecasts. References

• Z. Toth and E. Kalnay, Ensemble forecasts at NCEP and the breeding method, Mon. Wea. Rev. 125:3297--3319, 1997. doi:10.1175/1520-0493(1997)125<3297:EFANAT>2.0.CO;2
• J. S. Frederiksen, M. A. Collier and A. B. Watkins, Ensemble prediction of blocking regime transitions, Tellus A 56:485--500, 2004. doi:10.1111/j.1600-0870.2004.00075.x
• S. C. Yang, M. Cai, E. Kalnay, M. Reinecker, G. Yuan and Z. Toth, ENSO bred vectors in coupled ocean-atmosphere general circulation models, J. Clim.,19:1422--1436, 2006, . doi:10.1175/JCLI3696.1
• C. S. Frederiksen, J. S. Frederiksen and R. C. Balgovind. Dynamical variability and seasonal predictability in an intermediate complexity coupled ocean-atmosphere model. ANZIAM J. 54:C34--C55, 2013. http://journal.austms.org.au/ojs/index.php/ANZIAMJ/ article/view/6296
• J. S. Frederiksen and G. Branstator, Seasonal and intraseasonal variability of large-scale barotropic modes, J. Atmos. Sci. 58:50--69, 2000. doi:10.1175/1520-0469(2001)058<0050:SAIVOL>2.0.CO;2
• J. S. Frederiksen and G. Branstator, Seasonal variability of teleconnection patterns, J. Atmos. Sci. 62:1346--1365, 2005. doi:10.1175/JAS3405.1