13 March 20074th C20C Workshop1 Interannual Variability of Atmospheric Circulation in C20C models Simon Grainger 1, Carsten Frederiksen 1 and Xiagou Zheng 2 1. Bureau of Meteorology, Melbourne, Australia 2. National Institute of Water and Atmospheric Research, Wellington, New Zealand Acknowledgments: C20C Modelling groups, David Straus
13 March 20074th C20C Workshop2 Motivation What are the distributions of the components of variability? How well do models reproduce observed variability? What are the sources of these patterns? How does the interannual variability change over time? In observed data? In models – including different forcing scenarios? To investigate the properties of the interannual variability of seasonal mean climate data
13 March 20074th C20C Workshop3 Theory x = monthly anomaly of climate variable = external forcings (eg SST) assumed to be constant over a season = slowly varying internal dynamics internal to the atmosphere and are potentially predictable at long range (> 1 season) = intraseasonal component weather events that are not predictable at long range (eg blocking) and given by variability between ensemble members (m = 1,2,3 months, y = 1,Y years, s = 1,S members, r = points)
13 March 20074th C20C Workshop4 Components of variability (o = seasonal mean) Rowell et al. (1995) separate external and internal components Cannot separate , and monthly anomalies, but can for the interannual variability of seasonal mean Zheng and Frederiksen (1999) separate intra-seasonal component ♦ and hence can deduce slow-internal component V( sy )
13 March 20074th C20C Workshop5 Estimating Intraseasonal Variability Zheng and Frederiksen (2004) estimated intraseasonal variance as a function of monthly differences using moment estimation (m = 1,2,3) Assumes that: x can be modelled by a first-order autoregressive process Implies that intermonthly correlations can be constrained Variances V( sym ) are stationary across the season Reasonable assumption for summer and winter
13 March 20074th C20C Workshop6 Total Variability – DJF NCEPBOM (S=10)CSIRO (S=10) COLA (S=10)GSFC (S=14)UKMO (S=12)
13 March 20074th C20C Workshop7 Intraseasonal Variability – DJF NCEPBOM (S=10)CSIRO (S=10) COLA (S=10)GSFC (S=14)UKMO (S=12)
13 March 20074th C20C Workshop8 Potential Predictability (%) – DJF NCEPBOM (S=10)CSIRO (S=10) COLA (S=10)GSFC (S=14)UKMO (S=12)
13 March 20074th C20C Workshop9 Potential Predictability (%) – JJA NCEPBOM (S=10)CSIRO (S=10) COLA (S=10)GSFC (S=14)UKMO (S=12)
13 March 20074th C20C Workshop10 NCEP Covariability – NH DJF TotalSlowIntraseasonal
13 March 20074th C20C Workshop11 Slow PC Regression – NH DJF BOM C(x oyo,x yo )C( y,x yo )C( y, y + sy ) NAO PNA W. Pacific E. Atlantic TNH CSIRO C(x oyo,x yo )C( y,x yo )C( y, y + sy ) NAO PNA W. Pacific E. Atlantic TNH COLA C(x oyo,x yo )C( y,x yo )C( y, y + sy ) NAO PNA W. Pacific E. Atlantic TNH GSFC C(x oyo,x yo )C( y,x yo )C( y, y + sy ) NAO PNA W. Pacific E. Atlantic TNH UKMO C(x oyo,x yo )C( y,x yo )C( y, y + sy ) NAO PNA W. Pacific E. Atlantic TNH
13 March 20074th C20C Workshop12 ENSO Composites NCEP Covariability – SH JJA Slow UEOF-S1 (32.0%)UEOF-S2 (14.7%) UEOF-S3 (8.7%)UEOF-S4 (7.9%) UEOF-I1 (23.1%)UEOF-I2 (16.6%) UEOF-I3 (10.2%)UEOF-I4 (8.3%) Intraseasonal
13 March 20074th C20C Workshop13 Slow PC Regression – SH JJA BOM C(x oyo,x yo )C( y,x yo )C( y, y + sy ) High Latitude ENSO Warm ENSO Cold SP Wave CSIRO C(x oyo,x yo )C( y,x yo )C( y, y + sy ) High Latitude ENSO Warm ENSO Cold SP Wave COLA C(x oyo,x yo )C( y,x yo )C( y, y + sy ) High Latitude ENSO Warm ENSO Cold SP Wave GSFC C(x oyo,x yo )C( y,x yo )C( y, y + sy ) High Latitude ENSO Warm ENSO Cold SP Wave UKMO C(x oyo,x yo )C( y,x yo )C( y, y + sy ) High Latitude ENSO Warm ENSO Cold SP Wave
13 March 20074th C20C Workshop14 COLA Variability – DJF Slow V( y + sy )Slow External V( y )Slow Internal V( sy )
13 March 20074th C20C Workshop15 Conclusions C20C models are generally able to reproduce most of the large-scale observed grid point variability Although subtle differences at smaller scales are likely to be important C20C Intraseasonal covariability modes resemble observed, although relative importance changes For NH DJF, C20C models reproduce the PNA, but do not generally reproduce other observed modes of slow covariability Particularly not the NAO For SH JJA, C20C models reproduce both ENSO modes, but not necessarily other slow modes In some C20C models, separation of slow variability components reproduces expected internal modes
13 March 20074th C20C Workshop16 Australian Potential Predictability (%) DJFMAMJJASON T max Precip T min