Relationships between Systematic Errors in Medium Range Numerical Forecasts and Some of the Principal Modes of Low-Frequency Variability of the Northern Hemisphere 700 mb Circulation

Abstract The relationship between the existence of low-frequency 700 mb height anomalies in the initial conditions of NMC's MRF global spectral model and subsequent 5-, 7-, and 10-day forecasts of 700 mb height from 1982 to 1988 is explored. Low-frequency 700 mb flow regimes are specified in each of four two-month seasons by performing a rotated principal component analysis (RPCA) on 38 or 39 year time series of daily, low-pass filtered 700 mb height analyses. In a given season, the amplitude time series (ATS) for each mode is used to decide which MRF forecast error maps should be used in forming a composite map corresponding to either the “+” phase or the “−” phase of the given mode. Several methods, including Monte Carlo simulations, are used to evaluate the statistical significance of the composite maps. Many modes, including the Pacific North American (PNA) mode in winter and the leading summer mode, are found to be related to either unusually strong or unusually weak systematic error signature. Two d...