| Summary: | This video is a supplement to Chapter 6, Ghil, M., A. Groth, D. Kondrashov, and A. W. Robertson, 2018: Extratropical sub-seasonal–to–seasonal oscillations and multiple regimes: The dynamical systems view, in The Gap between Weather and Climate Forecasting: Sub-Seasonal to Seasonal Prediction, A. W. Robertson and F. Vitart (Eds.), Elsevier. It provides substantial evidence for the generation of 40–50-day atmospheric variability in the Northern Hemisphere (NH) mid-latitudes as a result of an oscillatory instability in the flow of the prevailing westerlies over large-scale topography. History The video was originally prepared as part of the research published in the Journal of Atmospheric Sciences, in a set of two papers: 1. Marcus, S. L., M. Ghil, and J. O. Dickey, 1994: The extratropical 40–50-day oscillation in the UCLA general circulation model. Part I: Atmospheric angular momentum. J. Atmos. Sci., 51, 1431–1446. 2. Marcus, S. L., M. Ghil, and J. O. Dickey, 1996: The extratropical 40-day oscillation in the UCLA general circulation model. Part II: Spatial structure. J. Atmos. Sci., 53, 1993–2014. The video could not be published in the mid-1990s, at a time when the journals of the American Meteorological Society did not allow the inclusion of multimedia supplements. Its scientific value is still considerable today, and the video is published here with the explicit consent of its original authors, M. Ghil, S. L. Marcus, J. O. Dickey and C. L. Keppenne. The video was produced by the Audiovisual Services Office of the Jet Propulsion Laboratory in 1991. Full credits appear at the end of the video, and its contents are described below. Description To test the theory of NH extratropical oscillations developed in simpler models, Marcus et al. (1994, 1996) studied a 3-year perpetual-January simulation that had been performed with a version of the UCLA general circulation model (GCM) in which no self-sustained Madden-Julian Oscillation (MJO) was apparent in the tropics. A robust 40-day oscillation in atmospheric angular momentum (AAM) was found to arise in the model's NH mid-latitudes when standard topography is present. Three shorter runs with no topography produced no intraseasonal oscillation; this result is consistent with a topographic origin for the NH extratropical oscillation in the standard model. The spatial structure of the circulation anomalies associated with the model's extratropical oscillation is shown in the present video, as displayed in several model-simulated fields, namely 500-hPa geopotential heights, 250-hPa streamfunction, and surface pressure torques. The oscillation is dominated by a standing wavenumber-2 pattern, which undergoes a predominantly barotropic, tilted-trough vacillation. High values of AAM are associated with low 500-hPa heights over the northeast Pacific and the North Atlantic oceans, and vice versa. These flow patterns resemble the configurations seen in the Charney and DeVore (1979) simple model, shown as Fig. 1b in the main text of Ch. 6 above. The NE–SW-tilting and NW–SE-tilting phases in the video's 500-hPa fields are strongly reminiscent of the extremes and intermediate phases of the 40-day oscillation that arises by Hopf bifurcation from the blocked equilibrium in the Legras and Ghil (1985) model; see discussion of Ghil et al. (2003, Fig. 3). References Charney, J. G., and J. G. DeVore, 1979: Multiple flow equilibria in the atmosphere and blocking. J. Atmos. Sci., 36, 1205–1216. Ghil, M., D. Kondrashov, F. Lott, and A. W. Robertson, 2003: Intraseasonal oscillations in the mid-latitudes: observations, theory and GCM results. In: Proc. ECMWF/CLIVAR Workshop on Simulation and Prediction of Intra-Seasonal Variability with Emphasis on the MJO, 3–6 Nov. 2003, ECMWF, Reading, UK. 35–53. Legras, B., and M. Ghil, 1985: Persistent anomalies, blocking and variations in atmospheric predictability. J. Atmos. Sci., 42, 433–471.
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