Atmospheric Regimes: The Link between Weather and the Large-Scale Circulation
Abstract
Recurrent atmospheric flow patterns are those that occur more often than expected by chance, and their identification has been an underlying theme in atmospheric science on time scales ranging from weather forecasting to inter-annual fluctuations. This chapter focuses on identifying recurrent, and also persistent flow patterns on intra-seasonal time scales. Recurrent blocking structures seen in five-day means of the 500 hPa geopotential height (Z500) field motivate the use of a new scalar planetary wave index, whose probability distribution function (pdf) is very non-Gaussian, and hints at bi-modality. The search for maxima in pdfs in higher dimensions is best done in the reduced dimensional coordinates defined by principal component analysis, with pdf density maxima assessed as significant against a multi-dimensional Gaussian null hypothesis.