Low-frequency nonlinearity and regime behavior in the Northern Hemisphere extratropical atmosphere

The extra-tropical atmosphere is characterized by robust cir- culations which have time scales longer than that associated with develop- ing baroclinic systems but shorter than a season. Such low frequency vari- ability is governed to a large extent by non-linear dynamics, and hence is chaotic. A useful aspect of this low-frequency circulation is that it can often be de- scribed by just a few quasi-stationary regime states, broadly defined as re- current or persistent large scale structures, that exert a significant impact on the probability of experiencing extreme surface weather conditions. We review a variety of techniques for identifying circulation regimes from reanalysis and numerical model output. While various techniques often yield similar regime circulation patterns, they o?er di?erent perspectives on the regimes. The regimes themselves are manifest in planetary scale patterns. They a?ect the structure of synoptic scale patterns. Extra-tropical flow regimes have been identified in simplified atmospheric models and comprehensive coupled climate models and in reanalysis data sets. It is an ongoing challenge to accurately model these regime states and high horizontal resolutions are often needed to accurately reproduce them. The regime paradigm helps to understand the response to external forcing