A new characterisation of the North Atlantic eddy-driven jet using two-dimensional moment analysis

Abstract

Abstract. We develop a novel technique for characterising the latitude (ϕ‾), tilt (α) and intensity (Umean) of the North Atlantic eddy-driven jet using a feature identification method and two-dimensional moment analysis. Applying this technique to the ERA5 reanalysis, the distribution of the daily winter ϕ‾ is unimodal, which is in contrast to the trimodal distribution of the daily jet latitude index (JLI) described by Woollings et al. (2010). We show that our method exhibits a higher persistence than the JLI, casting doubt on the previous interpretations of the trimodal distribution as evidence for regime behaviour of the North Atlantic jet. It also explicitly and straightforwardly handles days where the jet is split. Although climatologically α is positive, indicating a tilt from south to east, around a fifth of winter days show negative α. When plotted as a function of the North Atlantic Oscillation and East Atlantic pattern indices, there is a higher fraction of explained variance in the daily ϕ‾ within each quadrant of the phase space than is found for JLI, supporting the conclusion that ϕ‾ has smoother variations than JLI and has a closer relationship with these indices. Our method is simple, requiring only the daily 850 hPa zonal wind as input data, and diagnoses the jet in a more informative and robust way than other methods using low-level wind fields.