Several studies have shown a clear link between weather and climate in the more frequent occurrence of blocking weather regimes in the North Atlantic during negative periods of the North Atlantic Oscillation (NAO). The key question is whether negative NAO conditions cause increased blocking occurrence or whether the negative NAO signal is simply a reflection of a more frequent occurrence of blocking.

Blocking is known to arise from the breaking of upper level Rossby waves, so here we investigate this question by applying a wave-breaking index to the ERA40 reanalysis data. This reveals two key areas of wave-breaking in the Northern Hemisphere, where cyclonic wave-breaking acts to deform the climatological troughs lying to the north-west of the Atlantic and Pacific basins. In the Atlantic these wave-breaking events are shown to cause the anomaly patterns associated with the negative phase of the NAO, rather than vice versa. In fact the set of Atlantic wave-breaking days is almost synonymous with the set of negative NAO days. This leads us to hypothesise that the positive NAO phase represents the basic state of the atmosphere in the North Atlantic, and the negative phase the wave-breaking state.

In the Pacific the same relation holds for a pattern of variability referred to as the West Pacific Pattern (WPP). These results suggest that it is variations in the occurrence of blocking-like wave-breaking events which cause variability associated with the NAO and WPP.

In both sectors storm track variations and downstream blocking act as precursors to wave-breaking events. In the Atlantic an additional precursor is seen in the form of a quasi-stationary Rossby wave-train stretching across North America from the Pacific. This opens up a mechanism by which variability or change in the Tropical Pacific could affect Atlantic wave-breaking and hence the NAO.