The North Atlantic Oscillation (NAO) response to the northeast Pacific climate variability is examined using the ERA-40 dataset. The main objective is to validate a mechanism involving downstream wave propagation processes proposed in a recent idealized companion study: a low-frequency planetary-scale ridge (trough) anomaly located in the eastern Pacific–North American sector induces more equatorward (poleward) propagation of synoptic-scale wave packets on its downstream side, which favors the occurrence of anticyclonic (cyclonic) wave breakings in the Atlantic sector and the positive (negative) NAO phase. The mechanism first provides an interpretation of the canonical impact of the El Niño–Southern Oscillation on the NAO in late winter. The wintertime relationship between the Pacific–North American oscillation (PNA) and the NAO is also investigated. For out-of-phase fluctuations between the PNA and NAO indices (i.e., the most recurrent situation in late winter), the eastern Pacific PNA ridge (trough) anomaly modifies the direction of downstream wave propagation, triggering more anticyclonic (cyclonic) wave breakings over the North Atlantic. For in-phase fluctuations, the effect of the eastern Pacific PNA anomalies is cancelled out by the North American PNA anomalies. The latter anomalies being deeper and more centered in the latitudinal band of downstream wave propagation, they are able to reverse the direction of wave propagation just before waves enter the Atlantic domain. The contrasting relationship between the PNA and NAO is similar to what occurs for the two leading hemispheric EOFs of geopotential height: the northern annular mode (NAM) and the cold ocean–warm land (COWL) pattern. The proposed mechanism provides a physical meaning for the NAM and COWL patterns.

The formation of summer blocking events appears to have been mostly studied for a few individual events often associated with heat waves. Here we investigate summer blocking event dynamics in three areas over western Eurasia in order to draw some more general conclusions, mostly in terms of high‐ and low‐frequency processes. A 2‐D blocking event detection algorithm is applied to the 500‐hPa‐geopotential field from the ERA‐40 and ERA‐Interim reanalyses over the 1958–2017 period. It is shown that both high‐ and low‐frequency processes are important to initiate blocking events over southern central Europe. Blocking events over western Russia are preceded by a significant low‐frequency large‐scale wave train, and their formation and maintenance are dominated by low‐frequency processes only. Finally, it is shown that the risk of extreme seasons such as summer 2010 cannot be accurately estimated from the Poisson statistics of past events.

Summer 2018 was one of the driest and hottest experienced over northwestern Europe. In contrast, over southern Europe, it was marked by cooler and wetter conditions with flooding over Greece and Spain. This contrasting pattern was particularly enhanced over a 3‐week period starting on 21 June. Two atmospheric patterns are thought to have largely contributed to this anomalous weather: the positive North Atlantic Oscillation (NAO+) and a Wave‐7 pattern. Using linear regressions on detrended data, we show that the NAO+ was mainly responsible for the observed seasonal anomalies. However, during the 3‐week period, the rare combination of the NAO+ and Wave‐7 is necessary to explain the pattern of the observed anomalies. The global warming trend and, to a lesser extent, nonlinear processes are shown to have furthermore strongly modulated the anomalies associated with these two patterns.

Considerable uncertainties remain about the expected changes of ENSO and associated teleconnectivity as the climate is warming. Two ensembles of pacemaker experiments using the CNRM-CM5 coupled model are designed in a perfect model framework to contrast ENSO-forced teleconnectivity between the preindustrial period versus a warmer background state (obtained from a long stabilized simulation under late-twenty-first-century RCP8.5 constant forcing). The most notable sensitivity to the mean background state is found over the North Atlantic, where the ENSO–NAO teleconnection is considerably reinforced in a warmer world. We attribute this change to (i) a stronger and eastward-extended mean upper-level jet over the North Pacific, (ii) an eastward-shifted ENSO teleconnection over the North Pacific, and (iii) an equatorward-shifted and reinforced mean jet over the North Atlantic. These altogether act as a more efficient waveguide, leading to a better penetration of synoptic storms coming from the Pacific into the Atlantic. This downstream penetration into the North Atlantic basin forces more systematically the NAO through wave breaking. The reinforcement in the teleconnection is asymmetrical with respect to the ENSO phase and is mainly sensitive to La Niña events. Even though the Pacific jet tends to retract westward and move northward during cold events, mean changes are such that both Pacific and Atlantic jets remain connected in a warmer climate by contrast to the preindustrial period, thus ensuring preferred anticyclonic wave breaking downstream over the North Atlantic leading ultimately to NAO+ events.