The Link between the North Pacific Climate Variability and the North Atlantic Oscillation via Downstream Propagation of Synoptic Waves

Abstract

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.

Pacific Modulation of the North Atlantic Storm Track Response to Sudden Stratospheric Warming Events

“…SSW events with an equatorward Atlantic jet shift, however, do not exhibit robust geopotential height anomalies in the eastern Pacific (Figures d and S3d). These large‐scale flow patterns are in agreement with the influence of Pacific trough and ridge anomalies shown in Drouard et al () and Rivière and Drouard (), which are associated with differences in the synoptic wave propagation further downstream, and may result in either an equatorward or a poleward Atlantic jet anomaly, respectively.…”

On cold spells in North America and storminess in western Europe

“…The wave activity flux displayed in Figures g–i and S11 indeed suggests that the drivers of the anomalous cold spell circulation might be different from those associated with the climate modes. During the days prior to the North American temperature minima (Figures g and S11a and S11b), a wave train can be seen emanating from the subtropical central Pacific, arching over North America and turning north toward Greenland, tying together the Pacific, North American, and Atlantic circulation anomalies discussed earlier into a single hemispheric pattern [ Drouard et al , ]. At around lag 0, however, as the cyclonic anomaly over eastern North America reaches its maximum amplitude, the wave activity flux emanating from it turns south and enters the western subtropical Atlantic, generating an anticyclonic anomaly there and leading to the changeover from negative to neutral NAO conditions (Figure h).…”

Impact of North Atlantic Teleconnection Patterns on Northern European Sea Level

“…Our objective has been to understand the impact of North Atlantic teleconnections on northern European sea levels with less focus on the forcing mechanisms of these teleconnections, and the reader is referred to, e.g., Cassou et al [48] for details (see also [54][55][56][57][58]). But, to shed some light on this issue, we apply a similar composite analysis as in Figure 13 on additional key variables and show the hemispheric-scale linkages (Figure 15).…”