The NAO is not the only mode of variability with significant effects on SEWP. The tropical North Atlantic SST is related to the winter precipitation of south-eastern Europe through the atmospheric circulation processes. Also, many studies have investigated the influence of ENSO on South Europe, which is modulated by North Pacific Climate. However, the analysis of South Pacific Ocean on winter Europe is not evaluated considering that preceding boreal autumn SSTA in South Pacific Ocean have significant influences on boreal winter precipitation in the North America region and East Asia through interhemispheric Rossby wave train simulated by SSTA over South Pacific Ocean. This motivates us to ask whether tropical and south Pacific SON SSTA and DJF NAO have a synergistic effect on SEWP to better understand the influencing mechanism of SEWP variability. The concept of synergistic effect defined in Li et al. (2019) has been widely used to understand the variability of climate. However, the synergistic effect interpreted by former studies have not involved the contribution of linear and nonlinear effects. The climate, being a nonlinear system, prompts us to investigate whether the combined events contribute more significantly than the superposition of their isolated events. If this is the case, there may be an unexpected amplification due to non-linearity, which we define as a nonlinear effect in this study. Therefore, we explore the synergistic effect of DJF NAO and SON SSTA in tropical and South Pacific on SEWP through both linear and nonlinear contributions.

Firstly, we need to look for SON SST pattern that has synergistic effect with negative NAO (NAO-) on SWEP increase. Considering nonlinear effects cannot be taken into account using linear correlation, composite analysis of SON SSTA over Tropical and South Pacific and NAO- is performed. We found that DJF NAO- with positive phase of tripole pattern of SON SSTA over Tropical and South Pacific (TSPT+) will produce stronger SEWP anomalies than that with the SON TSPT- (first row in Figure 1). Hereafter, NAO- is refer to DJF NAO-, and TSPT+ is refer to SON TSPT+. The SEWP during the joint events of NAO- and TSPT+ (NAO-⊕TSPT+) is stronger than those during the isolated events of NAO- (NAO-\TSPT+) and TSPT+ (TSPT+\NAO-) (second row in Figure 1). The strength of SEWP anomalies during NAO-⊕TSPT+ are about two times that of NAO-\TSPT+ events, which is attributed to that the nonlinear effect of NAO- and TSPT+ is comparable to their linear superposition (third row in Figure 1).

Dynamical mechanisms reveal that SON TSPT+ can persist into DJF, and DJF TSPT+ with NAO- excites obviously stronger Rossby wave energy propagating from South Pacific to the NAE region than that without NAO- (fourth column in Figure 1). This suggests that TSPT+ can synergistically influence the NAE region with NAO- through atmospheric teleconnection. Moreover, numerical simulation confirms that TSPT+ forcing with NAO- sees stronger NAO- like circulation anomaly, and the nonlinear effect on the abnormal low over midlatitude North Atlantic and southern Europe is stimulated. This confirms that NAO- and TSPT+ have a synergistic effect on the NAE circulation.

The synergistic effect on the mean circulation over the NAE region accompanies with intensification of subtropical westerly jet from Atlantic to South Europe and intensified stationary waves propagating southeastward from Greenland to southern Europe and the Mediterranean. In addition, the synergistic effect of NAO- and TSPT+ on the anomalous enhanced cyclonic circulation over East Atlantic and Britain Isles enhancing the water vapor transports from North Atlantic to the Mediterranean and southern Europe. The intensified westerly jets during NAO-⊕TSPT+ conducive to the storm activities over southern Europe, which favors the increase of SEWP.

Overall, the synergistic effect of NAO- and TSPT+ revealed in this study is valuable in terms of our understanding of SEWP variability. The synergistic effect in this study is detected by linear superposition and nonlinear effect. This finding also enhances our understanding of the complex relationships between multiple climate variables and the specified dynamical process.

Reference:

1. Li, J. P., Zheng, F., Sun, C., Feng, J. & Wang, J. Pathways of Influence of the Northern Hemisphere Mid-high Latitudes on East Asian Climate: A Review. Atmos. Sci. 36, 902–921 (2019).