The Distinct Pacific-North American Teleconnection Pattern Induced By The South China Sea

The Pacific–North American(PNA)teleconnection pattern is one of the dominant lowfrequency modes of the midlatitude atmosphere in the boreal winter.A better understanding of PNA will improve the intraseasonal climate prediction that is of growing importance.The generation of PNA has been extensively studied,with particular emphasis laid upon the role of tropical forcing.The forcing role of El Ni(?)o-Southern Oscillation(ENSO)on the PNA has long been studied,but more unknown external forcings are yet to be explored.This study for the first time reveals that South China Sea(SCS)may also impact PNA.The major results are as follows:(1)Through causal inference with Liang–Kleeman information flow,a real physical notion which has just been rigorously derived from first principles,the PNA pattern is shown to be remarkably influenced by the SCS.The information flow rate,which measures the strength of causality from the Pacific sea surface temperature anomaly(SSTA)to the PNA index,shows a distinct local causal pattern within the SCS.This pattern is rather robust;it exists with different datasets,and lasts through time.On the other hand,the information flow from the SCS regional SSTA to the geopotential height anomalies shows two centers of causality that closely resemble the PNA centers over North America.The multivariate information flow further unravels the complex causalities in the system,and shows that the SCS exerts its own direct and independent causal impact on the PNA.(2)Based on simulation and observational analysis,the newly revealed SCS effect is verified,and remarkable PNA-like response induced by the SCS air-sea interaction is revealed.Conducted with Community Earth System Model(CESM),the sensitivity experiments have been designed with purported SCS warming and cooling.By comparing the forced PNA evolutions to the standard case,the atmospheric responses are analyzed.The airsea interaction is also diagnosed.It shows that,positive(negative)SCS SSTA regulates the atmospheric diabatic heating and convection through the anomalous surface heat flux,and then arouses anomalous ascending(descending)motion and upper-level divergence(convergence).Together with the Asian jet effect,this generates a unique negative(positive)wave source near the jet core to the north of SCS,and then the precursory anticyclonic(cyclonic)center leading to the response train is induced.Through propagating along the jet waveguide,and through being dispersed at the jet exit,the responded wave train carries energy toward Pacific and North America,and hence affects the PNA growth.(3)It is revealed that the PNA response induced by the SCS possesses unique features.Through composite analysis,we show that the SCS-induced PNA pattern and its evolution are distinctly different from the ENSO-induced and conventional counterparts,hence it should be a new type belonging to the PNA continuum.Generally,the centers of the SCS-induced PNA are more scattered in distribution.Most of all,they seem to follow well the great circle route as predicted by the classical theory,and such consistency is attributed to the configuration of the SCS forcing source and the jet stream effects.We remark that,although the SCS SSTA is relatively small in amplitude,it has an effect on PNA almost as conspicuous as that from ENSO.This study suggests that the SCS effect should be included in interpreting the formation and growth of PNA.It is expected that,by taking the SCS effect into account,a better model can be built for this climate mode toward more accurate predictions.