Impacts of Strong El Ni?o-Southern Oscillation Events on Wintertime Northern Hemisphere Storm Tracks in Two Pacific Decadal Oscillation Phases during 1950-2010

Northern Hemisphere storm track activities (NHSTs) not only influence the weather and climate along their paths, but they also play a crucial role in climate systems by systematically transporting heat, momentum, and moisture. Distinguish from previous studies focusing on atmospheric circulation anomalies, this study provides further evidence of wintertime NHSTs variation under the influence of strong El Nino-Southern Oscillation (ENSO) events and Pacific Decadal Oscillation (PDO) variation with ERA-20C reanalysis data, from two mathematical aspects of linear superposition and nonlinear modulation. While ENSO warm/cold events lead the entire NHSTs to a general south/north shift, the ENSO impact in two PDO phases exhibits a remarkable difference with the PDO phase. The Pacific storm track (PST) anomalies largely strengthen equatorward and downstream when both ENSO and PDO are in phase, but its anomalies tend to be weakened when ENSO and PDO are out of phase. Generally, the opposite situation occurs with Atlantic storm track (AST) anomalies, which display a strengthening dipole pattern when ENSO and PDO are out of phase. Apparently, the result is roughly a linear superposition of ENSO and PDO-only impacts. Nevertheless, further analyses indicate that the nonlinear modulation of PDO on the ENSO impact on NHSTs exists. With respect to the PST, it exhibits approximately parallel bands of south-north dipole anomalies when ENSO is in the PDO positive phase, but only the south branch remains when ENSO is in the PDO negative phase. Generally, a similar situation occurs to AST anomalies over the Atlantic Ocean. The modulation may be mainly associated with the atmospheric mean flow anomalies and the midlatitude sea surface temperature anomalies to ENSO in different PDO phases. To some extent, the results may be beneficial for understanding the variation of extreme weather events brought by NHSTs.