Colorado State University

Refereed Publications

Song, J., and P. J. Klotzbach, : Relationship between the Pacific-North American pattern and the frequency of tropical cyclones over the western North Pacific. Geophysical Research Letters, 46, 6118-6127 ,

  • Boreal summer western North Pacific tropical cyclone frequency is inversely linked to the concurrent Pacific-North American pattern
  • A greater number of tropical cyclones form north (south) of 12.5°N during low (high) Pacific-North American pattern phases
  • The Pacific-North American pattern influences tropical cyclone genesis predominately by modulating large-scale dynamic conditions

  • Plain Language Summary

    Tropical cyclones (TCs) are the most devastating natural disasters in many coastal regions, including over the western North Pacific (WNP). Therefore, the characteristics and driving mechanisms of WNP TC activity have been studied extensively over the past several decades. Previous publications indicated that El Niño-Southern Oscillation (ENSO) played the most important role on an interannual basis in modulating WNP TC formation, movement, and intensity. However, the interannual frequency of WNP TCs is only weakly influenced by ENSO. Here we find that there exists a significant negative correlation between the annual number of boreal summer WNP TCs and the concurrent Pacific-North American (PNA) pattern. Distinct from other climate modes (e.g., the North Atlantic Oscillation and the Arctic Oscillation), the inverse relationship between WNP TC frequency and the PNA is relatively stable and independent of the period analyzed. Furthermore, the PNA influences the formation of WNP TCs through a modulation of the large-scale environment, particularly the dynamic conditions. During low PNA years, the large-scale dynamic conditions are more favorable for TC development, compared to high PNA years. Our results imply that one may be able to improve the prediction of WNP TC frequency by considering variations in the PNA pattern.


    The frequency of tropical cyclones (TCs) over the western North Pacific during June–November has a significant inverse correlation with the concurrent Pacific-North American (PNA) pattern from 1965 to 2016. During low PNA years, more TCs form north of 12.5°N, with significantly greater TC occurrences from 15 to 20°N, compared to high PNA years. The difference in TC genesis location can be explained by the differences in the genesis potential index derived from the environmental variables in both PNA phases. The PNA influences TC formation primarily by modulating large-scale dynamic conditions, with thermodynamic conditions playing a lesser role. In low PNA years, low-level anomalous cyclonic lows over the Philippines and in the subtropical central Pacific provide significant positive relative vorticity anomalies favorable for TC genesis. Additionally, there is also less vertical wind shear to the north of the Philippines due to enhanced winds at low levels and weaker winds at upper levels.

    Key Figure

    Key Figure

    Fig. 2. (a) Time series of WNP TC number and the PNA index over theperiod from 1965 to 2016. (b) Thirty-year moving correlation coefficients between WNP TC number and the PNA (black line), the NAO (red line), and the AO (blue line) from 1965 to 2016.


    This work was jointly funded by the National Key Research and Development Program of China (2018YFC1507305 and 2018YFA0606003) and the National Grand Fundamental Research 973 Program of China (2015CB452800). Klotzbach would like to acknowledge financial support from the G. Unger Vetlesen Foundation. We thank the two anonymous reviewers for very useful comments, which significantly improved the quality of the manuscript.