Modulation of the Influence of ENSO on Northward-Moving Tropical Cyclones in the Western North Pacific by the North Atlantic Tripole SST Anomaly Pattern

The frequency characteristics of northward-moving tropical cyclones (NTCs) in the western North Pacific (WNP) are analyzed, and the possible combined effect of El Nin similar to o-Southern Oscillation (ENSO) and the North Atlantic tripole (NAT) sea surface temperature anomaly (SSTA) is investigated. Results show that the NTC frequency in summer shows obvious interannual and decadal variations. The SSTA in the eastern tropical Pacific has an effect on the NTC fre-quency, but this relationship is modulated by the NAT on the decadal time scale. During positive NAT phases, the effect of ENSO on NTCs is clear. There are fewer NTCs in El Nin similar to o-following years, whereas in La Nin similar to a-following years the NTC frequency is higher. However, during negative NAT phases, only El Nin similar to o has an effect on the NTC frequency, whereas there is no obvious feature found for La Nin similar to a, which may be related to the asymmetry of ENSO. The combined effect of La Nin similar to a and positive NAT phases presents an anomalous meridional dipole circulation at the low latitudes and mid-high latitudes near East Asia, which leads to TCs moving northward. The cold SSTA response in the tropical Indian Ocean may contribute to an anomalous cyclone in the WNP. The negative-positive-negative NAT SSTA mode can persist into the ensuing summer and favor wave pattern propagating eastward along the high-level jet waveguide so that there ex-ists an anomalous anticyclone in Northeast Asia, which helps TCs move farther north. The influence of El Nin similar to o modulated by negative NAT phases is roughly opposite.

Automated summary

The frequency characteristics of northward-moving tropical cyclones (NTCs) in the western North Pacific (WNP) are analyzed, and the possible combined effect of El Niño-Southern Oscillation (ENSO) and the North Atlantic tripole (NAT) sea surface temperature anomaly (SSTA) is investigated. The results show that the NTC frequency in summer exhibits interannual and decadal variations. The NAT modulates the relationship between ENSO and NTCs. During positive NAT phases, the effect of ENSO on NTCs is clear, while during negative NAT phases, only El Niño has an effect on the NTC frequency.