Seasonality in the ENSO-Independent Influence of Tropical Indian Ocean Sea Surface Temperature Anomalies on Western North Pacific Tropical Cyclone Genesis

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This study investigates the seasonality in the influence of tropical Indian Ocean (IO) sea surface temperature (SST) anomalies independent of El Niño-Southern Oscillation (ENSO) on tropical cyclone (TC) genesis over the western North Pacific (WNP) from 1979–2022. We focus on the dominant pattern of Indian Ocean SST variability – the Indian Ocean basin mode (IOBM) independent of ENSO. We separate the WNP TC-active period into the early season April–June (AMJ), the peak season July–September (JAS), and the late season October–December (OND). When the preceding and simultaneous influence of ENSO is removed, the correlation between WNP TC frequency and the ENSO-independent IOBM has considerable seasonality, with a significant correlation during JAS but an insignificant correlation during AMJ and OND. In warm ENSO-independent IOBM phases, WNP TC genesis is significantly suppressed over the region of [15–25°N, 140–155°E] during JAS, while there are insignificant changes in WNP TC genesis during AMJ and OND.

In a warm IOBM independent of ENSO, significant TC genesis reductions during JAS are primarily driven by significantly decreased 850-hPa relative vorticity and significantly increased 500-hPa vertical velocity (e.g., downward motion), while insignificant TC genesis changes during AMJ and OND are consistent with weak environmental changes over the WNP. These features can be linked to the seasonality in Indo-Pacific large-scale circulation anomalies induced by the ENSO-independent IOBM. In analogy to the mechanism of the Indo-western Pacific ocean capacitor mode, warm IO SSTAs independent of ENSO can lead to an anomalous low-level anticyclone over the WNP during both AMJ and JAS, with their remote influence dissipating during OND. The ENSO-independent warm IO-driven anomalous WNP anticyclone is weak and insignificant during AMJ but is strong and significant during JAS, likely due to a smaller amplitude of IOBM-induced SSTAs independent of ENSO during AMJ than during JAS.