资源环境科技发展态势分析平台

Previous studies have shown that a combination mode (C‐mode) is formed by nonlinear interactions between the western Pacific warm pool annual cycle and El Niño–Southern Oscillation variability. We find a significant inverse relationship between western North Pacific (WNP) tropical cyclone (TC) frequency and the C‐mode index on monthly timescales from 1970–2019. During positive C‐mode phases, TC formation is significantly suppressed over a region spanning 5°∼20°N and 140°∼175°E, primarily due to reduced 850‐hPa relative vorticity and increased 850‐200‐hPa vertical wind shear. By contrast, there is significantly enhanced TC genesis over the region spanning 25°∼30°N and 125°∼135°E, mainly due to increased 850‐hPa relative vorticity and 600‐hPa relative humidity. All of these changes in atmospheric conditions are further linked to an anomalous large‐scale anticyclone over the WNP generated by the C‐mode. Our results suggest that C‐mode variability can serve as a potential predictor for forecasting monthly WNP TC activity.