Abstract
In the Western North Pacific (WNP), the atmospheric low-level convergence is one of the main factors that influences the genesis of tropical cyclones (TC). It has been observed that the timing of the seasonal maxima in the low-level convergence and TC genesis has shifted since the mid-1990s from mid-August to late-July, with this shift having also affected the number of TC. A multidecadal frequency of 20 years was revealed in the timing variation of the tropical intraseasonal oscillation (ISO) in the Western Pacific, in which a weak WNP low-level convergence in spring may trigger an advanced ISO phase in summer and vice versa. The present diagnostic analysis does not identify any prominent oceanic variations associated with these multidecadal variations in the summer ISO or in the spring setup of the ISO. The atmospheric circulation does show an anomaly, which suggests an intensified extension of the subtropical high. The possible mechanism may be related to stochastic low-frequency variability of the atmosphere, which acts to influence the seasonal evolution of the WNP low-level convergence.
Highlights
The Western North Pacific (WNP) features the highest population of tropical cyclones (TCs) compared to any other ocean basin [1]
The low-level convergence in WNP is the major factor of the atmospheric circulation influencing the TC genesis in this basin
The analysis shows that the timing of peak low-level convergence has undergone a multidecadal variation that shifts between July and August, affecting the seasonality of TC genesis
Summary
The Western North Pacific (WNP) features the highest population of tropical cyclones (TCs) compared to any other ocean basin [1]. TC genesis in the WNP is affected by multiple factors, including the warm sea surface, upper-ocean heat content, tropospheric water vapor, intensity and eastward extension of monsoon trough, weak vertical wind shear and large-scale low-level convergence [2,7,8]. All these factors are influenced by the intraseasonal oscillation (ISO). InIn(e,f), line shows a discernable timing shift within the timescale manifest of a multidecadal variation in a value presents the convergent/divergent VP and high/low TC genesis anomalies.
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