Abstract
What causes the uncertainty in simulations of seasonal tropical cyclone (TC) activity remains unclear. Here, we uncovered the important contribution of intraseasonal variability to the uncertainty in simulating TC genesis frequency (TCGF) over the western North Pacific (WNP). This is demonstrated by analyzing 50-member ensemble simulations for the period June–November in 1998 using a high-resolution regional climate model. The uncertainty spans from +1 to −1.75 standard deviations (STDs), considerably larger than the observed deviation of −1.5 STDs. The uncertainty in simulations stems from two atmospheric dynamic modes, namely the Monopole and Dipole modes. The Monopole mode is characterized by a basin-uniform anomalous circulation pattern and controls TC genesis over the whole WNP, while the Dipole mode is marked by a zonal dipole of circulation anomalies and redistributes TC genesis locations among the ensemble members. Both modes are dominated by the intraseasonal variability, accounting for over 62% of the total uncertainty in the simulated TCGF. The intraseasonal variability observed over the WNP in 1998 was relatively weak, leading to significant uncertainty in simulating intraseasonal variability east of the Philippines among the ensemble members, which in turn caused uncertainty in the simulation of TCGF. By conditionally constraining the intensity of intraseasonal variability, we can significantly reduce model uncertainties in simulating TCGF. The finding provides new insights into understanding the simulation uncertainty and predictability of seasonal TC activity over the WNP.
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