Subseasonal activities of tropical cyclones over the western North Pacific in GloSea6

Tropical cyclones (TCs), which often form over the western North Pacific (WNP), have a large socioeconomic impact and result in destructive damage in East Asian countries. Therefore, it is crucial to estimate TCs characteristics and predict TCs using the model. This study analyzed the subseasonal predictability of TC activities over the WNP region from June to September, using 24 years (1993–2016) of 21-member ensemble hindcasts generated by the Global Seasonal Forecast System version 6 (GloSea6). We analyzed TC activities using dynamic genesis potential index (DGPI) developed by Wang and Murakami, and tracking algorithm (i.e., TempestExtremes (TE)). Compared to IBTrACS best track data, these two methods captured TC genesis points well and showed high correlation in TC genesis density. However, particularly in the South China Sea (SCS), a negative bias was observed in TE, while GPI exhibited a positive or zero bias. Despite using the same input data, different results were observed in this region, and we analyzed the reasons for this discrepancy in two parts. First, why does such a bias occur in DGPI? Second, what causes the differences between DGPI and TE? We used ERA5 data to analyze the relative error and bias of DGPI and examined how westerly wind biases in GloSea6 influenced wind shear and omega errors. In conclusion, one of the key reasons for the differences between the two methods was attributed to the wind shear error induced by the westerly wind bias.