Deterministic and ensemble forecasts of the Kuroshio south of Japan

Abstract We conducted 6-month deterministic and ensemble forecasts initialized from the ensemble mean analyses and 128-member ensemble analyses, respectively, on the first day of each month from 2016 to 2018 (i.e., 36 cases in total) to compare the forecast accuracies of the Kuroshio south of Japan. The results showed that the ensemble mean forecasts captured the Kuroshio paths better in general compared with the deterministic forecasts, especially during the Kuroshio large meander period after the summer of 2017. For 2016–2018, the predictability limit for the ensemble mean forecast is estimated to be 108 days on average, demonstrating for the first time that the ensemble mean forecasts achieve a significantly longer predictability limit than the 74-day limit of the deterministic forecasts for the Kuroshio. We also investigated the sensitivity of the forecast accuracy to the ensemble size using a resampling method. The results indicated that for the Kuroshio prediction, the ensemble mean forecasts are more accurate than the deterministic forecasts when the ensemble size exceeds 32 members. When the ensemble size is limited to 32 members or less, combining the deterministic and ensemble forecasts could improve the short-term Kuroshio forecasting. This study also identified substantial northward shift biases in the Kuroshio forecasts, suggesting that further system development is necessary to achieve more significant improvements in the forecast accuracy.