The reliable sampling interval for monitoring interannual variability of the Kuroshio transport at 18°N

The sampling interval for reliably monitoring interannual variability of the Kuroshio transport was studied using satellite altimeter data and outputs from HYbrid Coordinate Ocean Model (HYCOM). Interannual variations of the Kuroshio velocity and transport crossing 18°N were derived with different sampling interval, and their deviations from the ‘true’ interannual signals derived from original time series were estimated. The results indicate that sampling with an interval shorter than 20 days is required to obtain the reliable interannual signals, i.e., the Root Mean Square of the deviation is <50% of the interannual signal. However, the longer sampling intervals will lead to serious aliasing and degradation of the true interannual signals, which is mainly attributed to the energetic intraseasonal oscillations in this region. Power spectrum analysis reveal that there are significant 80-day intraseasonal variations as well as semiannual fluctuations in the Kuroshio transport at 18°N, which brings substantial sampling deviations relative to the true interannual variation. An ideal experiment is constructed to confirm that the reliable sampling interval of around 20 days, which can capture interannual variations and resolve the intraseasonal oscillation of the Kuroshio reliably at 18°N. This study provides an important reference for the future monitoring of the interannual variation of the Kuroshio.