Interannual Variability of Near-Inertial Energy in the South China Sea and Western North Pacific

Interannual variability of surface mixed-layer near-inertial energy (NIE, representing the intensity of near-inertial waves) in the South China Sea and western North Pacific (WNP) is investigated using satellite-tracked surface drifter data set. It is found that NIE in the study region correlates negatively with El Nino-Southern Oscillation (ENSO) with a correlation coefficient of R = -0.44 and a time lag of 5 months, mainly because the variation of local wind stress lags behind El Nino by 4 months. By separating summer and winter seasons, the correlation is significantly improved. The summer NIE correlates positively with El Nino (R = 0.62), since tropical cyclones over the WNP tend to be stronger and longer-lived during the El Nino developing phase. The winter NIE correlates negatively with El Nino (R = -0.65), since the winter monsoon is weakened by the ENSO-related WNP anomalous anticyclone. This is the first time that interannual variability of NIE is studied by direct current velocity observations.