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 Niño‐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 Niño by 4 months. By separating summer and winter seasons, the correlation is significantly improved. The summer NIE correlates positively with El Niño (R = 0.62), since tropical cyclones over the WNP tend to be stronger and longer‐lived during the El Niño developing phase. The winter NIE correlates negatively with El Niño (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. Plain Language Summary: Interannual climate variability, especially El Niño‐Southern Oscillation (ENSO), can trigger a series of flood, drought, and wildfires, resulting in a huge loss of people's property around the world. It has been demonstrated that ENSO can modulate large‐scale oceanic and atmospheric processes, such as sea surface temperature, circulation, monsoon, tropical cyclones (TCs), etc. Near‐inertial waves (NIWs) are ubiquitous in the upper ocean and usually contain the most energy of internal waves. Since NIWs are generated by time‐varying wind stress, they could also be influenced by ENSO. Using current velocity observations from satellite‐tracked surface drifter data set with hourly resolution, interannual variability of the surface mixed‐layer near‐inertial energy (NIE) is investigated. It is found that NIE in the western North Pacific (WNP) is indeed modulated by ENSO, with the monthly NIE lagging behind ENSO by 5 months. Since the monsoon reverses direction seasonally, summer and winter NIEs are discussed separately. The summer NIE is mainly generated by TCs while the winter NIE is mainly generated by winter monsoon, which are all tightly connected to and modulated by ENSO. Since NIWs play an important role in upper‐ocean turbulent mixing and air‐sea interaction, their interannual variations have important implications for the climate system. Key Points: Mixed‐layer near‐inertial energy in the study region correlates negatively with El Niño‐Southern Oscillation (ENSO) with a time lag of 5 monthsSummer near‐inertial energy is mainly generated by tropical cyclones and correlates positively with ENSOWinter near‐inertial energy is mainly generated by the winter monsoon and correlates negatively with ENSO [ABSTRACT FROM AUTHOR]