90 / 2023-07-29 17:07:15

Interannual variability of near-inertial energy in the South China Sea and western North Pacific

Near-inertial waves,Interannual variability,ENSO

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 dataset 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) correlates negatively with ENSO with a correlation coefficient of R=–0.44 and a time lag of five months, mainly because the variation of local wind stress lags behind El Niño by four months. Since the monsoon reverses direction seasonally, summer and winter NIEs are discussed separately. 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. 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.