The Influence of ENSO on the Structure of Internal Tides in the Xisha Area

Based on mooring data in the Xisha area of the South China Sea from August 2007 to December 2016, we found that the diurnal internal tides (ITs) had a tendency to weaken from 2008 to 2016, while the semidiurnal ITs were enhanced from 2011 to 2015. The diurnal and semidiurnal ITs in the Xisha area were negatively and positively correlated with the Niño 3.4 Index, respectively. It was also found that the large‐scale El Niño–Southern Oscillation (ENSO) can influence ITs. The link between ENSO and ITs is stratification. The upper‐ocean stratification of the Xisha area in El Niño years was stronger than that in La Niña years. Under the effects of stratification, the structures of diurnal and semidiurnal ITs showed different patterns during El Niño and La Niña years; both were dominated by low modes in La Niña years but displayed more signals of high modes in El Niño years. Plain Language Summary El Niño–Southern Oscillation (ENSO), originates in the tropical Pacific Ocean, is the strongest large‐scale climate fluctuation with worldwide impacts. ENSO can influence the sea surface temperature of the South China Sea. In so doing, it changes the stratification of the South China Sea, which can affect the vertical structure of internal tides (ITs). High‐mode ITs usually dissipate locally and thus generate local mixing, whereas low‐mode ITs can propagate for a long distance before being dissipated. This paper studied the modulation of ITs by ENSO events basing on a total of 9 years of observation data. The results showed that, in La Niña years, the diurnal and semidiurnal ITs were dominated by low modes. However, in El Niño years, the diurnal and semidiurnal ITs displayed more signals of high modes. Key Points Diurnal internal tides in the Xisha area of the South China Sea showed a weakening trend over the 2008–2016 period, while semidiurnal internal tides had opposite trend Upper‐ocean stratification of the Xisha area in the South China Sea in El Niño years was stronger than in La Niña years Internal tides were dominated by high modes during El Niño years but displayed more signals of low modes in La Niña years