Persistent Mixing Bursts in the Equatorial Pacific Thermocline Induced by Persistent Equatorial Waves

A recent study by Liu et al.(2020) suggested that due to the saturation of equatorially trapped planetary waves with different dynamical types, temporal periods, meridional and baroclinic modes, complex layer structures of vertical velocity shear and hence turbulent mixing could frequently occur in the thermocline of the eastern equatorial Pacific. We investigated the occurrence of the interior turbulent mixing as indicated by shear instabilities, above the Equatorial Undercurrent(EUC) core at three equatorial sites along 140°W, 170°W, and 165°E, respectively, based mainly on data from the Tropical Atmosphere and Ocean(TAO) mooring array. We found that turbulent mixing bursts persisted in the thermocline of all three sites. Specifically, the interior turbulent mixing layers(ITMLs) could occur in probability of approximately 68%, 53%, and 48% at the three sites, respectively. The overall occurrence probability shows obvious and similar biannual variations at 140°W and 170°W, which is higher in boreal from late summer to winter and lower in spring. Vertically, the ITMLs are primarily located above the EUC core and prevail in deeper(shallower) layers from late summer to winter(spring). Most ITMLs(70%) lasted for hours to 3 days, and a few of them(15%) for more than 7 days. The thicknesses of ITMLs were concentrated between 15 and 55 m. At 165°E, the vertical distribution of ITML occurrence probability was different from that at 140°W and 170°W, as it did not show a preference for depths; the durations of ITMLs are short(also from hours to several days) and their thicknesses were between 5 and 25 m. These properties, particularly the high occurrence probability, and short durations demonstrated the persistence of thermocline mixing in the western to eastern equatorial Pacific thermocline and confirmed the generation mechanism by persistent equatorial waves as well.

Journal of Oceanology and Limnology