Turbulent Mixing in the Barrier Layer of the Equatorial Pacific Ocean

The barrier layer (BL) is an expansive halocline layer commonly found in the western equatorial Pacific, long been thought to inhibit entrainment of colder thermocline water into the surface mixed layer (ML), consequently facilitating the development of El Nino. But here we find frequent turbulent mixing in the BL from both direct turbulence measurements and indirect mixing estimates within an 11-year-long Argo profile data set. The observed BL mixing is as strong as in the ML, yielding effective heat transfers across the isothermal layer (IL) base. The estimated BL mixing is ubiquitous, with occurrence ranging 20%-60% spatially and peaking at around 160 degrees W, 0 degrees N; it occurs more frequently in La Nina than El Nino years. The BL mixing is associated with thicker ML, BL, and IL, weaker BL stratification, and lower temperature and higher salinity in the IL. How the BL mixing may impact the El Nino development deserves further exploration.

Automated summary

The barrier layer (BL) is a common expanded halocline layer in the western equatorial Pacific that inhibits the entrainment of colder thermocline water into the surface mixed layer (ML), facilitating the development of El Nino. However, recent studies have found frequent turbulent mixing in the BL, as strong as in the ML, leading to effective heat transfers across the isothermal layer (IL). The occurrence of BL mixing is widespread, ranging from 20% to 60%, and is more frequent during La Nina years compared to El Nino. Thicker ML, BL, and IL, weaker BL stratification, and lower temperature and higher salinity in the IL are associated with BL mixing. Further research is needed to understand the impact of BL mixing on El Nino development.