F. M. Bingham, Z. Li, Shota Katsura, Janet Sprintall

Published in Journal of Geophysical Research: Oceans, NOV 2020

This study examines salinity barrier layers (BLs) in the eastern tropical North Pacific in the region of the Salinity Processes in the Upper ocean Regional Studies‐2 field campaign. We utilize a high‐resolution numerical model to study BLs and their relationship to frontal features and small‐scale ocean variability, focusing on two specific events. One is associated with a large outbreak of BL presence near 7°N along 125°W. The other is a relatively isolated but persistent BL that forms near 13°N, again along 125°W. In both cases we find that the BL is proximate to a salinity frontal feature in which isohalines tilt toward the fresh side of the front at its base. The BLs studied are associated with divergent flow at the surface on the fresh side of the front and convergent flow on the salty side. Tilting of the front is invoked to explain this, with an additional mechanism involving a vertical circulation which causes the base of the front to tilt preferentially.

Plain Language Summary

Salinity barrier layers (BLs) are common in the eastern tropical North Pacific (ETP) ocean. They consist of a surface mixed layer that is homogeneous in temperature, but with a high salinity layer at the base. They may play an important role in regulating the transfer of heat, momentum, and freshwater across the ocean surface and from there into the interior. This study examines BLs in the ETP in the region of the Salinity Processes in the Upper ocean Regional Studies‐2 (SPURS‐2) field campaign using a high‐resolution numerical simulation. SPURS‐2 took place in 2016–2017 and was designed to study processes connecting salinity and rainfall in the ETP in the vicinity of the intertropical convergence zone. Two specific examples of BLs are studied. In one, there is a large outbreak of such layers throughout the ETP along a large‐scale sea surface salinity (SSS) front. In the other example, the BL was an isolated feature associated with another sharp SSS front. So frontal features are an important part of the process of BL formation. In addition, we find surface flow which converges toward the front on the salty side and diverges away from it on the fresh side. BL formation at the front is a result of the front tilting away from the vertical, and in the case of the fronts studied here, that tilting is concentrated at the front's base. It is also likely associated with vertical circulation on either side of it.

FIG. SSS from the ROMS domain on (left panel) 25, (center panel) 26 and (right panel) October 27, 2017. Color scale is at the top. Red line is along 125°W. The thick green line represents the BLT at 125°W at a given latitude, where zero BLT is the red line and 25 m BLT is indicated by the scale bar at 10.5°N in the left panel. BLT, barrier layer thickness; ROMS, Regional Ocean Modeling System; SSS, sea surface salinity.

Bingham, F. M., Li, Z., Katsura, S., & Sprintall, J. (2020). Barrier layers in a high‐resolution model in the eastern tropical Pacific. Journal of Geophysical Research: Oceans, 125, e2020JC016643. https://doi.org/10.1029/2020JC016643