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.

The region between a shallow mixed layer and a deep isothermal layer resulting from salinity stratification is called a barrier layer (BL). Since BLs hinder the surface heat and momentum exchange with the ocean subsurface, they play an important role in air-sea interaction. Synoptic features and formation of BLs and associated temperature inversions (TIs) in the Eastern Pacific Fresh Pool (EPFP) were investigated using shipboard observations. BLs and TIs were found in the high precipitation EPFP, a frontal transition zone (FTZ) and in the sea surface salinity (SSS) front during a 2016 boreal summer cruise. During a 2017 boreal autumn cruise, BLs and TIs were found from the southern part of the EPFP to the SSS front. The BLs in the SSS front and the FTZ were meridionally displaced from the precipitation maximum during both the 2016 and 2017 cruises. Freshening and cooling by tilting of both the isohalines and isotherms of the SSS front occurred via Ekman and geostrophic flow during both cruises, and also within the EPFP during 2016. The SSS front was a dominant contributor to the density gradient in the horizontal plane. A high dissolved oxygen content observed within BLs confirmed that the surface flow plays an important role in the BL and TI formation through the tilting process. Patchy distribution of freshening and cooling by tilting implies an intermittent BL and TI formation with short time scales.