What caused barrier layer formation observed during the SPURS-2 Field Campaign?

Abstract:

The salt stratified region between a shallow mixed layer and a deeper isothermal layer 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. When BLs contain temperature inversions (TIs) of warmer water than the sea surface, this warmer water entrained into the overlying surface mixed layer will have a more significant impact. Two shipboard surveys collected high spatial resolution concurrent measurements of temperature, salinity and velocity profiles in the eastern tropical Pacific during the 2016 boreal summer and 2017 boreal autumn as part of the Salinity Processes in the Upper-ocean Regional Study 2 (SPURS-2). BLs and TIs were found in the high precipitation Eastern Pacific Fresh Pool (EPFP) region as well as in the sea surface salinity (SSS) front during the 2016 boreal summer cruise, and from the southern part of the EPFP to the SSS front during the 2017 boreal autumn cruise. The BLs in the SSS front 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 occurred in the EPFP and the SSS frontal regions due to Ekman flow and geostrophic flow during both boreal summer and autumn. Precipitation was also found to be important for the BL formation in boreal summer. The dominant contribution of the SSS front to the density gradient in the horizontal plane and the high dissolved oxygen content observed within BLs corroborated the BL and TI formation through tilting processes. Patchy distribution of freshening and cooling related to the tilting events implies intermittent BL and TI formation with short time scales in the EPFP.