Formation Mechanism of Barrier Layer in the Subtropical Pacific

Abstract:

Formation mechanism of barrier layers (BLs) in the subtropical Pacific was investigated by using Argo profiling float data and shipboard hydrographic section data. In this region, BLs were formed mainly in winter in association with the sea surface salinity (SSS) front, which was located on the equator side of the SSS maximum region. While BLs from gridded Argo data were broadly distributed, their distribution from raw Argo profiles was patchy and their temporal scale was shorter than 10 days. Formation mechanism of BLs was attributed to two processes: freshening near the sea surface and salinification in the subsurface. As for the former process, poleward Ekman advection of fresher water from the tropics across the SSS front was dominant, while the effect of precipitation was small. As for the latter process, inflow of high salinity water into mixed layers associated with the SSS front can contribute to the BL formation, but actually the core of subducted Tropical Water was too deep to affect salinity structure in the mixed layers across the seasonal thermocline. These features strongly suggest that tilting of the SSS front is essentially important for the BL formation in the subtropical Pacific. This tilting process can only occur where the contribution of horizontal SSS gradient to the horizontal density gradient is strong, and explains why spatial distribution of BLs corresponded to the SSS front. Seasonal variation of BLs corresponded well to that of mixed layer depth, indicating that the deeper mixed layers are before the BL formation, the thicker BLs are formed when the SSS front is tilted.