Formation and Variability of Barrier Layer in the South Pacific

The oceanic barrier layer (BL), dominated by salinity stratification, plays an important role in regulating heat exchange between the mixed layer and the interior ocean. However, the salinity contribution is ignored or underestimated when analyses relying only on temperature threshold methods. Here we introduce the Turner Angle method to quantify the salinity contribution and to obtain relatively accurate BL thickness (BLT), and further to investigate the spatial-temporal distribution, thickness variation and formation mechanism of the BL in the South Pacific using the updated Argo data during 2005-2021. With the successful usage of this method, we eliminate 41% of the invalid BLs and therefore improve the assessment of the BLs in the South Pacific. Formation of the thick BLs in the South Pacific is attributed to the faster deepening rate of the isothermal layer than the mixed layer. The deepening rate of the isothermal layer is facilitated by subsurface subantarctic mode water while that of the mixed layer is constrained by the subsurface salinity maximum in the South Pacific. On the interannual scale, the BLT in the South Pacific manifests a quasi-biennial variability, which is closely connected with the variability in the isothermal layer depth and is primarily caused by the anomalous winds driven by the Southern Annular Mode (SAM). The SAM-related anomalous winds regulate the isothermal layer and further the BL by exerting influences air-sea heat fluxes and the Ekman layer advection. The South Pacific is a major ventilation area in the world ocean with a barrier layer (BL) underneath the mixed layer for most of the year; and the BL plays a crucial role in heat exchange with the mixed layer and the interior ocean. This study investigated the interannual variability and formation mechanism of the BL in the South Pacific using Argo data. As previous analyses ignored or underestimated the salinity stratification, here we introduced the Turner Angle Method to quantify the salinity contribution in the BL. With the usage of this method, we eliminated 41% of the invalid BLs and largely improved the assessment of the BLs in the South Pacific. Our analysis showed that the barrier layer thickness in the South Pacific exhibits a quasi-biennial variation, which is related with the changes in isothermal layer depth and can be primarily ascribed to the Southern Annular Mode related anomalous winds. Turner angle method is used to identify the barrier layer (BL) in the South Pacific The formation of thick BL in the South Pacific is related to the subantarctic mode water and subsurface salinity maximum The thickness of BL manifests a quasi-biennial variability and is regulated by Southern Annular Mode related anomalous winds