Formation and subduction of the North Pacific Tropical Water (NPTW), its interannual variability, and its associated mechanisms were investigated by using gridded Argo-profiling float data and various surface flux data in 2003–11. The NPTW has two formation sites in the center of the North Pacific subtropical gyre, corresponding to two regional sea surface salinity maxima. Mixed layer salinity variations in these two NPTW formation sites were found to be significantly different. While seasonal variation was prominent in the eastern formation site, interannual variation was dominant in the western site. The mixed layer salinity variation in the eastern site was controlled mainly by evaporation, precipitation, and entrainment of fresher water below the mixed layer and was closely related to the seasonal variation of the mixed layer depth. In the western site, the effect of entrainment is small due to a small vertical difference in salinity across the mixed layer base, and excess evaporation over precipitation that tended to be balanced by eddy diffusion, whose strength varied interannually in association with the Pacific decadal oscillation. After subduction, denser NPTW that formed in the eastern site dissipated quickly, while the lighter one that formed in the western site was advected westward as far as the Philippine Sea, transmitting the interannual variation of salinity away from its formation region.

was further investigated. In addition to the surface freshwater flux in the KE region pointed out by previous studies, the decadal KE variability in association with the Pacific Decadal Oscillation likely contributes to the decadal salinity variability through water exchange between the subtropics and the subarctic across the KE. Interdecadal change in both the surface freshwater flux and the KE state, however, failed to explain the rapid freshening for the last 20 years.

No abstract

Seasonal and interannual variations of the barrier layer (BL) and its formation mechanism in the subtropical North and South Pacific were investigated by using raw and gridded Argo profiling float data and various surface flux data in 2003–12 and hydrographic section data from the World Ocean Circulation Experiment Hydrographic Programme. BLs detected by raw Argo profiles, which existed within the sea surface salinity (SSS) front located on the equator side of SSS maxima, were thickest and most frequent in winter and had a temporal scale shorter than 10 days, indicating their transient nature. Surface and subsurface processes for the BL formation suggested by previous studies were evaluated. Poleward Ekman advection of fresher water was dominant as the surface freshening process but cannot explain the observed seasonal variations of the BL. Subsurface equatorward intrusion of high-salinity tropical water was too deep to produce salinity stratification within isothermal layers. These results strongly suggest that BLs in the subtropical Pacific are formed mainly through tilting of the SSS front due to the poleward Ekman flow near the sea surface and the equatorward geostrophic flow in the subsurface. This idea is supported by the dominant contribution of the meridional SSS gradient to the meridional sea surface density gradient within the SSS front and the correspondence between the seasonal variations of the BL and isothermal layer depth. On an interannual time scale, the winter BL thickness in the North and South Pacific was related to the Pacific decadal oscillation and the El Niño–Southern Oscillation, respectively, through the intensity of trade winds controlling isothermal layer depth.

Seasonality and formation of barrier layers (BLs) and associated temperature inversions (TIs) in the eastern tropical North Pacific Ocean were investigated using raw and gridded Argo profiling float data, satellite data, and various sea surface flux data. BLs were observed frequently in boreal summer and autumn along the sea surface salinity (SSS) front south of the eastern Pacific fresh pool. TIs were found within the gap between the western and eastern Pacific warm pools in autumn when BLs were thickest. A mixed layer salinity budget was constructed to determine the formation mechanism responsible for BLs with TIs. This budget revealed that Ekman advection works to both freshen and cool the eastern tropical North Pacific in autumn and contributes to the formation of the thickest BLs with the warmest TIs through the tilting of the SSS front. Precipitation is a secondary contributor to BL formation in autumn. The BLs are also prevalent during summer but are thinner, are without associated TIs, and are primarily formed through precipitation. The largest rainfall associated with the intertropical convergence zone mostly occurred north of the band of thickest BLs in both summer and autumn. The geostrophic advection of salinity did not coherently contribute to the formation of BLs or TIs. The idea that Ekman advection contributes most to the formation of the thickest BLs with warm TIs was further corroborated because the horizontal salinity gradient was the dominant contributor to the density gradient and so is favorable for BL and TI formation.