Summertime surface and subsurface temperature variability in the North Pacific in the last decade

Vertical structures of summertime ocean temperature variability on interannual and longer time scales in the North Pacific (NP) are investigated based on observational data obtained by the Argo. In the central and especially eastern part of the NP, temperature variance is large but limited to the surface layer. Given shallow mixed layer isolated by strong stratification from the subsurface layer due to strong short wave radiation in summer, the limitation to the surface layer is expected. On the contrary, temperature variability in the western NP region frequently extends several hundred meters depth, associated with salinity variability. Longer time scale variability of temperature and salinity is also apparent as their difference before and after 2008. Solutions of an eddy-resolving ocean general circulation model strongly suggest that those temperature and salinity variability is associated with changes in the subsurface oceanic frontal structures: enhancement of the Kuroshio Extension northern bifurcation and associated weakened meridional temperature and salinity gradients to the south and north of the current after 2008. The deep structure of temperature variability apparently indicates that it is caused not by atmospheric thermal forcing but by subsurface oceanic changes. Also, it is hinted that the ocean-induced temperature anomalies in the western NP may affect heat release from the ocean to the atmosphere via sensible and latent heat fluxes.