Early summertime interannual variability in surface and subsurface temperature in the North Pacific

Vertical structures of early summer 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 NP regions, temperature variance is large but limited to the shallower 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 shallower layer is expected. On the contrary, temperature variability in the western NP region frequently extends several hundred meters in depth. In the western NP, longer time scale variability of temperature is also apparent as temperature difference before and after 2008. Solutions of an eddy-resolving ocean general circulation model strongly suggest that the temperature variability is associated with changes in the oceanic frontal structures that extend to subsurface layer: enhancement of the northern branch of Kuroshio Extension and associated weakened meridional temperature 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 oceanic structure changes, and it is corroborated by the similar variability in the subsurface salinity field. Also, it is shown that atmospheric thermal forcing strongly affects early summer sea surface temperature variability in the eastern NP, but not in the western NP.