Poleward shift and intensified variability of Kuroshio-Oyashio extension and North Pacific Transition Zone under climate change

The climate variability of the Kuroshio-Oyashio Extension (KOE) and North Pacific Transition Zone (NPTZ) exerts a strong control on marine populations that are sensitive to the strong productivity gradients between the subtropical and subpolar recirculation gyres. In observations, the relationship between KOE, NPTZ and productivity is evident in the first two dominant covariability modes between sea surface height and Chlorophyll-a anomalies, which are associated with a meridional shift in the location of the KOE (e.g. shift mode 1) and an intensification of the mean circulation of the KOE (e.g. intensification mode 2). To understand the projected impacts of anthropogenic forcing on the NPTZ, we examine these two dominant modes of variability in the Community Earth System Model Large Ensemble (CESM-LE) and an ensemble of climate models from the Coupled Model Intercomparing Project (CMIP5-E) under the Representative Concentration Pathways (RCP8.5). A significant poleward shift has been found in the KOE mean location associated with an equivalent shift of the Aleutian Low atmospheric pressure system. Superimposed to the changes of the mean, we find a significant increase (15–20%) in the variability of the shift mode in both the CESM-LE and CMPI5-E suggesting that variation in the meridional position of the NPTZ are also becoming stronger. These changes in variance of the shift mode are linked to an increase variability of the atmospheric forcing.