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Climate change has had large impacts on marine animals, including neon flying squid Ommastrephes bartramii (O. bartramii) in the North Pacific Ocean. O. bartramii statoliths from 2012, 2015, and 2016 were used to evaluate the variations in life cycle events. The relationship between mantle length and body weight showed significant differences between years and gender. The oldest squid was collected in 2016 at 271 days old, further proving that O. bartramii has nearly a 1-year life span. The hatching season ranged from December to May and the peak hatching time in 2015 was one-half month later than in 2012 and 2016. Growth rates varied with environmental factors such as sea surface temperature (SST) and chlorophyll a concentration (chl. a), indicating that higher SST and chl. a concentrations led to faster growth. An extreme El Niño event with lower SST in 2015 also led to younger age class and slower growth rates. The occurrence of differences in body size and growth rates between years, caused by the interannual variations of environmental factors, makes it necessary to use separate growth curves for different years when analyzing North Pacific O. Bartramii populations. [ABSTRACT FROM AUTHOR]

The western North Pacific Subtropical Countercurrent (WSTCC), anchored by the western subtropical front (WSTF), varies from seasonal to multidecadal time scales and affects the climate variability in the North Pacific. However, conflicting views exist for physical mechanisms of the decadal to multidecadal variability of the WSTF/WSTCC. While many previous studies attribute the WSTF/WSTCC decadal variability to the anomalous wind forcing by the Pacific Decadal Oscillation, by synthesizing observations, reanalysis products and the pre‐industrial model simulation experiment, the present study demonstrates that they are controlled by the Atlantic Multidecadal Oscillation (AMO) through the Subtropical Mode Water (STMW) variability. The AMO induced decadal temperature and volume anomalies of the STMW are stored in the subsurface ocean through the subduction process south of the Kuroshio Extension. Then these anomalies propagate southwestward along the thermocline circulation and reaches the northern flank of the WSTCC in about 5 years. During the AMO‐positive phase, the anomalous warm and thin STMW flattens the upper thermocline, reduces the meridional temperature gradient and weakens the WSTF intensity, lagging 5 years behind the AMO index. Subsequently, the WSTCC decreases and moves southward, and vice versa for the AMO‐negative phase. Our results provide a potential predictability for the WSTF and WSTCC decadal to multidecadal variability and are important for the North Pacific climate change and the biogeochemical processes. Plain Language Summary: The North Pacific subtropical countercurrent (STCC) is a shallow eastward jet stretching from the western Pacific to the north of Hawaii, accompanied by the subtropical front (STF) in the upper 200 m. Specifically, the western branch (western North Pacific Subtropical Countercurrent [WSTCC] and western subtropical front [WSTF]) is much stronger and broader than the eastern part. Decadal to multidecadal variability of the WSTCC and WSTF is known to have significant effects on the North Pacific climate change and biogeochemical processes. However, conflicting views exist for physical mechanisms of the decadal to multidecadal variability of the WSTF/WSTCC. While many previous studies attribute the WSTF/WSTCC decadal to multidecadal variability to the anomalous wind forcing by the Pacific Decadal Oscillation, the present study demonstrates that they are controlled by the Atlantic Multidecadal Oscillation (AMO) through the Subtropical Mode Water (STMW) variability. The STMW anomalies caused by the AMO are stored in the subsurface ocean and would propagate southwestward along the circulation, reach the WSTF/WSTCC region in about 5 years and influence the current's decadal to multidecadal variations. Our results provide a potential predictability for the WSTF and WSTCC variability on decadal to multidecadal time scales and are important for the North Pacific climate change and the biogeochemical processes. Key Points: The decadal‐multidecadal variation of the western North Pacific subtropical front and countercurrent is controlled by the Atlantic Multidecadal Oscillation (AMO)The AMO modulates the Subtropical Mode Water (STMW) variability, which controls the upper pycnocline slope and dominates the intensity of the countercurrentIt takes ∼5 years for the STMW to reach the countercurrent region, corresponding to the time lag of the frontal changes with the AMO [ABSTRACT FROM AUTHOR]