Variability of the transition zone chlorophyll front and its relationship to large-scale climate phenomena

The Transition Zone Chlorophyll Front (TZCF) is considered a foraging corridor for many commercially important predator species in the North Pacific, including tunas and swordfish, as well as for protected turtles. Here we use nearly three decades of satellite remotely sensed ocean color data to characterize the trends and variability of the TZCF properties, including its location, west/east slope, and the degree of its meandering, and investigate the relationships of these properties with large-scale climate phenomena in the tropics and in the North Pacific. We found that the interannual variability of TZCF properties is associated with the El Niño-Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), and the North Pacific Gyre Oscillation (NPGO) to various degrees. Variability in the latitude of the front during winter is associated with the PDO and ENSO and is related to anomalies in zonal wind strength that are typical of these climate patterns. Consistent relationships are also found between the climate modes and TZCF slope, and the degree of meandering is best correlated to the NPGO. There has been a significant northward movement of 8.25 km/year of the TZCF, as defined by the 0.2 mg/m³ chlorophyll contour, over the past 26 years. The long-term trend in TZCF location corresponds to a weakening in westerly winds near the front and weakened gyre circulation as manifested by more frequent negative NPGO since 2011. The study extends our previous understanding of the TZCF variability and its linkage to the large-scale climate modes, providing a more robust basis to explore climate impacts on the fisheries linked to TZCF variability in the Northeast Pacific.