Weakening and Poleward Shifting of the North Pacific Subtropical Fronts from 1980 to 2018

Recent evidence shows that the North Pacific subtropical gyre and the Kuroshio Extension (KE) and Oyashio Extension (OE) fronts have moved poleward in the past few decades. However, changes of the North Pacific Subtropical Fronts (STFs), anchored by the North Pacific subtropical countercurrent in the southern subtropical gyre, remain to be quantified. By synthesizing observations, reanalysis, and eddy-resolving ocean hindcasts, we show that the STFs, especially their eastern part, weakened (20% +/- 5%) and moved poleward (1.6 degrees +/- 0.4 degrees) from 1980 to 2018. Changes of the STFs are modified by mode waters to the north. We find that the central mode water (CMW) (180 degrees-160 degrees W) shows most significant weakening (18% +/- 7%) and poleward shifting (2.4 degrees +/- 0.9 degrees) trends, while the eastern part of the subtropical mode water (STMW) (160 degrees E-180 degrees) has similar but moderate changes (10% +/- 8%; 0.9 degrees +/- 0.4 degrees). Trends of the western part of the STMW (140 degrees-160 degrees E) are not evident. The weakening and poleward shifting of mode waters and STFs are enhanced to the east and are mainly associated with changes of the northern deep mixed layers and outcrop lines-which have a growing northward shift as they elongate to the east. The eastern deep mixed layer shows the largest shallowing trend, where the subduction rate also decreases the most. The mixed layer and outcrop line changes are strongly coupled with the northward migration of the North Pacific subtropical gyre and the KE/OE jets as a result of the poleward expanded Hadley cell, indicating that the KE/OE fronts, mode waters, and STFs change as a whole system.

Automated summary

Recent evidence suggests that there have been poleward movements of the North Pacific subtropical gyre and the KE/OE fronts in the past few decades. However, changes in the STFs, especially in their eastern part, still need to be quantified. By analyzing various data sources, this study shows that the STFs have weakened and shifted poleward from 1980 to 2018, with the central mode water and subtropical mode water being the most affected.