Changes in the North Pacific subtropical gyre under 1.5 °C low warming scenario

Abstract

Climate response to low warming scenarios that meet 2015 Paris Agreement is distinct from that to the medium/high-emissions scenarios analyzed in previous studies. The present study investigates changes in the North Pacific subtropical gyre and the underlying mechanisms under 1.5 °C low warming scenario by 11-member ensemble simulations from the Community Earth System Model. Specifically, atmospheric CO2 concentration peaks in 2036 and then decreases, with global mean surface temperature (GMST) first increasing and then stabilizing after 2045. The changes of the lower thermocline depth are consistently weaker in the subtropics than in the tropics or high latitudes through 2100. During GMST increasing stage, the subtropical circulation strengthens in the upper 1000 m north of 30° N as a result of the enhanced wind stress. When GMST stabilizes, the subtropical circulation strengthens in the surface layer (0–150 m) and the main thermocline (150–600 m) but displays insignificant change in the intermediate layer (600–1000 m). Wind and stratification changes are both important in driving the North Pacific subtropical gyre changes, because sea surface warming is much weaker in the low warming scenario than that in the medium/high-emissions scenarios. In addition, as the surface wind change displays substantial natural variability and is strongly model-dependent, it dominates the differences in projected subtropical circulation changes across models. This study highlights the importance of wind changes in projections of the subtropical circulation changes under low warming scenarios.