Increasing Shortwave Penetration Through the Bottom of the Oceanic Mixed Layer in a Warmer Climate

Abstract

AbstractShortwave penetration (Qpen) through the bottom of the oceanic mixed layer (ML) profoundly affects the thermal structure in the upper ocean and consequently contributes to sea surface temperature (SST) change, which has not been adequately understood under global warming. Here, using ensemble earth system model simulations under a high‐emissions scenario (SSP5‐8.5), we investigate the Qpen change and related effects on some oceanic parameters, which are regionally dependent. It is found that globally averaged Qpen typically increased by 2.49 ± 0.83 W m−2 in the second half of the 21st century, which is comparable to an increase in the net surface heat flux (3.01 ± 0.31 W m−2), corresponding to an increase in SST by 3.07 ± 0.83°C. Although there exist substantial intermodel uncertainties in the projected chlorophyll change, the shoaled ML contributes the most of the increase in Qpen in the global ocean, whereas in the tropical ocean, the reduction in the chlorophyll concentration plays an equivalent role with the mixed layer depth in determining Qpen change. The ML heat budget indicates that the enhanced Qpen leads to surface cooling through a decrease in the surface net surface heat flux. However, the cooling is compensated for by a warming effect from ocean dynamical change due to more shortwave penetration into the subsurface layer, leading to a small net effect on the ML heat budget. It is suggested that the impact of Qpen on the global oceanic ML heat balance needs to be adequately recognized.