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

Shortwave penetration (Q(pen)) 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 Q(pen) change and related effects on some oceanic parameters, which are regionally dependent. It is found that globally averaged Q(pen) 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 degrees C. Although there exist substantial intermodel uncertainties in the projected chlorophyll change, the shoaled ML contributes the most of the increase in Q(pen) 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 Q(pen) change. The ML heat budget indicates that the enhanced Q(pen) 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 Q(pen) on the global oceanic ML heat balance needs to be adequately recognized. Plain Language Summary Understanding the mechanism for ocean warming induced by anthropogenic effects (e.g., global warming) is important to study global climate change. The accumulated heat into the ocean (heat gain) mainly consists of heat fluxes at the air-sea interface absorbed with the oceanic mixed layer (ML). Additionally, a small part of shortwave radiation (similar to 20 W m(-2)) can penetrate out of the bottom of the ML and enters the subsurface ocean, which is referred to as Q(pen). As Q(pen) is instantaneously determined by the ML depth and chlorophyll concentration, it also reflects optical characteristics associated with the redistribution of net surface heat flux and biomass. However, the Q(pen) change characteristics and its climatic effect under anthropogenic warming have not been quantified yet. Here, we found that the globally averaged Q(pen) typically increased by 2.49 +/- 0.83 W m(-2) in the second half of the 21st century. Furthermore, the enhanced Q(pen) leads to less heat absorbed within the ML and consequently contributes to surface cooling. Meanwhile, the enhanced Q(pen) makes the subsurface gain more heat, which subsequently warms the ML by ocean dynamical change. Therefore, recognizing the role played by the enhanced penetration is essential to understanding the global oceanic ML heat balance.

Automated summary

Shortwave penetration (Q(pen)) through the bottom of the oceanic mixed layer has a profound impact on the thermal structure in the upper ocean and contributes to sea surface temperature change. Using ensemble earth system model simulations, we found that globally averaged Q(pen) increased in the second half of the 21st century, leading to surface cooling but also warming the mixed layer through ocean dynamical change. Recognizing the role played by Q(pen) is essential for understanding the global oceanic mixed layer heat balance.