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

Abstract

Shortwave 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.

Emerging Hotspots of Surface Chlorophyll Trend in the Tropical Oceans

“…Given that phytoplankton change plays a vital role in marine biogeochemical cycles and thus ocean carbon storage, figuring out where the SChl is changing and what dynamical processes are responsible for this trend is essential for the global climate change study, and is also helpful for the ocean conservation and governance to identify regions of the open ocean where is necessary to establish marine protected areas or not as supposed by Cael et al (2023). Meanwhile, the decreasing SChl also leads to stronger shortwave penetration, which in turn affects the energy balance in the upper ocean under the global warming scenario (Kang et al, 2017;Tian & Zhang, 2023b;Zhang et al, 2020).…”

“…(2023). Meanwhile, the decreasing SChl also leads to stronger shortwave penetration, which in turn affects the energy balance in the upper ocean under the global warming scenario (Kang et al., 2017; Tian & Zhang, 2023b; Zhang et al., 2020).…”

“…Albeit with substantial model biases in reproducing the SChl variability (Séférian et al., 2020), the CMIP6 models can well simulate the spatial pattern of decreasing trend in SChl during 1998–2020 (Figures S6 and S7 in Supporting Information ). A more detailed evaluation can be found in Tian and Zhang (2023b). To perform statistical analysis with consistent spatial resolution, all data (observation, reanalysis, and model data) are interpolated onto 1° × 1° horizontal grids with the Climate Data Operators (CDO).…”