Increasing summertime penetrative shortwave radiation and its weakening effect on the seasonal cycle in mid-latitude oceans

The seasonal cycle of sea surface temperature (SST) in the mid-latitude ocean is projected to be strengthened with greater warming in the summer hemisphere under global warming. Influenced by the phytoplankton pigment and mixed layer depth, a fraction of the shortwave radiation penetrates out of the bottom of the mixed layer (hereafter Q _pen ), which significantly affects the seasonal cycle of the SST. However, how the Q _pen will change under a warmer climate and its effect on the seasonal cycle of SST remain unknown. Here, we show that the summertime Q _pen increases by 3.9 (3.7) ± 1.9 (1.6) W m ^− 2 in the northern (southern) mid-latitude oceans during the second half of the 21st century by analyzing state-of-the-art Earth System Models simulations. This remarkable increase in Q _pen is mainly due to the shoaling of the mixed layer and partly to the reduction in chlorophyll concentration, which contributes significantly to the increase in Q _pen due to its greater sensitivity to shortwave penetration depth in mid-latitude oceans. The enhanced summertime Q _pen tends to efficiently weaken the amplitude of the summertime SST by ~ 0.3°C month ^− 1 , whereby it mitigates the increase in net surface heat flux associated with greenhouse gas warming. These significant changes in Q _pen highlight the significance of quantifying Q _pen effects and future ocean phytoplankton-induced heating feedbacks in mid-latitude seasonal cycles.