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

Abstract 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 Qpen), which significantly affects the seasonal cycle of the SST. However, how the Qpen will change under a warmer climate and its effect on the seasonal cycle of SST remain unknown. Here, we show that the summertime Qpen 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 Qpen 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 Qpen due to its greater sensitivity to shortwave penetration depth in mid-latitude oceans. The enhanced summertime Qpen 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 Qpen highlight the significance of quantifying Qpen effects and future ocean phytoplankton-induced heating feedbacks in mid-latitude seasonal cycles.