The team led by Associate Prof. Liu Fukai, Prof. Song Fengfei and Prof. Luo Yiyong of the College of Oceanic and Atmospheric Sciences and Frontiers Science Center for Deep Ocean Multispheres and Earth System from OUC made significant progress in the research of seasonal cycle of Sea Surface Temperature (SST), revealing the enhancement of seasonal amplitude of SST since the 1980s, and elucidating its controlling mechanism. The research outcome entitled “Human-induced intensified seasonal cycle of sea surface temperature” was published in Nature Communications.

SST is an important indicator of changes in ocean thermal processes and a central object of physical ocean and climate change research. Numerous studies have been carried out to investigate the changes in its mean state. However, even if the mean state remains constant, SST can still affect various aspects of the climate system through changes in its annual cyclic amplitude (i.e., summer-winter temperature difference). Present studies on seasonal cyclic variations of SST focus on predictions under future scenarios. However, there are still significant challenges in understanding the spatial and temporal evolution of SST. 

Through the analysis of different observational data, it is found that the amplitude of the seasonal cycle of the global SST has increased by about 3.9% over the past four decades, implying colder SSTs in winter and warmer SSTs in summer. In hotspots such as the subpolar North Pacific and the North Atlantic, the enhancement of the seasonal amplitude even exceeds 12%. These signals are not only confined to the ocean surface, but also extended to the bottom of the mixed layer, leading to a significant increase in the seasonal oscillations in the upper ocean. The research further quantifies the contributions of different physical processes to the SST seasonal amplitude increase, and finds that on the global scale, the shallowing of the marine mixed layer due to the enhanced vertical stratification is the main mechanism for the SST seasonal amplitude increase, while the increased ocean heat uptake and the enhanced oscillations of the heat fluxes at the ocean surface also have a significant regional contribution in the North Pacific Ocean and the North Atlantic Ocean.

Enhanced seasonal amplitude of SST makes significant climatic effects and leads to increased seasonal differences in dissolved oxygen content in the upper ocean. Meanwhile, it contributes to the increased seasonal amplitude of CO2 fluxes at the air-sea interface.  Particularly, considering that the oceanic DO content has been declining steadily over the past four decades, the increase in seasonal amplitude makes the summer hypoxia situation even more severe. The research reveals for the first time the vertical characteristics of the observed SST seasonal amplitude changes, elucidates the controlling mechanism of the SST seasonal oscillation changes, and points out the impacts of the SST seasonal changes on important ecological indicators.