Projected Sea Surface Temperatures over the 21st Century: Changes in the Mean, Varaibility, and Extremes

SST is a key variable in the climate system, regulating thermal and dynamical interactions between the ocean and atmosphere. In addition, SSTs are often a leading indicator and/or important driver of marine ecosystem fluctuations. Climate change may not only manifest in mean warming of SSTs but also in changes in the variability and extremes.

We used global climate models to assess changes in the mean, variability and extreme sea surface temperatures (SSTs) in large marine ecosystems (LMEs) adjacent to North America, Europe, and the Arctic Ocean. Results were obtained from 26 models in the Community Model Intercomparison Project Phase 5 (CMIP5) archive and 30 simulations from the NCAR Community Earth System Model large ensemble (CESM-LENS). All of the simulations used the observed greenhouse gas concentrations for 1976-2005 and the RCP8.5 “business as usual” scenario for greenhouse gases through the remainder of the 21^st century. In general, differences between models are substantially larger than among the simulations in the CESM-LENS, indicating that the SST changes are more strongly affected by model formulation than internal climate variability.

The annual SST trends over 1976-2099 in the 18 LMEs examined here are all positive ranging from 0.05 to 0.5°C decade^-1. SST changes by the end of the 21^st century are primarily due to a positive shift in the mean with only modest changes in the variability in most LMEs, resulting in a substantial increase in warm extremes and decrease in cold extremes. The shift in the mean is so large that in many regions SSTs during 2070-2099 will always be warmer than the warmest year during 1976-2005. The SST trends are generally stronger in summer than in winter, as greenhouse gas heating is integrated over a much shallower climatological mixed layer depth in summer than in winter, which amplifies the seasonal cycle of SST over the 21^st century. In the Arctic, the mean SST and its variability increases substantially during summer, when it is ice free, but not during winter when a thin layer of ice reforms and SSTs remain near the freezing point.