Aerosol and Solar Influences on Tropical Pacific Decadal Variability

Decadal variability in the tropical Pacific plays a key role in modulating global warming rates and driving climate impacts in many regions throughout the globe. However, the drivers of tropical Pacific decadal variability, including the relative importance of internal variability and external forcing, are poorly understood. Here we use ensembles of global climate models driven by different external forcings to highlight the potential influences of anthropogenic aerosols and solar variations on the tropical Pacific. In particular we show that the pattern of anthropogenic aerosol emissions in the early part of this century, involving decreased emissions from the USA and Europe and increased emissions from Asia, likely contributed to the negative phase of the Pacific Decadal Oscillation and the slowdown in global surface warming. We also present evidence that multi-decadal variations in tropical Pacific rainfall throughout the 20th century, along with associated influences on the extra-tropics including the North Atlantic Oscillation, may have been influenced by small variations in solar irradiance. However, the signal-to-noise ratio in the model simulations is smaller than in reality, in agreement with recent seasonal forecasting results. This highlights an important deficiency in the models, and affects conclusions on the relative roles of internal variability and external forcing.