Solar radiation is the only relevant energy source of the climate system and has a significant 11-year periodicity. The footprint of this 11-year solar cycle forcing was found in regional scales, such as in North Atlantic Oscillation and in tropical Pacific decadal variability (TPDV). However, the influence of the 11-year solar cycle forcing is still under debate and the strong internal variability reduces the detectability of the solar signal.
For the TPDV, it can be internally generated by the coupled atmosphere-ocean system or also a response to external forcings. Here, we systematically investigated the role of the 11-year solar cycle in the TPDV based on the observed/reanalysis datasets and sensitivity experiments driven by solar-cycle and non-solar-cycle forcings. In the solar maximum years, we found a lagged warming response confined in the upper-ocean layers above the thermocline and it is amplified by the atmosphere-ocean coupling processes in the following years. In addition, a coupled decadal variation of the tropical Pacific system is phased locked to the 11-year solar cycle. An El Niño-like pattern tends to appear in the solar cycle declining phase, accompanied by an eastward shifting of the Pacific Walker circulation. The opposite is true in the solar cycle ascending phase. These phase-locking coupled responses are absent in the non-solar-cycle experiments. The responses of the tropical Pacific to the 11-year solar cycle may modulate the central Pacific El Niño at a decadal timescale and synchronize the internal variability. To explain these coupled responses, we proposed a new mechanism that the heating effect of the accumulative solar irradiation during the 11-year solar cycle can be “integrated” into the tropical Pacific Ocean heat content and then provide a “bottom-up” effect on the atmosphere at decadal timescales.

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(*) In collaboration w/ Ziniu Xiao, State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China.