From Internal Variability to Aerosol Effects: Physical Mechanisms Behind Observed Decadal Trends in Surface Solar Radiation in the Western Pacific Ocean

The Pacific Ocean, spanning over 30% of the Earth's surface, provides an ideal setting for studying the surface radiative balance due to its relatively pristine atmospheric conditions, far from anthropogenic emission sources. In this study we investigated the causes for the decadal trends of surface solar radiation (SSR) observed at eight stations scattered across seven islands in the Western Pacific Ocean, and extrapolated the results to the whole Western Pacific region based on the understanding of physical processes. Our results show a contrast between the causes for SSR trends in the northwestern and in the southwestern Pacific. In the Southwestern Pacific region, changes in cloud cover play a major role in the SSR decadal trends and interannual variability. The cloud cover in these areas is strongly associated with sea surface temperature (SST) anomalies, especially those induced by El Nino Southern Oscillation (ENSO) and the Interdecadal Pacific Oscillation (IPO). Modes of variability such as ENSO and IPO affect evaporation and convection, impacting the large-scale dynamics of the atmosphere, therefore influencing the distribution of the regions of deep convection. This consecutively impacts the cloud cover on a regional level and therefore SSR. In the Northern Hemisphere, conversely, a strong influence of these modes on cloudiness and SSR was not found. Instead, indirect evidence suggests that anthropogenic aerosol transported from Eastern Asia plays a major role in the decadal SSR trends. These results contribute to an improved understanding of the physical processes relevant for the long-term SSR trends in remote regions.