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. Plain Language Summary: The majority of the Pacific Ocean is relatively far away from anthropogenic pollution sources. This makes it an interesting region for studying the potential effects of natural variations in the climate system and of long‐range aerosol transport on the amount of solar radiation that reaches the surface (also known as surface solar radiation—SSR) in remote regions. In this study we analysed the SSR variability on the decadal timescale at eight in situ stations scattered in the western Pacific Ocean. The aim was to identify the underlying causes for the observed decadal trends. We observed that in the Equatorial and Southwestern Pacific, the SSR variability, both on interannual and decadal timescales, is strongly associated with changes in cloud cover, and the changes in cloud cover are strongly associated with changes in sea surface temperature. In the Northwestern Pacific Ocean, on the other hand, the results suggest a significant impact of aerosols transported from Eastern Asia on the SSR decadal trends. These aerosols can take part in cloud formation and also attenuate radiation in cloud‐free conditions. An aerosol reduction in the 1990s could have played a major role for the positive decadal SSR trends observed in the Northwestern Pacific Ocean. Key Points: Surface solar radiation interannual and decadal variability in the southwestern Pacific Ocean is strongly tied to changes in cloud coverIn the Southwestern Pacific, cloud cover, and consequently surface solar radiation, follow the variability of oceanic modes, such as IPOEvidence suggests that aerosols play a major role in the surface solar radiation decadal trends in the northwestern Pacific Ocean [ABSTRACT FROM AUTHOR]