Impacts of the Ocean Modes on the Surface Solar Radiation decadal variability over the western Pacific

Sea Surface Temperature (SST) plays a major role in the unforced variability of the climate system on decadal scales via the interplay between ocean and atmosphere, and associated changes in cloud cover and water vapor. The ocean modes, such as El Nino Southern Oscillation (ENSO) and Interdecadal Pacific Oscillation (IPO), are major coherent manifestations of SST variability. This means that the impacts of the ocean modes might be reflected in several components of the climate system, such as the energy budget. At the surface, this can be observed in the decadal trends in surface solar radiation (SSR). In this study we investigated the impacts of IPO and ENSO and associated cloud cover and water vapor variability on the decadal trends in SSR at 6 island stations scattered in the western Pacific (two stations in Fiji, one in New Caledonia, Nauru, Papua New Guinea and Marshall Islands). We combined between 15 and 40 years of daily SSR observations (depending on the station) with cloud cover from ERA5 reanalysis, aerosol optical depth (AOD) from CAMS reanalysis, and time series of IPO and ENSO. The comparison between clear-sky and all-sky SSR trends show that the all-sky trends strongly dominate in 5 out of the 6 stations. The exception is New Caledonia, where the clear-sky seems to also play an important role in the overall trend. This is the least cloudy station, and also the closest station to eastern Australia, an important source of aerosols in the region. Maps of cloud cover trends show two distinct regions which can be approximately separated by the average climatological position of the South Pacific Convergence Zone (SPCZ): one where cloud cover trends follow the phase of the IPO (positive IPO phase = positive cloud cover trend; N-NE of the SPCZ) and one where the opposite happens (positive IPO phase = negative cloud cover trend; S-SW of the SPCZ). The direct comparison between annual time series of all-sky SSR and IPO shows correlations stronger than 0.5 at two stations in Fiji and the one in New Caledonia (SW of the SPCZ). At the station in Nauru (North of the SPCZ) there is a negative correlation stronger than -0.5. When comparing annual all-sky SSR to the ENSO index, significant negative correlations are found at Momote-Papua New Guinea (-0.41) and at Nauru (-0.96), both located in the Western Pacific Warm Pool, near the Equator. In all cases strong negative correlations (<-0.7) between SSR and cloud cover using both annual and 11-year moving means time series support the hypothesis of strong cloud control over SSR interannual and decadal variability. The results indicate that IPO and ENSO play a major role in the SSR variability over the Western Pacific by controlling different cloud cover regimes in the region. This reveals a real world case of the importance of unforced internal variability to the SSR decadal and sub decadal changes.