Interannual variation of cloud optical properties at ACRF Manus and Nauru sites from MFRSR measurements

• Long term MFRSR measurements are used to develop the cloud climatology in TWP region.
• At Nauru site, cloud properties are significantly correlated with PDO index.
• At Nauru site, cloud fraction, overcast cloud occurrence and COD decrease with SOI.
• At Manus site, cloud fraction, overcast CO and COD are bifurcately correlated with SOI.
• The drifting of tropical convection center can explain the impacts of SOI on cloud.

The long-term measurements of Multi-Filter Rotating Shadowband Radiometers and other instruments at two Atmospheric Radiation Measurements Climate Research Facility sites of Manus and Nauru have been processed to develop the climatology of cloud properties in the tropical warm pool region. Due to their unique geolocations and associated large-scale circulation patterns, cloud properties at these two sites exhibit distinctive characteristics. At the Nauru site, cloud properties are statistically significantly correlated with Pacific Decadal Oscillation index; the monthly anomaly values of cloud fraction, overcast cloud occurrence and optical depth decrease with Southern Oscillation Index (SOI). At the Manus site, monthly anomaly values of cloud fraction, overcast cloud occurrence and optical depth, to some extent, are bifurcately correlated with SOI, depending on the phase of El Niño/Southern Oscillation. The correlation of SOI and the MFRSR retrieved cloud optical properties can be explained by the drifting of tropical convection center in the equatorial area.