Assessment of FY-3C OLR Anomalies over the Tropical Pacific: Implications for ENSO Phase Tracking and Convection–SST Coupling

Abstract

Outgoing longwave radiation (OLR) from satellite observations is a key indicator of tropical deep convection and a valuable proxy for monitoring El Niño-Southern Oscillation (ENSO) evolution. This study evaluates the performance of OLR retrievals from Fengyun-3C (FY-3C), a Chinese polar-orbiting meteorological satellite, against the interpolated OLR product from the National Oceanic and Atmospheric Administration (NOAA) over the tropical Pacific Ocean for 2014–2019. The analysis focuses on distinguishing coupled and uncoupled El Niño events. To quantify the convection–sea surface temperature (SST) relationship, we developed a Coupling Strength Index (CSI) based on FY-3C OLR and Niño-3.4 SST anomalies. Temporal correlations between FY-3C and NOAA OLR exceed 0.98, confirming high overall consistency; FY-3C OLR faithfully captures convective responses to Niño-3.4 SST anomalies, especially during the strong 2015–2016 El Niño and 2017–2018 La Niña. Spatial correlations range from 0.76 to 0.97, with root-mean-square errors of 4.04–8.36 W m −2 , further demonstrating strong agreement with NOAA OLR, albeit with localized biases emerging during rapid convective transitions. CSI analysis reveals robust ocean–atmosphere coupling during events such as the 2015–2016 El Niño (CSI < −5 W m −2 °C −1 ) and identifies uncoupled El Niños characterized by SST warming without matching tropical convection (e.g., late 2014 and late 2018). Notably, CSI enables early detection of convection–SST decoupling, signaling OLR reversals roughly one month in advance. These results demonstrate that FY-3C OLR combined with the CSI provides a concise quantitative metric for ENSO air–sea coupling strength and improves operational monitoring of ENSO phase transitions.