Investigating SST Diurnal Variation and its Response to the MJO in the North Indian Ocean Using FY-4A Data

Xianliang Zhang^1,2,3Mao Zhihua^1,2*Longwei Zhang²Xuan Sang²

• ¹Shanghai Institute of Technical Physics, Chinese Academy of Sciences (CAS), Shanghai, Shanghai Municipality, China
• ²Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou, Zhejiang Province, China
• ³University of Chinese Academy of Sciences, Beijing, Beijing, China

This study investigates sea surface temperature (SST) diurnal variation in the North Indian Ocean by employing hourly data from the FY-4A geostationary satellite, in conjunction with reanalysis products and the Madden-Julian Oscillation (MJO) index. Validation against in situ SST measurements shows a bias of -0.33°C, an absolute bias of 0.73°C, an RSD of 0.33°C, an RMSE of 0.94°C, and a correlation coefficient of 0.90. The results demonstrate consistent accuracy throughout the diurnal cycle. Diurnal warming is most pronounced in the northern Arabian Sea and northern Bay of Bengal, especially from March to May prior to the monsoon onset.Wind speed and shortwave flux emerge as key drivers, although wind speed exerts a stronger regional influence on diurnal warming. Furthermore, the SST diurnal variation responds to the MJO within 1-2 days, as its active phase increases cloud cover and intensifies wind, thereby suppressing daytime warming. Despite the high temporal resolution of FY-4A, which allows for detailed sub-daily observations, monsoonal circulation and cloud cover can reduce data availability, complicating quantitative studies of SST variability and MJO feedbacks. Therefore, integrating multiple data sources is crucial for comprehensive analyses. Overall, these findings underscore the potential of geostationary satellites for monitoring diurnal SST fluctuations and emphasize the necessity of accounting for complex atmospheric-oceanic interactions in the North Indian Ocean when examining the role of diurnal variation in broader climate processes.