Contrasting temporal trends in low-level clouds and mid- and high-level clouds over the Indian Ocean in the last four decades (1979–2018)

Understanding the climatic trends in cloud fraction (fc) and its drivers is critical in climate science. Here, we analyzed 40 years (1979–2018) of hourly fc data at 0.25° × 0.25° spatial scale from ERA5 to examine the trends in the 3D distribution of fc over the oceanic region adjacent to the Indian Subcontinent in view of its drivers. We found that the mid-level fc (MCF) and high-level fc (HCF) have increased by 0.05 and 0.05–0.12 fraction in the last four decades in this region. On the contrary, the low-level fc (LCF) decreased by 0.04 fraction, resulting in a net marginal increase in total fc. The observed contrasting trends in LCF, MCF and HCF are manifestations of the changes in sea surface temperature and meteorological conditions. Although LCF showed a regionally averaged declining trend, it has increased over the south-southwestern part of the domain. The increasing trend of MCF and HCF can be attributed to the increase in natural convection due to surface warming. Our results suggest that the observed contrasting trends might have resulted in positive radiative feedback on the Indian Ocean warming.