How do tropical active fires respond to intra-annual climate change in the early 21st century?

• Trends of climate variables of 20-year MODIS active fire occurrences are diverse. • Correlation direction and strength between fires and climate differ geographically. • Active fire incidence is negatively correlated with rainfall and relative humidity. • High fire season swings with change in monthly minimum rainfall across the tropics. • Varied fire responses to other climate variables obscure fire-climate interactions. The interactions between fire, ecosystems, and climate are complex. Tropical ecosystems have dominated global active fires nowadays, yet its causes, mechanisms, and consequences remain relatively poorly understood. To investigate temporal response of remotely-sensed active fires to intra-annual climate change, several 1-km datasets, including the Moderate-resolution Imaging Spectroradiometer Collection 6 (MODIS C6) active fires and the Climatologies at High Resolution for the Earth's Land Surface Areas (CHELSA) climate variables, were gathered to examine the climatic characteristics of active fire incidences, fire-climate correlations, and the average monthly response of active fire occurrences to climate change using the Geographic Information System (GIS) Fishnet tool, Theil-Sen Median slope estimation, Mann-Kendall significance test, and Pearson's correlation. We concluded that climate variables’ trends of nearly two-decade active fires displayed varied degrees of increment in precipitation (Pre), temperature (Tas), and surface downwelling shortwave radiation (Rsds) and inconsistent decrement in near-surface relative humidity (Hurs) and near-surface wind speed (sfcWind). MODIS multi-year (2003–2018) active fires were moderately to strongly correlated negatively with Pre and Hurs at 10 km grid-resolution but positively with sfcWind and Rsds, showing marked geographical variations in correlation direction and strength. The most significant finding is the newly observed inverse relationship between active fires and precipitation on both sides of the equator. High occurrence areas of active fires regularly appear back and forth along with latitudinal changes (at one-degree intervals) in monthly minimum precipitation between the tropical Northern and Southern Hemispheres. The present study contributes to exploring the underlying mechanism of fire-climate interactions against the backdrop of climate warming.