Variability and Trends in Tropical Precipitation Derived from Satellite-Based Precipitation and Comparison with Climate Model Outputs

Decadal-scale variations and long-term trends in tropical precipitation, encompassing the region between 30^oN and 30^oS, have been examined for the satellite era spanning from 1979 to the present. This investigation utilizes the satellite-derived monthly precipitation data from the Global Precipitation Climatology Project (GPCP) and the precipitation outputs from CMIP6/AMIP models. To portray the spatial distribution of precipitation intensity across the tropical domain and various sectors within it, both over land and ocean, monthly precipitation percentiles are employed, alongside disparities between high (wet) and low (dry) percentiles in relation to respective mean precipitation. These metrics are then analyzed to ascertain variations and trends, which are intricately linked to shifts in precipitation patterns, encompassing the contraction and expansion of tropical and regional wet/dry areas, as well as changes in precipitation intensity. Notably, notwithstanding substantial interannual fluctuations mainly attributed to the El Niño-Southern Oscillation (ENSO), increases (decreases) in high (low) percentiles or precipitation occur over wet (dry) grid cells across tropical land and ocean. Concurrently, there are positive (negative) trends in the differences between wet (dry) percentiles and mean precipitation over tropical land and ocean, thus corroborating the broader spatial distribution of precipitation intensity on the monthly scale. This trend suggests a propensity towards strengthening of wet and dry zones. In general, regional outcomes tend to uphold this conclusion. These observational results are also generally corroborated by the CMIP6 historical and AMIP6 simulations, particularly in terms of directional changes.

Further exploration into precipitation and intensity changes in GPCP entails an emphasis on specific tropical zones, classified based on dynamic monthly precipitation percentiles derived from GPCP data, and also by vertical velocity at 500 hPa from two reanalysis products: MERRA-2 and ERA-5. Increased precipitation over wet/ascent zones and reduced precipitation over dry/descent zones are found during the study period, confirmed by the changes/trends in global summer monsoon precipitation and in the zonal-mean Intertropical Convergence Zone (ITCZ) intensity. Notably, there are parallels between changes in the intensity of the northern hemisphere summer land monsoon and precipitation over the wet/ascent zone within tropical land areas. Furthermore, a distinct augmentation in zonal-mean ITCZ intensity emerges over the study period, marked by a positive trend magnitude that surpasses the increase in precipitation over the wet/ascent zone situated over tropical ocean.