Interannual-Interdecadal Precipitation Variability in Tropical Wet and Dry Zones

Interannual-to-interdecadal precipitation variability in the tropics (30oN-30oS) is explored by means of the satellite-based monthly precipitation product from the Global Precipitation Climatology Project (GPCP). Four tropical zones are first classified based on the percentiles (Pct) of monthly rain rates, i.e., the wet (Pct ≥ 70th), intermediate (70th > Pct ≥ 30th), dry (30th > Pct ≥5th), and no rain (5th> Pct ≥ 0) zones. Variations in their sizes and precipitation over wet and intermediate zones are then examined to better understand variations in dry and wet regimes, not just mean values. On the interannual time scale, precipitation over wet zones shows strong sensitivity to ENSO, with more (less) precipitation over oceanic (land) wet zones following warm events. However, there is only a weak ENSO effect on precipitation over intermediate zones. ENSO can also modulate the sizes of these zones. Over tropical ocean, warm (cold) events tend to increase (reduce) the size of wet zones and reduce (increase) the size of dry zones, while their impact on the sizes of intermediate and no rain zones is weak. Over tropical land, ENSO has a strong impact on the sizes of wet and intermediate zones, with shrunk wet and enlarged intermediate zones following warm events; However, there is no consistent ENSO impact on the sizes of dry and no rain zones. On the interdecadal time scale, a decadal-scale shift appears around 1998 in the sizes of intermediate and dry zones specifically over tropical ocean, albeit the signs of change being opposite, while decadal-scale changes in the sizes of wet and no rain zones are relatively weak. Furthermore, precipitation over wet zones shows much more intense interdecadal variability than tropical mean precipitation does, specifically by manifesting a more prominent decadal-scale shift around 1998. For precipitation over intermediate zones, even though the decadal-scale change is in general weak, a moderate decrease is still discernible. AMIP simulations from multiple CMIP5 models can reproduce relatively well these observed variations on both the interannual and interdecadal time scales. Furthermore, comparisons with the CMIP5 historical runs suggest that these decadal-scale variations during the GPCP period are dominated by the Pacific Decadal Oscillation (PDO)-related climate shift.