Interdecadal Precipitation Variability in Distinct Tropical Zones during the Satellite Era

Tropical (30^oN-30^oS) precipitation variability on the interdecadal/trend time scales is investigated by focusing on precipitation intensity distribution derived from monthly rain rates from the Global Precipitation Climatology Project (GPCP). Monthly precipitation percentiles (Pct) for the tropical domain are first estimated; temporal variations in these percentiles are then examined, including linear trend estimation and significant tests using both the Student’s t and the Mann-Kendall trend test. An upward increase is found for the upper one third of the percentiles (Pct ≥ 70^th), with significant, positive trends being found for the 80^th, 90^th, and 95^th percentiles. Nevertheless, a downward decrease appears roughly for the middle one-half percentiles (approximately 20^th-65^th), with significant downward trend for the 20^th, 30^th, and 40^th percentiles. It is also noted that a decadal-scale shift appears around 1998 in the percentiles with opposite signs of change between these two groups. For those lower percentiles there is generally a weak increase.

Furthermore, the four precipitation categories are classified based on the percentiles for the entire record, i.e., wet (Pct ≥ 70^th), intermediate (70^th > Pct ≥ 30^th), dry (30^th > Pct ≥5^th), and no rain (5^th > Pct ≥ 0). Variations and changes in the occurrence frequency of the four precipitation categories, the locations and sizes of the four corresponding precipitation zones, and precipitation (intensity) over wet and intermediate zones are then explored. Precipitation intensity over wet zones shows much stronger trend/variability than mean precipitation including a more prominent decadal-scale change around 1998. A decadal shift also appears in the sizes of intermediate and dry zones especially over tropical ocean. But, interdecadal changes/trends in the sizes of wet and no rain zones are generally weak. Epoch differences in the occurrence frequency of the four precipitation categories between the two time periods: 1988-1997 and 1998-2015, also show intense variations/changes on the interdecadal/trend time scale. Spatial features of the occurrence frequency changes tend to reflect the dominant impact of decadal/interdacadal internal variability specifically the Pacific Decadal Oscillation (PDO). Comparisons with AMIP simulations and CMIP5 historical full-radiative forcings experiments further confirm the dominant role of the PDO, though the effects of anthropogenic forcings are discernible as well.