Wednesday, 25 January 2017
During the satellite era (1979-2015) in which the satellite-based global precipitation measurements are available, global mean surface temperature rapidly increased up to the late 1990s, followed by a temperature hiatus since about 1998/1999. It has been shown that the anthropogenic greenhouse-gases (GHG) might have played an important role in global mean temperature trends during the period, in addition to the anthropogenic aerosols and other natural forcings including the two large volcano eruptions (El Chichon, March 1982 and Mount Pinatubo, June 1991). However, differences in spatial patterns and magnitudes of surface temperature trends between observations and the outputs from the CMIP5 historical experiments also suggest the critical impact from decadal-to-interdecadal-scale internal modes specifically the Pacific Decadal Oscillation (PDO). Efforts have thus been made to estimate and separate the effects of these mechanisms on global precipitation. It is noted that the PDO has in general shaped the spatial patterns of observed precipitation trends specifically in the Pacific basin, including precipitation reductions in the tropical central-eastern Pacific and precipitation increases in the tropical western Pacific and along the South Pacific Convergence Zone. Furthermore, removing the PDO effect from the total observed precipitation trends makes the spatial structures of precipitation trends more similar to the ones simulated by CMIP5 historical full radiative forcing experiments especially in the context of zonal-mean results, confirming a combined effect of PDO and the anthropogenic GHG during the time period.
To further our understanding of the variations/changes not only in mean precipitation but also in the climatological wet and dry zones, four tropical climatological zones are classified based on the percentiles (Pct) of GPCP monthly rain rates, i.e., the wet (Pct ≥ 70th), intermediate (70th > Pct ≥ 30th), dry (30th > Pct ≥5th), and no rain (5th > Pct ≥ 0) zones. It is found that precipitation averaged over the wet zones shows much more intense interdecadal variability than tropical mean precipitation does, specifically by manifesting a more prominent decadal-scale shift around 1998, though the decadal-scale changes in the size of the wet zones are generally weak. On the other hand, evident decadal-scale changes exist in the sizes of the intermediate and dry zones specifically over tropical ocean, though signs of their change being opposite. Since about 1998, the tropical dry zones have become enlarged, while the intermediate zones have shrunk. Comparing with both the AMIP and CMIP5 historical simulations further confirms the critical role of the PDO in these decadal-scale changes.
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