Recent trends of tropical hydrological cycle inferred from GPCP and ISCCP data

December 1, 2011 — Click on image to enlarge.

Among the greenhouse gas–induced climate change projections, tropical hydrological cycle changes can be expected to cause shortage or excess of precipitation in many regions, and that in turn would impact all life on earth. We have examined decadal trends of the tropical hydrological cycle in the GPCP precipitation and ISCCP cloud and radiation data to determine if such trends can provide an observation-based benchmark for model predictions of the ongoing climate change. The results show (1) intensification of tropical precipitation in the rising regions of the Walker and Hadley circulations and weakening over the sinking regions – showcasing the “wet-getting-wetter, dry-getting-dryer” phenomena; (2) poleward shift of the subtropical dry zones (up to 2° decade−1 in June-July-August (JJA) in the Northern Hemisphere and 0.3–0.7° decade−1 in JJA and September-October-November (SON) in the Southern Hemisphere consistent with an overall broadening of the Hadley circulation; and (3) significant poleward migration of cloud boundaries of Hadley cell and plausible narrowing of the high cloudiness in the Intertropical Convergence Zone (ITCZ) region in some seasons. These trends indicate a strengthening of the tropical hydrological cycle with intensification of extremes of dry and wet conditions.

Figure 1 (top left): GPCP climatology (contour) and linear trends (shading) indicate that precipitation increases at climatologically wet regions and decreases at climatologically dry regions.

Figure 2 (bottom): Time-longitude distribution of meridionally averaged (30°S–30°N) GPCP annual mean precipitation (a) and corresponding linear trend with longitude (b) show that the rising (sinking) areas of the Pacific Walker circulation are represented with positive (negative) precipitation trends.

Figure3 (top right): Linear trends of the latitude of minimum precipitation, ITCZ, and Hadley cell boundaries inferred from GPCP for each season and the year for the Northern Hemisphere (a) and the Southern Hemisphere (b) show poleward shifts of the Hadley cell and subtropical dry zones. Total expansion of the tropics in these three indices are shown in (c). For quantities significant at the 90% level, bars are shaded green, blue, and orange, respectively.

References:
Zhou, Y. P., K.-M. Xu, Y. C. Sud, and A. K. Betts (2011), Recent trends of the tropical hydrological cycle inferred from Global Precipitation Climatology Project and International Satellite Cloud Climatology Project data, Journal of Geophysical Research - Atmospheres, doi:10.1029/2010JD015197.