Recorded presentation14A.1
Changes and Variations in Continental Freshwater Discharge from 1949-2004
Aiguo Dai, NCAR, Boulder, CO; and T. Qian and K. E. Trenberth
A new data set of continuous monthly streamflow from 1948-2004 at the farthest downstream stations for world's 925 largest ocean-reaching rivers has been created for community use. Available new gauge records are added to a network of gauges that covers ~80 x 106 km2 or ~68% of global actively drained land areas and accounts for about 73% of global total runoff. The data gaps are filled through linear regression .with streamflow simulated by a land surface model (namely the Community Land Model Version 3, CLM3) forced with observed precipitation and other atmospheric forcings that is significantly (and often strongly) correlated with the observations for most rivers. The new data set has improved homogeneity and enables more reliable assessments of decadal and long-term changes in continental freshwater discharge into the oceans. The model-simulated runoff ratio over drainage areas with and without gauge records is used to estimate the contribution from the areas not monitored by the gauges in deriving the total discharge into the global oceans. Results revealed large variations in continental discharge at interannual to decadal time scales. These variations are correlated with the Southern Oscillation Index (SOI) for the discharge into the Pacific, Atlantic, Indian, and global oceans as a whole (but not with discharges into the Arctic Ocean and the Mediterranean and Black Seas), consistent with previous regional analyses. Consistent with previous reports, there is a large upward trend in the water-year discharge into the Arctic Ocean (~0.21 x 10-3 Sv yr-1) from 1949-2004. For the other ocean basins and the global oceans as a whole, the discharge data show downward trends, which are statistically significant for the Pacific (-0.43 x 10-3 Sv yr-1) and global oceans (-0.58 x 10-3 Sv yr-1). Except for the Arctic discharge, precipitation changes are found to be the main cause for the discharge trends and large interannual to decadal variations, although the CLM3 simulation also suggests influences of surface temperature and other atmospheric forcing (e.g., through enhanced evaporation). For the Arctic drainage areas, the upward trends in streamflow are not accompanied by increasing precipitation, especially over the Siberia. The CLM3 simulation suggests that recent surface warming has induced decreasing trends in snow cover and soil ice water content over the northern high-latitudes, which contribute to the runoff increases in these regions. Our results contradict the notion that global continental runoff has increased during the recent decades. Recorded presentation
Session 14A, Detection and attribution of climate change: Part III
Thursday, 24 January 2008, 11:00 AM-12:15 PM, 215-216
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