The Hydrologic Cycle of Major Continental Drainage and Ocean Basins: A Simulation of the Modern and Mid-Holocene Conditions and a Comparison with Observations

Abstract

Each continental grid box of the National Center for Atmospheric Research (NCAR) community climate model (CCM1) is assigned to a particular continental drainage basin based on river basin extent and river flow direction. Boundaries are similarly assigned for the Arctic, Atlantic, Indian, and Pacific Oceans. The hydrologic variables from general circulation model simulations representing modern (control) and interglacial-6000 yr before present (6 ka bp)?climates are then summarized for continental drainage basins and oceans in order to examine the regional response of the hydrologic cycle to these climatic extremes. The NCAR CCM1 simulation of the modem climate reproduces the general features of the observed hydrologic cycle. The simulations for the individual oceans are generally in good agreement with the observational estimates; however, the magnitude of the precipitation, evaporation, and runoff for nearly all continental regions is 20% to 30% greater than the observed. Comparisons of the CCM1 control and 6 ka bp simulations reveal an increase in the simulated monsoon precipitation, evaporation, and runoff of South Asia in the 6 ka bp experiment relative to the control. There is also an increase in the freshwater surplus of the Indian Ocean basin, a larger freshwater deficit for the Pacific Ocean basin, and an enhanced hydrologic cycle in the high latitudes in the 6 ka bp experiment. An order of magnitude calculation suggests that the increased runoff simulated for South Asia is capable of altering the salinity of the northern Bay of Bengal to values consistent with observational estimates for this time period.