The Fundamental Global Water Cycle
The understanding and prediction of local, regional, and global water cycles are very important for understanding and prediction of societal impacts of water, especially water availability for domestic and industrial uses, and agriculture. There is substantial progress in understanding local and regional water cycles; but global water cycles, their variability, and changes are poorly quantified and understood. A major reason for this lack of progress is the non-availability of long-term observations of evaporation, precipitation, and water vapor transport, especially over the oceans. Among all global-scale water cycles, the most important one is the fundamental global water cycle (FGWC), defined as the annual cycle of evaporation, precipitation, and water vapor transport between the Northern and Southern Hemispheres.
Figure 1: The FGWC based on precipitation, GSSTF2 over-ocean evaporation, and over-land evaporation based on combined S05 EmP and GPCP precipitation during 1988-2000. Arrows and numbers across the hemispheric interface denote the direction and amount of net moisture flux transport. All quantities are in units of 106 m3/s.
We quantified the average FGWC over 13 years with remote-sensing-based precipitation estimates from the Global Precipitation Climatology Project (GPCP) Version 2 data sets and over-ocean evaporation estimates from the Goddard Satellite Surface Turbulent Fluxes Version 2 data set from 1988 to 2000. Over-land evaporation was estimated from the remote-sensing-based precipitation estimates combined with a global evaporation minus precipitation (EmP) data set.
Results show (Mehta et al., 2005) that 75% to 85% of the total global evaporation and approximately 70% of the total global precipitation occurs over the oceans in each season. In the GPCP-based FGWC estimate, there is a remarkable balance in the inter-hemispheric import-export of atmospheric moisture in December-January-February and June-July-August.
The dominant cross-equatorial atmospheric moisture transports in the atmospheric branch of the FGWC supply a significant amount of moisture to precipitation regions and are from the Northern Hemisphere to the Southern Hemisphere in December-January-February and the Southern Hemisphere to the Northern Hemisphere in June-July-August, with approximately 3×106 m3 s-1 net annual transport from the Southern Hemisphere to the Northern Hemisphere in the GPCP-based FGWC estimate (Fig. 1). These unique results show that evaporation, precipitation and atmospheric moisture transport over the oceans play a very important role in the FGWC and that they should be a significant component in national and international research programs on the global water cycle.