Chapter 5 - The Oceanic Heat Budget

5.8 Meridional Fresh Water Transport

The Earth's water budget is dominated by evaporation and precipitation over the ocean. Baumgartner and Reichel (1975) report that 86% of global evaporation and 78% of global precipitation occur over the ocean. A map of the net evaporation (Figure 5.12) shows that evaporation exceeds a meter per year in the trade-wind regimes in the eastern parts of the oceans. The transport of fresh water by the ocean can be calculated in the same ways heat transports are calculated, with similar uncertainties (Figure 5.13). Knowledge of water fluxes and transports is important for understanding the global hydrological cycle, ocean dynamics, and global climate. For example, the variability of fresh water fluxes may have played an important role in the ice ages as discussed in §13.3.

Figure 5.12 Evaporation minus precipitation in mm per day calculated from ECMWF 40-year reanalysis. From From Kallberg et al 2005.

Figure 5.13 Meridional transport of fresh water by the Atlantic from three surface-flux calculations: BR-Baumgartner and Reichel (1975); SBD-Schmitt et al., (1989); and IH-Isemer and Hasse's (1987) evaporation estimates combined with Dorman and Bourke's (1981) precipitation values. Also shown are direct measurements at 24°N by Hall and Bryden (1982) and 11°N by Friedrichs and Hall (1993). All are summed relative to an estimated Arctic southward export due to the Bering Strait through flow and the water budget of the Arctic itself. From Schmitt (1994).

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