Time Scales of Mixing in the Atmosphere and Oceans

It is often important to know how long an element spends in one environment before it is transported somewhere else in the Earth system. For example, if a time scale characterizing a chemical or physical transformation process in a region has been estimated, a comparison with the time scale characterizing the transport away from the region will tell which process is likely to dominate. 

Time scales of transport can also be applied to situations when no well-defined reservoirs can be defined. If the dominant transport process is advection by mean flow or sedimentation by gravity, the time scale characterizing the transport between two places is simply tadv = L/V where L is the distance and V the transport velocity. Given a t)q)ical wind speed of 20 m/s in the mid-latitude tropospheric westerlies, the time of transport around the globe would be about 2 weeks. 

Some important time scales characterizing the transport within the oceanic and atmospheric environments are summarized in Fig. 4-17. In view of the somewhat ambiguous nature of the definitions of these time scales, the numbers should not be considered as more than indications of the magnitudes. 

Acknowledgement. I wish to thank Anders Bjdrkstrom and Michael Jacobson for valuable comments on the manuscript. 

4-2 Calculate the turnover time of carbon in the various reservoirs given in Fig. 4-3. 

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