Note on Times 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. 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 this 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 rg., — UV, where L is the distance and V the transport velocity. Given a typical wind speed of 20m/s in the mid-latitude tropospheric westerlies, the time of transport round the globe would be about 2 weeks. 

In situations where the transport is governed by diffusive processes, a time-scale of transport can be defined as  

PACIFIC 1. ATLANTIC DEEP OCEAN 000 YEARS 100 YEARS  

Some important time-scales characterizing the transport within the oceanic and atmospheric environments are summarized in Fig. 4-14. 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. 

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