Ocean Science www.ocean-sci.net 1812-0784 1812-0792 2 1 2006 10.5194/os-2-1-2006 http://www.ocean-sci.net/2/1/2006/ http://www.ocean-sci.net/2/1/2006/os-2-1-2006.html http://www.ocean-sci.net/2/1/2006/os-2-1-2006.pdf 1 9 2006-01-09 Transient residence and exposure times E. J. M. Delhez University of Liège, Mathematical Methods and Modelling, Belgium The residence time measures the time spent by a water parcel or a pollutant in a given water body and is therefore widely used in environmental studies. The adjoint method introduced by Delhez et al.&nbsp;(2004) to compute this diagnostic is revised here to take into account the effect of the initialization and of the boundary conditions. <br /><br /> In addition to the equation for the mean residence time, it is suggested to solve a simple advection-diffusion problem to quantify the effect of the initialization and clarify the interpretation of the results. <br /><br /> Using the two same equations but with modified boundary conditions, the method can also be used to quantify the accumulated time spent by water/tracer parcels in a control domain. This diagnostic is called "exposure time". <br /><br /> Analytical and realistic model results are used to illustrate the concepts.