The interaction of the ocean with the rest of the earth system happens primarily at the sea-surface through air-sea fluxes that imprint on fluid parcels the current physical or chemical state of the atmosphere. When fluid parcels are transported below the surface, they become shielded from the atmosphere until they resurface at a later time to communicate past physical or chemical climate conditions to the atmosphere. The delay between successive visits to the surface mixed layer imparts to the climate system an important long-term memory.

Using an ocean general circulation model we explore the time for fluid parcels to be transported from the surface mixed layer to the deep ocean through a multiplicity of pathways by advective and mixing processes. The diagnostic tool used to summarize the transport properties of the modeled ocean circulation is the probability density function (pdf) of transit time computed as a function of position in the ocean interior. Through the combined use of a forward and adjoint global tracer transport model, the location and time at which water masses formed in the surface mixed layer are efficiently mapped out. The method is also used to efficiently map out where and when fluid parcels in the deep ocean will make next contact with the surface mixed layer.