I am looking for a few options of particle tracking for ocean modeling to try to do a comparison of different codes.
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The main difference between online and offline trajectory calculation is that the online is implemented as part of the model, and the trajectories are part of the model output. Offline trajectory calculation is a separate piece of software that takes Eulerian model output fields as input and calculates the trajectories.
The main advantage of online particle tracking is that the displacement of a particle or tracer can be evaluated every time step, no matter how small it is. As a result, you can obtain a true particle trajectory based on what model produces. The disadvantage of the online tracking is that the user needs to know the particle release location and time ahead of running the model. The models listed below (at least) support online Lagrangian trajectory calculation:
On the other side, offline particle tracking can only use the Eulerian fields as often as they are output. Thus, there will be an error in the calculated trajectory depending on the frequency of the output. On the other side, the main advantage of offline calculation is that the user can experiment and play around with deployment locations, times, area of coverage etc., because trajectory calculation typically takes much less computational effort than the actual model. There are several options here:
All models and tools listed in this answer are freely available for download under different open source licenses.
We are in the final stages of a new book on numerical particle tracking for the coastal ocean. Originally as part of that book and now separate from it, we have developed an evolving list of routines for particle tracking. It can be access at http://www.nefsc.noaa.gov/drifter/particles.html.
I don't mean to detract from the answer by @IRO-bot. I just wanted to complement his results.
Edits and extensions can be emailed to James Manning @NOAA