Problems about efdc output files U3DTS001

I am a researcher in university and there are some questions when I use efdc,especially the output files.Among the output files there comes a velocity output called “U3DTSxxx”.I am confused that when I plot the time series in Matlab and I find that during the latter time fraction the velocity appears positive at one moment and negative at the next moment.And it appears that periodically.
Any answers will be helpful!

I have attached some figures of U3DTS021. Attached files


There are three kind of current velocities that can be exported from the EFDC model results. These are magnitude velocity, x-component and y-component velocities. The magnitude velocity should be possible always while the x- and y-component have both positive and negative values. Please make sure that you extract and plot the correct value of velocity. Your plot seems like x- and y-components.

Hope this helps.