Short answer: I would look to find iso-surfaces (or surface values) of temperature/salinity/potential density propagating downwards into different water masses.

It all depends on what kind of data you have. In case of a vertical profile time-series, you could make a contour plot of one of these properties, with depth in the y-axis, and time in the x-axis. Then, downwelling would be indicated by spikes of surface properties propagating downward.

In case you have several spatially distributed vertical profiles, at a single moment in time, you could look for downward "dents" on the iso-surfaces.

Hope it helps.

Stamatios Petalas Thanks a lot.

using the temperature and salinity profile, you can compute the potential density at each points and draw the isopycnal, the downward movement of isopycnals indicate the downwelling phenomina

During downwelling heat energy transfer and distribute ind deep ocean and dwelling organisms can get heat energy through this process