Lots and lots of physical measurements. One large scale example is the Global Drifter Program. The US contribution is largely funded by NOAA.

http://www.aoml.noaa.gov/phod/dac/index.php

The array is usually maintained at above 1000 discrete instruments deployed by various cargo ships, and research vessels globally. These measure ocean currents centered at 15m depth, which helps eliminate bias from surface currents driven by winds. These drifters also measure surface temperature some with sea level air pressure.