It is a very very good question... And even though there are several different methods being used, I wonder if they are all corresponding with each other? Or if the results are being cherry picked? Something I will have to dig into more for sure.
But I’m thinking even if there is a rise in temp using just the Sea Surface Temperatures they could be fairly accurate in my theory because that is where the thermal exchange with the atmosphere might indeed happen as the evaporation layer in the system cycle?
1. Thermometers under buoys or ships Since about 1990 an extensive array of moored buoys across the equatorial Pacific Ocean has beamed temperature data from a 1 meter depth up to a satellite. Lots of ships are also recording their intake water temperatures but the depths and locations vary making this data harder to use.
2. Satellite remote sensing NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) SST satellites have been providing global SST (Sea Surface Temperature) data since 2000. Unlike buoys, the satellites can sense the surface temperature everywhere. The temp measured is of the surface only, though. The surface “skin” temp can be quite different than the temp of the water below because of things like evaporation, wind, sunshine, and humidity. Also, cloud cover prevents satellites from sensing surface temperatures.
3. Acoustic Tomography Sound, especially low frequencies, can travel long distances under water. Since the speed of sound under water varies with temperature, measuring how long sound takes to travel a certain distance will give you the average temperature of the water over that distance. Acoustic Thermometry of Ocean Climate (ATOC) is using trans-basin acoustic transmissions to observe the world’s oceans, and the ocean climate in particular.
4. Ocean Surface Topography By bouncing microwaves off the the ocean surface and using GPS location, satellites can precisely measure the height of any spot on the ocean surface. Reasoning that water expands and contracts as it heats and cools, then so too would the height of the sea surface. I think Ocean Surface Topography is the easiest and best technique for measuring ocean temperature.
Thanks for the informative response.
I totally disagree with you on #4. There are just too many other factors that would undermine getting an accurate reading.