[seszett]

Well actually, the total amount of ocean water stays constant during a storm. The quantity of rainwater that falls is absolutely negligible, and the surge caused by wind means the water level falls very slightly in the areas that are not affected by the very localized surge.

That's why you don't measure "three points in the US and one point in Japan". You measure thousands of points all around the world, so if storms affect two or three of them for a day, the global result isn't significantly affected. For instance, just the French observation network is made up of 90 stations that cover enough of the globe to be enough on their own for measuring a 2 mm rise over two months (near real-time data available on ftp://ftp.sonel.org/tidegauge/rmsl/Demerliac/RAW/).

It's really not as complicated as you think it is, the basic science behind global sea level measurements was established in the 18th century.