Ocean: role in the climate system

Physical
To store energy
Annual cycle: mixed layer (〜100 m?)
Decadal and centennial scale: upper ocean (〜500 m?)
Millenial scale: whole ocean
To transport energy
Upper ocean (〜500 m?): wind-driven circulation
driven by frictional stress by winds at the surface
Deep ocean: thermohaline circulation
driven by inhomogeneity of density due to temperature and salinity
Source of water to evaporate into atmosphere
[difference from land ... see next section]

Bio-chemical [not directly discussed here]
To store carbonates (matter related to CO2)
To make part of them into sediments
etc.