The meridional overturning circulation (MOC) of an ocean basin is its circulation in the latitude–depth plane, obtained by adding up the northward flow across the whole width of the basin. At a given latitude its strength is the maximum of the overturning streamfunction, the northward transport integrated from the surface down to a given depth, in sverdrups; in the Atlantic it is the AMOC. The older name thermohaline circulation describes the same deep circulation as one driven by differences of temperature and salinity. Deep-water formation is the sinking of surface water, made dense by cooling and by the salt left behind when sea ice forms, to intermediate or abyssal depths.