According to climatological data, the low salinity of near-surface water in the northern North Pacific, which reduces its density so much as to prevent sinking to great depth there, is due to the small regional evaporation rate (which allows a substantial net freshwater input to the surface layer from precipitation and runoff), in combination with the small rate of flow through the surface layer (which amplifies the effect of the freshwater flux on the salinity). The low evaporation rate is due in turn to the relatively low surface temperature (decreasing the specific humidity of the air at the air-sea interface), which seems to be caused mainly by the relatively large proportion of cold upwelling water in the inflow to the surface layer, contrasting with that in the northern North Atlantic where warm surface water to the south is the principal component of inflow. The reduction in southern inflow—and thereby in through-flow as well—results somewhat from the absence of a sink for surface water in any analogue to the Norwegian Sea, but probably in greater part from the more southerly extent of the subpolar gyre in the North Pacific than in the North Atlantic, whereby little water from the subtropical gyre passes through the northern North Pacific. The latter feature indicates a linkage between deep-water formation in the northern hemisphere and the distribution of wind-stress curl. Some aspects of this process by which lowering the temperature of seawater can reduce its density—by decreasing its salinity through diminished evaporation—are illustrated in a simple model.