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The Labrador Sea is a North Atlantic basin between Greenland and northeastern North America, studied as a site of deep ocean convection. Winter surface heat loss can increase near-surface water density until mixing reaches much deeper layers, forming Labrador Sea Water.

Repeated temperature and salinity profiles distinguish deepening convection from ordinary seasonal surface changes. The resulting water spreads into the wider circulation and helps ventilate the ocean interior. The depth of mixing varies between years; a single winter or one observing station cannot characterize the entire overturning circulation.^[ref]

Labrador Sea Water is a water mass formed by deep winter mixing in the Labrador Sea, identifiable by its temperature, salinity, density and ventilation history. Surface heat loss increases density and can deepen convection, producing a relatively uniform water column that subsequently spreads into the surrounding North Atlantic.^[ref]

A motivating measurement combines shipboard profiles of temperature, salinity and dissolved oxygen with repeated profiling-float observations. A deep layer with weak vertical property gradients indicates mixing; oxygen and other tracers help reveal recent contact with the atmosphere. Comparing successive winters distinguishes renewed formation from the later movement of an older water mass. Surface freshening can preserve stable stratification even during substantial cooling, so heat loss alone does not determine the depth reached by convection.^[ref]

The observational lesson is that a water mass is identified through a combination of properties and history. A cold reading at one depth is insufficient to establish its origin or to measure the whole ocean overturning circulation.