Deep convection: A Source or Sink for Atmospheric CO2?

The Labrador Sea, together with the Nordic Sea Overflow Water, produce the intermediate and deep waters of the North Atlantic. Labrador Sea Water (LSW) is formed every winter by deep convection. Depths of winter convection in the Labrador Sea are variable, exceeding 2000 m in severe winters of the early 1990’s, but restricted to around 1000 m in many of the recent years. In 2008, 2014 and 2015, convection depths returned to over 1500 m. LSW spreads at intermediate depths in the North Atlantic. Some of the LSW contributes to an upper part of North Atlantic Deep Water and is incorporated into the meridional overturning circulation. In this way, atmospheric CO₂ sequestered in the Labrador Sea is stored in the deep ocean, potentially for hundreds of years, and the Labrador Sea provides a conduit for long-term storage of atmospheric CO_2. Since the waters which overflow from the Nordic Seas pass through the deep Labrador Sea, the Labrador Sea is an ideal locale to study the ventilation processes of the deep North Atlantic. A time-series study from 1993 to 2015 shows a steady increase of dissolved inorganic carbon (DIC) concentration in all water masses in the Labrador Sea. In the newly-ventilated LSW, DIC concentration increased by 0.76 µmol/kg³/year and pH_total decreased by 0.0030/year. The Labrador Sea DIC inventory increased at a rate of 0.04 PgC/year over the study period. Previous studies show the surface waters approach equilibrium with atmospheric CO₂ during deep convection in winter, and identified the biological pump as a major mechanism for uptake of atmospheric CO₂ in the region. The significance of variable depth convection for atmospheric CO₂ sequestration is discussed on the basis of the hydrographic studies.