Rates of North Atlantic Deep Water formation calculated from chlorofluorocarbon inventories

Chlorofluorocarbon (CFC) inventories provide an independent method for calc ulating the rate of North Atlantic Deep Water (NADW) formation. From data c ollected between 1986 and 1992, the CFC-11 inventories for the major compon ents of NADW are: 4.2, million moles for Upper Labrador Sea Water (ULSW), 1 4.7 million moles for Classical Labrador Sea Water (CLSW), 5.0 million mole s for Iceland-Scotland Overflow Water (ISOW), and 5.9 million moles for Den mark Strait Overflow Water (DSOW). The inventories directly reflect the inp ut of newly formed water into the deep Atlantic Ocean from the Greenland, I celand and Norwegian Seas and from the surface of the subpolar North Atlant ic during the time of the CFC-11 transient. Since about 90% of CFC-11 in th e ocean as of 1990 entered the ocean between 1970 and 1990, the formation r ates estimated by this method represent an average over this time period. F ormation rates based on best estimates of source water CFC-11 saturations a re: 2.2 Sv for ULSW, 7.4 Sv for CLSW, 5.2 Sv for ISOW (2.4 Sv pure ISOW, 1. 8 Sv entrained CLSW, and 1.0 Sv entrained northeast Atlantic water) and 2.4 Sv for DSOW. To our knowledge, this is the first calculation for the rate of ULSW formation. The formation rate of CLSW was calculated for an assumed variable formation rate scaled to the thickness of CLSW in the central Lab rador Sea with a 10:1 ratio of high to low rates. The best estimate of thes e rates are 12.5 and 1.3 Sv, which average to 7.4 Sv for the 1970-1990 time period. The average formation rate for the sum of CLSW, ISOW and DSOW is 1 5.0 Sv, which is similar to (within our error) previous estimates (which do not include ULSW) using other techniques. Including ULSW, the total NADW f ormation rate is about 17.2 Sv. Although ULSW has not been considered as pa rt of the North Atlantic thermohaline circulation in the past, it is clearl y an important component that is exported out of the North Atlantic with ot her NADW components. (C) 2000 Elsevier Science Ltd. All rights reserved.