Abstract
AbstractHydrographic measurements from ships, autonomous profiling floats, and instrumented seals over the period 1986–2016 are used to examine the temporal variability in open-ocean convection in the Greenland Sea during winter. This process replenishes the deep ocean with oxygen and is central to maintaining its thermohaline properties. The deepest and densest mixed layers in the Greenland Sea were located within its cyclonic gyre and exhibited large interannual variability. Beginning in winter 1994, a transition to deeper (>500 m) mixed layers took place. This resulted in the formation of a new, less dense class of intermediate water that has since become the main product of convection in the Greenland Sea. In the preceding winters, convection was limited to <300-m depth, despite strong atmospheric forcing. Sensitivity studies, performed with a one-dimensional mixed layer model, suggest that the deeper convection was primarily the result of reduced water-column stability. While anomalously fresh conditions that increased the stability of the upper part of the water column had previously inhibited convection, the transition to deeper mixed layers was associated with increased near-surface salinities. Our analysis further suggests that the volume of the new class of intermediate water has expanded in line with generally increased depths of convection over the past 10–15 years. The mean export of this water mass from the Greenland Sea gyre from 1994 to present was estimated to be 0.9 ± 0.7 Sv (1 Sv ≡ 106m3s−1), although rates in excess of 1.5 Sv occurred in summers following winters with deep convection.
Highlights
The Nordic seas (Fig. 1) are a key region for dense water formation that impacts climate on a global scale (e.g., Gebbie and Huybers 2010)
Warm Atlantic water (AW) flows northward into the Nordic seas, releases heat to the atmosphere, and transforms into cold and dense waters that spill across gaps in the Greenland– Scotland Ridge as overflow plumes that feed the lower limb of the Atlantic meridional overturning circulation (AMOC)
While the primary source of the Denmark Strait Overflow Water (DSOW) was initially thought to be dense water formed by open-ocean convection in the Iceland and Greenland Sea gyres (Swift et al 1980; Swift and Aagaard 1981; Strass et al 1993), later studies argued that modified AW transported by the East Greenland Current (EGC) is the main source (Mauritzen 1996; Eldevik et al 2009)
Summary
The Nordic seas (Fig. 1) are a key region for dense water formation that impacts climate on a global scale (e.g., Gebbie and Huybers 2010). Moore et al (2015) found that the magnitude of the atmospheric heat fluxes over the Greenland Sea have decreased by 20% since the end of the 1970s They further suggested that if this trend continues, the mixed layer depth could be limited in the future such that only shallow convection occurs, which in turn could impact the production of dense water. The main focus of the present study is to examine the interannual variability of convection and dense water formation in the Greenland Sea. Using a combination of hydrographic observations and a one-dimensional mixed layer model, we document the evolution of the convective product for the period 1986– 2016 and explore its sensitivity to changes in hydrographic and atmospheric forcing conditions. That there was hardly sea ice within the gyre during the time period covered here [except the winters between 1986 and 1990 and in 1997–98; see Fig. 2b in Moore et al (2015)]
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