Water mass transformation in the Iceland Sea: Contrasting two winters separated by four decades

Dense water masses formed in the Nordic Seas flow across the Greenland–Scotland Ridge and contribute substantially to the lower limb of the Atlantic Meridional Overturning Circulation. Originally considered an important source of dense water, the Iceland Sea gained renewed interest when the North Icelandic Jet — a current transporting dense water from the Iceland Sea into Denmark Strait — was discovered in the early 2000s. Here we use recent hydrographic data to quantify water mass transformation in the Iceland Sea and contrast the present conditions with measurements from hydrographic surveys conducted four decades earlier. We demonstrate that the large-scale hydrographic structure of the central Iceland Sea has changed significantly over this period and that the locally transformed water has become less dense, in concert with a retreating sea-ice edge and diminished ocean-to-atmosphere heat fluxes. This has reduced the available supply of dense water to the North Icelandic Jet, but also permitted densification of the East Greenland Current during its transit through the presently ice-free western Iceland Sea in winter. Together, these changes have significantly altered the contribution from the Iceland Sea to the overturning in the Nordic Seas over the four decade period.publishedVersio

A Comparison of Arctic Ocean Sea Ice Export Between Nares Strait and the Canadian Arctic Archipelago

Nares Strait and the channels of the Canadian Arctic Archipelago (CAA) act as conduits for sea ice export from the Arctic Ocean but have never been directly compared. Here, we perform such a comparison for both the sea ice area and volume fluxes from October 2016 to December 2021. Nares Strait provided the largest average seasonal (October through September) ice area flux of 95 ± 8 × 103 km2 followed by the CAA regions of the Queen Elizabeth Islands (QEI) at 41 ± 7 × 103 km2 and M’Clure Strait at 2 ± 8 × 103 km2 with corresponding ice volume fluxes of 177 ± 15 km3, 59 ± 10 km3, and 8 ± 8 km3, respectively. Larger Arctic Ocean ice export at Nares Strait was associated with a shorter ice arch duration (237 days) compared to M’Clure Strait (163 days) and QEI (65 days). Seasonal Arctic Ocean ice export was dominated by Nares Strait in 2017–2019 and 2021 but was remarkably exceeded by the QEI in 2020. Large-scale atmospheric circulation patterns were found to influence the ice area flux in the absence of ice arches but no occurrence of coherent Arctic Ocean ice export events coinciding across all gates were observed. Average net seasonal Arctic Ocean ice area and volume export were 138 × 103 km2 and 245 km3, which represent ∼16% of the area and ∼25% of the volume of sea ice export from Fram Strait. Divergent Arctic Ocean export ice trajectories are apparent for Nares Strait and the QEI when compared to Fram Strait

Water mass transformation in the Greenland Sea during the period 1986-2016

Hydrographic 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 ≡ 10^6 m^3 s^−1), although rates in excess of 1.5 Sv occurred in summers following winters with deep convection

What causes the location of the air-sea turbulent heat flux maximum over the Labrador Sea?

The Labrador Sea is a region of climatic importance as a result of the occurrence of oceanic wintertime convection, a process that is integral to the Atlantic Meridional Overturning Circulation. This process requires large air-sea heat fluxes that result in a loss of surface buoyancy, triggering convective overturning of the water column. The Labrador Sea wintertime turbulent heat flux maximum is situated downstream of the ice edge, a location previously thought to be causal. Here we show that there is considerable similarity in the characteristics of the regional mean atmospheric circulation and high heat flux events over the Labrador Sea during early winter, when the ice is situated to the north, and midwinter, when it is near the region of maximum heat loss. This suggests that other factors, including the topography of the nearby upstream and downstream landmasses, contribute to the location of the heat flux maximum

Irminger Sea deep convection injects oxygen and anthropogenic carbon to the ocean interior

Deep convection in the subpolar North Atlantic ventilates the ocean for atmospheric gases through the formation of deep water masses. Variability in the intensity of deep convection is believed to have caused large variations in North Atlantic anthropogenic carbon storage over the past decades, but observations of the properties during active convection are missing. Here we document the origin, extent and chemical properties of the deepest winter mixed layers directly observed in the Irminger Sea. As a result of the deep convection in winter 2014–2015, driven by large oceanic heat loss, mid-depth oxygen concentrations were replenished and anthropogenic carbon storage rates almost tripled compared with Irminger Sea hydrographic section data in 1997 and 2003. Our observations provide unequivocal evidence that ocean ventilation and anthropogenic carbon uptake take place in the Irminger Sea and that their efficiency can be directly linked to atmospheric forcing

Airborne measurements of a polar low over the Norwegian Sea

We will report an airborne measurement of a polar low over the Norwegian Sea on 9 March 1998 during the Arctic Airborne Measurement Program 1998 (AAMP 98). Global objective analysis suggests that the low formed due to an interaction between an upper level potential vorticity anomaly and a low-level baroclinic zone. Seven constant height transects in the zonal direction were made in the convective line clouds associated with the polar low. A strong horizontal temperature gradient in the zonal direction was observed in the lower boundary layer. Gale force northerly winds, in thermal wind balance with this baroclinity, were also observed in the lower atmosphere