Search Results

You are looking at 1 - 4 of 4 items for

  • Author or Editor: Kjetil Våge x
  • All content x
Clear All Modify Search
Ailin Brakstad, Kjetil Våge, Lisbeth Håvik, and G. W. K. Moore

Abstract

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 ≡ 106 m3 s−1), although rates in excess of 1.5 Sv occurred in summers following winters with deep convection.

Open access
Lisbeth Håvik, Mattia Almansi, Kjetil Våge, and Thomas W. N. Haine

Abstract

Dense water masses transported southward along the east coast of Greenland in the East Greenland Current (EGC) form the largest contribution to the Denmark Strait Overflow. When exiting Denmark Strait these dense water masses sink to depth and feed the deep circulation in the North Atlantic. Based on one year of mooring observations upstream of Denmark Strait and historical hydrographic profiles between Fram Strait and Denmark Strait, we find that a large part (75%) of the overflow water (σθ ≥ 27.8 kg m−3) transported by the EGC is of Atlantic origin (potential temperature θ > 0°C). The along-stream changes in temperature of the Atlantic-origin Water are moderate north of 69°N at the northern end of Blosseville basin, but southward from this point the temperature decreases more rapidly. We hypothesize that this enhanced modification is related to the bifurcation of the EGC taking place close to 69°N into the shelfbreak EGC and the separated EGC. This is associated with enhanced eddy activity and strong water mass modification reducing the intermediate temperature and salinity maxima of the Atlantic-origin Water. During periods with a large (small) degree of modification the separated current is strong (weak). Output from a high-resolution numerical model supports our hypothesis and reveals that large eddy activity is associated with an offshore shift of the surface freshwater layer that characterizes the Greenland shelf. The intensity of the eddy activity regulates the density and the hydrographic properties of the Denmark Strait Overflow Water transported by the EGC system.

Open access
Kjetil Våge, Robert S. Pickart, G. W. K. Moore, and Mads Hvid Ribergaard

Abstract

The impact of the Greenland tip jet on the wintertime mixed layer of the southwest Irminger Sea is investigated using in situ moored profiler data and a variety of atmospheric datasets. The mixed layer was observed to reach 400 m in the spring of 2003 and 300 m in the spring of 2004. Both of these winters were mild and characterized by a low North Atlantic Oscillation (NAO) index. A typical tip jet event is associated with a low pressure system that is advected by upper-level steering currents into the region east of Cape Farewell and interacts with the high topography of southern Greenland. Heat flux time series for the mooring site were constructed that include the enhancing influence of the tip jet events. This was used to force a one-dimensional mixed layer model, which was able to reproduce the observed envelope of mixed layer deepening in both winters. The deeper mixed layer of the first winter was largely due to a higher number of robust tip jet events, which in turn was caused by the steering currents focusing more storms adjacent to southern Greenland. Application of the mixed layer model to the winter of 1994–95, a period characterized by a high-NAO index, resulted in convection exceeding 1700 m. This prediction is consistent with hydrographic data collected in summer 1995, supporting the notion that deep convection can occur in the Irminger Sea during strong winters.

Full access
Stefanie Semper, Kjetil Våge, Robert S. Pickart, Héðinn Valdimarsson, Daniel J. Torres, and Steingrímur Jónsson

Abstract

The North Icelandic Jet (NIJ) is an important source of dense water to the overflow plume passing through Denmark Strait. The properties, structure, and transport of the NIJ are investigated for the first time along its entire pathway following the continental slope north of Iceland, using 13 hydrographic/velocity surveys of high spatial resolution conducted between 2004 and 2018. The comprehensive dataset reveals that the current originates northeast of Iceland and increases in volume transport by roughly 0.4 Sv (1 Sv ≡ 106 m3 s−1) per 100 km until 300 km upstream of Denmark Strait, at which point the highest transport is reached. The bulk of the NIJ transport is confined to a small area in Θ–S space centered near −0.29° ± 0.16°C in Conservative Temperature and 35.075 ± 0.006 g kg−1 in Absolute Salinity. While the hydrographic properties of this transport mode are not significantly modified along the NIJ’s pathway, the transport estimates vary considerably between and within the surveys. Neither a clear seasonal signal nor a consistent link to atmospheric forcing was found, but barotropic and/or baroclinic instability is likely active in the current. The NIJ displays a double-core structure in roughly 50% of the occupations, with the two cores centered at the 600- and 800-m isobaths, respectively. The transport of overflow water 300 km upstream of Denmark Strait exceeds 1.8 ± 0.3 Sv, which is substantially larger than estimates from a year-long mooring array and hydrographic/velocity surveys closer to the strait, where the NIJ merges with the separated East Greenland Current. This implies a more substantial contribution of the NIJ to the Denmark Strait overflow plume than previously envisaged.

Open access