Search Results

You are looking at 1 - 2 of 2 items for

  • Author or Editor: Steingrímur Jónsson x
  • All content x
Clear All Modify Search
Wilken-Jon von Appen, Dana Mastropole, Robert S. Pickart, HéÐinn Valdimarsson, Steingrímur Jónsson, and James B. Girton

Abstract

Time series data from a mooring in the center of Denmark Strait and a collection of shipboard hydrographic sections occupied across the sill are used to elucidate the mesoscale variability of the dense overflow water in the strait. Two dominant, reoccurring features are identified that are referred to as a bolus and a pulse. A bolus is a large, weakly stratified lens of overflow water associated with cyclonic rotation and a modest increase in along-stream speed of 0.1 m s−1. When a bolus passes through the strait the interface height of the overflow layer increases by 60 m, and the bottom temperature decreases by 0.4°C. By contrast, a pulse is characterized by anticyclonic rotation, a strong increase in along-stream speed of >0.25 m s−1, a decrease in interface height of 90 m, and no significant bottom temperature signal. It is estimated that, on average, boluses (pulses) pass through the strait every 3.4 (5.4) days with no seasonal signal to their frequency. Both features have the strongest along-stream signal in the overflow layer, while the strongest cross-stream velocities occur above the Denmark Strait overflow water (DSOW). In this sense neither feature can be characterized as a simple propagating eddy. Their dynamics appear to be similar to that ascribed to the mesoscale variability observed downstream in the deep western boundary current. Strong correlation of bottom temperatures between the mooring in Denmark Strait and a downstream array, together with a match in the frequency of occurrence of features at both locations, suggests a causal relationship between the mesoscale variability at the sill and that farther downstream.

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