One of the key open issues regarding the climate system is how changes in atmospheric and freshwater forcing in the Northern Hemisphere will influence the large scale ocean circulation often referred to as the Atlantic Meridional Overturning Circulation. In particular, the Nordic Seas are a vital region for the circulation of the North Atlantic and for the moderation of our climate. Warm, surface sub-tropical waters flow northward in the Atlantic Ocean and pass into the Nordic Seas where their properties are modified by intense air-sea fluxes and they release heat to the atmosphere before returning south at depth as dense water that spills through the gaps in the ridge that runs between Greenland and Scotland. This process regulates our climate by providing a mechanism for the northward flow of heat in the Atlantic Ocean. The largest and densest of the outflows from the Nordic Seas occurs through the Denmark Strait, located between Iceland and Greenland. This Denmark Strait Overflow Water (DSOW) accounts for roughly half of the total dense water feeding the deep ocean current flowing southward along the east coast of the U.S. The source waters and dynamics upstream of the Denmark Strait affect the downstream fate of the overflow and the sensitivity of the deepest limb of the Atlantic Meridional Overturning Circulation to changes in climate forcing. This study will significantly improve our understanding of the circulation north of Denmark Strait by identifying and quantifying the different pathways, how they interact with each other, and their underlying dynamics. Furthermore, the project will shed light on the nature and causes of the pronounced variability of the DSOW at the sill. This has been observed in many previous studies but is yet to be fully explained using observational data. The international collaborators will contribute additional observations they collected in the Nordic Seas to the project. All of the mooring data used in the study (including those provided by the European collaborators) will be archived in a publicly accessible data portal. An outreach program will be carried out to share the results in the national media and to create science lesson content appropriate for public schools. The study will be led by a post-doctoral investigator.
The aim of the proposed project is to quantify the upstream sources of the Denmark Strait Overflow Water (DSOW) and relate the variability in the different sources to that observed at the sill. To accomplish this, the investigators will use time series data provided by four different institutions who together deployed 12 moorings spanning the strait 200 kilometer north of the sill from August 2011 to August 2012, plus a mooring at the sill itself. While parts of the data have been analyzed previously by the individual investigators, never before have all of the data been considered in tandem. The first objective is to quantify a newly discovered pathway of dense water in the central portion of the strait, known as the Separated East Greenland Current. This pathway was recently revealed in a small number of shipboard crossings and is thought to be a free-jet located between the boundary current on the Greenland slope (the Shelfbreak East Greenland Current) and the Iceland slope (the North Icelandic Jet). The mooring data will be used to quantify the structure, transport, and dynamics of this newly recognized feature over a year-long period. Following this, the second objective is to produce the first strait-wide picture of the time-varying pathways carrying dense water into the Denmark Strait, and how the transport is partitioned between the three overflow pathways. Once this is established, the third objective is to elucidate the dynamics of the combined upstream circulation system and assess how this impacts the transport and hydrographic variability at the sill. The aim is to determine if the ubiquitous synoptic scale variability of the DSOW at the sill is driven by upstream fluctuations in any of the pathways, and/or if hydraulic processes at the sill influence the upstream circulation. The final objective of the study is to devise a sparse mooring array that could be implemented in the future to optimally monitor the upstream sources of DSOW over interannual to decadal time scales.
