The subpolar region of the North Atlantic is crucial for the global climate system. It is where coupled atmosphere-ocean processes, on a variety of spatial scales, require an integrated approach for their improved understanding and prediction. This region has enhanced ‘communication’ between the atmosphere and ocean.
Here large surface fluxes of heat and moisture make the surface waters colder, saltier and denser resulting in a convective overturning that contributes to the lower limb of the Atlantic Meridional Overturning Circulation (AMOC). The AMOC is an ocean circulation that carries warm water from the tropics northward with a return flow of cold water southwards at depth; it is instrumental in keeping Europe’s climate relatively mild. The Iceland Sea – to the north and east of Iceland – is arguably the least studied of the North Atlantic’s subpolar seas. However new discoveries are forcing a redesign of our conceptual model of the North Atlantic’s ocean circulation which places the Iceland Sea at the heart of this system and suggests that it requires urgent scientific focus. The recently discovered North Icelandic Jet is thought to be one of two pathways for dense water to pass through the Denmark Strait – the stretch of ocean between Iceland and Greenland – which is the main route for dense waters from the north to enter the Atlantic. Its discovery suggests a new paradigm for where dense water entering the North Atlantic originates. However at present the source of the North Icelandic Jet remains unknown. It is hypothesized that relatively warm Atlantic-origin water is modified into denser water in the Iceland Sea, although it is unclear precisely where, when or how this happens. We will test this hypothesis and investigate this new ocean circulation paradigm. We will examine wintertime atmosphere-ocean processes in the Iceland Sea by characterising its atmospheric forcing, i.e. observing the spatial structure and variability of surface heat, moisture and momentum fluxes in the region and the weather systems that dictate these fluxes. We will make in situ observations of air-sea interaction processes from several platforms (an aircraft; and via project partners an unmanned airborne vehicle, a meteorological buoy and a research vessel) and use these to evaluate meteorological analyses and reanalyses from operational weather forecasting centres. These meteorological analyses and reanalyses are a blend of observations and model output and represent the atmosphere as best we know it. We will carry out numerical modelling experiments to investigate the dynamics of selected weather systems which strongly influence the region, but appear not to be well represented; for example, the boundary layers that develop over transitions between sea ice and the open ocean during cold-air outbreaks; or the jets and wakes that occur downstream of Iceland. We will use our unique observations to improve model representation of these systems. We will also carry out new high-resolution climate simulations. A series of experiments will cover recent past and likely future situations; as well as some idealised situations such as no wintertime sea ice in the Iceland Sea region. We will use a state-of-the-art atmospheric model with high resolution over the Iceland Sea to investigate changes in the atmospheric circulation and surface fluxes. Finally, in collaboration with our international partners, we will analyse new ocean observations and establish which weather systems are important for changing ocean properties in this region. We will use a range of ocean and atmospheric models to establish how current and future ocean circulation pathways function. In short, we will determine the role that atmosphere-ocean processes in the Iceland Sea play in creating the dense waters that flow through Denmark Strait and feed into the lower limb of the AMOC.