Atmospheric Forcing of the Iceland Sea

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.

The main impacts from this project will be:

(1) Improved weather forecasts for Iceland, the Iceland Sea and the North Atlantic region by the inclusion of additional radiosonde soundings into operational forecasting systems. This impact will be for the period of the field campaign and will be on all global forecasting agencies and relevant regional agencies. Beneficiaries of this impact will include residents of Iceland and other forecast users in this region.

(2) An assessment of meteorological analyses and reanalyses products for the subpolar North Atlantic specifically, but with relevance to climatologically similar locations too; for example near marginal-ice-zones and steep mountains. Evaluating the quality of these products will have long-lasting benefits for all their users. Note these products are very widely used; for example, the paper describing the European Centre for Medium-range Weather Forecast's (ECMWF) ERA Interim product (Dee et al. 2011) has been cited over 2500/3600 times (Web of Science/Google Scholar), implying it has been used at least this many times - in all likelihood many more. We will establish any systematic biases or errors, and seek the cause and possible solutions to these. Consequently one potential impact would be an improvement in future reanalyses products (the PI has a track record of impact in this regard - see Pathways to Impact). A variety of analyses and reanalyses will be assessed, including the UK, Icelandic and ECMWF operational analyses and several reanalyses products including from the ECMWF and the Arctic System Reanalyses (ASR). It is planned for some of our project partners, namely the UK Met Office and the Icelandic Met Office, to be directly involved in this work. In addition we will exploit opportunities provide by the WMO's Polar Prediction Project to directly engage with other forecast providers.

(3) Improved numerical weather and climate prediction models, primarily through testing and development of parameterization schemes and model configurations. This will also be a long-lasting benefit to society as a whole through improved weather and climate prediction. Our project partners, the UK Met Office and the Icelandic Met Office have already been involved in developing some aspects of our plans, so are already engaged and will collaborate here. Particular areas of model deficiency are outlined in the Case, the Pathway to Impact and in project partner Letters of Support. These include poor representation of boundary-layer structure and mixed-phase cloud development over sea ice and during cold-air outbreaks. Note some parameterization deficiencies are also known weaknesses in many models, so there should be impact on other national and private meteorological service providers and numerical weather- or climate-prediction model development teams.

(4) Improved coupled forecasting systems via model development and testing against a unique coupled atmosphere-ocean observational data set for a critical region for the climate system. Many forecasting centres are currently developing or trialling coupled forecasting systems for the short-term (they are already in use for the few centres that carry out seasonal prediction) and this presents a host of new challenges, particularly around coupled model initialisation and the representation of sea ice. Our unique data set will be invaluable in testing these systems in the challenging environment of the subpolar seas. This data set is eagerly anticipated by the Met Office (see their Letter) and is likely to be sought after by the international community too. This impact will also lead to a long-lasting benefit to society as a whole through improved weather and climate prediction.

(5) The broader Iceland Sea Project provides an excellent outreach opportunity for us to engage with the public about the science we do. We plan to contribute to a multifaceted outreach programme being led by our US project partners.