Elsevier

Progress in Oceanography

Volume 90, Issues 1–4, July–September 2011, Pages 62-89
Progress in Oceanography

Closing the loop – Approaches to monitoring the state of the Arctic Mediterranean during the International Polar Year 2007–2008

https://doi.org/10.1016/j.pocean.2011.02.010Get rights and content

Abstract

During the 4th International Polar Year 2007–2009 (IPY), it has become increasingly obvious that we need to prepare for a new era in the Arctic. IPY occurred during the time of the largest retreat of Arctic sea ice since satellite observations started in 1979. This minimum in September sea ice coverage was accompanied by other signs of a changing Arctic, including the unexpectedly rapid transpolar drift of the Tara schooner, a general thinning of Arctic sea ice and a double-dip minimum of the Arctic Oscillation at the end of 2009. Thanks to the lucky timing of the IPY, those recent phenomena are well documented as they have been scrutinized by the international research community, taking advantage of the dedicated observing systems that were deployed during IPY. However, understanding changes in the Arctic System likely requires monitoring over decades, not years. Many IPY projects have contributed to the pilot phase of a future, sustained, observing system for the Arctic. We now know that many of the technical challenges can be overcome.

The Norwegian projects iAOOS-Norway, POLEWARD and MEOP were significant ocean monitoring/research contributions during the IPY. A large variety of techniques were used in these programs, ranging from oceanographic cruises to animal-borne platforms, autonomous gliders, helicopter surveys, surface drifters and current meter arrays. Our research approach was interdisciplinary from the outset, merging ocean dynamics, hydrography, biology, sea ice studies, as well as forecasting. The datasets are tremendously rich, and they will surely yield numerous findings in the years to come. Here, we present a status report at the end of the official period for IPY. Highlights of the research include: a quantification of the Meridional Overturning Circulation in the Nordic Seas (“the loop”) in thermal space, based on a set of up to 15-year-long series of current measurements; a detailed map of the surface circulation as well as characterization of eddy dispersion based on drifter data; transport monitoring of Atlantic Water using gliders; a view of the water mass exchanges in the Norwegian Atlantic Current from both Eulerian and Lagrangian data; an integrated physical–biological view of the ice-influenced ecosystem in the East Greenland Current, showing for instance nutrient-limited primary production as a consequence of decreasing ice cover for larger regions of the Arctic Ocean. Our sea ice studies show that the albedo of snow on ice is lower when snow cover is thinner and suggest that reductions in sea ice thickness, without changes in sea ice extent, will have a significant impact on the arctic atmosphere. We present up-to-date freshwater transport numbers for the East Greenland Current in the Fram Strait, as well as the first map of the annual cycle of freshwater layer thickness in the East Greenland Current along the east coast of Greenland, from data obtained by CTDs mounted on seals that traveled back and forth across the Nordic Seas. We have taken advantage of the real-time transmission of some of these platforms and demonstrate the use of ice-tethered profilers in validating satellite products of sea ice motion, as well as the use of Seagliders in validating ocean forecasts, and we present a sea ice drift product – significantly improved both in space and time – for use in operational ice-forecasting applications.

We consider real-time acquisition of data from the ocean interior to be a vital component of a sustained Arctic Ocean Observing System, and we conclude by presenting an outline for an observing system for the European sector of the Arctic Ocean.

Introduction

iAOOS (integrated Arctic Ocean Observing System) is a research-based step toward creating a sustainable ocean observing system in the Arctic. This initiative is led by the Arctic Ocean Sciences Board. It attempts to coordinate and optimize ongoing observation efforts at high latitudes in the North.

iAOOS-Norway: Closing the loop is the Norwegian contribution to iAOOS, funded by the Norwegian Research Council for the years 2007–2010. It aims to coordinate, modernize, and utilize ongoing Norwegian monitoring of the Arctic Mediterranean (the Arctic Ocean and the Barents, Greenland, Iceland and Norwegian Seas). Our main objective is to contribute to a modern operational observing system for the Arctic, with particular emphasis on meeting society’s needs for knowledge and information about, and safety in, the High North. We have performed process studies (biospheric, oceanographic and cryospheric), maintained and improved long-term monitoring sites, and tried to reduce the temporal lag between obtaining new research results and applying them in operational public services providing forecasts of arctic conditions.

“The loop” refers to the large-scale ocean circulation that transports warm water poleward from the equator. In the Atlantic, this circulation is primarily a vertical overturning structure (thus, it is commonly referred to as the “Meridional Overturning Circulation”, the MOC). In the Nordic Seas and the Arctic Ocean, on the other hand, the circulation becomes more of a horizontal loop: warm waters flow northward in the east, and cold, dense waters flow towards the Atlantic in the west. The loop is at its narrowest in the Nordic Seas and broadens out in the Arctic. Norwegian contributions to monitoring this system will naturally be focused primarily on the Nordic Seas, thus our project subtitle is “closing the loop”.

In this paper, we touch upon a broad range of activities carried out and results obtained in iAOOS-Norway, and in the simultaneous Norwegian IPY projects POLEWARD (surface drifters) and MEOP (animal-borne platforms). To serve the purpose of providing an overview of the state of the Nordic Seas, we also include many contributions from outside these three projects, this being reflected in the long list of authors. The paper is arranged as follows: in Section 2 we provide an overview of the methods and tools used; in Section 3 we discuss “the loop”, its forcing mechanisms, its strength, its structure and its hydrographic properties; in Section 4 we present sensitivity and process studies of sea ice; in Section 5 we use the contrasting Arctic and Atlantic biological regimes to investigate possible ecosystem changes in a warming climate. In Section 6, we show how IPY data are used to improve operational ocean and ice forecasts through improving data products, model validation and data assimilation. The paper ends with a discussion of ocean observation systems in Section 7. Note that iAOOS-Norway and the other IPY projects are still in progress and that not all results are published yet. We therefore list in Table A1 the contact information of the investigators responsible for the individual research topics.

Section snippets

Tools

The iAOOS-Norway sampling region covers both open ocean and ice-covered seas. It covers the in- and outflows of the Arctic Ocean as defined by the boundary currents of the region: the Norwegian Atlantic Current (splitting into the West Spitsbergen Current and the Barents Sea inflow), and the East Greenland Current (Fig. 1). Information about key state variables of the boundary currents has been collected using both conventional methods (moored instruments yielding high-resolution time series of

The loop

Ocean circulation in the Nordic Seas and the Arctic Ocean is dominated by the large-scale MOC. The Norwegian Atlantic Current carries warm water northwards in the Nordic Seas whereafter dense overflows across the Greenland–Scotland Ridge return it to the North Atlantic. The northernmost limb of this overturning circulation extends throughout the Arctic Ocean itself.

The MOC is made possible by the large north–south temperature gradient imposed by solar forcing, by the large scale wind fields,

Sea ice

Arctic sea ice has undergone rapid changes in recent years, resulting in a dramatic reduction in summer sea ice extent (e.g. Stroeve et al., 2007), a significant thinning of the remaining ice (Giles et al., 2008), and a change of the predominant ice type from multi-year ice (MYI) to first-year ice (FYI; Maslanik et al., 2007, Nghiem et al., 2007). FYI tends to be thinner and more saline than MYI with different surface characteristics that lead to larger, shallower melt ponds, which affect the

Biology

The tremendous physical contrasts across the Fram Strait, with warm Atlantic Water on the eastern side and cold ice-covered arctic water masses on the western part of the Strait, set the scene for the biological productivity and community structure. Primary productivity – and with it community structure and ecosystem functioning – in arctic marine systems is strongly related to the presence of ice, modifying the seasonal light cycle, as well as the supply of nutrients at the onset of and

Forecasting ocean and ice conditions in the High North

One aspect of public service has received direct attention in iAOOS-Norway, namely short-term ocean and ice forecasting. Activities at sea in northern regions depend not only on reliable weather forecasts, but also on good forecasts of the sea ice movement and ocean currents. Safe navigation, safe operations, and successful rescue operations depend crucially on good forecasts of all these three regimes. We have therefore made a particular effort in iAOOS-Norway to ensure that the public

Concluding remarks

This paper has presented a status report of the research activities of the Norwegian IPY projects iAOOS-Norway, POLEWARD and MEOP at the official end of the IPY. Highlights of the research include: a quantification of the Meridional Overturning Circulation in the Nordic Seas (“the loop”) in thermal space, based on a set of up to 15-year-long series of current measurements; a detailed map of the surface circulation as well as characterization of the eddy dispersion based on drifter data;

Acknowledgements

The iAOOS-Norway, POLEWARD and MEOP teams would like to thank the Norwegian Research Council for its support through the International Polar Year and NORKLIMA programs (grant numbers 176096/S30, 178559/S30 and 176477). The projects are described on the web at www.iaoos.no and www.meop.no. We would like to thank our home institutes and universities (the Norwegian Meteorological Institute, the Norwegian Polar Institute, the Institute of Marine Research, the University Centre in Svalbard, and the

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