Research

Introduction

Human activities have been transforming the face of the planet for thousands of years. While life on Earth has adjusted to what we perceive as the benign and stable climate of the Holocene, a threat to that stability has emerged as the result of human activities over the past few centuries since the start of the Industrial Revolution. The planet is now warming as a result of anthropogenic greenhouse gas emissions. The prospects for future climate are troubling, as climate change is predicted to change radically the environment in which we live. Global mean temperature is predicted to increase and sea level to rise; and there are serious likely consequences for habitability of coastal zones, human health, and biodiversity.

A recent summary of the global implications of Arctic climate feedbacks[1] identifies four broad groups of impacts. Two of these we do not consider here: the impact of warming on sea level rise via increased ice loss from the Greenland ice sheet, and on marine and terrestrial carbon cycles and reservoirs. The third is the albedo feedback, which is expected to contribute in future to reduced sea ice cover, reduced snow cover and permafrost degradation on land, and to changed weather patterns. The fourth is the ocean thermohaline (THC) circulation feedback, in which the effects of warming on dense water production alter both Arctic and global ocean circulation.

Arctic temperature is increasing due to ice-albedo feedback, the first signs of which have already been identified[2]. Observations show a continuing reduction in sea ice cover, leading to prediction of a seasonally sea-ice-free Arctic Ocean by the end of the century or sooner[3,4,5]. Considerable progress has been made in the past decade on albedo feedback and its consequence[eg. 6], and there are many active research topics in this area (eg. cloud convective feedback[7]).

Hence the most significant unresolved impact on the Earth’s climate, in particular on the UK and the rest of north-western Europe, of Arctic warming will be through the potential for increased Arctic freshwater export, leading to perturbation of the oceanic thermohaline circulation and subsequent regional cooling. Recent evidence shows that the storage of freshwater is closely linked to the wind-driven circulation, with recent increases in freshwater storage attributed to the spin-up of the Arctic Ocean[8,9,10].

However our ability to represent the Arctic ocean, sea ice and climate system in models depends on understanding in detail the processes of surface momentum transfer, formation and fate of dense water, diapycnal and isopycnal mixing, and net ocean fluxes of heat and freshwater. We aim in this proposal to use new (and historical) pan-Arctic observations to address these deficiencies directly. The use of model simulations in understanding these changes is compromised by the inability of both ocean-ice general circulation models and coupled climate models to reproduce accurately important features of the Arctic ocean and sea ice system, and wider impacts on the Earth’s climate.

We aim through the use of models to examine the impact of these processes on the Arctic atmosphere and climate, and to a future Arctic climate with reduced sea ice. We recognize that there are important uncertainties that govern the Arctic atmosphere itself. Nonetheless it is in the ocean that the greatest uncertainties lie.

 



Figure 1 (a): Observed Arctic Dynamic Ocean Topography (DOT) in 2010 from the CryoSat and Envisat radar altimeters. The main features of the Arctic circulation, including the Beaufort Gyre, Transpolar Drift and the east Greenland current are clearly visible. The DOT is calculated by subtracting the EGM-08 geoid from a combined CryoSat and Envisat mean sea surface. The CryoSat data are preliminary and show some artifacts at small scale above 81.5oN (the latitudinal limit of Envisat). (b): Mean sea surface height (SSH) from the NEMO-CICE ice-ocean GCM for March 1979, global mean resolution 1/12o, local Arctic resolution ca. 1/30o. Allowing for the different durations and time periods, the similarities are remarkable: eg. the SSH and DOT differences between the Beaufort Gyre and Nansen / Amundsen Basins is ~0.5 m in both.

Aims

  1. To improve understanding of contemporary Arctic ocean and sea ice variability & trends
  2. To reduce uncertainty of future (seasonal–decadal) predictions of Arctic climate

Objectives

  1. Identification & quantification of critical present-day Arctic ocean & sea ice processes, specifically issues related to momentum transfer, dense water formation, mixing, and heat and freshwater fluxes
  2. Diagnosis of present & prognosis of future Arctic ocean and sea ice circulation, fluxes, & dynamics, using a hierarchy of inverse, idealized, GCM and coupled climate models, tested with new observations
  3. Evaluation of future Arctic regional climate & wider consequences of change, using a coupled IPCC-class climate model, taking into account the improved knowledge and understanding of the consequences of sea ice reduction on Arctic ocean circulation and fluxes.

Methodology & Approach

To achieve the project objectives, we propose a linked suite of studies employing new and existing measurements, process models and coupled climate models. To evaluate, improve and validate the representations of key processes in models related to momentum, energy and freshwater exchanges (Objective 1) requires a combination of new observations with the development of new inverse models and parameterisations. Our programme will employ new (and historical) satellite and in-situ observations to provide the quantitative constraints necessary to produce new and accurate parameterizations of surface momentum flux and turbulence.

To estimate seasonally-resolved boundary fluxes requires the integration of in-situ and remote-sensed measurements using an inverse model. The Arctic ocean and sea ice system is complex, and the result of interaction between mechanical and buoyancy fluxes, local and remote forcing, and topography.

To isolate the roles of individual processes in controlling Arctic ocean heat and freshwater fluxes, both in present and future (ice-free) conditions (Objective 2), we will conduct a series of idealized model experiments, incorporating new parameterisations.

To improve predictions of future climate using a coupled climate first requires evaluation of the climate model under present-day conditions. We will test an IPCC-class climate model (Objective 2) against both the new observations and the results of the idealized model experiments to determine the model’s skill in reproducing present-day Arctic ocean-ice system. We will then conduct a series of experiments to determine the future impact of the sea ice retreat and increases in momentum and freshwater fluxes on Arctic and regional climate (Objective 3).

References

  1. M. Sommerkorn, S. J. Hassol, Arctic Climate Feedbacks: Global Implications. (WWF International Arctic Programme, Oslo, 2009).
  2. M. C. Serreze, A. P. Barrett, J. C. Stroeve, D. N. Kindig, M. M. Holland, The Cryosphere 3, 11 (2010).
  3. IPCC, “Climate Change 2007: The Physical Science Basis - Summary for Policymakers” (2007).
  4. J. Stroeve, M. M. Holland, W. Meier, T. Scambos, M. Serreze, Geophys Res Lett 34, (May 1, 2007).
  5. M. Y. Wang, J. E. Overland, Geophys Res Lett 36, (Apr 3, 2009).
  6. C. Deser, R. Tomas, M. Alexander, D. Lawrence, Journal of Climate 23, 333 (Jan, 2010).
  7. D. S. Abbot, C. C. Walker, E. Tziperman, Journal of Climate 22, 5719 (Nov 1, 2009).
  8. I. V. Polyakov et al., Journal of Climate 21, 364 (Jan 15, 2008).
  9. A. Y. Proshutinsky, M. A. Johnson, J. Geophys. Res.-Oceans 102, 12493 (Jun 15, 1997).
  10. A. Proshutinsky, R. H. Bourke, F. A. McLaughlin, Geophys Res Lett 29, (Dec 5, 2002).

 


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