Acoustic transient signal from sea ice deformation in Eastern Beaufort Sea 

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Arctic Ocean circulation

The Arctic Ocean (Fig. 2.1) is an enclosed ocean. The upper layer circulation in each ocean basin is driven by the large-scale distribution of wind stress [Siedler01]. The Arctic circulation is structured within two main circulation patterns: the Beaufort Gyre (BG) and the transpolar drift (TPD) (Fig. 2.2). The BG, a clockwise circulation centered at ∼(80 °N, 155 °W) [Barry93] constitutes the main circulation pattern in the Canadian Basin [Stein88, Asplin09, Lukovich09]. It contains approximately 45,000 km3 of fresh water in the upper layer, “ a volume 10-15 times larger than the total annual river runoff to the Arctic Ocean, and larger than the amount of fresh water stored in the sea ice” as mentioned by Proshutinsky et. al. [Proshutinsky02, Proshutinsky05]. The fresh water in the BG is separated from a more saline upper layer found in the Eurasian Basin (Nansen and Amundsen basins) by a primary front defining the baroclinic structure of the TPD [Dickson08].
During the winter, except the landfast ice, the Arctic Sea ice cover is in perpetual motion under five main forcings: wind, currents, Coriolis force, internal stresses, and sea ice surface tilt [Thorndike82, Barry93, Rotschky11]. The TPD, the second circulation pattern in the Arctic, tends to pile up the multi year ice (MYI) against the Canadian Arctic Archipelago (CAA) and the Greenland coasts, resulting in the thickest ice in this region of the Arctic Ocean [Barry93]. The Arctic ice is exported to the Atlantic through the Fram Strait (the main discharge gate), in the East coast of Greenland [Kwok09a, Smedsrud11, Tsukernik10]. In the CAA, the sea ice is emptying through the Straits of Amundsen Gulf, McClure and Queen Elizabeth Islands [Kwok06, Spielhagen12].

Arctic Ocean in the global system

The deep water circulation, called the global conveyor belt or thermohaline circulation [Siedler01], involves more complex processes over the global ocean. The deep and bottom waters produced by the polar oceans form part of this global thermohaline circulation [Thomas10]. The ice-ocean exchanges of heat and salt/freshwater appear to be crucial for this deep water formation [Goosse99]. Indeed, cold dense waters resulting from the combined effects of intense heat losses from the ocean surface to the atmosphere and salt rejection from the formation of sea ice, convect to form the deep and bottom waters of the global ocean [Grassl01] (Fig. 2.2).
According to Hartmut [Grassl01] “these water masses then spread to fill the deep ocean and volume conservation requires surface waters to flow poleward into these regions to replace this spreading deep water.”
The Arctic Ocean than plays an important role in the Global Ocean thermohaline circulation, especially through its exchanges with the North Atlantic. Dieckmann and Hellmer [Dieckmann10] summarizes these exchanges as follows: “In the Central Arctic Ocean, convection is restricted to the upper 50–100 m due to the strong stratification of the water column, the deeper layers being renewed by advection of water masses of Atlantic origin entering through the Fram Strait and across the Barents Sea”. The strength of the overturning circulation is related to this convective activity in the deep-water formation regions, most notably the Labrador Sea [Dickson08] which the variability is sustained by an interplay between the storage and release of freshwater from the central Arctic.

Arctic Ocean in the global warming context

The earth’s near-surface temperature has increased by a linear trend of 0.74°C on average over the 100-years between 1906 and 2005 [IPCC07]. The rate of warming since 1976 was the most important during the last 1000 years [IPCC01]. The IPCC 2007s report also mentioned that 11 of the 12 years between 1995 and 2006 are among the 12 warmest years observed since 1850. The Arctic regions temperature has increased twice than almost any other region of the globe [IPCC07, Stroeve12a].
In the 1950s, the Arctic Ocean was sea ice covered all through the year [Langehaug13]. Sea ice controls, but is also controlled by the fluxes of heat, moisture and momentum across the ocean–atmosphere interface [Thomas10]. Indeed, the ice cover reflects 50–80% of the sun’s energy, contributing to maintain the high latitudes cold [Stroeve12a]. Because it is relatively thin, sea ice is vulnerable to small perturbations within the ocean and/or the atmosphere and changes of sea ice coverage are commonly taken as an indicator for climate change monitoring [Haas10a]. Since 1979, the Arctic summer sea ice extent, often defined as the area with ice concentration ≥15% in a grid cell [Wang12], has declined at a rate of >11% per decade [Kattsov10]. The Arctic also experienced in the last decades, a drastic decline of its ice thickness. In the central Arctic, the sea ice has lost 40% of its winter thickness within the last 30 years, from 3.6 m in the 1980s to 1.8 m in 2008 [Kwok09b].
The climates models projection suggest that the Arctic could be sea ice free during summer season in the current century, between 2040 and 2100 [Stroeve12b, Wadhams12]. Because of the complex interactions between ice, ocean, atmosphere and Earth climate, the sea ice is a major scientific concern of this century. The consequences of a complete melting of the arctic ice are still unknown. However, some scientific evidences of the global warming at all levels in the boreal regions are already occurring, sometimes more rapid than model predictions [Walther02, Parmesan06, Soja07, Serreze07b].

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Landscapes and marine ecosystems

The most visible impact of climate change is the continuous decrease in the ice cover and the reduction of snow in the Polar Regions. These changes in the recent years are accompanied by major changes in all aspects of the arctic regions, whose consequences are still poorly understood. Soja et al. [Soja07] noted that some scientific evidence of the transformation of landscapes due to changes in climate is mounting throughout the circum boreal zone in Alaska, Canada and Russia. The major impact and the most widespread in arctic land is the change of permafrost occurrence and distribution [Hinzman05]. Permafrost is perennially frozen ground which underlies 20–25% of the exposed land surface of the earth in regions with cold climates [Serreze00]. In the Arctic regions, the temperatures at the top layer of the permafrost have increased by up to 3°C since the 1980s [IPCC07].
Permafrost controls the local hydrography processes, as it prevents surface water from infiltrating to deeper groundwater zones, causing surface soils to be very wet [Hinzman05]. Permafrost also plays an important role in the sequestration of the carbon and the methane production [Jorgenson01]. Permafrost response to global warming depends on complex biophysical factors (e.g. thawing, soil, vegetation, fire), making its predictions difficult [Jorgenson10]. However, the disappearance of permafrost would result in soil drying accompanied by ecosystems reorganization, high fire frequency, and impact on latent and sensible heat fluxes [Hinzman05, Walther02, Soja07, Henry12]. Important shifts in vegetation at the regional scale were already found in the Arctic regions, indicating an increase in plant growth [Serreze00, Henry12]. The alteration of such ecosystem moisture could result in the modification of population abundance, persistence, and distribution of the arctic terrestrial animals [Henry12, Hinzman05].

Table of contents :

1 Résumé 
1.1 L’Arctique et les changements climatiques
1.1.1 L’océan Arctique
1.1.2 Les changements climatiques dans l’Arctique
1.2 Observation de l’environnement par acoustique passive (PAM)
1.2.1 Les paysages acoustiques sous-marins
1.2.2 Contexte de l’Arctique canadien
1.3 Positionnement de la thèse
1.3.1 Objectifs
1.3.2 Matériels et méthodologie
1.3.2.1 Données acoustiques et vitesse des courants marins
1.3.2.2 Les données environnementales
1.3.3 Synoptique du traitement des données
1.3.4 Résumé des résultats
1.3.4.1 Cycle annuel du bruit ambiant
1.3.4.2 Les transitoires acoustiques
1.3.5 Conclusions
1.3.6 Perspectives
1.4 Production scientifique
2 Introduction 
2.1 The Arctic Ocean
2.1.1 Bathymetry and topography of the Arctic Ocean
2.1.2 Arctic Ocean circulation
2.1.3 Arctic Ocean in the global system
2.1.4 Arctic Ocean in the global warming context
2.1.4.1 Landscapes and marine ecosystems
2.1.4.2 Societal issues
2.1.4.3 Economic issues
2.2 Marine soundscape and passive acoustic monitoring (PAM)
2.2.1 Marine soundscape
2.2.2 Passive acoustic monitoring methodology
2.3 Canadian Arctic monitoring
2.3.1 PAM in the Canadian Arctic context
2.4 Objectives of the thesis
2.4.1 Objectives
2.4.2 Outlines of the thesis
3 Arctic Ocean noise: background 
3.1 Under ice Ocean noise sources
3.1.1 The thermal cracking
3.1.2 Wind-generated noise
3.1.3 Noise from the mechanical behavior of the ice cover
3.2 Environmental correlate, space and time scale
3.3 Ambient noise in the Marginal Ice Zone (MIZ)
4 Arctic pristine underwater soundscape 
4.1 Background noise estimation
4.1.1 Robust noise estimation
4.1.2 Non–acoustic noise cancellation
4.2 Increase of Arctic ocean noise from winter sea ice melting alone
4.2.1 Context
4.2.2 Experiment and data analysis
4.2.3 Results
4.2.4 Discussion
4.3 Under-ice background noise and its relation with environmental forcing
4.3.1 Context
4.3.2 Material and methods
4.3.2.1 The data set
4.3.2.2 Data analysis
4.3.3 Results
4.3.4 Discussion
4.4 Conclusion
5 Acoustic transient events under Arctic ice cap 
5.1 Arctic ice cap
5.1.1 The arctic sea ice decline
5.1.2 Mechanical behavior of the Arctic ice cape
5.2 Acoustic transient signal from sea ice deformation in Eastern Beaufort Sea
5.2.1 Context
5.2.2 Numerical analysis
5.3 Results
5.3.1 Class 1: wideband transient
5.3.2 Class 2: pure tone modulation transients
5.3.3 Class 3: High frequency noise transient
5.3.4 Transient sounds and leads opening
5.3.5 Environmental correlates of under-ice transients
5.4 Conclusion
6 Conclusions and perspectives 
6.1 Conclusions
6.1.1 Methods and tools provided
6.1.2 Properties of the Canadian Arctic soundscapes
6.1.2.1 The first component: Ambient noise in [10 Hz-500 Hz] frequency band
6.1.2.2 Second component: Transient signal generated by the sea ice
6.1.3 Limits of our work
6.1.4 How passive acoustics can be complementary with other studies and other means of observation
6.2 Perspectives
6.2.1 SAea ice acoustic observation
6.2.2 Impact of human activities in the Arctic
Appendix: Shipping noise

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