Under-ice background noise and its relation with environmental forcing

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Bathymetry and topography of the Arctic Ocean

The Arctic Ocean is the smallest of the word’s five ocean regions, with approximative total area of 9.4×106 km2 [Jones01] and covers ∼3% of the Earth’s total surface area. It en-compasses four basins in the Central Arctic (Canadian Basin, Makarov Basin, Amundsen Basin, and Nansen Basin) surrounded by seven seas (Beaufort, Chukchi, Siberian, Laptev, Kara, Bar-ents, Greenland-Norway seas) [Jakobsson02, Dickson08]. The Arctic Ocean is also the shallowest ocean region with an mean depth of 1800 m, where shelf seas with a depth below 100 m, take up about one-third of the ocean area [Thomas10]. The Arctic is connected to the Pacific Ocean through the Bering Strait and the Bering Sea. For the Atlantic Ocean, two gateways connect it to the the Arctic Ocean. There are the Greenland-Norwegian seas through Fram Strait and the Barents Sea in the East (Fig. 2.1) [Jones01, Zhang03] and the Baffin Bay in the Southeast through a network of shallow channels in the Canadian Arctic Archipelago. The shallow channels network consists of ∼16 major passages that vary from 10 to 120 km in width and from a few meters to more than 700 m in depth [Archambault10, Spielhagen12]. The Fram Strait, ∼500 km wide, is the only deep–water passage with a depth of >2500 m while the Barents Sea is a shallow seaway (50–500 m) [Spielhagen12]. The Arctic Ocean presents the particularity of being covered with ice of variable thickness and age most of the year [Marsan04, Maslanik11, Marsan12, Stroeve12b]. In mid-winter (Febru-ary/March) at its maximum extent, sea ice covers the entire Arctic Ocean extending from the North Pole to about 44°N in the Sea of Japan [Thomas10]. The minimum sea ice cover extent is observed in September [Thomas10, Stroeve12b, Wadhams12]. The seasonal sea ice cover in the Arctic then varies from ∼15.5×106 km2 in the northern winter decreasing to ∼6×106 km2 in the summer [Thomas10]. We then distinguish the multi-year ice (MYI) that covers permanently the central arctic and the first-year ice (FYI) which is the new formed ice covering the surrounding seas.

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 cir-culation, especially through its exchanges with the North Atlantic. Dieckmann and Hellmer [Dieckmann10] summarizes these exchanges as follows: “In the Central Arctic Ocean, convec-tion 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.

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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].

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 Données acoustiques et vitesse des courants marins Les données environnementales
1.3.3 Synoptique du traitement des données
1.3.4 Résumé des résultats Cycle annuel du bruit ambiant 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 Landscapes and marine ecosystems Societal issues 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 The data set 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 The first component: Ambient noise in [10 Hz-500 Hz] frequency band 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 Sea ice acoustic observation
6.2.2 Impact of human activities in the Arctic
A Appendix: Shipping noise


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