The Arctic Ocean
Geography, Hydrography, Flora and Fauna, and Climate
The Arctic Ocean is a marginal sea (or small ocean) confined by the North American, European and Asian continents (Figure 1). Its only connection to the Pacific Ocean is a through the Bering Strait, located to the south of the Chukchi Sea, while the Arctic Ocean is connected to the Atlantic on both sides of Greenland. The deepest passage to the Atlantic is the Fram Strait, at approximately 80°N between Greenland and Svalbard (Figure 1).
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Figure 1. Bathymetric and topographic map of the Arctic (USGS), with important geographic and hydrographic features labeled. Red line represents hydrographic section from the Bering Strait to Svalbard (Figure 3), similar the the path of the 2015 U.S. Arctic Ocean Section (About the Cruise).
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The Arctic Ocean is divided into two major basins, separated by the Lomonosov Ridge, which extends from north of Greenland to the Asian continental shelf, nearly intersecting the North Pole (Figure 1). To the left of the Lomonosov Ridge in Figure 1 is the Canadian Basin, which is partitioned at around 85°N latitude by a bathymetric mound, or ridge, known as the Alpha Ridge. The Makarov Basin is to the north of the Alpha Ridge, while the Canada Basin is to its south, where much of the U.S. GEOTRACES Arctic Expedition (referred to here as the AOS or Arctic Ocean Section) will occur (About the Cruise). To the right of the Lomonosov Ridge in Figure 1 is the slightly deeper Eurasian Basin, which is also subdivided like the Canadian Basin, however, its sub-basins won't be described here since the AOS won't occupy the Eurasian Basin. Considering the Arctic Ocean as a whole, over half of its area consists of expansive continental shelves (Talley et al., 2011), which in many cases, act as boundaries for ocean currents in the region.
In the Arctic Ocean, along with all of Earth's oceans, there are surface currents (Figure 2), which transport water and ice around the Arctic, and the flow of these currents is not bounded by the ridges that exist at the seafloor. An important current, especially in the region where the AOS will be sampling, is the Bering Strait inflow, where Pacific water enters the Arctic Ocean (Figure 2). After this current enters, it hugs the Alaskan coast as it moves northeastward, becoming the Alaskan Coastal Current, which travels along the North American shelf before leaving the Arctic through the Canadian archipelago (Figure 2). Towards the interior of the Canadian Basin, in the region of the Arctic known as the Beaufort Sea, is the Beaufort Gyre (Figure 2), which is an anticyclonic (clockwise in the northern hemisphere) current. Just north of the Beaufort Gyre is one of the major Arctic currents, known as the Transpolar Drift, responsible for transporting water and ice from Russia to North America, approximately along the Lomonosov Ridge (Figure 2; Talley et al., 2011).
While ice drift differs somewhat from the surface circulation of water, the Beaufort Gyre and Transpolar Drift are both evident in ice movement, as depicted by ice age in Video 1. There are also intermediate and deep circulation features within the Arctic Ocean, however, they are not extensively included in this overview. Important features of the subsurface water that will be described are water masses (or layers) that form as a result either sea ice formation, melting, river input or evaporation. These layers exist within the interior of the ocean because temperature and salinity affect density, which results in water sinking to and residing at its neutral density. An example of this is that cold, salty water is much denser than warm, fresh water. If these two "water masses" are mixed together and their respective temperatures and salinities are conserved, the cold, salty water will sink below the warm, fresh water. This example is illustrated in Video 2, and can also be carried out at home using kitchen salt and food coloring.
The water masses of the Arctic Ocean are illustrated as sections of (A, B) potential temperature and (C, D) practical salinity in Figure 3, extending from the Bering Strait (on the left) to Svalbard (on the right). The section path is illustrated by the red line in Figure 1. Polar Surface Water (PSW) occupies the upper 200 m of the water column, which is comprised of other layers, generally having temperatures that range from -1.9 to -1.5°C and salinities from 28 to 33.5 (Figure 3A, C). The upper-most layer in the PSW is the Polar Mixed Layer (PML), which extends to about 50 m from the surface and is strongly influenced by the freezing and melting of ice, containing some of the freshest water in the Arctic Ocean (Figure 3C). Underneath the PML in the Canadian Basin is a local temperature maximum that extends to the north, composed of two unique water masses. One of these water masses is the Alaskan Coastal Water (ACW) at a depth range of 50-70 m, with temperatures that range from -1.2 to -1.1°C and salinities of 31-32 (Figure 3A, C), having similar properties as its neighboring Bering Strait Water (BSW). The BSW originates from Pacific water entering the Arctic Ocean through the Bering Strait, and this water can be identified its salinity that ranges from approximately 32 to 33 in the Canadian Basin (Figure 3C). The temperature of the BSW is strongly influenced by seasons, resulting in unique summer (sBSW) and winter (wBSW) features. The sBSW is the temperature maximum tongue that extends northward below the PML at about 100 m, while below it is the slightly colder wBSW, which composes the bottom portion of the PSW in the Canadian Basin (Figure 3A).
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Figure 2. Bathymetric and topographic map of the Arctic (USGS), with schematic of surface currents (adapted from Talley et al., 2011).
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Video 1. Arctic ice age, 1984-2016 (NASA), also depicting surface currents and circulation. The video also illustrates that Arctic sea ice age has been decreasing.
Video 2. A demonstration to illustrate the formation of oceanic water masses based on density, which is a function of temperature and salinity.
The water masses of the Arctic Ocean are illustrated as sections of (A, B) potential temperature and (C, D) practical salinity in Figure 3, extending from the Bering Strait (on the left) to Svalbard (on the right). The section path is illustrated by the red line in Figure 1. Polar Surface Water (PSW) occupies the upper 200 m of the water column, which is comprised of other layers, generally having temperatures that range from -1.9 to -1.5°C and salinities from 28 to 33.5 (Figure 3A, C). The upper-most layer in the PSW is the Polar Mixed Layer (PML), which extends to about 50 m from the surface and is strongly influenced by the freezing and melting of ice, containing some of the freshest water in the Arctic Ocean (Figure 3C). Underneath the PML in the Canadian Basin is a local temperature maximum that extends to the north, composed of two unique water masses. One of these water masses is the Alaskan Coastal Water (ACW) at a depth range of 50-70 m, with temperatures that range from -1.2 to -1.1°C and salinities of 31-32 (Figure 3A, C), having similar properties as its neighboring Bering Strait Water (BSW). The BSW originates from Pacific water entering the Arctic Ocean through the Bering Strait, and this water can be identified its salinity that ranges from approximately 32 to 33 in the Canadian Basin (Figure 3C). The temperature of the BSW is strongly influenced by seasons, resulting in unique summer (sBSW) and winter (wBSW) features. The sBSW is the temperature maximum tongue that extends northward below the PML at about 100 m, while below it is the slightly colder wBSW, which composes the bottom portion of the PSW in the Canadian Basin (Figure 3A).
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| Figure 3. Sections of (A, B) potential temperature and (C, D) practical salinity to illustrate the Arctic Ocean water masses. The upper 300 m of the water column are expanded in panels A and C so layers are visible. The cross-basin section is illustrated by the red line in Figure 1, produced using data from NOAA's World Ocean Database (Boyer et al. 2013) using Ocean Data View (Schlitzer, 2015). |
Below the PSW and the various layers that compose it, there is a temperature maximum layer (ranging from 0 to 3°C), having a depth range of 200-1000 m, which is known as Atlantic Water (AW) due to its origin (Figure 3B). Below the AW (i.e., below 1000 m) is the deep water of the Arctic Ocean, which represents approximately 60% of the basin's volume (Talley et al., 2011). The upper portion of the deep water is referred to as Upper Polar Deep Water (UPDW), which is the deepest layer that exchanges water between the Canadian and Eurasian basins (Figure 3B, D). Below the UPDW, water cannot exchange over the Lomonosov Ridge (below 1700 m), resulting in the Canadian Basin Deep Water (CBDW) and the Eurasian Basin Deep Water (EBDW) being confined to their respective basins (Figure 3B, D). The EBDW is slightly colder and fresher than the CBDW (Figure 3B, D), and they both have residence times (i.e., the amount of time water resides in these deepest layers before leaving) of hundreds of years. The CBDW has a residence time of about 450 year, while the EBDW's residence time is 250 years, or about half as long as the CBDW (Talley et al., 2011).
In addition to the interesting geographical and hydrographical features previously described, the Arctic Ocean is also home to many unique plants and animals that only exist in the cold environment of the Arctic. Green algae and other phytoplankton (plant) communities flourish in the ice-covered and ice-free environments, creating productive waters that support zooplankton (animal) communities. There are a variety of marine mammals and seabirds that survive in these productive waters, and more can be learned about them by watching Video 3.
Video 3. Exploring Oceans: Arctic, narrated by Sylvia A. Earl for the National Geographic. This video introduces some of the flora and fauna, along with other aspects of the Arctic Ocean.
The animals and plants introduced in Video 3 are all adapted to the Arctic climate, which has been changing in recent years, resulting in younger sea ice (Video 1), which is more vulnerable to changing climate. To learn more about the Arctic climate in 2014, watch Video 4, and to learn more about how sea ice extent has changed since the beginning of the satellite record, visit Arctic News. Video 4 also explains how variability in Arctic climate (weather) can affect those of us who live at lower latitudes.
Video 4. Arctic Report Card 2014 (NOAA). This is an overview of Arctic climate and environmental change at the close of 2014, based on 10 essays prepared by a team of 63 scientists from 13 countries.
References
References
- Boyer, T.P., Antonov, J.I., Baranova, O.K., Coleman, C., Garcia, H.E., Grodsky, A., Johnson, D.R., Locarnini, R.A., Mishonov, A.V., O'Brien, T.D., Paver, C.R., Reagan, J.R., Seidov, D., Smolyar, I.V., Zweng, M.M. 2013. World Ocean Database 2013, NOAA Atlas NESDIS 72, Levitus, S., Ed., Mishonov, A., Technical Ed. Silver Spring, MD, 209 pp.
- Schlitzer, R.. 2015. Ocean Data View, odv.awi.de.
- Talley, L.D, Pickard, G.L., Emery, W.J., Swift, J.H. 2011. Descriptive Physical Oceanography: An Introduction, 6th Edition. Academic Press, Boston, MA, 555 pp.
