Physical oceanography

Figure 2.7 shows typical vertical profiles of temperature and salinity for the Beaufort Sea (north of Alaska, see Figure 2.2) and for near the North Pole, collected during the August-September 1993 cruise of the USS Pargo. While the two profiles show some obvious differences, which will be addressed shortly, several common features stand out. Note first the existence of a low-salinity surface layer. In these two examples, surface salinities are about 28 psu (Beaufort Sea) and 31 psu (near the Pole). Standard ocean water is around 35 psu. Temperatures in the surface layer are near the salinity-adjusted freezing point (salt in solution depresses the freezing point to below 0 °C). The region below the surface layer, extending to about 200-300 m depth, is characterized by

Low Visibiliy Barents Sea

Figure 2.6 Sea ice field in the Barents Sea near Franz Josef Land (see Figure 2.3) for July 6, 2001, based on visible-band Moderate Resolution Imaging Spectroradiometer (MODIS) imagery. The horizontal resolution is about 250 km. The image is about 526 km by 376 km. The scene shows the transition from high ice concentration to open-ocean waters (the marginal ice zone). Note the large individual ice floes and polynyas on the right-hand side of the image and the ice caps on the islands of Franz Josef Land (courtesy of T. Haran, NSIDC, Boulder, CO).

Figure 2.6 Sea ice field in the Barents Sea near Franz Josef Land (see Figure 2.3) for July 6, 2001, based on visible-band Moderate Resolution Imaging Spectroradiometer (MODIS) imagery. The horizontal resolution is about 250 km. The image is about 526 km by 376 km. The scene shows the transition from high ice concentration to open-ocean waters (the marginal ice zone). Note the large individual ice floes and polynyas on the right-hand side of the image and the ice caps on the islands of Franz Josef Land (courtesy of T. Haran, NSIDC, Boulder, CO).

Sea Temperature Profile North Sea
Figure 2.7 Temperature (T) and salinity (S) profiles for the Beaufort Sea and near the Pole. The y-axis is decibars (dbar), which closely approximates depth in meters (courtesy of J. Morison, Polar Science Center, University of Washington, Seattle, WA).

a rapid increase in salinity. This is attended by an increase in temperature to maximum (and above-freezing) values at around 300-500 m depth. While temperature falls off at greater water depths, from about 400 m downward (not shown) salinity stays nearly uniform at 34.5-35.0 psu.

The layer of rapid salinity (temperature) increase is termed a halocline (thermo-cline). Over most of the global ocean, a stable upper-ocean stratification (less dense water at the top) is maintained by higher water temperatures closer to the surface. However, at the low water temperatures found in the Arctic Ocean, the density stratification is determined not by temperature, but by the vertical profile of salinity -lower salinity means lower density. Consequently, the halocline is associated with a strong increase in density with depth, known as a pycnocline. As density increases strongly with depth, this means that the upper Arctic Ocean is very stably stratified, which inhibits vertical mixing. The importance of the strong "cold Arctic halocline" -so-called because of the associated low temperatures - cannot be overstressed. As will be examined further in Chapter 7, the fresh surface layer and limited vertical mixing with the warmer waters below is the key feature of the Arctic Ocean that, along with low winter air temperatures, allows sea ice to form readily.

Maintenance of the fresh, low-density surface layer (and, in turn, the cold Arctic halocline) is in large part determined by river input. The importance of this river inflow is demonstrated by the fact that the Arctic Ocean contains only about 1% of the global volume of seawater, yet it receives about 11% of the world's river flow, which peaks in June due to melt of the terrestrial snowpack (see Chapter 6). The Eurasian drainages

of the Ob, Yenisey and Lena, along with the Mackenzie River in Canada, supply two-thirds of the total river discharge. Figure 2.8 gives the boundaries of the Arctic terrestrial drainage and these four major individual watersheds. The Arctic drainage, which extends well into middle latitudes, is defined in Figure 2.8 by all areas draining into the Arctic Ocean as well as into Hudson Bay, James Bay, Hudson Strait, Bering Strait and the northern Bering Sea. The influence of river inflow is immediately evident in the map of mean surface salinity for summer (Figure 2.9). The freshwater discharged by the major rivers during early summer gives rise to very low salinity values along the Siberian continental shelves and near the Mackenzie Delta - this freshwater eventually mixes outward to impact much of the central Arctic Ocean. The contrast with the winter mean surface salinity field when river discharge is small (Figure 2.10) is obvious.

Another important contributor to the fresh, low-density surface layer is the influx of relatively low salinity waters from the Pacific into the Arctic Ocean through the Bering Strait. It is the influence of closer proximity to Pacific-derived waters and river

Figure 2.9 Mean surface salinity (psu) for summer (adapted from Arctic Climatology Project, 1998, by permission of NSIDC, Boulder, CO).

runoff that gives rise to the lower surface salinities in the Beaufort Sea example in Figure 2.7. A further significant contribution to the fresh surface layer is made by net precipitation (precipitation less evaporation) over the Arctic Ocean itself. The salinity of the surface layer is also influenced by the growth and melt of the sea ice cover. As sea ice forms, brine is rejected. The density of the surface layer increases as does the depth of vertical mixing. In contrast, as ice melts in summer, the surface layer becomes fresher, and vertical mixing is inhibited.

Understanding the observed vertical ocean structure also requires us to consider the inflow of Atlantic-derived waters. Figure 2.11 is a simple schematic of the mean surface and deep ocean currents in the Arctic Ocean. First a brief word on the surface currents, setting the stage for more detailed investigations in Chapter 7. Immediately

Figure 2.10 Mean surface salinity (psu) for winter (adapted from Arctic Climatology Project, 1997, by permission of NSIDC, Boulder, CO).

obvious is the surface/near surface flow entering the Arctic Ocean from the Bering Strait. Note also the clockwise motion of the upper ocean (and hence the overlying sea ice cover) over much of the central Arctic Ocean, known as the Beaufort Gyre, and the motion from the Siberian shelves, across the Pole and through Fram Strait, known as the Transpolar Drift Stream. South of Fram Strait, this becomes the East Greenland Current, which extends to a considerable depth. The temperature maximum layer at about 300-500 m depth seen in the profiles in Figure 2.7 manifests the inflow of warm Atlantic-derived waters. This Atlantic inflow is provided by two branches, one west of Sptisbergen (the West Spitsbergen Current) and one through the Barents Sea (the Barents Sea Branch). Atlantic water sinks below the Arctic surface layer in the northern Barents Sea. It follows that the temperature maximum layer is known as the Atlantic Layer. The stronger temperature maximum in the North Pole example

Figure 2.11 Major surface (hatched arrows) and deep currents (thin black arrows) of the Arctic Ocean, along with the locations of discharge from the four largest Arctic-draining rivers (Ob, Yenisey, Lena and Mackenzie). Shading corresponds to bathymetry (courtesy of G. Holloway, Institute of Ocean Sciences, Sidney, BC).

Figure 2.11 Major surface (hatched arrows) and deep currents (thin black arrows) of the Arctic Ocean, along with the locations of discharge from the four largest Arctic-draining rivers (Ob, Yenisey, Lena and Mackenzie). Shading corresponds to bathymetry (courtesy of G. Holloway, Institute of Ocean Sciences, Sidney, BC).

of Figure 2.7 manifests the stronger influence of Atlantic-derived waters in this area. Note also from Figure 2.11 how the deep ocean currents in the Arctic Ocean are allied with the bottom topography.

Given the great importance of freshwater in the Arctic Ocean, efforts have been made to quantify its major sources and sinks. Mean annual estimates assembled from a variety of sources are summarized in Table 2.1. The table lists freshwater contributions, relative to a reference salinity of 34.80 psu, expressed in terms of millimeters of freshwater averaged over the Arctic Ocean. In a steady state condition, the freshwater sinks must balance the freshwater sources. The fairly small net imbalance of — 29 mm shown in Table 2.1 must be viewed with the strong caveat that estimates for individual terms range widely. In addition to the freshwater sources discussed in preceding paragraphs, there is a small source provided by the Norwegian Coastal Current, which represents the inflow of waters of Baltic origin. The major freshwater sink is provided by the flux of sea ice and relatively low salinity water through Fram Strait associated with the East Greenland Current (300 mm and 120 mm respectively, 420 mm total). As developed in later chapters, this flux has potential implications for so-called deep-water production in the northern North Atlantic. The next largest sink represents the outflow of ice and low-salinity Arctic waters through the channels of the Canadian Arctic Archipelago (370 mm collectively). Because of its high salinity, the Atlantic (Spitsbergen-Barents Sea) inflow represents a small freshwater sink. Efforts through ASOF and other programs are revising these estimates. For example, Woodgate and

Table 2.1 Annual mean freshwater budget (mm) for the Arctic Ocean

Source

Inflow

Outflow

River runoff

Bering Strait inflow

Precipitation less evaporation

Norwegian Coastal Current

East Greenland Current (Fram Strait ice flux)

East Greenland Current (Fram Strait water flux)

Canadian Arctic Archipelago (water)

Canadian Arctic Archipelago (sea ice)

Atlantic inflow

Total

Net imbalance

Notes: Values in parentheses are estimated root mean square errors Source: Barry and Serreze, 2000, based on various original sources.

Aagaard (2005) suggest that the Bering Strait inflow is considerably larger given in Table 2.1.

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