2) Introduction: Freshwater sources

There are many different sources of freshwater in the Arctic Ocean:

  • River runoff from Alaska, Canada, and Siberia

o   10% of annual global river runoff enters the Arctic Ocean despite its small size (the Arctic Ocean represents only about 1 % of the global ocean volume)

o   River runoff contributes 38% of the total annual input of freshwater to the Arctic Ocean

  • Precipitation (rain & snow)

o   Net precipitation (that is, precipitation falling over the Arctic Ocean minus evaporation of freshwater from it) contributes 24% of the total annual input of freshwater to the Arctic

  • Melting sea ice

o   Sea ice melts in summer (freshwater source) but the freezing of seawater into sea ice during the fall and winter actually removes freshwater from the liquid ocean (freshwater sink). On average, the net effect of this melt/freeze cycle actually removes more freshwater from the Arctic than it adds

  • Melting glacier ice

o   Glaciers in Greenland and the Canadian Arctic Archipelago have been melting at an accelerated rate, potentially increasing their contribution to the total freshwater volume of the Arctic region

  • Pacific seawater

o   Wait, what? How does water from the Pacific Ocean qualify as a source of “freshwater”? Pacific seawater has a lower salinity (about 32-33 ppt) compared to Atlantic seawater (about 35 ppt). Most researchers use Atlantic seawater as a reference for comparison. Any water source with a salinity lower than 35 ppt can be considered a source of freshwater. We can even calculate how much freshwater is “contained” in Pacific seawater. Using Atlantic seawater salinity (SATL) as the reference and assuming a Pacific seawater salinity (SPAC) of 32.5, we calculate that typical Pacific water is 7% freshwater and 93% seawater:

(SATL – SPAC) / SATL = (35 – 32.5) / 35 = 0.07 = 7%

o   Pacific water is the second largest source of freshwater to the Arctic Ocean, contributing 30% of the total annual input 

Except for Pacific water, all the other source waters on the list have a very low or zero salinity. As these sources mix into the Arctic Ocean, they decrease the salinity of the ocean where they enter. As these waters then mix and spread out over the Arctic Ocean, the lower salinity will be spread wider. In this sense, we can learn more about the currents in the Arctic Ocean if we can track the movement of these low salinity waters across the ocean.

 

Figure 2.  Map of mean, annual surface salinity of the Arctic, North Pacific and North Atlantic Oceans according to the PHC 3.0 Climatology (PHC 3.0, updated from:  Steele, M., R. Morley, and W. Ermold (2001). PHC: A global ocean hydrography with a high quality Arctic Ocean, J. Climate 14, 2079-2087).  Graph copied from:  http://psc.apl.washington.edu/nonwp_projects/PHC/AnnualGraphicsP3.html#here.  Warmer colors indicate higher salinities whereas cooler colors represent lower salinities (more freshwater).  The intrusion of Atlantic water into the Arctic is shown in the figure as the spread of red color northward through Fram Strait and Barents Sea (bottom of the figure).  Freshwater is shown as purple and blue colors entering the Arctic from the shelf seas on the periphery.

Figure 2. Map of mean, annual surface salinity of the Arctic, North Pacific and North Atlantic Oceans according to the PHC 3.0 Climatology (PHC 3.0, updated from: Steele, M., R. Morley, and W. Ermold (2001). PHC: A global ocean hydrography with a high quality Arctic Ocean, J. Climate 14, 2079-2087). Graph copied from: here. Warmer colors indicate higher salinities whereas cooler colors represent lower salinities (more freshwater). The intrusion of Atlantic water into the Arctic is shown in the figure as the spread of red color northward through Fram Strait and Barents Sea (bottom of the figure). Freshwater is shown as purple and blue colors entering the Arctic from the shelf seas on the periphery.

River runoff is the largest source of freshwater to the Arctic Ocean.  Since different rivers enter the ocean at specific locations, tracking the distribution of river runoff throughout the Arctic Ocean can be very valuable to the study of ocean currents.  But there are a few problems associated with tracking river waters in the Arctic.  First, there are hundreds of rivers flowing into the ocean from the surrounding lands of Alaska, Canada, and Siberia.  Second, rivers all have a salinity of about zero.  How do we tell the different river waters apart?  For that matter, how can we tell river water apart from other freshwater sources such as sea-ice melt or snowmelt?