Variability of freshwater input and sea ice formation/melting in the Arctic Ocean can affect on the arctic and global climate through affecting the heat transport by changing the extent of surface stratification and also the deep convection. The historical and recently acquired data of two chemical tracers, oxygen isotope ratio (δ¹⁸O) and alkalinity in seawater, are used in this study 1) to distinguish the freshwater sources and 2) to investigate their variability in the Arctic Ocean.

The isotope ratio in seawater has been successfully used to separate contributions of meteoric water and sea ice melt water in the Arctic Ocean in the past, because the arctic meteoric water has very low δ¹⁸O value, while sea ice melt water is close to standard sea water composition. The total alkalinity (TA) is found to be also different among freshwater sources. Arctic river contains relatively high TA and TA in natural sea ice is low. Therefore, TA data is examined in order to distinguish freshwater sources in the Arctic Ocean.

In the Arctic Ocean, the TA and δ¹⁸O data are available since 1935 and since 1966, respectively. By combining both tracer data together, we can cover whole Arctic Ocean, and longer time length to reveal freshwater distribution and its spatial and temporal variability in the Arctic Ocean. We have solved mass balance equations of three components mixing of Atlantic water (ATW), sea ice melt water (SIM), and other freshwater (OF) from each pair of two conservative elements, salinity-δ¹⁸O and salinity- TA. The OF includes river runoff, precipitation and salinity deficit of inflowing Pacific water. Estimated mixing ratio of each water mass from two chemical tracers agreed well when appropriate end-member values. Therefore, results from salinity-TA and salinity-δ¹⁸O are combined together to estimate SIM and OF distributions in the entire Arctic Ocean.

The surface waters in Chukchi Sea and in the region from the Fram Strait to the Kara Sea contain relatively large fraction of SIM (more than 3 % in summer) and the Chukchi Sea has the small amount of the Arctic meteoric water. On the other hand, the surface water in Siberian and American coastal areas contains more than 20 % of OF and large fraction of OF (>10 %) distributes over the Canadian Basin (Canada and Makarov Basins) and over the Lomonosov Ridge. In the Fram Strait, the fraction of OF is high in the western part and low in the eastern part, showing the significant influence from the outflow of rivers in the East Greenland Current. The calculated freshwater inventory in upper 300 m of water column suggests that the water inflowing from the Atlantic Ocean through the eastern part of the Fram Strait will exit along the western part of the Strait after experiencing 5 m water depth equivalent of freshwater removal due to net ice formation and receiving 10 m water depth equivalent input of OF within the Arctic Ocean. The large inventory and deeper penetration of OF and brine formed during freezing sea ice are found in the Canada Basin as compared with the Eurasian Basin. Our calculation shows that the water at 150 m depth of the Canada Basin is formed from relatively fresh surface water having initial salinity of 31-32 but rising to 33 by brine injection. This means that the water from shallow shelves enters into the intermediate depths to renew the halocline layer and characterize the Canada Basin as the reservoir of both OF and brine. In this Basin, changes between cyclonic and anticyclonic regimes are found in the size of distribution of low salinity and high OF water from the surface to the 150 m depth. At 150 m depth, content of brine is also change with regime changes. Higher contents of OF and brine in anticyclonic years may suggest more active formation of dense shelf water in the anticyclonic regime than cyclonic regime.

We appreciate Drs. Murata and Shimada of JAMSTEC for providing unpublished data from the pastArctic expeditions by R/V Mirai.