Abstract
An important yet still not well documented aspect of recent changes in the Arctic Ocean is associated with the advection of anomalous sub-Arctic Atlantic- and Pacific-origin waters and biota into the polar basins, a process which we refer to as borealization. Using a 37-year archive of observations (1981–2017) we demonstrate dramatically contrasting regional responses to atlantification (that part of borealization related to progression of anomalies from the Atlantic sector of sub-Arctic seas into the Arctic Ocean) and pacification (the counterpart of atlantification associated with influx of anomalous Pacific waters). Particularly, we show strong salinification of the upper Eurasian Basin since 2000, with attendant reductions in stratification, and potentially altered nutrient fluxes and primary production. These changes are closely related to upstream conditions. In contrast, pacification is strongly manifested in the Amerasian Basin by the anomalous influx of Pacific waters, creating conditions favorable for increased heat and freshwater content in the Beaufort Gyre halocline and expansion of Pacific species into the Arctic interior. Here, changes in the upper (overlying) layers are driven by local Arctic atmospheric processes resulting in stronger wind/ice/ocean coupling, increased convergence within the Beaufort Gyre, a thickening of the fresh surface layer, and a deepening of the nutricline and deep chlorophyll maximum. Thus, a divergent (Eurasian Basin) gyre responds altogether differently than does a convergent (Amerasian Basin) gyre to climate forcing. Available geochemical data indicate a general decrease in nutrient concentrations Arctic-wide, except in the northern portions of the Makarov and Amundsen Basins and northern Chukchi Sea and Canada Basin. Thus, changes in the circulation pathways of specific water masses, as well as the utilization of nutrients in upstream regions, may control the availability of nutrients in the Arctic Ocean. Model-based evaluation of the trajectory of the Arctic climate system into the future suggests that Arctic borealization will continue under scenarios of global warming. Results from this synthesis further our understanding of the Arctic Ocean’s complex and sometimes non-intuitive Arctic response to climate forcing by identifying new feedbacks in the atmosphere-ice-ocean system in which borealization plays a key role.
Highlights
The role that oceanic warmth penetrating the Arctic Ocean from the lower latitude regions with major oceanic currents plays in the state of polar ocean and sea ice was realized over a century ago
Why is this? The incoming AW provides the base of the halocline complex while the inflowing PW interleaves the halocline complex. This sets the stage for the ’double estuary’ state of the Arctic Ocean, such that AW leaving the Arctic is fresher and cooler than the AW that entered. As both subarctic (Atlantic and Pacific) sources are changing, and because an underwater barrier largely confines the effects the PW to the AB, we argue that the AB experiences pacification and the Eurasian Basin (EB) experiences atlantification
Steered slope currents will play a big role here, as transport and pathways may change (Bluhm et al, 2020, this issue)
Summary
The first instrumental observations on this topic carried out by Nansen and his crew aboard Fram in the Eurasian Basin (EB; explanation of abbreviations used in the text are given in Table 1) in the early 1890s revealed the major features of water mass structure in the polar basins (Figure 2) They found warm (temperature > 0◦C, Figure 2) and salty water of Atlantic origin (Atlantic Water, AW) which was distributed throughout the deep basins of the EB at intermediate depths (∼150-900 m). They observed nearfreezing and relatively fresh water in the ∼50 m surface layer and beneath, within the 50–150 m depth range, large vertical salinity and density gradients associated with halocline overlying the AW. See a nice overview of earlier works by Rudels (2011)
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