Changing ventilation of the Arctic water column, by Pacific Winter Water (PWW) and Pacific Summer Water (PSW).
For full caption, see Figure 3 below.
and Freshening of the Pacific Inflow to
the Arctic from 1990-2019 implying
dramatic shoaling in
Pacific Winter Water ventilation of
the Arctic water column
Part of the AON (Arctic Observing Network)
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The Pacific inflow to the Arctic
traditionally brings heat in summer, melting sea ice;
dense waters in winter, refreshing the Arctic's cold
halocline; and nutrients year-round, supporting Arctic
Bering Strait moorings from 1990-2019
find increasing (0.010+-0.006Sv/yr) northward
flow, reducing Chukchi residence times by ~1.5 months
over this period (record maximum/minimum ~7.5 and ~4.5
Annual mean temperatures warm
significantly (0.05+-0.02degC/yr), with faster change (~0.1degC/yr) in
warming (June/July) and cooling (October/November)
months, which are now 2-4degC above climatology.
water duration increased from 5.5 months (1990s) to over
7 months (2017), mostly due to earlier warming (1.3+-0.7days/yr).
Dramatic winter-only (January-March)
freshening (0.03psu/yr), makes winter waters
fresher than summer waters. The resultant winter
density change, too large to be compensated by Chukchi
sea-ice processes, shoals the Pacific Winter Water
equilibrium depth in the Arctic from 100-150m to
50-100m, implying Pacific Winter Water no longer
ventilates the Arctic's cold halocline at 33.1psu.
The Bering Strait is the only oceanic
link between the Pacific and Arctic Oceans. The
typically northward flow through the strait carries
Pacific oceanic nutrients to the Arctic, vital
for ecosystems. The flow varies seasonally in
temperature and salinity. In spring/summer, it brings warm
waters that start the melt-back of Arctic sea ice.
In winter, it carries cold waters that traditionally
sink deeper (100-150m) into the Arctic, well below
the summer waters.
Annually-serviced instrumentation moored to
the sea floor measured (hourly) the flow and properties
in the strait from autumn 1990 to summer 2019.
We find the flow is increasing
significantly, reducing by ~1.5 months the time
taken to reach the Arctic from the strait (now ~5
Summer waters are now 2-4degC
warmer than typical in the 1990s and warm for longer (7
months compared to 5.5 months).
In winter, waters are dramatically
fresher than before, now fresher than in summer.
This change means the winter waters can no longer
sink so deep in the Arctic - now only 50-100m, the
same depth as the summer waters. This not only means oceanic
nutrients are available closer to the surface, but
may also restructure how the upper Arctic Ocean mixes.
Figure 1. Annual mean Bering Strait properties. (a) Summer satellite (MODIS) Sea Surface Temperature (SST) image of the Bering Strait region showing moorings (black dots) and NCEP wind points (X) [from Woodgate et al., 2010]. (b) Total northward volume transport estimated from A2 (grey) and from A3 with corrections (black, with uncertainty dashed), the latter split into volume colder than (blue crosses) or at/warmer than (red pluses) 0degC, and into the pressure-head (green circles) or local wind-driven (brown triangles) contributions. From A3 (black) and A2 data (grey), annual mean (c) near-bottom temperature with SST (red); (h) salinity; (g,i) heat and freshwater transports respectively, with corrections (red) for the Alaskan Coastal Current (ACC) and surface layer/stratification. From 30-day smoothed A3 data, first (d) and last (e) Julian day (JD) above 0degC and number of days above 0degC (f), showing (blue) when 30-day smoothed SSM/I ice concentration at A3 first/last falls below 20% (melt-back) (d); rises above 20% (freeze-up) (e), and open water time between these dates (f).
Figure 2. Trends and recent change in Bering Strait properties of A3 (a) transport, (b) temperature, (c) salinity, (d) density, (e) heat transport, (f) freshwater transport (FWT) and (g) % SSM/I ice concentration at A3. First column: trend per month (large squares if significant) from 1990 (black), 1998 (blue), and 2000 (red) to end of present data (summer 2019). Last three columns: 30-day A3 data for 2016, 2017 and 2018 (red); all prior data (grey), and the 1990-2004 climatology of ice or water properties [Woodgate et al., 2005a] (black).
Figure 3. (a) Thirty-day smoothed A3 near-bottom density for waters at/warmer than (red) or colder than (blue) 0degC. (b) For 1991 (top) and 2018 (bottom) volumetric temperature-salinity (TS) plot (left) and volume in salinity (middle) and density (right) classes, summed for waters at/warmer than (red) or colder than (blue) 0degC. (a) and (b) mark salinity and density thresholds discussed in Section 5. (c) Annual mean transports (from A3) in density classes: total (grey), < 26.2kg/m3 (magenta pluses), > 26.2 kg/m3 (black dashed) and >26.5 kg/m3 (green circles). (d) TS plot for 2002 waters north of the Chukchi slope colored by silicate (indicating Pacific influence) [modified from Woodgate et al., 2005c], marking changing TS of Pacific Winter Waters (PWW). (e) Schematic of changing ventilation from PWW and Pacific Summer Water (PSW).
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