Warming 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

Rebecca A Woodgate and Cecilia Peralta-Ferriz

Geophysical Research Letters, April 2021.

doi: 10.1029/2021GL092528

Corresponding Author:  Rebecca Woodgate (woodgate@uw.edu)

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 ecosystems.

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 months).

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. Warm (≥0degC) 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 months).

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).