Observed increases in Bering Strait oceanic fluxes from the Pacific to the Arctic from 2001 to 2011 and their impacts on the Arctic Ocean water column
Rebecca A Woodgate, Thomas
J Weingartner and Ron Lindsay
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Mooring data indicate the Bering Strait throughflow increases ~50% from 2001 (~0.7Sv) to 2011 (~1.1Sv), driving heat and freshwater flux increases. Increase in the Pacific-Arctic pressure-head explains two-thirds of the change, the rest being attributable to weaker local winds. The 2011 heat flux (~5x1020J) approaches the previous record high (2007) due to transport increases and warmer lower layer (LL) temperatures, despite surface temperature (SST) cooling. In the last decade, warmer LL waters arrive earlier (1.6±1.1 days/yr), though winds and SST are typical for recent decades. Maximum summer salinities, likely set in the Bering Sea, remain remarkably constant (~33.1psu) over the decade, elucidating the stable salinity of the western Arctic cold halocline. Despite this, freshwater flux variability (strongly driven by transport) exceeds variability in other Arctic freshwater sources. Remote data (winds, SST) prove insufficient for quantifying variability, indicating interannual change can still only be assessed by in situ year-round measurements.
© Polar Science Center,
University of Washington, 2012
(b-h) Bering Strait annual mean (AM) time-series from 1991 – 2011 of:
b) transport calculated from A3 (blue) or A2 (cyan), adjusted for changes in instrument depth (black) with error bars (dashed) calculated from variability;
c) near-bottom temperatures from A3 (blue) and A4 (magenta-dashed), and the NOAA SST product (red diamonds);
d) salinities from A3 (blue) and A4 (magenta);
e) heat fluxes: blue - from A3 only; red – including ACC correction (1×1020J) and contributions from surface layer of 10m (lower bound) or 20m (upper bound) at SST, with black x indicate heat added from 20m surface layer;
f) freshwater fluxes: blue – from A3 only; red – including 800-1000km3 (lower and upper bounds) correction for stratification and ACC;
g) transport attributable to NCEP wind (heading 330º, i.e., northwestward) at each of 4 points (coloured X in (a)) and the average thereof (black); and
h) transport attributable to the pressure-head term from the annual (black) or weekly (green) fits.Uncertainties are order 10-20%. Red lines on (g) and (h) indicate best fit for 2001-2011 (trends=m(er), in Sv/yr, (er) being the 95% confidence limit from a 1-sided Student’s t-test).
Figure 2: Time-series of:
a) 30-day (grey) and 365-day (red) smoothed NOAA SST data for the Bering Strait region;
b) annual mean SST (red diamonds) and A3 lower layer (LL) temperature (blue dots);
c) first day in year when 30-day smoothed SST (red diamonds) or A3 LL temperature is above 0ºC, with trends (as per Figure 1) in days/yr for 1998-2011 for SST (top left) and A3 (top right);
d) as per (c) for last day in year when temperatures above 0ºC (trends are not significant);
e) as per (c) for number of days above 0ºC;
f) and (g) annual mean NCEP winds at heading 330º (i.e., northwestward) at each of 4 points (coloured X in Figure 1a) and the average thereof (black) for 1982-2011 (f) and 1948-2011 (g).
First row: All A3 lower layer (LL) data from 1998 to 2011. Rows 2-5: A3 LL data from cold years 1991 (row 2) and 2001 (row 3) and warm years 2007 (row 4) and 2011 (row 5).
First column: Temperature-salinity (TS) plot with sigma-0 contours showing that year’s data (in color) and 1998-2011 data (grey). Black box marks extract of TS plot shown in column 2. Yellow line is a western Arctic TS profile from 2002 (CBL2002 station 14 - 76º31.4’N, 168º51.8W in 750m water north of the Chukchi slope [Woodgate et al., 2005c]).
Second column: Volume of water (in TS classes) for that year (or group of years) infered from moored velocity measurements. Positive/Negative values indicate net northward/southward transport.
Third column: Volume of water per salinity class for that year (or group of years) above 0ºC (red), below 0ºC (blue), and total (black). Grey lines indicate totals from each year 1998-2011.
Fourth column: Heat flux per sigma-0 class for that year (or group of years). Grey lines indicate totals from each year 1998-2011.
In first 3 columns, black dashed line marks 33.1 psu.
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