Bering Strait Freshwater paper GRL 2004

ONR N00014-99-1-0345

NSF 0125082

The freshwater flux through the Bering Strait into the Arctic Ocean is important regionally and globally, e.g. for Chukchi Sea hydrography, Arctic Ocean stratification, the global freshwater cycle, and the stability of the Atlantic overturning circulation. Aagaard and Carmack [1989] estimated the Bering Strait freshwater flux as 1670 km3/yr (relative to 34.8 psu), assuming an annual mean transport (0.8 Sv) and salinity (32.5 psu). This is ~ 1/3rd of the total freshwater input to the Arctic. Using long-term moored measurements and ship-based observations, we show that this is a substantial underestimate of the freshwater flux. Specifically, the warm, fresh Alaskan Coastal Current in the eastern Bering Strait may add ~ 400 km3/yr. Seasonal stratification and ice transport may add another ~ 400 km3/yr. Combined, these corrections are larger than the interannual variability observed by near-bottom measurements and near-surface measurements will be necessary to quantify this flux and its interannual variability.

Figure 1. The Bering Strait region, with mooring locations (A1 - 65.90°N 169.43°W, water depth 50 m; A2 - 65.78°N 168.59°W, 53 m; A3 - 66.29°N 168.97°W, 56 m; A3' - 68.17°N 168.97°W, 59 m; A4 - 65.75°N 168.26°W, 59 m) as black dots, showing sea surface temperature for 26th August 2004 (MODIS/Aqua level 1 image courtesy of Ocean Color Data Processing Archive, NASA/Goddard Space Flight Center). Arrow marks the Diomede Islands ~ 65.8°N 168°W. White areas indicate clouds.

Figure 2. Thirty-day running mean of hourly time-series of salinity 9 m above bottom at sites A1 (western channel, cyan), A2 (eastern channel, blue), A3 (north of Bering Strait, black), and A4 (the Alaskan Coastal Current, red). Not all moorings are deployed each year. Between summer 1992 and summer 1995, A3' (thin black line) was deployed instead of A3 (see Figure 1). Record mean (i.e., approximately annual mean from summer to summer) salinities (S, black) (errors ~ 0.2 psu) and freshwater transports (errors ~ 300 km3/yr) are estimated from A3 records, assuming no stratification in velocity or density (FW, green) and with the correction of 800 km3/yr (FW+, red).

Figure 3. July 2003 CTD salinity section across the eastern channel of the Bering Strait from the Diomede islands (left) to Alaska (right). Black dots indicate moored instrument locations. Note the fresh (warm) ACC on the Alaskan coast.

Figure 4. Hourly time-series data from A2 and A4 for July 2002 to 2003. (a) Principal component of velocity (~ northward velocity) at A2 (47 m, blue) and A4 (34 m, red; 24 m, yellow; and 14 m, green). (b) Salinity at A2 (48 m, blue) and A4 (39 m, red). Thick black line marks times when near- bottom temperatures at A2 and A4 are at the freezing point (Julian days 360 to 482, i.e., late December 2002 to late April 2003).

Revising the Bering Strait Freshwater Flux into the Arctic Ocean

Rebecca A Woodgate and Knut Aagaard