Changes in Freshwater Content of the Arctic Ocean from the 2008 Winter Survey Miles McPhee McPhee Research Company

Starting in 2005, Ice-Tethered Profilers (data collected and made available from Woods Hole Oceanogr. Inst.: have collected upper ocean profiles of T and S, nominally twice per day from 8 – 700 m. Near surface salinity, 1 Oct 07 to 8 Jan 08

March-April, 2008, IPY NPEO/BGEP/Switchyard Aerial Survey

Estimates of freshwater volume distribution from Aagaard & Carmack (1989, J. Geophys. Res., 94). A & C assumed 3 m thick ice

Banks Island Alaska Chukchi

Dickson et al. (1988, Prog. Oceanogr. 20) estimated the salt deficit of the of Great Salinity Anomaly as it passed the Labrador coast in to be about 7 x tonnes, roughly equivalent to 2,000 km 3 FW. The change from climatology in the observation box is about 3-4 times this.

Dynamic height difference across the gridded study area has increased by about 75%

Surface geostrophic currents range from 0 to 2.7 cm s -1

dyn-m

TO#4 TO#5

TO#4 TO#5

dyn-m

Why so much freshening in the Canada Basin? 1.Local melting unlikely: 11 m change in FWC would require about 17 m net sea ice melt. 2.Proshutinsky et al. (2002, Geophys. Res. Lett., 29): Seasonal and decadal variation in Beaufort Gyre FWC is associated with changes in atmospheric circulation: when SLP is anticyclonic FW collects in BG by Ekman transport convergence. This also increases the potential energy of the water column and “spins up” the Gyre. With the reverse regime (cyclonic SLP), the BG spins off FW toward Fram Strait and Canadian Archipelago conduits to the North Atlantic. 3.Ogi et al. (2008, Geophys. Res. Lett, 35): In 2007 anomalously high SLP over the Canada Basin during Jul-Aug-Sep caused ice drift away from the margins, and contributed to the record low ice extent (this would also cause FW convergence). They found that a similar (although less pronounced) regime has dominated over the past decade.

Free-drift force balance

International Arctic Buoy Program (data courtesy UW Polar Science) displacements for the month of September, 2007.

Volume transports associated with the ice displacements using a Rossby similarity expression for stress in terms of ice velocity.

Ogi, Rigor, McPhee & Wallace. [GRL, 2008] Summer 2007 Sea-level pressure and buoy drift tracks

Ogi, et al. form an index by regressing the gridded summer (JAS) atmospheric pressure patterns onto the May-minus-Sep ice extent for years The resulting spatial pattern resembles the JAS pressure anomaly in 2007, and indicates that low ice extent years are correlated with anomalously high pressures SLP Anomaly Regression of JAS sea-level Pressure on Sep-May ice extent

Each year can then be projected onto this pattern, and scored to show how well a particular year matches the SLP anomaly pattern. Since 1998, this index has remained above the long term normalized value in 8 of the 11 years, with consistently high values in the last 4 years Circles: May-Sep sea ice extent; Squares: Jul-Aug-Sep sea-level pressure anomaly

Ekman Pumping

Trajectories of three ITPs in the Canada Basin in 2007 & There are three periods when separate buoys traversed the same region. Look at changes the ITPs were within the 20-km radius circle at different times (shown in days of 2007)

The downward displacement of the upper temperature maximum corresponds to an Ekman pumping velocity of about 3.5 meters per month.

V ice ww  km An average difference in velocity of roughly a centimeter per second over a separation of 100 km is enough to induce an average downwelling velocity of 3.5 m/mo.

Is the freshening an accerelating trend? Is the freshening an accelerating trend?

1.In boxes encompassing the 2008 survey, FWC has increased by 26% on the Pacific side of the Lomonsov Ridge and decreased by 26% on the Eurasian side. Applying these ratios to the overall Canadian and Eurasian Basin totals estimated by Aagaard and Carmack (1989, J. Geophys. Res.) implies a net increase of about 7,700 km 3 over the deep basins. According to their estimates, this is equivalent to roughly four times the salt deficit associated with the Great Salinity Anomaly (1960s, 70s) in the Nordic Seas/North Atlantic. 2.Significant freshening in the SE Canada Basin has changed the dynamic topography. An 800 km long section across the traditional Beaufort Gyre shows no southward return surface current, although summer data suggests that between the easternmost station and Banks Island, there may be southward surface flow. 3.There is now a large net NNE freshwater transport across the traditional BG (section a—a’) in contrast to small net southward transport in the climatological baroclinic regime. Southward FWT in the eastern Canada Basin, and westward transport north of Alaska (section b—b’) have intensified dramatically. Summary:

Challenges: 1.Properly describe the impact of increased stratification on IOBL exchanges, including: (i) deposition of solar radiation; (ii) mixing across the upper pycnocline; (iii) transport of heat and fresh water in the IOBL. 2.Model the impact of changes in FWC on the baroclinic structure and circulation in the Arctic. 3.Understand how the changing IOBL conditions are affecting the combined ice/liquid fresh water export from the Arctic. 4.The increasing dominance of first-year ice probably changes many of the “canonical” rules, e.g., drift speed at 2% of the surface wind, and the ability of the pack to form underice fresh water deposits in summer. 5.There is now much more open water during high sun angles early in the summer. This probably means more horizontal heterogeneity, e.g., many small “marginal ice zones” with large horizontal temperature gradients. Our understanding of how this impacts ice/ocean exchanges is rudimentary.