Ocean

A. Proshutinsky¹, J. Morison², I. Ashik³, E. Carmack⁶, I. Frolov³, J. C. Gascard⁴, M. Itoh⁵, R. Krishfield¹, F. McLaughlin⁶, I . Polyakov⁷, B. Rudels⁸, U. Schauer⁹, K . Shimada⁵ , V. Sokolov³, M. Steele², M.-L. Timmermans¹, and J. Toole¹

¹Woods Hole Oceanographic Institute, Woods Hole, MA
²Polar Science Center, Applied Physics Laboratory, University of Washington, Seattle, WA
³Arctic and Antarctic Research Institute, St. Petersburg, Russia
⁴Université Pierre et Marie Curie, Paris, France
⁵Institute of Observational Research for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan
⁶Institute of Ocean Sciences, Sidney, Canada
⁷International Arctic Research Center, Fairbanks, Alaska
⁸Finnish Institute of Marine Research, Helsinki, Finland
⁹Alfred-Wegener-Institut für Polar- und Meeresforschung, Bremer-haven, Germany

Summary

In general, the Arctic Ocean continued to warm and freshen in 2007 under the influence of unusual atmospheric forcing and continued dramatic sea ice melt. These changes were accompanied by an unprecedented rate of sea level rise.

Surface circulation regime

The circulation of the sea ice cover and ocean surface layer are closely coupled and are primarily wind-driven (Proshutinsky and Johnson, 1997). Data from satellites and drifting buoys indicate that the entire period of 2000–2006 has been characterized by an anticyclonic (clockwise) circulation regime due to a higher sea level atmospheric pressure over the region north of Alaska, relative to the 1948–2005 mean, and the prevalence of anticyclonic winds (Figure O1). Under these conditions, the clockwise circulation pattern in the Beaufort Sea region (the Beaufort Gyre) tends to be relatively strong. Conversely, in the cyclonic regime the clockwise circulation pattern in the Beaufort Sea region weakens, and the flow across the basin, from the Siberian and Russian coasts to Fram Strait (the Transpolar Drift), shifts poleward. The cyclonic pattern dominated during 1989–1996; the anticyclonic pattern has prevailed since 1997. The dominance of the anticyclonic regime during the last decade of 1997–2007 is consistent with the Arctic Oscillation (AO) index which fluctuated about zero, indicating a relatively low level of influence from the Atlantic on these Arctic processes (Rigor et al., 2002) .

Figure O1. Circulation patterns of the simulated upper-ocean wind-driven circulation in (left) winter and (right) summer of 2007. Updated following Proshutinsky and Johnson (1997).

Water temperature and salinity

Marginal Seas

Sea Surface Temperature (SST ) trends over the past 100 yr in the Arctic marginal seas (White, Kara, Laptev, East Siberian, Chukchi, and Beaufort) were analyzed by Steele et al. (2008). They found that many areas cooled up to ~0.5°C decade^–1 during 1930–65 as the AO index generally fell (see Fig.A.5), while these areas warmed during 1965–95 as the AO index generally rose. Warming is particularly pronounced since 1995, and especially since 2000 when the AO index exhibited relatively low and fluctuating values. Summer 2007 satellite-derived data indicate that SST anomalies were up to 5°C in ice-free regions (Fig.O2).

Figure O2. (top left) Mean satellite-derived summer (Jul–Aug) SST (Reynolds et al. 2002) and anomalies from this mean over 2000–07. Latitudes 70° and 80°N and longitudes 0°/180° and 90°E/270°E are shown. For 2007, extra contours for 3° and 4°C are provided. Also shown is the Sep mean ice edge (blue contour) for each year (from Steele et al. 2008).

Long-term (1930–95) salinity trends (Steele and Ermold 2004) show that since 1930, the White Sea has gained freshwater while the East Siberian Sea has lost it, consistent with river discharge trends over this period. Over the past 20 yr, increases in both river discharge and direct precipitation can explain observed salinity decreases in the White Sea, but not in the Kara Sea. Salinity trends in the Laptev Sea and East Siberian Sea indicate that ocean circulation plays a dominate role in these areas, where in recent years freshwater has been diverted eastward along the coast, rather than northward toward the deep ocean. Polyakov et al. (2008) confirmed these results and extended the data analysis up to 2003. The result shows that the freshwater content trend in the Siberian Seas (upper 50-m layer) for the period 1920–2003 is 29 ± 50 km³ decade^–1. In 2007, all expeditions working in the marginal seas (I. Polyakov et al. 2007, personal communication) indicated an unusual freshening of the surface layer due to an extreme rate of sea ice melt.

Central Arctic (Nansen, Amundsen, and Makarov basins)

In spring 2007 near surface salinities at the North Pole (NP) were increased by about 1 unit and the Atlantic Water (AW) core temperature was increased by 0.5°C, respectively, above pre-1990s climatology. Data collected since 2000 at the North Pole Environmental Observatory (NPEO , http://psc.apl.washington.edu/northpole/) indicate that in 2000 and 2001 the spring salinities in the upper 150 m near the pole and in the northern Makarov Basin were elevated 1–2 units above climatology and temperatures in the AW core along the Lomonosov Ridge were elevated 1°–2°C. These conditions were nearly the same as observed in 1993 (Morison et al. 2000). In spring of 2004 and 2005, NP region hydrographic conditions largely returned to climatology (Morison et al. 2006). In spring of 2006, temperature and salinity anomalies near the NP region began to move away from climatological norms, again, repeating their behavior in spring of early 2000–03. The 2007 International Polar Year expansion of NPEO airborne surveys, combined with observations of the Switchyard project (W. Smethie 2007, personal communication), yielded a springtime section across 90°W–90°E. Results from this survey were consistent with the NP data, a further indication that in 2007 upper-ocean salinity structure and Atlantic Water temperatures in the central Arctic Ocean moved away from climatological norms, with increased salinity and temperature.

Atlantic Water enters the Arctic Ocean through the Barents Sea and Fram Strait, where it transitions from surface water to water of intermediate depth. The Atlantic Water temperature increase can be partially explained by other observed changes in the AW layer circulation. The first evidence of strong warming within the AW layer was found in the Nansen Basin in 1990 (Quadfasel et al. 1991). Positive AW anomalies of up to 1°C were carried along the continental margins into the Arctic Ocean interior (Woodgate et al. 2001; Schauer et al. 2002). Schauer et al. (2004) and Polyakov et al. (2005) have also shown that since the late 1990s, AW temperature has increased. Polyakov et al. (2007) and Dmitrenko et al. (2008) found that the 2000–05 Atlantic Water warming along the Siberian continental slope has propagated as a series of several AW warm impulses that penetrated into the Arctic Ocean through Fram Strait in 1999–2000 with a mean speed of 2.5 cm s^–1. The 2007 results suggest one of these pulses has reached the central Arctic Ocean. Preliminary reports from the summer hydrographic surveys in the central Arctic (U. Shauer et al. 2007, personal communication) indicate that, relative to 2004–05, in the Nansen and Amundsen Basins the temperature of the Atlantic Water core increased by approximately 0.5°С, and the thickness of this layer increased by 100–150 m mostly due to the propagation of a warming signal into the deeper ocean layers. In the Makarov Basin, no changes in the parameters of the Atlantic Water layer were detected. In contrast to the NPEO springtime results, in all basins the summer surface salinity was 1–2 units less than in 2003–05. It is speculated that the decrease in salinity in these regions is related to the extent of the massive sea ice melt in 2007.

Canada Basin and Beaufort Gyre

The 2007 Canada Basin and the Beaufort Gyre summer conditions exhibited very strong freshening relative to 2006 and previous years of observations (Richter-Menge et al. 2006). Data collected as part of the Beaufort Gyre Environmental Observatory (BGEO, www.whoi.edu/beaufortgyre/index.html) show that in 2000–07, the total freshwater content in the Beaufort Gyre has not changed dramatically relative to climatology (although the absolute maximum was observed in 2007), but there was a significant change in the freshwater distribution (Fig. O3(c,d)). The center of the freshwater maximum shifted toward Canada and significantly intensified relative to climatology. This region of the Beaufort Gyre is much fresher than 30 yr ago.

Figure O3. (a), (c) Summer heat (1 x 1010 J m–2) and (right) freshwater (m) content in the Arctic Ocean based on 1980s climatology (Timokhov and Tanis 1997). (b), (d) Heat and freshwater content in the Beaufort Gyre in 2007 based on hydrographic survey (black dots depict locations of hydrographic stations). For reference, the Beaufort Gyre region is outlined in black in panels a and c. The heat content is calculated relatively to water temperature freezing point in the upper 1,000-m ocean layer. The freshwater content is calculated relative to a reference salinity of 34.8.

Significant changes were observed in the heat content of the Beaufort Gyre (Fig. O3(a,b)). It has increased relative to the climatology, primarily because of an approximately twofold increase of the Atlantic layer water temperature (Shimada et al. 2004). In the late 1990s, Atlantic Water with temperatures as much as 0.5°C warmer than previous records was observed in the eastern Canada Basin (McLaughlin et al. 2004). These observations signaled that warm-anomaly Fram Strait waters, first observed upstream in the Nansen Basin in 1990, had arrived in the Canada Basin, and confirm the cyclonic circulation scheme. The 2007 observations manifested that these processes are still in progress and the AW layer warming signal propagated farther east. The surface layer water in the Beaufort Gyre also accumulated a significant amount of heat in 2007, due to the significant retreat of the ice cover causing its exposure to the direct solar heating. This was similar to the conditions observed in the regions free of ice in the Amundsen, Nansen, and Makarov Basins.

Sea Level

Figure O4 contains sea level (SL) time series from nine coastal stations having representative records for the period of 1954–2007 in the Siberian Seas (from the Arctic and Antarctic Research Institute data archives). There is a positive SL trend along Arctic coastlines of 1.94 ± 0.47 mm yr ^–1 for 1954–89, after correction for Glacial Isostatic Adjustment (GIA). This compares to an estimated rate of 1.85 ± 0.43 mm yr^–1 along the Arctic coastlines over the same period, based on 40 Arctic coastal stations available at the time (Proshutinsky et al. 2004). The addition of 1990–2007 data increases the estimated rate of SL rise for the nine stations in the Siberian Seas, beginning in 1954, to 2.61 ± 0.45 mm yr^–1 (after correction for GIA). The rate of 2.61 ± 0.4 mm/yr is considerably larger than the rate of 1.94 ± 0.47 mm/yr. Both estimates are higher than the sea level rise rate for the global ocean estimated by the Intergovernmental Panel on Climate Change (IPCC) as ~1.8 mm/yr, for 1961 to 2003 (IPCC, 2007). Note the time period included in our estimate (1954–2007) is longer than the IPCC period (1961–2003) and that the sea level in the Arctic rose significantly during 2000–2007.

Figure O4. Five-year running mean time series. The black line is the annual mean sea level at nine tide gauge stations located along the Kara, Laptev, East Siberian, and Chukchi Sea coastlines. The red line is the annual mean AO index anomaly multiplied by 3. The blue line is the sea level pressure at the North Pole (from NCAR – NCEP reanalysis data) multiplied by –1.

From the beginning of the record until 1996, SL correlates relatively well with the time series of the AO index and SLP at the North Pole (Fig.O4). In contrast, from 1997–2007 SL generally increased despite the relatively stable behavior of AO and sea level pressure, indirectly indicating that after 1996 something other than the inverted barometric effect dominated sea level rise in the region. Among possible candidates are ocean expansion due to heating, freshening, and wind-driven effects.

References

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October 3, 2008

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