Ocean

A. Proshutinsky¹, M.-L. Timmermans², I. Ashik³, A. Beszczynska-Moeller⁴,
E. Carmack⁵, J. Eert⁵, I. Frolov³, M. Itoh⁶, T. Kikuchi⁶, R. Krishfield¹,
F. McLaughlin⁵, B. Rabe⁴, U. Schauer⁴, K. Shimada⁷, V. Sokolov³, M. Steele⁸,
J. Toole¹, W. Williams⁵, R. Woodgate⁸, S. Zimmermann⁵

¹Woods Hole Oceanographic Institution, Woods Hole, MA, USA
²Yale University, New Haven, CT, USA
³Arctic and Antarctic Research Institute, St. Petersburg, Russia
⁴Alfred Wegener Institute, Bremerhaven, Germany
⁵Institute of Ocean Sciences, Fisheries and Oceans Canada, Sidney, British Columbia, Canada
⁶Japan Agency for Marine-Earth Science and Technology, Tokyo, Japan
⁷Tokyo University of Marine Science and Technology, Tokyo, Japan
⁸University of Washington, Seattle, WA, USA

November 9, 2011

Highlights

• The wind-driven circulation regime was anticyclonic, supporting a high volume of freshwater in the Beaufort Gyre region.
• Upper ocean temperature and salinity were comparable with previous years, indicating some stabilization in warming and consistent with sea ice conditions described in the Sea Ice essay.
• The state of the deep ocean layers also shows some stabilization without pronounced inter-annual changes.
• Rates of sea level rise did not change significantly, following previously observed patterns of decadal variability in which multiyear periods of significant sea level rise alternate with periods of sea level decrease.

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Wind driven circulation

In 2010, the annual wind-driven circulation regime was anticyclonic (clockwise) with a well-organized Beaufort Gyre and relatively weak Transpolar Drift (Fig. SIO6a). In 2011 (January-August), the anticyclonic wind-driven circulation intensified (Fig. SIO6b) relative to conditions in 2010, and the Transpolar Drift was much stronger than in 2010. Circulation in 2011 (January - August) more closely resembled 2007 conditions, although a stronger Arctic High and anticyclonic circulation was observed in 2007 (Fig. SIO6c). The anticyclonic circulation regime similar to that shown in Fig. SIO6a-c has persisted between 1997 and 2011, with only one short-lived reversal to a cyclonic regime in 2009, the consequences of which were important, as discussed below. The anticyclonic regime has dominated for at least 14 years instead of the typical 5-8 year pattern (as reported in Proshutinsky and Johnson [1997, 2010], who analyzed statistics of Arctic circulation regimes between 1948 and 2010). It may be that after the anomalous 2007 conditions (a historical minimum of September sea-ice extent, and maximum upper-ocean warming and freshening) the Arctic climate system bifurcated towards a new state characterized by a more persistent anticyclonic regime and with relatively small changes from year to year.

Fig. SIO6a. Simulated wind-driven ice motion (arrows) and observed sea level atmospheric pressure (hPa, black lines) for 2010. Results are from a 2-D coupled ice-ocean model (Proshutinsky and Johnson, 1997; 2011) forced by wind stresses derived from 2010 NCEP/NCAR reanalysis 6-hourly sea level pressure fields.

Fig. SIO6b. Simulated wind-driven ice motion (arrows) and observed sea level atmospheric pressure (hPa, black lines) for January-August 2011. Results are from a 2-D coupled ice-ocean model (Proshutinsky and Johnson, 1997; 2011) forced by wind stresses derived from 2011 NCEP/NCAR reanalysis 6-hourly sea level pressure fields.

Fig. SIO6c. Simulated wind-driven ice motion (arrows) and observed sea level atmospheric pressure (hPa, black lines) for January-August 2007. Results are from a 2-D coupled ice-ocean model (Proshutinsky and Johnson, 1997; 2011) forced by wind stresses derived from 2007 NCEP/NCAR reanalysis 6-hourly sea level pressure fields.

Ocean temperature and salinity

Upper-ocean temperature

Upper ocean temperature anomalies in summer 2010 (Fig. SIO7 were comparable to those in 2009 (not shown) but remained lower than the record set in 2007, with no significant inter-annual changes in summer warming since 2008. In August 2011, there is a wide area of anomalously warm SSTs (sea surface temperature) in the western Arctic Ocean (north of NW Canada, Alaska and eastern Siberia), although maximum values do not reach those seen in 2007 (Fig. SIO7). Much of the eastern Arctic Ocean (north of western Russia and Europe) is also anomalously warm, with the exception of Fram Strait. For more information about water temperatures in Fram Strait, and the adjacent Greenland and Norwegian seas, see the essay on Cetaceans and Pinnipeds.

Fig. SIO7. SST anomalies in August of 2007 (left), 2010 (middle) and 2011 (right) relative to the August mean of 1982-2006. The anomalies are derived from satellite data according to Reynolds et al. (2002). The August mean ice edge (thick blue line) is also shown.

Inter-annual variations in SST anomalies reflect differences in the pace of sea ice retreat (see the essay on Sea Ice), as well as changing advection of warm ocean currents from the south (Steele et al. 2011). In recent years, solar radiation has penetrated more easily into the upper ocean under thinning and retreating ice cover to create warm near-surface temperature maxima (Jackson et al., 2010). In the Canada Basin, this maximum has descended to depths around 30 m because of increased downwelling in the convergent Beaufort Gyre during recent strongly-anticyclonic years (Yang et al. 2009), while surface mixing is decreasing as stratification increases (Toole et al. 2010; McPhee et al. 2009). Outside of the Beaufort Gyre, the temperature maximum does not survive through the winter (Steele et al. 2010).

Upper-ocean salinity

Relative to the 1970s, surface waters in 2009-10 (Fig. SIO8 were generally saltier in the Eurasian Basin and fresher in the Canada Basin, with the maximum freshwater anomaly centered in the Beaufort Gyre. Several key changes in salinity distribution of the upper layers occurred between 2007-08 and 2009-10 (not shown). The western Canada Basin surface waters were fresher in the latter two years, with saltier surface waters in the eastern Canada Basin in 2010-09 compared to 2007-08. The region between Greenland and the North Pole was generally fresher in 2009-10 than in 2007-08, while the upper ocean was saltier in the western Makarov Basin in the latter two years. These changes in upper-ocean salinity possibly result from a 2009 shift in the large-scale wind-driven circulation to some reduction in strength of the anticyclonic Beaufort Gyre and the Transpolar Drift (Timmermans et al. 2011).

Fig. SIO8. Salinity anomalies at 20 m depth in 2009-2010 (right) relative to 1970s climatology (left). The 500 and 2500 m isobaths have been plotted using the IBCAO grid.

Beaufort Gyre freshwater and heat content

The Beaufort Gyre is the largest reservoir of freshwater in the Arctic Ocean. In 2010, the magnitude of the freshwater content was comparable to 2008 and 2009 conditions, with the exception that freshwater tended to spread out from the 2007-2009 center (Fig. SIO9). In total, during 2003-2010 the Beaufort Gyre accumulated more than 5000 km³ of freshwater, a gain of approximately 25 percent (update to Proshutinsky et al. 2009) relative to climatology of the 1970s.

Fig. SIO9. Summer heat content (1 × 1010 J m-2) and freshwater content (m). The top row shows heat and freshwater content in the Arctic Ocean based on 1970s climatology (Timokhov and Tanis 1997, 1998). The centre and bottom rows show heat and freshwater content in the Beaufort Gyre based on hydrographic surveys (black dots depict hydrographic station locations) in 2010 and 2011, respectively. The Beaufort Gyre region is shown by black boxes in the top row. Heat content is calculated relative to freezing temperature in the upper 1000 m of the water column. Freshwater content is calculated relative to a reference salinity of 34.8.

Note that freshwater increases in recent years have not been limited to the Beaufort Gyre. Relatively recent changes in upper ocean freshwater content (Fig. SIO10) in all Arctic Ocean basins were reported by Rabe et al. (2011). They showed that between 1992-1999 and 2006-2008, the freshwater content in Arctic basins increased by 8400 ± 2000 km³, and suggested that this was largely due to enhanced advection of river water from the shelves and net melting of sea ice.

Fig. SIO10. Objectively-mapped observed freshwater inventory (meters) from the surface to the depth of the 34 isohaline for the deep Arctic Ocean during July, August and September: (left) 1992-1999, (center) 2006-2008 and (right) difference between the 2006-2008 and 1992-1999 periods. Positive differences mean freshening. The locations of measured salinity profiles used for mapping are shown as black dots. The thick gray line represents the 1 m contour of the combined (maximum) statistical error estimate for both mapping time periods. Figures were modified from Rabe et al. (2011).

The Beaufort Gyre heat content in 2010 increased relative to 2009 primarily due to surface heating by direct penetration of solar radiation into ice-free coastal regions (Fig. SIO9, left panels; and see the essay on Sea Ice). In August 2011, freshwater and heat conditions similar to 2010 were emerging (Fig. SIO9), although insufficient data were available to constrain the 2011 estimates at the time of this writing.

The Atlantic Water Layer

Warm water of North Atlantic origin, the Atlantic Water Layer (AWL), resides between approximately 200 and 900 meters and is characterized by temperatures >0°C and salinities >34.5. In 2009-2010, AWL maximum temperature anomalies were generally highest on the Eurasian side of the Lomonosov Ridge, with maximum values about 1°C along the boundaries of the Eurasian Basin (Fig. SIO11). Warming was less pronounced in the Canada Basin than in the Eurasian Basin. There was little to no temperature anomaly (<0.1°C) at the south-east boundary of the Canada Basin or in the basin boundary regions adjacent to Greenland and the Canadian Arctic Archipelago. Negative (cooling) temperature anomalies were detected in the vicinity of Nares Strait.

Fig. SIO11. Atlantic Water layer temperature maximum anomalies (right) relative to 1970s climatology (left). The 500 and 2500 m isobaths have been plotted using the IBCAO grid.

The characteristics of the AWL are regulated by the Atlantic Water (AW) properties and transport at the inflow in Fram Strait. After reaching a maximum in 2006, AW temperature in Fram Strait decreased until 2008 (Fig. SIO12). In 2009, AW temperature and salinity in northern Fram Strait increased, returning in summer 2010 to the long-term mean. The autumn and winter AW temperatures were slightly higher in 2009/2010 than the previous year, while in summer 2010 the mean temperature remained close to that observed in summer 2009, with typical substantial seasonal variability. With the exception of one strong event in December, the AW inflow with the West Spitsbergen Current was lower in 2009-2010 compared to the somewhat stronger inflows observed in winter 2008-2009. However, in summer 2010 the AW volume transport was relatively high and the typical seasonal summer minimum was not well pronounced (Fig. SIO12); this is the case at least for the months for which data are available. For more information about water temperatures in Fram Strait, and the adjacent Greenland and Norwegian seas, see the essay on Cetaceans and Pinnipeds.

Fig. SIO12. Atlantic water (defined by T >1°C) mean temperature and the volume inflow in the West Spitsbergen Current, northern Fram Strait, measured by moorings at 78°50'N. A mooring array has been maintained in the Fram Strait since 1997 as a joint effort of the Norwegian Polar Institute and the Alfred Wegener Institute for Polar and Marine Research.

The Pacific Water Layer

The Pacific Water Layer (PWL) is located in the Canada Basin at depths between approximately 50 and 150 meters (Steele et al. 2004) and originates from the Bering Strait inflow. The relatively warm and fresh PWL (S <33.5) comprises about two thirds of the Canada Basin halocline by thickness and about half by freshwater content (e.g., Aagaard and Carmack 1989). Some component of this water carries high nutrient content throughout the Arctic Ocean and even downstream into, for example, Baffin Bay (Tremblay et al. 2002). The freshwater flux from the North Pacific Ocean into the North Atlantic Ocean provides a ''short circuit'' for the global thermohaline ocean circulation (e.g., Wijffels et al. 1992). Important changes in Pacific Winter water spreading between 2002-2006 and 2007-2010 are shown in Fig. SIO13 In the period 2002-2006, this water penetrated into the Beaufort Sea from the southern end of the Northwind Ridge. In 2007-2010, it took a different path, spreading northward along the Chukchi Plateau directly from the Herald Canyon. The pathway change was possibly associated with changes in strength and spatial pattern of the wind-driven sea-ice motion (Fig. SIO13, bottom panels). While the extent to which the Pacific Winter water is influenced by wind forcing is unclear, it may be that in recent years the increased westward wind forcing (and increased westward ice transport) prevents the Pacific Winter water from spreading directly east. These changes in the physical environment cause changes in the biogeochemical environment in the Pacific sector of the Arctic Ocean (see the essay on Ocean Biogeophysical Conditions).

Fig. SIO13. Top row: potential temperature (°C) in the Canada Basin at the S=33.1 isohaline. Bottom row: sea level atmospheric pressure (hPa) and simulated wind-driven component of ice drift. Left and right columns are for 2002-2006 and 2007-2010, respectively. Large arrows show suggested spreading of Pacific Winter waters.

The characteristics of Pacific waters depend on water properties and transport in Bering Strait and on atmospheric conditions which modify these waters during their propagation over the shallow Chukchi Sea region. At this time, preliminary annual mean values (not shown) of the Bering Strait volume flux, temperature and salinity are available through 2009. The values suggest that 2008 and 2009 were slightly cooler than the period 2002-2007, but still warmer than measurements made between 1999 and 2001. The 2009 volume transport (~0.9Sv) was slightly higher than in 2008, but still less than the 2007 transport (>1Sv) which was the highest in the available record (spanning 1991-95 and 1998-2009). The 2009 heat flux was close to the long-term mean value, while the freshwater flux in 2009 was somewhat higher than the mean (due in part to the higher than average transports), but still less than previous maxima in 2004 and 2007. Mooring data are only available up to the summer of 2010. Temperatures in August 2010 were somewhat warmer than in August 2009, but the impact of this on the annual mean can only be assessed when an entire year of data is available. Similarly, the first half of 2010 was somewhat saltier than in 2009, although it is too soon to reach conclusions about annual mean freshwater flux.

Sea level

Sea level (SL) is a natural integral indicator of climate variability. It reflects changes in practically all dynamic and thermodynamic processes of terrestrial, oceanic, atmospheric and cryospheric origin. SL time series are available from nine coastal stations in the Siberian seas that have representative records for the period of 1954-2010 (Arctic and Antarctic Research Institute data archives). In 2010, SL along the Siberian coastline continued to decrease relative to 2008 and 2009 (Fig. SIO14). This caused a reduction in the estimated rate of SL rise for the nine stations since 1954 to 2.49 ± 0.45 mm yr^-1 (after correction for glacial isostatic adjustment, GIA; Proshutinsky et al., 2004). Until the late 1990s, the SL time series correlates relatively well with the Arctic Oscillation (AO, Thompson and Wallace, 1998) index and with the inverse of the sea level atmospheric pressure (SLP) at the North Pole. Consistent with these influences, sea level declined significantly after 1990 and reached a minimum in 1996-1997, when the atmospheric circulation regime changed from cyclonic to anticyclonic. In contrast, from 1997 to 2006, mean SL has generally increased while the AO and SLP have remained more or less stable. After 2008, SL has tended to decrease and shown no apparent correlation with the AO or SLP at the North Pole. Since SL change exhibits large inter-annual variability and is the net result of many individual effects of environmental forcing, it is difficult to evaluate the significance of the change in relative terms. Although not statistically robust, the tendency toward decreasing SL rise may be due to steric effects associated with some reduction of surface ocean warming and stabilization of ocean freshwater content and/or with the wind regime shift over the Siberian seas.

Fig. SIO14. Five-year running mean time series of: annual mean sea level at nine tide gauge stations located along the coasts of the Kara, Laptev, east Siberian, and Chukchi seas (black line); anomalies of the annual mean Arctic Oscillation index (AO, Thompson and Wallace, 1998) multiplied by 3 for better visual comparison with other factors (red line); sea surface atmospheric pressure at the North Pole (from NCAR-NCEP reanalysis data) multiplied by -1 (inverted barometer effect, dark blue line); and annual sea level variability (light blue line). Dotted lines depict trends for SL, AO and SLP.

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