M.-L. Timmermans1, A. Proshutinsky2, I. Ashik3, A. Beszczynska-Moeller4, E. Carmack5, I. Frolov3, R. Ingvaldsen6,
M. Itoh7, J. Jackson11, Y. Kawaguchi7, T. Kikuchi7, R. Krishfield2, F. McLaughlin5, H. Loeng6,
R. Pickart2, B. Rabe4, B. Rudels8, I. Semiletov9, U. Schauer4, N. Shakhova9, K. Shimada10,
M. Steele11, J. Toole2, T. Weingartner12, W. Williams5, R. Woodgate11, M. Yamamoto-Kawai10, S. Zimmermann5
1Yale University, New Haven, USA
2Woods Hole Oceanographic Institution, Woods Hole, MA, USA
3Arctic and Antarctic Research Institute, St. Petersburg, Russia
4Alfred Wegener Institute, Bremerhaven, Germany
5Institute of Ocean Sciences, Sidney, Canada
6Institute of Marine Research, Bergen, Norway
7Japan Agency for Marine-Earth Science and Technology, Tokyo, Japan
8Finish Institute of Marine Research, Helsinki, Finland
9International Arctic Research Center, University of Alaska Fairbanks, Fairbanks, USA
10Tokyo University of Marine Science and Technology, Tokyo, Japan
11Applied Physics Laboratory, University of Washington, Seattle, USA
12 University of Alaska Fairbanks, Fairbanks, USA
November 24, 2012
- The 2011 annual wind-driven circulation regime was anticyclonic, supporting continued high volumes of freshwater in the Beaufort Gyre region and consistent with a 2012 shift of the Beaufort Gyre freshwater center to the west.
- Sea surface temperatures in summer continue to be anomalously warm at the ice-free margins, while upper ocean temperature and salinity show significant interannual variability with no clear trends.
- Oceanic fluxes of volume and heat through the Bering Strait increased by ~50% between 2001 and 2011.
- Sea level exhibits decadal variability with a reduced correlation to sea level atmospheric pressure since the late 1990s.
Wind driven circulation
Sea ice and ocean surface layer circulation are closely coupled and primarily wind-driven (e.g. Proshutinsky and Johnson, 1997). The Proshutinsky and Johnson model is an effective tool to represent ice and upper-ocean velocities across the entire Arctic. Comparisons between model ice drift and daily-averaged buoy velocities for 39 Ice-Tethered Profilers drifting in different parts of the Arctic basin between 2004 and 2010 indicate average correlation coefficients (r values) between simulated and observed velocity components are 0.8, with no significant regional or temporal differences within the Arctic basin (see Timmermans et al., 2011).
In 2011 the annual simulated wind-driven ice and surface ocean circulation assimilated from satellite and drifting buoy data was anticyclonic (Fig. 2.6) with a well-organized clockwise Beaufort Gyre (BG) over the entire Canada Basin. To examine this in context with the most recent wind-driven circulation, we compare the average circulation pattern for September 2011-August 2012 to the average over the preceding 12 months (Fig. 2.7).
September 2010-August 2011. The overall sense of the ice and surface ocean circulation during September 2010-August 2011 was similar to the 2011 average (Fig. 2.6). Sea ice and surface freshwater transport through Fram Strait originated from the Laptev Sea and in a strong eastward current off northern Greenland. Surface waters from the Laptev Sea were also partly swept into the enlarged BG but flow was directed primarily towards Fram Strait. Near-surface waters and sea ice from the Chukchi and East Siberian Seas were driven westward by winds (Figs. 2.6 and 2.7 [left panel]).
September 2011-August 2012. The wind-driven anticyclonic BG circulation between September 2011 and August 2012 was weaker than the average over the preceding 12 months. In 2011-2012, the somewhat weaker wind stress curl over the BG does not appear to have affected freshwater accumulation by Ekman transport, i.e., in 2012 there was no evidence of reduced freshwater in the BG (Fig. 2.11). However, the circulation patterns are consistent with the slight shift of the BG freshwater center to the west in 2011-2012. That same year, cyclonic circulation was intensified over the Norwegian, Barents and Kara seas, and partially over the Laptev Sea. This drove intensified sea ice and surface water flow from the Kara and Laptev seas northward and then out of the Arctic Ocean via Fram Strait. This wind driving forced earlier ice-free conditions in these regions, and therefore increased solar absorption into the upper ocean (see Fig. 2.9 and discussion). In the Beaufort and Chukchi seas, unusually strong westward winds (see Fig. 1.6 in the Air Temperature, Atmospheric Circulation and Clouds essay) resulted in delayed ice loss from the Chukchi Sea and accumulation of sea ice in the vicinity of Wrangel Island, where some ice persisted through the end of summer (see the Sea Ice essay for more information on ice conditions in 2012). Surface-ocean temperatures in August were subsequently relatively cool in these regions (see Fig. 2.9 and discussion).
Wind-driven circulation in August. Ice drift patterns in summer are critical to summer sea ice conditions and upper-ocean properties, and influence ice conditions over the following seasons. When strong anticyclonic ice drift prevails in the summer (as in 2007), significant volumes of sea ice can be pushed out of the Arctic Ocean leaving vast areas of open water which can accumulate heat from direct solar radiation and delay autumn freeze up. By contrast, under cyclonic atmospheric forcing (as in 2009), sea ice outflow via Fram Strait can be reduced (Fig. 2.8). In these summers with more sea ice, less heat is accumulated in the upper ocean, allowing earlier autumn freeze-up. While the average wind-driven circulation in August over the years 1948 to 2012 is cyclonic (not shown), the August mean wind-driven ice and surface-ocean circulation since 2007 shows significant interannual variability (Fig. 2.8). Cyclonic circulation persisted in August 2008, 2009 and 2012, although with weaker Fram Strait outflows in 2008 and 2009.
Ocean Temperature and Salinity
Upper-ocean temperature. Mean sea surface temperature (SST) anomalies in August 2012, relative to the August mean of 1982-2006, were more than 2ºC higher in parts of the Beaufort, Laptev and Kara seas (Fig. 2.9). This excess heat, derived from solar radiation, can be stored below a strong summer halocline as a Near Surface Temperature Maximum (NSTM). Jackson et al. (2012) analyzed upper-ocean properties in the Canadian Basin through 2010 to demonstrate that the NSTM loses heat to the surface layer throughout winter, contributing to the surface-ocean heat budget year round.
While most Arctic boundary regions displayed anomalously warm SST in 2012, a strong cold anomaly was evident in the Chukchi Sea. This appears to be related to the unusual sea ice extent pattern, and in particular the persistence of sea ice in this area even as the main ice pack retreated northward (see the Sea Ice essay for more information on ice conditions in 2012). Further, a storm during the first week of August (see the essay on Air Temperature, Atmospheric Circulation and Clouds) caused rapid degradation of this southern ice patch (see Fig. 2.5 in the Sea Ice essay) and produced very cool SSTs, which persisted for at least a week. Preliminary analysis of in situ UpTempO buoy data from this area (http://psc.apl.washington.edu/UpTempO) indicates that SST returned to warmer values with further ice loss and solar absorption, and possibly some contribution from sub-surface heat mixed upward.
Upper-ocean salinity. Relative to the 1970s Environmental Working Group (EWG) climatology, the major upper-ocean salinity differences in 2011 (Fig. 2.10) are saltier central Nansen and Amundsen basins and a fresher Canada Basin, with the maximum freshwater anomaly centered in the BG. Another key feature of the upper ocean salinity, relative to climatology, is that the upper ocean is generally saltier around the southern boundary of the Canada Basin due to intensified upwelling at the basin boundaries associated with the large-scale wind-driven circulation in 2011 (Figs. 2.6 and 2.7). This circulation pattern shifted the position of the upper-ocean front between saltier waters of the Eurasian Basin and fresher Canada Basin waters. The magnitude of salinity difference from climatology is <1 in the Barents Sea, much smaller than in regions of the central Arctic basins. Upper-ocean salinity in 2011 is fresher than the 1970s climatology on the south side of the Barents Sea Opening (BSO) and to the east of Svalbard, while areas of the central Barents Sea and to the north of Svalbard exhibit higher salinity.
Beaufort Gyre freshwater and heat content. Arctic Ocean freshwater is concentrated in the BG of the Canada Basin, which has accumulated more than 5000 km3 of freshwater during 2003 to 2012. This is a gain of approximately 25% (update to Proshutinsky et al., 2009) relative to climatology of the 1970s. This strong freshwater accumulation trend in the Beaufort Gyre is linked to an increase in strength over the past decade of the large-scale anticyclonic wind forcing (Proshutinsky et al., 2009; Proshutinsky and Johnson, 1997, updated). Shifts in major freshwater pathways also influence BG freshwater (Morison et al., 2012), while freshwater transported offshore during storms in the southern Beaufort Sea can also account for a significant fraction of the observed year-to-year variability in freshwater content of the BG (Pickart et al., 2012).
In 2012, the BG freshwater content was comparable to that in 2011, with preliminary estimates for the 2012 summer average freshwater content over the Beaufort Gyre region (relative to a salinity of 34.8) of 22.6 m (cf. the 2011 summer average: 21.9 m) (Fig. 2.11, right panels). In 2012, the freshwater center appears to have shifted to the northwest, consistent with the large-scale wind forcing (Fig. 2.7, right panel). The BG heat content in 2012 also remained roughly comparable to 2011 conditions, with about 25% more heat on average in the summer compared to 1970s values (Fig. 2.11, left panels). As further hydrographic data become available from the 2012 field season, heat and freshwater content in the boundary regions in particular will be better constrained.
The Atlantic Water Layer. Warm water of North Atlantic origin, residing at depths below the Arctic halocline, is characterized by temperatures >0°C and salinities >34.5. In 2011, maximum Atlantic Water (AW) Layer temperature anomalies (relative to 1970s climatology) were generally highest on the Eurasian side of the Lomonosov Ridge, with maximum anomalies >2°C in Fram Strait (Fig. 2.12). 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 southeast boundary of the Canada Basin nor in the basin boundary regions adjacent to Greenland and the Canadian Archipelago. Atlantic Water temperatures are cooler now than in the 1970s in the vicinity of Nares Strait.
AW properties in the Arctic are regulated by the Atlantic water inflow through Fram Strait and via the BSO. The warmest AW temperatures in Fram Strait were observed in 2006, with a return of maximum temperatures to the long-term mean (2.7°C) by summer 2010 (Fig. 2.13). In summer 2011, the mean temperature remained close to that observed in 2010. In 2011 an anomalously warm and saline southward flow of AW was observed in western Fram Strait (not shown), possibly indicating that the warm AW anomaly, which had entered the Arctic Ocean in 2006, returned to Fram Strait after completing a loop in the Eurasian Basin. AW temperatures in the BSO were also maximal in 2006, and declined through 2011. The largest volume fluxes of AW through the BSO were measured in winter 2006, and were relatively low in the following years to 2011, although with strong seasonal variability; the lowest volume fluxes were observed in the spring and summer months.
The Pacific Water Layer. The Pacific Water Layer (PWL) in the Arctic originates from the Bering Strait inflow and resides in the Canada Basin at depths between about 50 and 150 m. PWL properties and circulation patterns depend significantly on the prevailing winds, which tended to drive Pacific waters north and westward (Figs. 2.6 and 2.7) in 2011. Excessively high Pacific Water (PW) temperatures (warmer than 6°C) were observed on the Chukchi shelf/slope, Northwind Ridge region in September 2010 (during the R/V Mirai expedition; Kawaguchi et al., 2012) and later in winter 2010/2011 measured by Ice-Tethered Profilers. Data from Ice-Tethered Profilers that sampled in the central Canada Basin during 2004-2012 indicate no clear trend in PW maximum temperatures (in the salinity range 29-33) over this time. There is significant interannual variability in both the salinity and temperature of PW in the central basin (temperature changes by as much as 1°C), with warming in recent years broadly congruent with freshening and the warmest temperatures observed in 2007 and 2010.
The properties of the PW inflow through the Bering Strait are measured by year-round in situ moorings, giving some information from 1990 and good coverage from 1999 to present. Recent results from these moorings (Woodgate et al., in press) show that the 2011 flow through the strait is ~1.1 Sv, significantly greater than the generally accepted climatological value of ~0.8 Sv (Roach et al., 1995), almost 50% more than the 2001 value of ~0.7 Sv (Woodgate et al., 2006), and comparable to previous high flow years of 2007 and 2004 (Woodgate et al., 2010).
The 2011 Bering Strait heat flux (~5x1020 J relative to -1.9°C, the freezing point of Bering Strait waters) is comparable with the previous record high in 2007. This high heat flux is due to increased flow and warming of the lower layers of the water column in the strait. Interannual change in these lower layer temperatures does not correspond to interannual change in satellite sea surface temperatures (SSTs) in the region (Woodgate et al., in press). A preliminary estimate of the freshwater flux through the strait relative to a salinity of 34.8 (Woodgate et al., in press) suggests the 2011 annual mean is 3000-3500 km3, roughly 50% greater than 2001 and 2005 values; interannual variability of the freshwater flux appears to be larger than the interannual variability in the other major freshwater sources to the Arctic (rivers and net precipitation). The ~50% increase in oceanic fluxes through the Bering Strait between 2001 and 2011 is mostly due to an increase in the far-field pressure head forcing of the flow (Woodgate et al., in press).
In 2011, sea level (SL) along the Siberian coast increased relative to previous years (Fig. 2.14). This caused an increase, to 2.66 ± 0.41 mm yr-1, in the estimated rate of SL rise since 1954 after correction for glacial isostatic adjustment (Proshutinsky et al., 2004). Until the late 1990s, sea level atmospheric pressure accounted for more than 30% of variability in SL (Proshutinsky et al., 2004) due to the inverse barometer effect. In contrast, from 1997 to 2011, mean SL has generally increased while sea level atmospheric pressure has remained stable. The tendency toward SL rise in this period may be due to steric effects associated with a reduction of sea ice and ocean surface warming (Henry et al., 2012). After 2008, SL decreased to a minimum in 2010 and then increased in 2011. These variable changes likely result from a combination of many forcing factors. One important factor is associated with Ekman transport directed toward coastlines; in 2011, Ekman transport drove positive sea level anomalies along the Siberian coast.
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