Ocean Temperature and Salinity
M.-L. Timmermans1, I. Ashik2, Y. Cao3, I. Frolov2, H.K. Ha4, R. Ingvaldsen5, T. Kikuchi6, T.W. Kim4,
H. Loeng5, S. Nishino6, R. Pickart7, I. Polyakov8, B. Rabe9, I. Semiletov8, U. Schauer9,
W.M. Smethie10, V. Sokolov2, M. Steele11, J. Su3, J. Toole7, W. Williams12,
J. Zhao3, W. Zhong3, S. Zimmermann12
1Yale University, New Haven, CT, USA
2Arctic and Antarctic Research Institute, St. Petersburg, Russia
3Ocean University of China, Qingdao, China
4Korea Polar Research Institute, Incheon, Republic of Korea
5Institute of Marine Research, Bergen, Norway
6Japan Agency for Marine-Earth Science and Technology, Tokyo, Japan
7Woods Hole Oceanographic Institution, Woods Hole, MA, USA
8International Arctic Research Center, University of Alaska Fairbanks, Fairbanks, AK, USA
9Alfred Wegener Institute, Bremerhaven, Germany
10Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY, USA
11Applied Physics Laboratory, University of Washington, Seattle, WA, USA
12Institute of Ocean Sciences, Sidney, BC, Canada
November 27, 2013
- Summer sea surface temperatures in 2013 were higher than previous years in the Barents and Kara seas and can be attributed to increased solar heating associated with the early retreat of the sea ice cover.
- A reduction in freshwater content by about 7% was observed in the Beaufort Gyre region in 2013 relative to 2012.
- Pacific Water transport through Bering Strait into the Arctic Ocean was reduced in 2012 by more than 25% relative to 2011, with consequent reductions in heat and freshwater fluxes.
Summer Sea Surface Temperature
Arctic Ocean mean sea surface temperatures (SST) in August 2013 ranged between ~0°C and 4°C, with even higher SSTs in some marginal seas (Fig. 23a). While most Arctic boundary regions displayed anomalously warm SSTs in August 2013, relative to the 1982 - 2006 August mean (Fig. 23b), cold anomalies were evident in the Chukchi and East Siberian seas; cooler SSTs are linked to later and less-extensive sea-ice retreat in these regions relative to recent years. Anomalously warm August SSTs in the Barents and Kara seas are related to earlier ice retreat in these regions and possibly also the advection of anomalously warm water from the North Atlantic. Wind stresses derived from NCEP/NCAR reanalysis sea level pressure fields suggest sea ice in summer 2013 was driven away from the Barents and Kara seas, opposite to August conditions of the preceding six years (see Fig. 2.8, Timmermans et al. 2012). See the essay on Sea Ice for information on ice extent, age and thickness.
Hydrographic data show surface waters in the vicinity of the Barents Sea Opening (BSO) in September 2013 were about 3°C warmer than in September 2012. SSTs in the southern Barents Sea in September 2013 were as high as 11°C; these anomalously high temperatures, up to 5°C above the 1977-2006 mean, were likely caused by increased heating during summer (Trofimov and Ingvaldsen 2013).
Upper Ocean Salinity
No appreciable differences in upper-ocean (at 20 m depth) salinity were observed between 2012 and 2013, although definitive statements are precluded by the spatial and temporal limitations of the available data. The central Canada Basin remains the freshest region of the Arctic Ocean, and the saltiest upper ocean is observed at the boundaries of the Eurasian Basin and the Barents Sea (Fig. 24a). Relative to the 1970s Environmental Working Group (EWG) climatology (Timokhov and Tanis 1997, 1998), the major upper-ocean salinity differences in 2012-2013 (Fig. 24b) were saltier waters in the central Eurasian Basin and fresher waters in the Beaufort Gyre region of the Canada Basin. The upper waters of the Barents and Kara seas were predominantly anomalously salty relative to climatology, although the magnitude of the salinity difference was smaller than in the central Arctic Basin.
The maximum liquid freshwater content anomaly is centered in the Beaufort Gyre (Fig. 25). The Beaufort Gyre accumulated more than 5000 km3 of freshwater, measured relative to a salinity of 34.8, during 2003-2012; this is a gain of approximately 25% (update to Proshutinsky et al. 2009) relative to climatology of the 1970s (see Fig. 5.24b, Timmermans et al. 2013). Most of this increase occurred between 2004 and 2008, with freshwater content remaining relatively stable between 2008 and 2012, although with a 2012 shift in the freshwater center to the northwest relative to previous years.
In 2013, a reduction in freshwater content by about 7% was observed relative to 2012 (Fig. 25). This reduction may be attributed in part to weaker wind-stress gradients in 2013 resulting in reduced wind-forced accumulation of surface-waters (and relaxation of downwelling) in the region. Aside from this change, wind stresses indicate the average circulation pattern for the September 2012-August 2013 period was generally similar to the average over the preceding 12 months (see Fig. 2.7, Timmermans et al. 2012). Strong anticyclonic (clockwise) wind forcing in winter 2013 was followed by anomalously weak forcing (in nearly the opposite sense) in summer 2013. It is of note that trends in Beaufort Gyre heat content generally follow freshwater trends; there is ~25% more heat on average in the summer now compared to the 1970s.
Pacific Water Layer
The Pacific Water Layer in the Arctic Ocean originates from the Bering Strait inflow and resides in the Canada Basin at depths between about 50 and 150 meters. As reported in Woodgate et al. (2012), 2011 was a high transport year for Pacific Water inflow through the Bering Strait, with transports being ~1.1 Sv, much higher than the accepted climatology (1991-2003) of ~0.8 Sv (Woodgate et al. 2005). This high transport resulted in heat fluxes comparable to 2007 (the previous record high since 1991), and record maximum freshwater fluxes since 1991. In contrast, preliminary data suggest that the 2012 annual means were much lower than in 2011; annual mean 2012 transport was close to the climatological mean of 0.8 Sv. The annual mean temperature of the Pacific Water Layer in 2012 was colder than the last decade, and comparable to the annual means of the 1991-2001 period. These two factors yield a heat flux in 2012 comparable to the record low in 2001. Freshwater transport was also reduced in 2012 compared to 2011; in general freshwater flux through the Bering Strait shows interannual variability that is larger than the interannual variability in the other major freshwater sources to the Arctic (i.e., rivers and net precipitation).
Pacific Water enters the Canada Basin via different mechanisms and pathways. Moored measurements of the Pacific water boundary current in the Beaufort Sea north of Alaska (the Beaufort shelfbreak jet) show an 80% decrease in volume transport in the current between 2002 and 2011 (Brugler et al. 2013), where this decrease is predominantly in the summer months. Brugler et al. (2013) attribute the decrease in transport to an increase in easterly winds associated with a stronger Beaufort High and deeper Aleutian Low. These authors propose that in recent years Pacific heat and freshwater is being advected directly north into the Canada Basin interior instead of progressing eastward in the Beaufort shelfbreak jet.
In the central Canada Basin, observations show heat and freshwater content in the Pacific Water Layer increased by about 40% during 2003-2013, with the largest increases in the southern Canada Basin before 2010. Freshwater content has been relatively stable since 2010. In 2013 maximum Pacific Water Layer temperatures over the abyssal plain of the Canada Basin were ~0.5°C.
Atlantic Water Layer
Warm water of North Atlantic origin, lying below the halocline at depths between about 200 m and 900 m (but nearer the surface in the vicinity of the Barents Sea Opening and Fram Strait), is characterized by temperatures >0°C and salinities >34.5. Maximum Atlantic Water temperatures are generally around 1-2°C cooler in the Canadian Basin than in the Eurasian Basin (see Fig. 5.22b in Proshutinsky et al. 2012). In 2012 and 2013, the warmest Atlantic Water temperatures (~5°C) were observed in the Barents Sea. The coolest temperatures (~0°C) were observed off the north coast of Greenland. No significant changes were observed in 2013 in the Atlantic Water Layer compared to 2012 conditions.
Relative to 1970s climatology, maximum Atlantic Water temperature anomalies were <0.5°C warmer in the Canadian Basin and ~0.5-1°C warmer in the Eurasian Basin. Maximum temperatures of the Atlantic Water flowing into the southern Barents Sea in 2013 were about 0.5°C higher than the 1977-2006 mean (Trofimov and Ingvaldsen 2013). 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.
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