Arctic Ocean-Sea Ice-Climate Interactions

Sea Ice Ice extent waxes and wanes with the seasons. Ice extent is at a maximum in March (typically 14 million square km, about twice the area of the contiguous United States) and about half that in September [data from NSIDC].

Ice extent is declining in all months, but most rapidly in September, the end of the melt season [courtesy NSIDC, Boulder, CO]. The modern satellite record starts in 1979. Ice extent for earlier years is determined from earlier types of satellite data (back to1972), aircraft and ship reports. Records back to 1953 are pretty good.

September ice extent (all colored regions) and anomalies in ice concentration (see color bar) for the years 2002 through 2009. Each panel shows the median September ice extent. Concentration anomalies and median extent are referenced to the period 1979-2000 [courtesy NSIDC, Boulder, CO]. All of these Septembers were characterized by extreme negative anomalies in ice extent.

Sea ice formation, types and morphology

Temperature versus density plot for fresh water and ice at standard atmospheric pressure [from Maykut, 1985, by permission of Applied Physics Laboratory, University of Washington, Seattle, WA]. Water density increases with decreasing temperature until 3.98 deg. C. With further cooling, density decreases. Hence as a fresh water column cools from the surface, it initially sets up convection (overturning). One the entire column is at the temperature of maximum density, further cooling leads to a stable (stratified) situation (with the colder, lighter water at the top), and ice can form.

Courtesy John Wettlaufer, presented at 2007 Woods Hole GFD summer program

The effect of salinity on the temperature of maximum density (dashed curve) and the freezing point temperature (solid curve) [from Maykut, 1985, by permission of Applied Physics Laboratory, University of Washington, Seattle, WA]. With salinities above 24.7 psu (most ocean water is higher than this), the temperature of maximum density equals the freezing point of water. Salt in solution also depresses the freezing point. At 35 psu, the freezing point of water is -1.8 deg. C. Cooling at the surface increases the water density, and there will be overturning. However, because of pre-existing strong density gradients, only the top 10-40 m of the water column needs to cool to the temperature of maximum density before ice can form.

At high water temperatures (> several C), density depends mainly on temperature. However, at water low temperatures near freezing (such as in the Arctic), density depends mainly on salinity http://topex-www.jpl.nasa.gov/files/archive/activities/ts2ssac3.pdf

Temperature and salinity profiles for the Beaufort Sea and near the pole. The y-axis is depth in decibars (dbar), which closely approximates depth in meters [courtesy of J. Morison, Polar Science Center, University of Washington, Seattle, WA]. Note the low salinity very near the surface (the fresh surface layer), and the rapid increase in salinity from the surface downwards. This makes the water column stable (low density water at the top), which allows sea ice to readily form.

Terminology of oceanographers Thermocline: Strong vertical temperature gradient Halocline: Strong vertical salinity gradient Pycnocline: Strong vertical density gradient In the Arctic, the halocline forms the pycnocline

Sea ice formation sequence: frazil ice (small platelets, needles, < 3-4 mm) grease ice (1-10 cm thick) pancake ice pack ice

Courtesy John Wettlaufer, presented at 2007 Woods Hole GFD summer program

Top: Frazil ice, small needle-like ice crystals, typically 3 to 4 millimeters in diameter, suspended in water, represent the first stages of sea ice growth; they merge under calm conditions to form thin sheets of grease ice on the surface. Bottom: Grease ice [courtesy NSIDC, see http://nsidc.org/cgibin/words/topic.pl?sea%20ice for more]

Grease ice and small pancake ice Virginia Institute of Marine Science, http://web.vims.edu/bio/microbial/nbpice.html?svr=www

Small and medium pancake ice Virginia Institute of Marine Science, http://web.vims.edu/bio/microbial/nbpice.html?svr=www

Medium pancake ice Virginia Institute of Marine Science, http://web.vims.edu/bio/microbial/nbpice.html?svr=www

Large pancake ice forming pack ice Virginia Institute of Marine Science, http://web.vims.edu/bio/microbial/nbpice.html?svr=www

Pack ice Virginia Institute of Marine Science, http://web.vims.edu/bio/microbial/nbpice.html?svr=www

Iceberg in pack ice Virginia Institute of Marine Science, http://web.vims.edu/bio/microbial/nbpice.html?svr=www

Ridged (left) and undeformed (right) first-year ice, near Barrow. Ridging results from velocity convergence [photo by M. Serreze].

Top: Multiyear ice, defined as ice that has survived at least one melt season. Multiyear ice has a very low salinity (2 psu or so) and is a suitable source of drinking water. Bottom: A summer melt pond on sea ice. This is a shallow one. A deep one on multiyear ice can drown a snow machine [courtesy NSIDC, see http://nsidc.org/cgibin/words/topic.pl?sea%20ice for more.

Courtesy John Wettlaufer, presented at 2007 Woods Hole GFD summer program

Ice concentration for the Arctic and peripheral seas (see color bar) for the period 5-11 January, 1994, based on the analysis of the National Ice Center [courtesy K. Knowles, NSIDC]

Sea ice motion, thickness and deformation

Mean annual sea ice drift in the Arctic, based on data from the IABP, the North Pole program and other sources with overlay of sea level pressure from NCEP/NCAR [ice drift field courtesy of I. Rigor, Polar Science Center, University of Washington, Seattle, WA]. Since ice responds to wind forcing, the mean drift pattern broadly relates to the sea level pressure distribution. Ice export from the Arctic Ocean to the North Atlantic is primarily through Fram Strait, between Greenland and Svalbard.

Ice motion can be quite variable at daily to monthly time scales. This map shows the mean pattern of sea ice drift in the Arctic for January 1991 based on SSM/I retrievals with overlay of mean sea level pressure from NCEP/NCAR [ice drift field courtesy of C. Fowler, University of Colorado, Boulder, CO, sea level pressure field by the authors].

Mean pattern of sea ice drift in the Arctic for January 1989 based on SSM/I retrievals with overlay of sea level pressure from NCEP/NCAR [ice drift field courtesy of C. Fowler, University of Colorado, Boulder, CO, sea level pressure field by the authors]. Note the very different pattern of ice motion compared to that shown on the previous slide.

MODIS satellite image showing the shear zone along the coast of the Canadian Arctic Archipelago. North is roughly to the right. Axel Heiburg Island is at the bottom. The image covers an area of approximately 526 km by 376 km, with a resolution of 250 m [courtesy of T. Haran, NSIDC, Boulder, CO].

Estimated distribution of mean winter sea ice thickness in the Arctic based in part on under-ice submarine sonar data [from Bourke and Garrett, 1987, by permission of Elsevier]. The ice is thickest along the coast of the Canadian Arctic Archipelago where the mean drift pattern promotes ridging.

Ice thickness estimated for the late winters of 2006, 2007 and 2008 based on measurements taken by NASA's ICESat laser altimeter [from NSIDC, courtesy of Ron Kwok, JPL]. Thickness can vary quite a bit from year to year. This variability seems to be superposed upon a downward trend in ice thickness over recent decades.

Time series of sea ice divergence (in percent per day) in the vicinity of the SHEBA station calculated at four different spatial scales (average sizes) centered on the station, based on the RADARSAT RGPS [from Stern and Moritz, 2002, by permission of AGU]. Divergence (positive values in the plot) results on openings (leads) that in winter may quickly refreeze. Convergence results in closing (increased ice concentration) and ridging. Convergence/divergence is quite episodic in nature, with big events typically associated with the passage of weather systems and their associated wind patterns.

A pressure ridge in the Chukchi Sea resulting from convergent ice motion. This one is about 3 m wide and about 3 m high. Virginia Institute of Marine Science, http://web.vims.edu/bio/microbial/nb Pice.html?svr=www See ridging animation: http://www.crrel.usace.army.mil/sid/hopkins_files/seaice/movies/ridgemovie.mpg

Lead in Chukchi Sea about 25 m wide Virginia Institute of Marine Science, http://web.vims.edu/bio/microbial/nbpice.html?svr=www

Case studies: September ice extent for 2007 vs. 2008

Sept 14, 2008: 4.51 million sq. km Ice extent for September 2007 and 2008 was the lowest and second lowest, respectively, recorded in the satellite area. The map shows ice extent for at the September 2008 minimum (bluish) and for 2007 (yellow line), The time series at the bottom snow September ice extent from 1979 through 2008 and daily extent for 2007, 2008 and from climatology [from Serreze et al., 2008]. Sept 16, 2007: 4.13 million sq. km

Even More Young, Thin Ice than in Spring 2007 Estimated Ice Age from Tracking Algorithm The setup: For both years the melt season started out with lots of young, thin ice prone to melting out in summer [courtesy J. Maslanik, Univ. CO Boulder]

As seen in fields of temperature anomalies at the 925 hpa level, the melt season (May-August) was warm in 2008 (lots of melt) but not as warm as in 2007. This helps to explain the slightly higher ice extent for September 2008 compared to 2007 [J. Stroeve, NSIDC, and NOAA Climate Diagnostics Center]. The Melt Season was Not as Warm as in 2007

The warmer conditions over the 2007 melt season are related to a dipole anomaly circulation of the atmosphere at sea level, with high pressure centered north of the Beaufort Sea and low pressure over Eurasia. Winds between these two pressure features were strong and from the south, hence bringing in lots of heat into the Chukchi and East Siberian seas. The southerly winds also pushed ice away from the shore, leaving areas of open water. There was a also a dipole circulation in summer 2008, but not as well developed [J. Stroeve, NSIDC and NOAA Climate Diagnostics Center].

Compare and contrast the previous slides with the 925 hpa temperature anomaly and sea level pressure fields averaged for May-August of 1996. Air temperature anomalies are negative over much of the Arctic Ocean. There is no dipole anomaly, but rather a mean low pressure cell over the central Arctic Ocean. Sea ice extent in September 1996 was the highest for that month in the satellite record [NOAA Climate Diagnostics Center].

The Fram Strait outflow, the thermohaline circulation and the Arctic back door

Blue paths represent deepwater currents, red paths represent surface currents [courtesy Wikipdia]. Warm surface waters in the north Atlantic travel northward and then cool and sink in the Greenland/Iceland/Norwegian seas (the GIN seas). Disruption of this sinking in the GIN seas could have widespread climate effects. Changes in freshwater export via Fram Strait is a potential source of disruption. See the animation: http://svs.gsfc.nasa.gov/vis/a000000/a003600/a003658/

The Mean Annual Freshwater Budget of the Arctic The biggest oceanic sinks of freshwater in the Arctic Ocean are the transports of ice and low salinity water through Fram Strait and of low salinity water through the straits of the Canadian Arctic Archipelago. The biggest oceanic source of freshwater is the import of low salinity water through Bering Strait. From Serreze et al., 2006

Mean annual cycle of the ice volume flux through Fram Strait. The dotted line shows the 1950-2000 parameterized mean monthly flux. The solid line shows the volume flux adjusted for the mean ice thickness observed from 1990-1996 using upwardlooking sonar [from Vinje, 2001, by permission of AMS]. There is about a factor of two difference in the flux between winter and summer.

Mean patterns of sea ice drift in the Arctic for winter (top) and for summer, (bottom) based primarily on data from the IABP, the North Pole program and other sources with overlay of sea level pressure from NCEP/NCAR [ice drift field courtesy of I. Rigor, Polar Science Center, University of Washington, Seattle, WA, sea level pressure field by the authors]. Note he stronger ice flux through Fram Strait in winter, consistent with the stronger sea level pressure gradient across the strait and hence stronger winds.

Time series of the parameterized monthly ice volume flux through Fram Strait (grey columns) and the 12-month running mean (black line) [from Vinje, 2001, by permission of AMS]. The flux is quite variable.

Global mean surface salinity The North Atlantic is saltier than the north Pacific, hence sea surface height is higher in the North Pacific. This means a flow of water downhill from the Pacific to the Atlantic across the Arctic Ocean NOAA, World Ocean Atlas, http://www.nodc.noaa.gov/oc5/woa05/ 2005

Conceptual model of the present-day atmospheric and oceanic transports and its consequences. DM is diapycnal mixing, PA (PP) is pressure in the Atlantic (Pacific) and S is salinity [from Stigebrandt, 2000, by permission of Springer-Verlag]. The upper conveyor is the back door, which transports low salinity water from the North Pacific through Bering Strait, through the Arctic Ocean and into the North Atlantic.