Variability Of Sea Ice Cover Research Articles

Variability of sea-ice conditions in the Fram Strait over the past 30,000 years

Sea ice is a critical component of the climate and oceanographic system in the North Atlantic Ocean. A biomarker record reveals millennial-scale and glacial–interglacial fluctuations in sea-ice coverage in the northernmost Atlantic Ocean over the past 30,000 years. Sea ice is a critical component of the climate system: variations in sea-ice cover affect the albedo of polar regions, and also the rate of deepwater formation1,2. Changes in the sea-ice cover of the North Atlantic Ocean are thought to have been related to abrupt climate changes throughout the last glacial termination3, but reconstructions of sea-ice conditions are rare. Here we use the sedimentary abundance of the IP25 and brassicasterol biomarkers, produced by sea-ice-associated diatoms and open-water phytoplankton, respectively, to generate a record of sea-ice conditions in the northernmost Atlantic Ocean for the past 30,000 years. Our reconstruction shows that a stationary margin between sea-ice cover and the open ocean existed during the Last Glacial, although perennial sea-ice cover prevailed for most of the Last Glacial Maximum. An early warming about 14,000 years ago was associated with ice-free conditions; however, seasonal sea ice was present throughout the Holocene. We find temporal links between our record of sea ice and reconstructions of the amount of relatively warm Atlantic water advected into the Nordic Seas4,5. We therefore conclude that changes in sea-ice conditions are linked to regional and global climate anomalies and oceanographic circulation in the North Atlantic.

Weddell Sea anomalies: Excitation, propagation, and possible consequences

Antarctic marginal seas are susceptible to significant decadal variability as revealed by the analysis of a 200‐year integration of a regional ice‐ocean model forced with the atmospheric output of the IPCC climate model ECHAM5‐MPIOM. The strongest signal occurs on the southern and western Weddell Sea continental shelf where changes in bottom salinity are initiated by a variable sea ice cover and modification of surface waters near the Greenwich meridian. Related zonal shifts of the western rim current guide deep waters with different temperature out of the Weddell Sea. With a deep boundary current the temperature signal propagates westward through southern Drake Passage and along the upper continental rise in the southeast Pacific thereby influencing the hydrographic conditions on the continental shelf of Bellingshausen, Amundsen, and Ross Seas.

Observations of sea ice interannual variations and spring bloom occurrences at the Japanese scallop farming area in the Okhotsk Sea using satellite imageries

The dynamics of ice formation and phytoplankton bloom development in the coastal region of the Okhotsk Sea, Hokkaido, where the Japanese scallop, Mizuhopecten yessoensis, are cultured were investigated using seven years (1998–2004) satellite data from the Special Sensor Microwave/Imager (SSM/I) and Sea-viewing Wide Field-of-view Sensor (SeaWiFS). The interannual variability of sea ice cover and timing of spring bloom occurrences were analyzed. Longer ice cover in 1999, 2001 and 2003 with the presence of ice until early April and shortened ice cover in 1998, 2000, 2002 and 2004 with the occurrence of ice until early March were recognized at this area. Variability in the timing of sea ice retreat and development of spring blooms at the scallop areas were observed. Progression of a single ice edge bloom showed higher Chl- a concentration compared to development of an initial ice edge bloom followed by a later open water bloom. Higher concentration of phytoplankton biomass was observed in the initial bloom when sea ice melting is delayed compared to when the sea ice leaves earlier. Wind events were also observed to affect the occurrences of spring bloom.

Variability of sea ice cover in the Chukchi Sea (western Arctic Ocean) during the Holocene

Dinocysts from cores collected in the Chukchi Sea from the shelf edge to the lower slope were used to reconstruct changes in sea surface conditions and sea ice cover using modern analogue techniques. Holocene sequences have been recovered in a down‐slope core (B15: 2135 m, 75°44′N, sedimentation rate of ∼1 cm kyr−1) and in a shelf core (P1: 201 m, 73°41′N, sedimentation rate of ∼22 cm kyr−1). The shelf record spanning about 8000 years suggests high‐frequency centennial oscillations of sea surface conditions and a significant reduction of the sea ice at circa 6000 and 2500 calendar (cal) years B.P. The condensed offshore record (B15) reveals an early postglacial optimum with minimum sea ice cover prior to 12,000 cal years B.P., which corresponds to a terrestrial climate optimum in Bering Sea area. Dinocyst data indicate extensive sea ice cover (>10 months yr−1) from 12,000 to 6000 cal years B.P. followed by a general trend of decreasing sea ice and increasing sea surface salinity conditions, superimposed on large‐amplitude millennial‐scale oscillations. In contrast, δ18O data in mesopelagic foraminifers (Neogloboquadrina pachyderma) and benthic foraminifers (Cibicides wuellerstorfi) reveal maximum subsurface temperature and thus maximum inflow of the North Atlantic water around 8000 cal years B.P., followed by a trend toward cooling of the subsurface to bottom water masses. Sea‐surface to subsurface conditions estimated from dinocysts and δ18O data in foraminifers thus suggest a decoupling between the surface water layer and the intermediate North Atlantic water mass with the existence of a sharp halocline and a reverse thermocline, especially before 6000 years B.P. The overall data and sea ice reconstructions from core B15 are consistent with strong sea ice convergence in the western Arctic during the early Holocene as suggested on the basis of climate model experiments including sea ice dynamics, matching a higher inflow rate of North Atlantic Water.

Numerical simulation of ice-ocean variability in the Barents Sea region

A dynamic–thermodynamic sea ice model has been coupled to a three-dimensional ocean general circulation model for the purpose of conducting ocean climate dynamical downscaling experiments for the Barents Sea region. To assess model performance and suitability for such an application, the coupled model has been used to conduct a hindcast for the period 1990–2002. A comparison with available observations shows that the model successfully tracks seasonal and inter-annual variability in the ocean temperature field and that the simulated horizontal and vertical distribution of temperature are in good agreement with observations. The model results follow the seasonal and inter-annual variability in sea ice cover in the region, with the exception that the model results show too much ice melting in the northern Barents Sea during summer. The spatial distribution of the winter simulated sea ice cover is in close agreement with observations. Modelled temperatures and ice concentrations in the central Barents Sea are biased too high and too low, respectively. The probable cause is too high inflow of Atlantic Water into the Barents. The seasonal and inter-annual fluctuations in temperature and sea ice cover in the central Barents are, however, in excellent agreement with observations. Salt release during the freezing process in the numerical simulation exhibits considerable inter-annual variability and tends to vary in an opposite manner to the net inflow volume flux at the western entrance of the Barents Sea. Overall, the model produces realistic ice-ocean seasonal and inter-annual variability and should prove to be a useful tool for dynamical downscaling applications.

Variation in sea ice cover on the east coast of Canada from 1969 to 2002: climate variability and implications for harp and hooded seals

There is growing evidence that global change and climate variability are affecting sea ice dynamics in northern and eastern Canada. Such variability could have serious consequences for harp seals Pagophilus groenlandicus and hooded seals Cystophora cristata, which congregate to whelp on ice every February and March in the Gulf of St. Lawrence and off Newfoundland. We com- bined a numerical and spatial analysis of weekly averages of ice data to examine the variability of ice cover in eastern Canada during February and March, 1969 to 2002. Sea ice cover varied cyclically in eastern Canada during that time and exhibited a period of light ice years between 1996 and 2002. Spring thaw generally results in a significant reduction of sea ice cover throughout much of the study area, although some regions exhibit increases associated with oceanographic phenomena. Heavy ice years correlate with positive spring North Atlantic Oscillation (NAO) conditions and an in-depth analysis of highly anomalous (+/-) years (1970, 1972, 1981, 1993 and 2001) revealed consistent spa- tial and numerical patterns in ice dynamics. During light ice years, a synchronous and dramatic reduction in sea ice cover occurred in the Gulf and off eastern Newfoundland in the first and second week of March, coinciding with peak pupping periods for harp seals. Light ice years and rapid reduc- tions in sea ice represent unquantified risks for pagophilic seals. These include increases in neonatal mortality, changes in food availability for pups and, possibly, increased risk of epizootics due to crowding on whelping patches. The magnitude of these risks may increase if observed changes in cli- mate reduce sea ice cover in eastern Canada as they have in the Arctic. Patterns in sea ice cover and NAO conditions can be incorporated into short- and long-term management schemes aimed at ensur- ing the sustainability of commercially exploited pagophilic seal populations.

Sea-ice switches and abrupt climate change.

We propose that past abrupt climate changes were probably a result of rapid and extensive variations in sea-ice cover. We explain why this seems a perhaps more likely explanation than a purely thermohaline circulation mechanism. We emphasize that because of the significant influence of sea ice on the climate system, it seems that high priority should be given to developing ways for reconstructing high-resolution (in space and time) sea-ice extent for past climate-change events. If proxy data can confirm that sea ice was indeed the major player in past abrupt climate-change events, it seems less likely that such dramatic abrupt changes will occur due to global warming, when extensive sea-ice cover will not be present.

A predator's perspective on causal links between climate change, physical forcing and ecosystem response

The mechanisms by which variability in sea ice cover and its effects on the demography of the Antarctic krill Euphausia superba cascade to other ecosystem components such as apex preda- tors remain poorly understood at all spatial and temporal scales, yet these interactions are essential for understanding causal links between climate change, ecosystem response and resource monitor- ing and management in the Southern Ocean. To address some of these issues, we examined the long- term foraging responses of Adelie penguins Pygoscelis adeliae near Palmer Station, western Antarc- tic Peninsula, in relation to ice-induced changes in krill recruitment and availability. Our results suggest that (1) there is a direct, causal relationship between variability in ice cover, krill recruitment, prey availability and predator foraging ecology, (2) regional patterns and trends detected in this study are consistent with similar observations in areas as far north as South Georgia, and (3) large- scale forcing associated with the Antarctic Circumpolar Wave may be governing ecological inter- actions between ice, krill and their predators in the western Antarctic Peninsula and Scotia Sea regions. Another implication of our analyses is that during the last 2 decades in particular, krill pop- ulations have been sustained by strong age classes that emerge episodically every 4 to 5 yr. This raises the possibility that cohort senescence has become an additional ecosystem stressor in an envi- ronment where ice conditions conducive to good krill recruitment are deteriorating due to climate warming. In exploring these interactions, our results suggest that at least 1 'senescence event' has already occurred in the western Antarctic Peninsula region, and it accounts for significant coherent decreases in krill abundance, predator populations and predator foraging and breeding perfor- mance. We propose that krill longevity should be incorporated into models that seek to identify and understand causal links between climate change, physical forcing and ecosystem response in the western Antarctic Peninsula region.

Sea-ice cover, sea-surface salinity and halo-/thermocline structure of the northwest North Atlantic: modern versus full glacial conditions

Seasonal sea-ice develops along the eastern continental margins in the northern North Atlantic, where freshwater and/or meltwater outflow are responsible for relatively low salinity in surface waters and very pronounced water mass stratification. Sea-ice constitutes a major parameter in the marine ecosystem since the duration and extent of its seasonal spreading constrain the plankton distribution and the related microfossil assemblages on the sea floor. Organic-walled dinoflagellate cysts that are highly resistant to dissolution were recovered from surface sediments of the northern North Atlantic, and used to develop transfer functions (best analogue method) for the reconstruction of the seasonal spreading and duration of sea-ice cover, in addition to salinity and temperature of the warmest month of the year. Application of the best analogue approach to cores from the Labrador Sea reveals large variations in sea-ice cover and sea-surface conditions throughout the last glacial stage and during the early Holocene. Isotopic analyses in epipelagic and mesopelagic planktonic foraminifers also suggest important changes in salinity and temperature gradients between the surface and sub-surface water masses. Specific study of the last glacial maximum LGM time slice (16–20 ka on a 14 C time scale) in the northwestern North Atlantic shows much more extensive sea-ice than at present, with perennial sea-ice lying along the continental margins of eastern Canada. Seasonal spreading of the cover of sea-ice offshore was accompanied by large seasonal contrasts in temperature, with very cold winters but relatively warm summers, a pattern linked to strong stratification between a buoyant low saline surface layer having a low thermal inertia, and the underlying intermediate oceanic waters.

A numerical model for interdecadal variability of sea ice cover in the Greenland-Iceland-Norwegian Sea

A coupled ocean-sea ice-atmosphere model is used to study interdecadal variability (∼40 years) of sea ice depth and concentration in the Greenland-Iceland-Norwegian Sea. This oceanic region is represented by a meridionally aligned channel on a β-plane with open zonal boundaries at 60 °N and 80 °N. The model consists of a one and a half layer reduced gravity ocean model, a thermodynamic/dynamic sea ice model and an energy balance model of the atmosphere. The coupled model is driven by prescribed surface wind stress, fluxes of heat, salt and ice at inflow points on the northern and southern open zonal boundaries and annual distribution of solar radiation. It is shown that the coupled model supports unforced modes of interdecadal oscillation resulting from a form of hydraulic control which regulates the total fluid volume in the oceanic active layer. The mechanism for the oscillations relies on the presence of three key features: (1) a region of intense oceanic entrainment located in the eastern part of the domain, (2) a vigorous southward flowing western boundary current, representing the East Greenland Current (EGC), which supports most of the meridional transport across the domain, and (3) a marked buoyancy contrast between the relatively salty domain interior and the much fresher western boundary region. During an oscillation excess water is pumped into the domain via entrainment, thereby creating an active layer depth anomaly, which then propagates westward via long baroclinic Rossby waves until it reaches the EGC where it is subsequently drained out of the domain across the southern open zonal boundary. As the depth anomaly traverses the basin, an anomalous geostrophic circulation is established in which cold fresh Arctic water enters the domain interior, and this eventually promotes enhanced thermodynamic sea ice growth. Consequently, the interdecadal oscillations of the coupled model are characterised by pulse periods, typically spanning 20 years, during which there is an abnormally large winter sea ice cover, separated by interpulse periods, lasting another 20 years, during which the winter sea ice extent is nearly uniform and significantly smaller than in a pulse maximum. The duration of the interpulse periods is dictated by the time it takes for the Rossby waves to traverse the basin. In addition to the interdecadal oscillation solution, the coupled ocean-sea ice-atmosphere model is found to also have a stable cyclostationary state, with no interannual variability. Stochastic forcing, in the form of randomly specified interannual anomalies of salinity (of maximum amplitude 0.1 ppt) or ice inflow (of maximum amplitude 0.1 Sv) at the northern open zonal boundary, in both cases is capable of driving the model from the cyclostationary state solution to the interdecadal variability one.

Year‐to‐year variations in Bering Sea ice cover and some consequences for fish distributions

The southernmost extension of winter ice cover varies interannually and on longer time scales, reflecting large‐scale changes in driving forces, especially in the position and intensity of the winter Aleutian Low Pressure System. A conspicuous pattern is alternating warm and cool periods of several years' duration. These variations in sea ice cover are reflected in the character of a subsurface cold pool, formed as stratification isolates the deeper cold waters from surface exchanges. The cold pool is better developed and more extensive in summers that follow deep southward penetration of winter sea ice. Interannual and decadal‐scale variations in the distributions of some fish stocks reflect those of ice and thermal conditions. In particular, the distribution of walleye pollock, Theragra chalcogramma, varies significantly with multiannual cool and warm years while Arctic cod, Boreogadus saida, is only present within the cold pool. The relation among climate variations, sea ice cover, subsurface thermal conditions, and fish distribution provides information on how climate affects marine ecosystems and may also have practical application in predicting fish distributions.

Evolution of the Norwegian Current and the Scandinavian Ice Sheets during the past 2.6 m.y.: evidence from ODP Leg 104 biogenic carbonate and terrigenous records

Records of biogenic and terrigenous components have been obtained from the interval corresponding to the last 2.6 m.y. of ODP Sites 643 and 644 in order to reconstruct surface and deep water regimes in the Norwegian Sea. Surface water regimes record long lasting moderate glacial conditions during the interval 2.6–1.0 Ma. Small intrusions of Atlantic water episodically penetrated into the Norwegian Sea forming a narrow tongue along the eastern margin, which is documented at Site 644. The polar front was most probably situated between the Site 644 and 643 locations on the outer Vøring Plateau during these time intervals. Deep water regimes reflect long-term persistent corrosive bottom waters, most probably due to a weakly undersaturated water column and a low rate of carbonate shell production in surface waters. Deep water production in the Norwegian-Greenland Sea may have operated in a different way, e.g. brine formation during winter sea ice growth. Bottom waters were oxygenated throughout the entire period, and deep water was exchanged persistently with the North Atlantic. Increased glacial/interglacial environmental contrasts are documented, reflecting a strengthening of the Norwegian Current and intensified glaciations on the surrounding land masses during the interval 1.0–0.6 Ma. During this time a major shift in the mode of deep water production occurred. The onset of large amplitudes in glacial/interglacial environmental conditions with maximum contrasts in surface water regimes, different modes of deep water production, and intensified exchange with the North Atlantic marks the last 0.6 Ma. A broad development of the Norwegian Current is observed during peak interglacials, while during glacials seasonally variable sea ice cover and iceberg drift dominate surface water conditions.

Sea Ice Cover and Related Atmospheric Conditions in Arctic Canada During the Summer of 1978

Abstract Maps are presented of sea ice cover and open water in the Canadian Arctic for the final day of each month, June–September, 1978. The maps are derived from NOAA satellite imagery and show an extent of open water which is considerably smaller than that in the summers of 1975–77. Areal and temporal variations in sea ice cover and open water in the Canadian Arctic are related to atmospheric conditions particularly the locations of high- and low-pressure cells and the prevailing winds over the region and to melting degree days at four selected stations. The prevailing winds bring warm or cold air masses which increase or decrease air temperatures/melting degree days and consequently speed up or delay the time of ice melting. Moreover, strong prevailing winds depending on whether offshore or onshore take ice away from the roast or push ice toward the coast and thus cause a real variations of open water and ice cover.