C51A-0518
Acceleration of sea-ice Melt Addition Into the Northern North Atlantic
Observations over the last decade suggest both a thinning of the Arctic sea-ice cover (Rothrock et al., 1999; Laxon et al., 2003) and a dramatic reduction in its spatial extent (Comiso, 2002; Stroeve et al., 2005). We detect a positive change of ~10 ‰ between 1998 and 2005 in the stable oxygen isotope composition of the net freshwater component in the East Greenland Current (EGC) and East Greenland Coastal Current (EGCC), key carriers of freshened surface waters out of the Arctic. This isotopic signal is unique in the context of oxygen isotope measurements in the northern North Atlantic region dating back to the early 1960s. We show that this signal reflects a remarkable increase in the sea-ice melt water transport within these currents, to the equivalent of a net minimum volumetric loss of multi-year ('permanent') Arctic sea-ice of 10 ± 3% per decade. This independent measurement therefore complements and corroborates the sea-ice reduction inferred from satellite data, and places it in a longer-term multi-decadal context. Our findings suggest that a large proportion of the sea-ice meltwater resulting from the rapid reduction of Arctic sea-ice between 1998 and 2005 is exported from the Arctic via the EGC/EGCC into the northern North Atlantic. Additionally it appears that this sea-ice meltwater export is not in phase with atmospheric circulation regimes such as the North Atlantic Oscillation and the Arctic Oscillation. These findings could have important ramifications for the global thermohaline circulation (e.g. Rahmstorf, 2005).
C51A-0519
Pacific water transport in the Arctic Ocean in a variable GM diffusivity model
One of the crucial problems in the present Arctic Ocean modeling is a significant high-salinity bias in the
central Canada Basin. Improvement of such an ocean structure in simulation by using sophisticated
numerical models is necessary in order to clarify mechanisms of recent drastic change of Arctic sea ice
extent, which is suggested to be induced by changes in not only wind stress and atmospheric warming but
also ocean heat transport. It is indicated that one of possible reasons for the salinity bias is insufficient
transport of the Pacific water from the Chukchi shelf to the basin by mesoscale eddies.
In this study, the Pacific water transport and corresponding salinity distribution in the Arctic Ocean in a
coupled sea ice-ocean model, which incorporates a modified Gent and McWilliams diffusion scheme, are
investigated. Three experiments are conducted as follows. First, a coefficient of isopycnal layer thickness
diffusion is uniformly fixed to a lower value. Second, the coefficient is set to a higher value. Third, the
coefficient temporarily and spatially varies depending on local baroclinicity. In the lower diffusivity case, a
high-salinity bias arises in the Canada Basin and a low-salinity bias appears in the Eurasian Basin. In this
simulation, a large amount of the fresh Pacific water is improperly transported westward over the Siberian
shelves, whereas it is scarcely involved into the Canada Basin. This pathway is inconsistent with the
observational estimates by using chemical properties. Therefore, insufficient transport of the Pacific water
across the Beaufort shelfbreak is specified as one of crucial factors for the salinity bias. In the variable
diffusivity case, the parameterized eddy-induced transport of the Pacific water into the Canada Basin is
locally promoted by reflecting the strong baroclinicity along the shelfbreak. The excessive intrusion of the
Pacific water into the Eurasian Basin significantly decreases. This improvement remarkably reduces the
seesaw-like salinity biases in the two basins. The uniform large diffusivity also promotes the shelf-to-basin
transport of the Pacific water. However, a bowl-shaped structure in the Canada Basin becomes flattened, and
then high-salinity bias rather increases. Consequently, an appropriate representation of the eddy-induced
shelf-to-basin transport is essentially important for further progress in the Arctic Ocean modeling.
http://research.iarc.uaf.edu/~ejnabe/
C51A-0520
The Beaufort Gyre Fresh Water Reservoir: State and Variability From Observations
We investigate basin-scale mechanisms regulating anomalies in fresh water content (FWC) in the Beaufort
Gyre (BG) of the Arctic Ocean using historical observations, data collected in 2003-2007 by the Beaufort
Gyre Exploration Project, and measurements obtained from drifting Ice-Tethered Profilers. The major cause
of the large FWC in the BG is the process of Ekman pumping associated with the climatological anticyclonic
atmospheric circulation over the Canada Basin centered in the BG. The mechanically-forced seasonal
variability of FWC in the central BG follows wind curl changes with a maximum in November – January and a
minimum in June-August tracking seasonal changes in the atmospheric circulation. The atmospheric and
oceanic thermal regimes regulate seasonal transformations of liquid FWC due to the seasonal cycle of sea
ice melt and growth. Combination of the two mechanisms, reflected in the seasonal cycle of total BG FWC,
has two pronounced peaks separated by approximately 3-4 months. The first peak (June-July) is observed
when the sea ice thickness reaches its minimum (maximum fresh water release from sea ice to the ocean)
and when the Ekman pumping is very close to its weakest. The second maximum is observed in November-
January when the wind curl is strongest (maximum Ekman pumping) and the salt flux from the growing sea ice
has not reached its maximum. One conclusion from this study is that the observational practice to sample the
Arctic Ocean hydrography in August-September (when the sea ice coverage is at its seasonal minimum and
the Arctic is accessible by research icebreakers) and April-May (using aircraft when the sea ice is sufficiently
strong and there is adequate daylight) misses natural FWC seasonal variability and underestimates the
seasonal variability of hydrographic fields, their gradients and circulation patterns.
http://www.whoi.edu/beaufortgyre
C51A-0521
Large Freshwater Anomalies in the Arctic Ocean: Results from the 2008 IPY/NPEO Hydrographic Survey
Based on an aerial hydrographic survey conducted in March and April, 2008, supplemented by unmanned drifting ice-tethered profilers (ITPs), we report that the precipitous decrease in minimum Arctic ice extent observed in the past few years has been accompanied by significant changes in upper ocean salinity, especially over the Canada Basin, where the anticyclonic Beaufort Gyre has traditionally maintained one of the major freshwater reservoirs of the world ocean. Our winter measurements corroborate and extend observations of increased summer freshwater content (FWC) first detected during the joint WHOI-IOS- JAMSTEC expedition in 2003 and monitored since. The survey comprised operations in the western Arctic that included 15 ice-landing stations and 8 airdropped expendable probes, plus 20 ice-landing stations in the eastern Arctic staged from ice station Barneo near the North Pole. We found that in the southeast quadrant of the Canada Basin, anomalous FWC (i.e., the change relative to PHC 3.0 winter [March-April-May] climatology based predominantly on conditions in the 1970s) has increased by as much as 11 m. Positive anomalies were found at all stations in the Pacific sector, including ITP profiles, but their magnitudes decreased to the west and north. In the eastern Arctic we found negative FWC anomalies on the Eurasian side of the Lomonosov Ridge, reaching values as low as -5 m. Smaller positive anomalies characterized water in the Makarov Basin. Freshening of the upper ocean over the Canada Basin has also substantially changed steric levels. A west- to-east line of stations extending about 800 km across the Basin, centered near 75°N, 150°W, nearly bisected the traditional Beaufort Gyre. In contrast to the domed climatological dynamic topography typical of the Gyre, the 2008 survey showed a monotonic rise in dynamic height as far east as 135°W, indicating a northward surface geostrophic flow component across the entire section, with large impact on freshwater transport.
C51A-0522
Intrusion of Warm Bering/Chukchi Waters Onto the Shelf in the Western Beaufort Sea
Wind-driven changes in the path of warm Bering/Chukchi waters carried by the Alaska Coastal Current (ACC) through Barrow Canyon during late summer are described from high-resolution hydrography, acoustic Doppler current profiler measured currents, and satellite-measured sea surface temperature imagery acquired mid-August to mid-September 2005-2007 near Barrow, Alaska. Numerical simulations are used to provide a multi-decadal context for these observational data. Four generalized wind regimes and associated circulation states are identified. When winds are from the east or east-southeast, the ACC jet tends to be relatively strong and flows adjacent to the shelf break along the southern flank of Barrow Canyon. These easterly winds drive inner shelf currents northwestward along the Alaskan Beaufort coast where they oppose significant eastward intrusions of warm water from Barrow Canyon onto the shelf. Because these easterly winds promote sea level set down over the Beaufort shelf and upwelling along the Beaufort slope, the ACC jet necessarily becomes weaker, broader, and displaced seaward from the Beaufort shelf break upon exiting Barrow Canyon. Winds from the northeast promote separation of the ACC from the southern flank of Barrow Canyon and establish an up-canyon current along the southern flank that is fed in part by waters from the western Beaufort shelf. When winds are weak or from the southwest, warm Bering/Chukchi waters from Barrow Canyon intrude onto the western Beaufort shelf.
C51A-0523
River Runoff is the Dominant Source of Freshwater to the Central Arctic Ocean and Beaufort Sea during Spring 2008
Bottle chemistry data (salinity, stable oxygen isotopes, nutrients, barium, and total alkalinity) were collected from twenty stations in the central Arctic Ocean and fifteen stations in the southern Canada Basin/Beaufort Sea during March and April of 2008 thanks to an expanded effort of the North Pole Environmental Observatory (NPEO) within the International Polar Year. Freshwater contributions from sea-ice melt, meteoric water, and Pacific water were quantified in these two regions to track the position of the front separating Atlantic and Pacific water as well as the influence of river runoff in the central Arctic Ocean. A partial return of the general hydrography toward near-climatological conditions was observed during NPEO 2004-2005, but was followed in 2006-2007 by a reversion to a regime more typical of the early 1990s. Continuing changes are expected as the Arctic Oscillation index has progressed back toward a more cyclonic mode during the winter of 2006-2007 and 2007-2008. River runoff was the largest contributor of freshwater to upper 120 meters of the central Arctic sections as Pacific water contributions were < 30%, except over the southern Alpha Ridge. The largest contributions of river water were located over the Makarov Basin and Mendeleev-Alpha Ridge system. River runoff was the largest source of freshwater (above 190 meters depth) at the Beaufort Sea stations, much larger than ice melt despite the record retreat in sea-ice summer 2007. Positive fractions of sea ice melt >3% were found at only five stations located in the easternmost section. The summer 2008 occupation of these stations (Beaufort Gyre Exploration Project) will offer a rare opportunity to compare seasonal changes in freshwater content and distribution. A preliminary analysis using barium to separate Siberian from North American river runoff varieties suggests North American river runoff contributions to the upper 80 meters of the water column were < 2% at all stations except 73N,140W (4%). However, recent studies have cast barium in doubt as a conservative tracer due to the decreasing ice extent. Ongoing total alkalinity analyses will answer the question of whether North American or Eurasian river waters dominate.
C51A-0524
Joint Effects of Boundary Currents, Thermohaline Intrusions and Gyre Circulation on the Recent Warming of Atlantic Water in the Canada Basin: 1993-2007
The 1990-91 influx Atlantic water, both anomalously warm and in greater volume than in the past, enveloped the Chukchi Borderland in the western Canada Basin by 2002 and has spread across the southeastern Canada Basin by 2007. Warmer, younger (more ventilated) and less dense Fram Strait Branch waters have displaced colder, older and denser ambient waters, increasing the temperature of the Fram Strait Branch core from a fifty-year or more mean of ~0.45 oC to ~0.7 oC. Physical and geochemical data collected from 1993-2007 show that the two main transport mechanisms are the boundary current and thermohaline intrusions, established by large thermal gradients. The boundary current operates in a cyclonic direction whereas the thermohaline intrusions operate in an anticyclonic direction due to the background of the Beaufort Gyre. The boundary current, a fully-pan Arctic structure, has slowed considerably in the Canada Basin, with effective speeds of ~ 0.5 cm/sec, similar to the effective spreading rate of the thermohaline intrusions (~0.3 cm/s). Our data also shows that the influence of the Beaufort Gyre on circulation extends into and perhaps deeper than the FSB of the Atlantic layer. The thermohaline intrusions show signs of dissipation near the Northwind Ridge in 2007 suggesting that, in the absence of similar influxes in the future, they would disappear from the Canada Basin with time.
C51A-0525
Mechanisms explaining anomalous ice conditions in the Beaufort Sea during 2006 and 2007
Unusual Beaufort Sea ice conditions, in Summers 2006 and 2007, are documented. Comparison of NASA Team ice concentration estimates against in-situ observations show that NASA team concentrations, were 30% lower than in situ observations for flooded ice, and 10% lower for refrozen ice. We show that the drift of sea ice into the Beaufort and divergence precondition recent Summer ice conditions. Intrusions of first year ice from the Chukchi Sea to the Northern Beaufort, and recent reduction in size of the Beaufort Gyre has led to reduced replenishment of older, multi-year ice in the western Beaufort, resulting in a younger, thinner ice pack in most of the Beaufort. However, during the Winter of 2006, an anomalous southward, then westward push of MY ice formed an ice tongue that survived the Summer melt season. To the north of this tongue of MY ice, there is a trend over the last decade towards increasing late Winter pack divergence. This leads to 20-30% thin ice area of melting out earlier in Summer, which may precondition the accelerated Summer ice loss observed in recent years. Late Winter opening in 2007 was two times greater than previously observed. Our results support the hypothesis (Perovich et~al. 2008) that Summer 2007 thinning of MY ice was caused by an increase in solar absorption in the upper ocean due to lower sea ice concentration than normal, as the low ice concentration was partially driven by an anomalous opening event in the Beaufort Sea perennial ice pack in Spring 2007.
C51A-0526
Arctic sea ice retreat in 2007 follows thinning trend
The minimum of arctic sea ice extent in the summer of 2007 was unprecedented in the historical record. A coupled ice–ocean model is used to determine the state of the ice and ocean over the past 29 years to investigate the causes of this ice extent minimum within an historical perspective. We find that even though the 2007 ice extent was strongly anomalous, the loss in total ice mass was not. Rather, the 2007 ice mass loss is largely consistent with a steady decrease in ice thickness that began in 1987. Since then, the simulated mean September ice thickness within the Arctic Ocean has declined from 3.7 to 2.6 m at a rate of –0.57 m decade–1. Both the area coverage of thin ice at the beginning of the melt season and the total volume of ice lost in the summer have been steadily increasing. The combined impact of these two trends caused a large reduction in the September mean ice concentration in the Arctic Ocean. This created conditions during the summer of 2007 that allowed persistent winds to push the remaining ice from the Pacific side to the Atlantic side of the basin and more than usual into the Greenland Sea, exposing large areas of open water, resulting in the record ice extent anomaly.
C51A-0527
Ice Thickness Distribution and Bottom Topography in Beaufort Sea in Winter Preceding 2007 Ice Minimum
The unprecedented sea ice retreat in the Arctic Ocean (especially Beaufort Sea) during the summer of 2007 was characterised by exceptional bottom melting. We were able to characterise the thickness distribution of the sea ice during the preceding winter in parts of the Beaufort Sea which subsequently became completely ice-free, as well as in regions of heavier multi-year ice, by means of a transect in March 2007 using a submarine equipped with upward-looking and multi-beam sonar. The transect involved extensive survey work under the APLIS 2007 ice camp from which the SEDNA project (led by University of Alaska) subsequently took place. The upward sonar generated statistical properties of the ice thickness distribution which showed that the ice cover was sensitised for subsequent break-up under enhanced bottom melt because of (a) the prevalence of first-year ice which was unusually thin, (b) the relative absence of deep ridges, (c) the relative prevalence of first-year ridges which melt quickly through water percolation around ice blocks. This is in contrast to the structure of the ice thickness distribution in earlier years. The multi-beam sonar (with additional information from high-resolution studies of individual ridges by small AUVs in 2007 and 2008) yields special insight into the role played by ridge melt in the overall break-up process.
C51A-0528
A Case Study of the Annual Evolution of the Cape Bathurst Polynya Complex in Western Amundsen Gulf, 2003-2004
A case study focused on the spatial and temporal interaction of dynamic and thermodynamic sea ice processes in the southern Beaufort Sea (SBS) and Amundsen Gulf (AG) was conducted to determine the mechanisms which form the Cape Bathurst flaw lead polynya complex over an annual cycle. This study is motivated by the need to further understand the complex interaction of the Beaufort Sea perennial sea ice pack and a seasonal sea ice zone of the SBS and AG as it is manifested in the operation of the Cape Bathurst flaw lead polynya complex. Understanding the present interplay of thermodynamic and dynamic forcing in the polynya is vital should we hope to determine how it will operate in a future Arctic dominated by seasonal sea ice. The study area has been the subject of the Canadian Arctic Shelf Exchange Study and the International Polar Year – Circumpolar Flaw Lead Study. The case study presented here further attempts to provide context in terms the physical operation of the polynya and associated flaw lead system for the biological and oceanographic measurements that were made in the course of those studies. We first describe the regional scale evolution of sea ice in western AG between fall 2003 and summer 2004 representing sea ice formation through both dynamics and thermodynamics. We investigate fine scale sea ice thickness in western AG to determine the relative contribution of leads, ridges and level sea ice in spring 2004 prior to the formation of the polynya proper. The region's potential thermodynamic sea ice growth is modeled between freeze-up in 2003 and break-up in 2004. Finally, sea ice motion in the region between freeze-up and break- up in 2004 was used to help determine the relative contribution and timing of dynamic processes on the sea ice in our case study region. It is concluded that the formation of the Cape Bathurst polynya in spring results from the interaction of the Beaufort Sea perennial pack with seasonal sea ice in AG, beginning in fall.
C51A-0529
Oceanic Forcing of Arctic Sea Ice
The recent warming and dramatic summer reduction of sea ice cover in the Arctic Ocean so far have been
primarily associated with the atmospheric forcing and ice-albedo feedback. We analyze numerical model
output validated with available observations to determine the relative importance of the internal oceanic
forcing of sea ice melt. In particular, the thermodynamic coupling at the ice-ocean interface in the western
Arctic Ocean is investigated. Under-ice ablation by anomalously warm water advected from the Chukchi
shelves and distributed at the subsurface layer in the western Arctic Ocean by mesoscale eddies is found to
explain at least 60% of the total variance of sea ice thickness. We hypothesize that the excess oceanic heat,
that in recent years has been accumulating below the surface during summer, is a critical initial factor in
reducing ice concentration and thickness in the western Arctic Ocean at the early melting season and
onwards the following year. Observational data and more realistic model representation of feedback
processes between the upper ocean and the atmosphere under diminishing ice cover are critical to test this
hypothesis and to advance Arctic climate prediction.
http://www.oc.nps.edu/NAME/name.html
C51A-0530
On the need to include sea-ice tide interaction in assessing Arctic Environmental Change
We present new modeling results demonstrating that sea ice motion in the Arctic is strongly influenced by oceanic tides, and that sea ice models that do not include tidal forcing cannot completely capture sea ice motion, atmosphere-ocean heat fluxes and sea ice growth and decay. Output from a barotropic ice-tide model forced with NCEP derived winds have been validated against GPS buoys in a range of locations across the Arctic Basin. Our results demonstrate strong rotary coherence between the ice-tide model and buoy-derived ice velocities in the semi-diurnal (M2/S2) anti-clockwise band where the tidal amplitude exceeds about 5cm. When tidal forcing is removed from the model runs, no such coherence exists, and air- ice-ocean fluxes also differ, as does ice growth, indicating that oceanic tides influence the ice mass balance of the Arctic and should be included in global climate models.
C51A-0531
Accelerating Impetus of Atmospheric Circulation for the Recent Rapid Arctic Climate System Change and the Extreme Sea Ice Loss in 2007
Arctic climate system change has accelerated tremendously since the beginning of this century, and a
strikingly extreme sea ice coverage loss occurred in summer 2007, in the context of global warming climate.
These apparently could not be solely explained by the gradually-enhanced surface radiative forcing due to
greenhouse-gas-emissions forcing. Although a variety of contributing factors have been examined from
different aspects, the underlying fundamental physical mechanism and processes, which orchestrate these
contributors and synthetically drive the acceleration and the extreme event, still remain unclear. In this study,
we detected rapid, systematic and drastic spatial changes in atmospheric circulations in the latest decade,
which may serves as an important route that global warming forcing takes effects. These changes
considerably intensified both lateral polar-subpolar interactions in atmosphere and ocean, and local
atmosphere-ice-ocean interfacial interactions. These intensified interactions significantly strengthened
poleward atmospheric and oceanic heat energy transport and, in turn, accelerated sea ice reduction,
including the extreme sea ice coverage loss in summer 2007. Our results also suggest that radical spatial
shift of atmospheric circulations can extremely speed up global warming forced changes, leading to a rapid
climate change event. The identification of the atmospheric circulation changes may also shed light on the
recent argument about a tipping point of Arctic climate change, and provide skillful information for future
prediction.
http://www.iarc.uaf.edu/people/indiv/iarc_all_staff.php?photo=xzhang
C51A-0532
Why Arctic sea ice retreat appears to be most rapid in September
The annual minimum (September) Arctic sea ice extent has retreated rapidly in recent decades, while the annual maximum (March) ice extent has receded far more slowly. This is typically attributed to different physical factors operating during different seasons, which is further evidenced by the observed asymmetrical seasonal cycle in Northern Hemisphere sea ice extent. Here we examine the latitude of the sea ice edge averaged zonally over locations where it is free to migrate, in contrast with the standard practice of viewing Arctic sea ice changes from the perspective of total extent or area. We show that (i) the enhanced rate of September retreat compared with other months and (ii) the asymmetry of the seasonal cycle are both primarily a consequence of the geometry of the continents, which block southward ice extension during winter but have relatively little effect at annual minimum when the ice cover is confined to the Arctic Ocean. We find that the zonal mean latitude of the free ice edge has been migrating northward during the 1978--2007 era of satellite measurements at an annually constant (but accelerating) rate of ~9km/year, following a nearly sinusoidal seasonal cycle. These results suggest a change in perspective of the most critical quantities for understanding changes in Arctic sea ice.
C51A-0533
The effect of retreating summer ice on upwelling across the shelf-break of the Arctic Ocean.
During 2007 and 2008 the reduction in summer Arctic sea-ice extent led to larger ice-free areas over the Arctic shelves and shelf-break. Surface-stress on the Arctic Ocean depends on the wind if ice-free and on a combination of the wind and ice drift if ice-covered. The reduction in summer sea-ice extent would have modified the surface-stress and so led to changes to vertical mixing over the shelves, upwelling at the shelf- break and the subsequent flux of nutrients into the euphotic zone. Numerical simulations of upwelling across the Arctic shelf-break show a depth of upwelling from near the nutrient maximum of the Pacific Water and a bottom intensified shelf-break jet; both features essentially due to the change in slope Richardson number across the shelf-break. We make estimates of the increased nutrient flux to the Arctic shelves and the consequences to productivity for 2007 and 2008 based on this depth of upwelling, NCEP wind and observed ice conditions.
C51A-0534
The Role of the Pacific- North American (PNA) Pattern in the 2007 Arctic Sea Ice Decline
The extreme loss of sea ice over the western Arctic during the boreal summer of 2007 (July-August- September) was accompanied by a very unusual atmospheric circulation pattern. Here we show that the anomalous circulation was linked to a leading climate mode of the Northern Hemisphere, the Pacific- North American (PNA) pattern. The PNA index was three standard deviations above the 1950-2007 mean, and its primary signal in the atmospheric circulation is a strong anomalous anticyclone that was collocated with the location of the greatest Arctic sea ice decline. Therefore, the record- strength PNA along with recent climate trends in the Arctic help to explain the sudden and extreme sea ice melt during the summer of 2007. While the observed PNA was unique, the large decline in sea ice may be further evidence of increased vulnerability to natural atmospheric variability due to a changing climate system.
C51A-0535
The Arctic Oscillation and its Influence on Sea Ice and Polar Precipitation
The Arctic Oscillation (AO) is thought to be the most significant influence on polar climate, affecting cloud cover, sea surface temperature, heat flux, frequency and intensity of storms, sea ice variability and precipitation patterns. In this study, we examined the relationship between the AO, sea ice extent, and polar precipitation anomalies. We performed correlation analyses for three time intervals (1978-2008, 1978-1997, 1997-2008), comparing the AO with 5x5 degree land-based precipitation anomaly data averaged over 5 regions, as well as sea ice extents for the Greenland Sea, the Barents and Kara Seas, and Baffin Bay. Our results show that from 1978-2008, there are positive anomalies 73% of the time when the AO is in its positive phase, and negative precipitation anomalies 56% of the time when the AO is in its negative phase. There was a significant positive correlation for 1978-1997 between AO indices and precipitation anomalies over Greenland/Iceland (r=0.78) and Scandinavia (r=0.71). For the same period there was a weak positive correlation between the AO and Baffin Bay ice extent (r=0.46) and a weak negative correlation between the AO and Barents Sea ice extent (r=-0.25) that was likely due to the dominant cyclonic circulation in the Arctic. Correlation coefficients for Greenland/Iceland and Scandinavia precipitation anomalies (r=0.58 and 0.56, respectively), and Baffin Bay ice extent (r=0.44) decreased for the period 1997-2008, during which the AO index fluctuated between positive and negative on an approximately bi-yearly timescale, a higher frequency than previously recorded. The lack of a persistent positive or negative AO phase contributed to the decrease in correlation values and may indicate a change in the dominant polar atmospheric circulations. Alternatively, the AO may be approaching a neutral state. If the AO goes to neutral, we may not be able to anticipate changes in precipitation or sea ice since the AO only seems to have a strong relationship with precipitation and ice extent when it's positive. As the AO has been fluctuating over the past decade while sea ice extent has been steadily decreasing, it is likely that there are additional factors at work beyond the natural cycle of the AO.
C51A-0536
Atmospheric Forcing of the Beaufort Sea Ice Gyre: Surface-Stratosphere Coupling
Circulation of sea ice in the Beaufort Sea plays an important role in the overall sea ice mass flux and reduction in the areal extent of sea ice in the northern hemispere. In this paper we examine the nature of correspondence between synoptic weather patterns and reversals in the Beaufort sea ice Gyre, based on a catalogue of daily synoptic weather types generated for the Beaufort Sea region from 1979 to 2006. We then extend this analysis to examine the role of stratospheric forcing on surface phenomena. Investigated in particular is the correspondence between reversals in stratospheric winds at 10mb during winter as defined by stratospheric sudden warmings (SSW) and mean sea level pressure synoptic types in the Beaufort Sea region. The results from this analysis show that cyclonic and anticyclonic synoptic types are associated with reversals in the Beaufort Gyre throughout the annual cycle. Furthermore, investigation of stratospheric wind gradients averaged over the Beaufort Sea region demonstrates a prevalence in anticyclonic activity during SSWs that persists for approximately 20 days. Examination of the evolution in synoptic types in the Beaufort Sea region also shows an increase in the number of synoptic types associated with anticyclonic activity during SSWs.
C51A-0537
Changes in the Arctic Sea Ice-Climate System: The Role of the Atlantic Multidecadal Oscillation
Recent satellite observations suggest an arctic sea ice-climate system in rapid transformation, with sea-ice extent in summers 2007 and 2008 being the lowest ever recorded, far exceeding IPCC model ensemble forecasts. The mismatch between models and observations can arise from: 1) Model underestimation of the sea-ice response to external greenhouse-gas (GHG) forcing and 2) Non-modelled natural variability and processes. Regarding natural variability, the historical context of sea ice on long time scales is poorly known compared to interannual modes of variability. Here, we synthesize a set of long historical and paleo proxy records of sea ice, together with numerical model data. We find multidecadal 60-80 yr fluctuations to be a robust feature, most pronounced in the Greenland Sea. We show an apparent linkage with the Atlantic Multidecadal Oscillation (AMO) during the instrumental record, including the present sea ice- / AMO+ phase. Similar behavior through previous centuries is suggested from historical and paleo proxy sea-ice records from the region. Moreover, the AMO appears to have a role in both the early 20th and 21st century warming events. These findings may have implications for understanding recent and future changes, implying a return to AMO- could decelerate the recent GHG-induced arctic warming and sea-ice losses, unless a tipping point has already been passed.
C51A-0538
The Arctic Dipole Anomaly (DA) and sea ice transport: A mechanism responsible for the record ice minimum in 2007
This study identifies an atmospheric circulation anomaly-Dipole Anomaly (DA) in the arctic atmosphere, and its relationship with sea ice motion using datasets from NCEP Reanalysis for the period of 1948-2007. The DA is the second-leading mode of EOF (empirical orthogonal function) of monthly mean SLP north of 70 N during the all four seasons and accounts for about 13 percent of the variance. One of its two anomaly centers is over the Canadian Archipelago; the other is situated over northern Eurasia and the Siberian marginal seas. Due to the DA's strong meridionality, it becomes an important dynamic mechanism to drive both anomalous sea ice out of the Arctic Basin and cold air outbreaks into the Barents Sea, the Nordic Seas and northern Europe. When the DA remains in its positive phase (that is, negative SLP anomalies appear in northern Eurasia and the Siberian marginal seas with the concurrent positive SLP over the northern American and Greenland), there are large-scale changes in the intensity and character of sea ice transport in the Arctic Basin. The significant changes include a weakening of the Beaufort Gyre, an increase in sea ice export out of the Arctic Basin through Fram Strait and the northern Barents Sea, and enhanced sea ice import from the Laptev Sea and the East Siberian Sea into the Arctic Basin. Consequently, more sea ice appears in the Greenland and the Barents Seas during the positive phase of the DA. The record low Arctic sea ice in the summer of 2007 is found to be triggered by the Arctic Dipole Anomaly pattern. This local, second-leading mode in the Arctic produced a strong meridional wind anomaly that drove more sea ice out of the Arctic Ocean during the summer of 2007 from the western to the eastern Arctic into the northern Atlantic.
C51A-0539
The Role of Arctic Clouds During Intervals of Rapid Sea Ice Loss
The Arctic climate system appears to be veering quickly toward a much warmer, less icy state. GCMs have long projected this kind of shift, though the simulated rate of change and the amount of polar amplification are model-dependent. Some climate models also produce periods of rapid future climate change superimposed on the long-term warming trend. These intervals coincide with abrupt reductions in sea ice and have been dubbed "rapid ice loss events" (RILEs). Previous work has identified the existence of RILEs in several GCMs and attributed them in part to pulse-like increases in meridional ocean heat transport and the positive surface albedo feedback. Here we present new evidence that clouds may also play an important role in RILEs, based on ensemble simulations using NCAR's Community Climate System Model (CCSM3). Consistent with most GCMs, the CCSM3 projects a gradual increase in future cloudiness over the Arctic, as greenhouse warming takes hold this century. Although the simulated cloud increase occurs in all seasons, it is greatest during autumn and winter (when polar clouds exert the strongest surface warming signal) and appears to be driven mainly by enhanced evaporation from the warming Arctic Ocean. During one simulated RILE in a transient greenhouse simulation, the cloud coverage in CCSM3 varies seasonally to optimize the surface warming and promote ice melt. In this event, cloud anomalies become positive (negative) during autumn-winter (summer), such that both longwave and shortwave radiation at the surface are enhanced. This favorable combination for ice loss results in a sustained interval of positive cloud radiative forcing (CRF) anomalies that hinder ice growth and promote ice melt. We suggest that increased surface evaporation associated with sea ice retreat may be important for initiating and/or maintaining RILEs, in addition to stochastic variations in atmospheric circulation that occasionally favor seasonally optimal combinations of anomalous cloud amount.
C51A-0540
Interannual Variability of the Ekman Layer Processes in the Arctic Ocean
The daily fields of the sea level pressure, surface wind, sea-ice concentration and sea-ice motion over the whole Arctic Basin have been analyzed for the period of 1979-2006. The responses of the upper Arctic Ocean, including the Ekman transport, upwelling and downwelling, to interannual varying forcing are examined in this study. In the western Arctic, the anti-cyclonic wind stress and sea-ice motion force offshore Ekman transport away from the southern boundary and result in strong downwelling in the interior Beaufort Sea. There was significant variability on interannual time scales during a 28-year period from 1979 to 2006. The changes were most pronounced in winter. The atmospheric and sea-ice conditions during two periods, 1979-1986 (weak downwelling) and 1997-2004 (strong downwelling), were analyzed. The annual mean SLP over the western Arctic Basin was lower in 1997-2004 than in 1979-1986, consistent with previous studies. The seasonality of SLP had changed between these periods. The winter high had become higher and summer low became lower in 1997-2004 as compared with that in 1979-1986. The speed of the surface geostrophic wind was considerably higher in the winter of 1997-2004 than in 1979-1984. The difference of the annual-mean speed, however, was smaller. This change of seasonality may have a considerable impact on the air-water and air-ice stresses. A larger seasonal variation may result in a larger annual-mean stress due to the nonlinearity of the bulk formula. In fact, the sea ice motion speed had increased considerably from 1979-1986 to 1997-2004. Meanwhile, the reduction of sea-ice coverage exposed the Arctic Ocean to the direct wind-stress forcing and thus may also have contributed to the change of the Beaufort Sea downwelling.
C51A-0541
Melt Duration Variability and Sea Ice Conditions within the Canadian Arctic Archipelago: 1979-2007
The links between melt duration and sea ice conditions within the Canadian Arctic Archipelago (CAA) and its sub-regions were explored from 1979 to 2007. Melt duration was derived from passive microwave brightness temperatures and sea ice conditions were extracted from the Canadian Ice Service Digital Archive. Melt duration in the CAA is increasing at 6.0 days decade-1 which is statistically significant at the 99 percent confidence level. The longest melt durations within the CAA were 1998 (123 days), 2006 (118 days), and 1994 (115 days). All sub-regions within the CAA also exhibited positive slopes for melt duration and only the Western Arctic Waterway was not statistically significant. Minimum sea ice coverage with the CAA has decreased by -2.42x103km2year-1 (-6.6 percent decade-1) but this trend has yet to reach statistical significance at the 90 percent confidence level. The years with the minimum sea ice coverage within the CAA were 1998 (131x103km2), 2007 (169x103km2), and 1999 (216x103km2). All sub-regions within the CAA are experiencing negative slopes in sea ice coverage but only Baffin Inlet is statistically significant at the 95 percent confidence level. Results however show a clear shift between decreases in the amount of first-year ice promoted to multi-year ice (- 1.67x103km2year-1) within the CAA compared to increases in the amount of multi-year ice imported into the CAA (2.04x103km2year-1). Longer melt seasons within the CAA may not yet bring about substantial reductions sea ice conditions because the CAA acts as a drain-trap for multi-year ice. As the melt season length continues to increase, and the transition to a summer-time sea ice free Arctic continues, the supply of multi-year ice from the Arctic Ocean to the CAA may reduce but it is unlikely to stop. With respect to practical utilization of the Northwest Passage it is apparent that as the seasonal ice breaks-up earlier, multi-year ice then begins to flow and fill the open water gaps resulting in only a minor lengthening of the shipping season.
C51A-0542
The Increase of the Ice-free Season as Further Indication of the Rapid Decline of the Arctic sea ice
The unprecedented depletion of sea ice in large sectors of the Arctic Ocean in the summer of 2007 has been
the subject of many publications which highlight the spectacular disappearance of the sea ice at the time of
minimum ice cover or emphasise the losses at very high latitudes. However, minimum values can be strongly
affected by specific circumstances occurring in a comparatively short time interval. The unusually clear skies
and the presence of a particular wind pattern over the Arctic Ocean may partly explain the record minimum
attained in September 2007.
In this contribution, instead of limiting ourselves to the September minimum or the March maximum, we
consider the ice conditions throughout the year, opting for a less used, and hopefully more convenient
approach.
We chose as variables to describe the evolution
of the sea ice situation in the Arctic Ocean and peripheral seas in the 1979-2007 period the length of the ice-
free season (LIFS) and the inverse sea ice index (ISII). The latter is a quantity that measures the degree of
absence of sea ice in a year and varies between zero
(when there is a perennial ice cover) and one (when there is open water all year round). We used sea ice
concentration data obtained from passive microwave satellite imagery and
processed with the Bootstrap algorithm for the SMMR and SSM/I periods, and with the Enhanced NASA Team
algorithm for the AMSR-E period.
From a linear fit of the observed data,
we found that the average LIFS in the Arctic
went from 118 days in the late 1970s to 148 days in 2006, which represents an average rate of increase of
1.1 days/year. In the period 2001-2007 the LIFS increased monotonically at an average rate of 5.5
days/year,
in good agreement with the general consensus
that the Arctic sea ice is currently in an accelerated decline. We also found that 2007 was the longest ice-
free season on record (168 days). The ISII also reached a maximum in 2007 .
We also investigated what happened at the regional level. For example, the Northwest Passage and the
Northern Sea Route are especially relevant to assess the maritime transport between the Atlantic and the
Pacific, changes in the ice cover in oil rich areas such as the north coast of Alaska will attract the attention of
the oil industry, and the disappearance of the sea ice in Hudson Bay will strongly affect its wildlife.
We divided the Arctic in 85 regions and examined how the LIFS and the ISII changed in each of them since
1979.
53 regions enjoyed their longest ice-free seasons in 2006 or 2007. 2006 was special for the Canadian Arctic
(longest ice-free season on record for about half of the regions) while 2007 was the year of the Russian
Arctic
(with the longest ice-free season in the period under study for more than half of the regions).
Some of the largest variations were observed in the Russian Arctic, where the average LIFS increased from
84 days in the late 1970s to 129 around 2006, to reach a maximum of 171 days in 2007. Let us quote the
changes in the White Sea
(105 days between 1979 and 2006), in the South Barents Sea (70 days), in the South East Siberian Sea (60
days)
and in the mid-latitude Chukchi Sea (66 days).
Other areas where important changes took place include
the Gulf of Finland (101 days), the Gulf of Riga (111 days)
and the West coast of Spitsbergen (61 days).
In the Canadian Arctic it is worth mentioning the increase of
62 days in Hudson Strait, 36 days in Hudson and Baffin Bays, and 52 days in Davis St. In almost all straits
and sounds of the High Canadian Arctic the increase has been clearly non-linear and we prefer to compare
the average LIFS in the periods 1979-1983 and 2002-2006. We quote an increase of 87 days in Lancaster
Sound and of 74 days in Coronation Gulf.
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C51A-0543
Influence of Thermokarst Failures on Hillslope and Stream Water Quality
The occurrence of thermokarst terrain has increased with the warming of climate and degradation of
permafrost. In 2007 we observed numerous hillslope thermokarst failures in the Noatak Preserve (western
Brooks Range), Alaska. The observed failures were shallow translational landslides that had displaced
massive amounts of active layer soils (often 10's m wide, 100's m long and a few m deep). Hillslopes had
been structurally reconfigured and surface layers of permafrost were thawing. Therefore, we expected
shallow subsurface water to be altered with respect to its biogeochemistry, becoming in general, richer in
dissolved solute concentrations compared with soil water from outside the thermokarst failure. We also
expected that headwater streams at the base of these hillslopes would reflect the changed chemical loading.
We sampled hillslope water upslope and within 5 thermokarst failure features. We also sampled 3 receiving
streams up and downstream of where we expected hillslope water affected by thermokarst to enter the
stream. Water samples were analyzed for total suspended sediments (TSS), major ions (Cl, SO4, Na, K,
Ca, Si), as well as TDN, TDP, DOC, and specific conductivity. TSS increased by more than 3 orders of
magnitude within thermokarst failures, and more than 2 orders of magnitude in receiving waters. Water
collected within thermokarst failures was consistently higher in concentrations of Cl, Na, TDN, and DOC (on
average, 341%, 263%, 88%, and 47%, respectively), consistently lower in specific conductivity (-18%).
Concentrations of SO4, K, and Si in thermokarst waters were, on average greater than above
thermokarst waters, whereas those for Ca and TDP were on average, lower. In receiving streams
concentrations of Na, TDN, and DOC were all consistently greater downstream (on average 30%, 43%, and
13%, respectively) of thermokarst-affected inflows. These results suggest that the greatest changes to
aquatic ecosystems resulting from thermokarst failure features are likely to be associated with increased
suspended sediment load, rather than changes to dissolved chemistry.
http://thermokarst.psu.edu
C51A-0544
The Atmospheric Response to Climate-Driven Arctic Boreal Forest Changes in a Coupled Atmosphere-Biosphere Model
Northern Hemisphere high latitude land areas are warming at rates greater than most areas of the globe, with portions of western Canada and Alaska having warmed by 3-4 degrees Celsius over the last 50 years. With anthropogenic climate change, the Arctic may warm an additional 4-7 degrees Celsius over the course of this century. Climate change has induced new species growth and migration in some high latitude ecosystems, but our understanding of how these changes might affect the climate system is limited. One Arctic ecosystem that is responding to this warming is the boreal forest where the vegetation is marching poleward to a more favorable environment. To address the climate implications of a greening-up of the Arctic biosphere by way of boreal forest migration, a coupled atmosphere-biosphere model, CAM3-CLM, was used to assess the sensitivity of the climate to potential land cover changes that may result from future warming. Two sensitivity experiments were performed to test the hypothesis that climate-driven high-latitude ecosystem changes will result in significant regional climate changes and contribute to additional warming. One simulation was performed to assess the climatic influence of a modest poleward shift of the boreal forest – a process that is slowly occurring throughout much of the northern edge of the boreal forest ecosystem. A second experiment was performed to test a more dramatic, although hypothetical, forest migration that many climate system models suggest may occur over several centuries. Results suggest that significant warming may occur from changes in the partitioning of the surface energy budget as the boreal forests migrate into tundra regions. More importantly, these experiments highlight potential dynamic changes in terms of teleconnections that may lead to climate change outside of the regions of land cover change.