C52A-01 INVITED 10:20h
Sea ice retreat and its consequences in greenhouse simulations of climate change
The retreat of sea ice is a key factor in the polar amplification of greenhouse warming. In this presentation, we compare the greenhouse warmings in the context of sea ice retreat in five global climate models used in the Arctic Climate Impact Assessment. Of these five models, one is relatively sensitive and one relatively insensitive to increases of greenhouse gas concentrations. In addition to the correlation between the magnitude of the Arctic warming and the area of sea ice retreat, there are correspondences between sea ice retreat and the seasonality and spatial pattern of the warming. The warming, which is largest over and near areas of sea ice retreat, is strongest in autumn, immediately following the summer months when the loss of sea ice is greatest (in four of the five models). The ice-related warming, in turn, drives the response of sea level pressure in high latitudes, where the largest decrease of pressure occurs during autumn and early winter. Corresponding changes of precipitation are less systematic, although there are indications of an increase of precipitation in summer and autumn over areas where the sea ice cover has been lost.
C52A-02 INVITED 10:35h
Explaining the Recent Decreases in Sea Ice on the Arctic Ocean
Three of the past six summers have exhibited record low sea-ice extent on the Arctic Ocean, and this summer appears to be on pace to set a new record minimum. Simultaneous decreases in sea ice thickness have also been observed (Rothrock, et al. 1999). Taken together, these observations imply a precipitous decline in the total volume of sea ice on the Arctic Ocean. Is this decline due to changes in the advection of heat into the Arctic, or due to a simple change in winds and the drift of sea ice on the Arctic Ocean? Rigor, et al. (2002) showed that the winter wind anomalies associated with the high-index AO conditions increases the advection of ice away from the Eurasian and Alaskan coasts. This advection increases the production of thin ice in the flaw leads along the coast during winter, and preconditions the sea-ice to be more prone to melt during the following spring and summer. During summer, low-index AO conditions favor southeasterly wind anomalies which increase the advection of ice away from the Alaskan coast and increase the advection of warm air onto the ocean, both of which act to decrease the amount of ice in the Beaufort and Chukchi seas. However, the impact of the summer AO on sea-ice extent appears to be preconditioned by the state of the AO during the previous winter, and in recent years the correlations between the summer AO-index and sea-ice extent are not as strong as they were in prior years. For example, during the summers of 2002 and 2003, the summer AO was in a high-index phase, which favors above normal ice concentrations along the Alaskan coast, and yet record minima were observed during both years. We hypothesize that the minima in Arctic sea ice extent may have been dynamically induced by changes in the surface winds. Based on results of a simple model that keeps track of the age of ice as it moves about on the Arctic Ocean, we argue that the areal coverage of thick multi-year ice decreased precipitously during 1989-1990 when the Arctic Oscillation was in an extreme "high index" state, and has remained low since that time. Under these conditions, younger, thinner ice anomalies recirculate back to the Alaskan coast more quickly, decreasing the time that new ice has to ridge and thicken before returning for another melt season. During the 2002 and 2003 summers this anomalously younger, thinner ice was advected into Alaskan coastal waters where extensive melting was observed, even though temperatures were locally colder than normal. The age of sea-ice explains more than half of the variance in summer sea-ice extent.
http://iabp.apl.washington.edu/IceAge&Extent/
C52A-03 INVITED 10:50h
Role of Sea Ice in a Mechanism of Arctic Decadal Variability
This study investigates decadal variability of the Arctic Ocean - Greenland, Iceland, Norwegian Seas atmosphere-ice-ocean system. Based on findings from previous observational and modeling studies of Arctic climate it is assumed that decadal variability in the region is associated with self-sustained oscillations in the climate system. We hypothesize that Arctic variability is regulated by heat and freshwater exchange between the Arctic Ocean and the GIN Sea. The interaction between basins is weak during anticyclonic circulation regimes (low AO/NAO) and strong during cyclonic circulation regimes (high AO/NAO). Regime shifts are controlled by the system itself through oceanic and atmospheric gradients (dynamic height and surface air temperature) that increase during the anticyclonic regime and decrease during the cyclonic regime. A simple model of the Arctic Ocean and Greenland Sea, coupled to a thermodynamic sea ice model and atmospheric model, has been used to investigate a possible mechanism of self-sustained climate oscillation. Periodic solutions obtained from simulations with seasonally varying forcing, for scenarios with high and low interaction between the regions, reproduce major anomalies in the ocean thermohaline structure, sea ice volume, and fresh water fluxes attributed to ACCR and CCR. Role of sea ice in the mechanism of decadal variability is discussed.
C52A-04 INVITED 11:05h
On large outflows of Arctic sea ice into the Barents Sea
We find an anomalously high outflow of Arctic sea ice through the passage between Svalbard and Franz Josef Land (S-FJL) during the winter of 2003. This is preconditioned by the unusually high concentration of thick perennial ice over the Nansen Basin at the end of the 2002 summer. Together with a Dec-Mar sea-level pressure pattern, a deep low situated over the eastern Barents Sea, the consequence is an enhancement of ice export through this passage. As well, this winter (Oct through May) saw the lowest Fram Strait ice area flux over the 25 year ice flux record - 288x10$^3$ km$^2$ compared to the mean of 491x10$^3$ km$^2$. The Oct-May ice area flux through the S-FJL passage, at 113x10$^3$ km$^2$, is not only unusual in magnitude but also remarkable in that almost ~70$%$ the area is multiyear ice. Using ice thickness from radar altimetry, we estimate this year's winter ice volume flux to be ~ 350 km$^3$. This can be compared to an export of ~1060 km$^3$ through the Fram Strait over the same period. Examining a 10-year record with satellite motion and thickness estimates, we find the mean volume export through the S-FJL passage to be ~63 km$^3$. Another anomalously large outflow of Arctic sea ice through this passage, comparable to that in 2003, is found in 1996. This southward flux of sea ice represents one of the two major sources of freshwater in the Barents Sea; the other is the eastward flux of water via the Norwegian Coastal Current. With model simulations, the consequences of variability in freshwater input on the Barents Sea hydrography and its impact on transformation of Atlantic Water along its way towards the Arctic Ocean are discussed.
C52A-05 11:20h
Comparison of Model- and Satellite-Derived Arctic Sea Ice Thickness
Sonar measurements suggest that Arctic sea ice has thinned in recent decades. The amount of thinning is uncertain because of the sparseness of observations and the difficulty of distinguishing the long-term signal from natural variability. Climate models predict a thinning ice cover, but until recently it has been impossible to validate models with synoptic-scale data. A new eight-year time series from satellite altimetry provides, for the first time, a basin-scale ice thickness data set for model validation. The satellite data, which cover about half the area of permanent Arctic sea ice, have been compared to ice thicknesses generated by the Los Alamos sea ice model, CICE, coupled to the POP ocean model. The model was run for 50 years on a 0.4-degree global grid using protocols and forcing data from the Arctic Ocean Model Intercomparison Project (AOMIP). Model- and satellite-derived wintertime ice thickness fields were compared for 1993-2001. The modeled mean thickness of 2.70 m over the region of satellite coverage is very close to the mean from the data. The spatial thickness patterns are generally similar, with the thickest ice adjacent to the Canadian Archipelago and Greenland. The model thickness is biased high in the Canadian Archipelago, possibly because of insufficient grid resolution, and low in the Barents Sea, probably because of excessively warm ocean temperatures. The standard deviation of wintertime ice thickness in the model is about 4% of the mean, compared to 9% for the satellite data. Much of the model variability results from dynamic processes. Thermodynamic variability is underestimated, at least in part because the AOMIP radiative forcing is based on monthly climatologies. Future work will aim to correct these biases.
http://climate.lanl.gov/
C52A-06 11:35h
Declining Arctic Landfast Ice between 1975 and 2003
Landfast ice constitutes about 15% of the total Arctic sea ice cover in winter. Because landfast ice rings the coastline of the Arctic Ocean, it plays a unique role in the Arctic climate system, with impacts on coastal communities and ecosystems, shipping routes, and the storage and release of freshwater. Using the weekly ice charts produced by the U.S. National Ice Center, we analyzed the interannual variation of landfast ice in the Northern Hemisphere from 1975 to 2003. We divided the Arctic and sub-Arctic seas into 17 regions and created a time series of winter landfast ice extent for each region. The duration of the ice season was also computed for each year by finding the dates in the fall and spring when the ice extent rose above or fell below a certain threshold. We find statistically significant decreases in landfast ice extent in the Laptev, Chukchi, and Beaufort Seas, as well as the Canadian Archipelago, over the period examined. Large declines were also found in the Kara and East Siberian Seas, and along the east coast of Greenland. On the other hand, landfast ice extent increased in the Barents Sea and the Sea of Okhotsk. However, the trend for the Northern Hemisphere as a whole is significantly negative. The duration of the landfast ice season has also shortened significantly in the East Siberian, Chukchi, and Bering Seas, but has increased in the Barents Sea. The trend for the Northern Hemisphere as a whole is close to zero. Several factors may be responsible for the observed changes in landfast ice, including surface air temperature, atmospheric circulation patterns associated with the Arctic and the Pacific Decadal Oscillations and increasing river discharge. We explore these various factors to explain the spatial and temporal changes in landfast ice extent and duration.
C52A-07 11:50h
The variability of Atlantic water transport in the Arctic Ocean and adjacent seas and its influence on arctic sea ice
A retrospective investigation is conducted to examine the variability of exchanges between the Arctic and Atlantic oceans during 1948-2003 using a global ice-ocean model. Model results indicate that the transports of volume, heat, and freshwater at the Faroe-Scotland Passage, Denmark Strait, Fram Strait, and the Barents Sea Opening are closely correlated with the North Atlantic Oscillation (NAO), as are ice exports at Fram Strait and Denmark Strait. There is a noticeable increase in Atlantic inflow at the Faroe-Scotland Passage since 1960s, which is related to the elevation of the NAO in recent decades. As a result, there is an increase in northward heat transport in the Greenland-Iceland-Norwegian (GIN) Sea and at Fram Strait and the Barents Sea Opening. The increase in heat transport is found to contribute to a continued thinning of arctic sea ice since 1966. The influence of the heat transport on sea ice is found to lag 2-3 years, which suppresses ice production even when the NAO temporarily dips. Because of this delay, the decline of arctic sea ice is likely to continue if the NAO does not shift to a sustained negative mode.
C52A-08 12:05h
Recent Change of Arctic Sea Ice Cover
A high resolution coupled ice-ocean model of the Pan-Arctic region forced with realistic atmospheric data is used to investigate causes and long-term variability trends of the Arctic Ocean and its sea ice. Model results suggest that the recent decrease of sea ice cover might be in part due to the delayed effect of thermodynamic interactions at the ice-ocean interface, in particular upward heat fluxes, resulting from increased advection of warm Atlantic and summer Pacific waters into the central Arctic Ocean during the 1990s. More importantly, the modeled rate of decrease of sea ice thickness and volume, especially during the last several years, appears to be larger than that of ice extent and concentration as determined from satellite data. Through air-sea-ice interactions the recent decrease of total sea ice volume leads to an increase of freshwater content, which when exported out of the Arctic Ocean into the active convection regions of the sub-polar North Atlantic can potentially provide an even larger freshwater signal than that of the 1980s to mid-1990s. Such changes will have major consequences to the global ocean thermohaline circulation as well as to the long term global ocean heat and salt transports and climate. The warming trend, if continued, will not only significantly affect global climate but will also change the strategic and economic importance of the Arctic Ocean through its use for commercial shipping routes and increased exploration of natural resources.