GC43A-01 INVITED 13:45h
Arctic Warming Signals From Satellite Observations
Global warming signals are expected to be amplified in the polar regions because of ice-atmosphere feedbacks associated with the high reflectivity of the ice and snow that blankets much of the region. Analysis of infrared satellite data reveals that the Arctic region has been warming at the rate of 0.5 °C per decade since 1981 but large spatial variability in the trends are apparent with the most positive occurring in North America and the Western Arctic and with some negative trends occurring in parts of Russia. During approximately the same period, the Arctic perennial ice cover declined at a rapid rate of 9.2 % per decade. While large interannual variability in the perennial ice area was observed in the 1980s and early 1990s, the perennial ice areas from 1998 to 2004 have been abnormally low compared to the average perennial ice area during the previous 20 years. Moreover, the length of melt temperatures has also been increasing by 13 days per decade over sea ice covered areas, suggesting concurrent thinning in the ice cover. In other regions, the length of melt has increased by 5 days per decade over Greenland, showing consistency with the observed thinning in the ice sheets and increasing extent of melt areas. The length of thawing at the permafrost areas of North America has also been increasing at 7 days per decade, which can be a major concern in inhabited regions. Furthermore, the areal extent of the snow cover in the entire Northern Hemisphere has been decreasing by about 2.6 % per decade while most glaciers in the Arctic region have been declining. The locations of most rapid changes are in same general areas as where the surface temperature data show considerable warming. The overall impact of aforementioned changes in the Arctic region can be profound, especially if the current trends continue.
GC43A-02 INVITED 14:10h
Recent Changes in High-Latitude Glaciers, Ice Caps, and Ice Sheets
The mass balance of high-latitude glaciers and ice sheets is highly variable on a wide range of spatial and temporal scales, but through a combination of remote sensing and in situ measurements, some significant changes have been observed in recent years. On the Greenland ice sheet most of the coastal regions have thinned substantially as melt has increased and some of its outlet glaciers have accelerated. Near the equilibrium line in West Greenland, we have seen evidence of summer acceleration that is linked to surface meltwater production, suggesting a relatively rapid response mechanism between the ice sheet and a warming climate. Throughout much of the rest of the Arctic, glaciers and ice caps have been shrinking in recent decades, with increased mass losses during the 1990s in parts of Canada and Alaska. The picture is more complicated in the southern hemisphere, where Antarctic ice is growing in some areas, shrinking dramatically in others, and is essentially in balance elsewhere. The West Antarctic Ice Sheet (WAIS) shows thinning along its northern margin, particularly in the glaciers that flow into the Amundsen Sea. The western portions of the WAIS, however, show thickening, but in the aggregate the mass loss is believed to exceed the gain. Changes in the East Antarctic Ice Sheet are small, but we don't know at this point whether it is growing or shrinking. On the Antarctic Peninsula, the rapid disintegration of the Larsen B ice shelf has resulted in acceleration and thinning of a small number of glaciers that once fed the ice shelf. This behavior raises questions about relatively near-term consequences of climate change and the Antarctic Ice Sheet's contribution to sea level rise. These recent observations offer only a snapshot in time of their long-term behavior, but they are providing crucial information about the current state of ice mass balance and the mechanisms that control it. As we continue to learn more through a combination of remote sensing observations, in situ measurements and improved modeling capabilities, it is important that we coordinate and integrate these approaches effectively in order to predict future changes and their impact on sea level.
GC43A-03 INVITED 14:35h
SubArctic Oceans and Global Climate
The passages connecting the Arctic Ocean with the Atlantic and Pacific, and their `mediterranean' basins, are focal points for the global meridional overturning circulation, and all of the climate impacts which this implies. It is also a difficult region to model accurately: the sensitivity of climate models to subpolar ocean dynamics is well-known. In this talk we stress the need to instrument and analyze the subpolar oceans, and some examples of sustained observations developing there. Results from satellite altimetry, recent Seaglider deployments from Greenland, and mooring arrays will be described. In particular we show the first Seaglider sections of hydrography and bio-optical profiles of the Labrador Sea (one of the first extended deployments of this autonomous undersea vehicle); we discuss the decline during the 1990s of the subpolar gyre circulation of the Atlantic from its great strength during the positive NAO period of the early 1990s, and its relevance to the salinity decline observed over a much longer period; we review observations of the flows at the Iceland-Scotland Ridge and Davis Strait, argued in terms of volume transport plots on the potential temperature/salinity plane; we display maps of the `convection resistance' (related to dynamic height) and its sensitivity to surface low-salinity water masses and their partition between shallow continental shelves and deep ocean. This is a particularly exciting time for climate studies, with fundamental properties of the atmosphere-ocean circulation under debate, even before one considers natural and human-induced variability. Is the four-decade long decline in subArctic salinity the result of increased hydrologic cycle, increased or altered Arctic outflow to the Atlantic, or slowing of the subpolar circulation? Is the basic intensity of the MOC more dependent on high-latitude buoyancy forcing, or wind- or tide-driven mixing in the upwelling branch, or possibly wind-stress at high latitude? Is the uptake of anthropogenic carbon from the atmosphere sensitive to changes in the gyre circulations and MOC? How does the oceanic circulation participate in the remarkably `tall' subArctic/Arctic climate system, that reaches up through the atmospheric storm tracks to the stratosphere? These questions are tied closely to the Arctic Basin itself, where thermodynamic warming competes with wind-induced anomalies and trends in sea-ice cover. The Arctic-SubArctic Ocean Flux (ASOF) program aims to narrow the uncertainty implicit in these questions, constructing a bridge between Arctic Ocean circulation dynamics and the dynamics of the subtropical Atlantic and Pacific. By defining a series of both flux array measurements and process studies, ASOF can illuminate what has long been {\it aqua et glacies incognita.}
http://www.ocean.washington.edu/research/gfd/gfd.html
GC43A-04 INVITED 15:00h
The Arctic Amplification Debate
Largely due to feedbacks involving snow and sea ice, rises in surface air temperature (SAT) in response to greenhouse gas (GHG) loading are expected to be most pronounced in the Arctic, especially for autumn and winter over the Arctic Ocean. Observations document recent warming, but this Arctic Ocean signal is not readily evident. This finding, along with large model-to-model scatter in SAT projections, pronounced variability in SAT over the 20th century, and links between observed SAT changes and atmospheric circulation, questions the concept of Arctic amplification. However, a closer evaluation shows that disparities between observations and models are large only for scenarios well into the future. We offer that the often-perceived mismatch between observations and models projections, and the ensuing debate about Arctic amplification, can be partly rectified by considering the Arctic in the context of an emerging greenhouse signal in which sea ice and snow cover are in a state of preconditioning . At present, SATs over the Arctic Ocean are still fundamentally constrained by the insulating effects of the sea ice cover. In the next several decades, we expect that a threshold will be reached in terms of sea ice loss, allowing Arctic Ocean SATs to increase. While part of this preconditioning is driven by GHG radiative forcing, its is presently more closely tied to low frequency variability on the Northern Annular Mode (NAM). There is in turn growing evidence that GHG forcing may favor the positive mode of the NAM that favors sea ice losses.
GC43A-05 15:25h
Significant thinning of the south Greenland ice sheet margin
The repeated laser altimetry by NASA between 1993 and 1999 documented that the southern ice sheet margin north of Qaqortoq to has been thinning significantly. In order to gain insight to the thinning a ground monitoring program was established. It became clear that thinning rates were nearly an order of magnitude higher than first observed by NASA i.e. 8 m/yr from 1985 to 2000. And, the calving front has been retreating in total 5 km since 1890 at increasing rate where 4 km of the retreat has been over the last 30 years. Modelling results show that the front has reached the flotation point and became instable as a result of mass balance adjustment to warmer climate after the little ice age ending around 1890. The disintegration of the front has resulted in reduction in backpressure and backward migration of the thinning up on the land-based part of the margin all the way to the equilibrium line. In addition to dynamically driven thinning (some 45%) increased melting has also provided a contribution to thinning which is the remaining 55%.