OS34A-01 16:00h
The Subpolar Gyre - a regulator for the inflow of Atlantic water to the Nordic Seas
As shown by Hakkinen & Rhines (2004), the sea level within the Subpolar Gyre, as determined by altimetry, has been increasing throughout the nineties and in the beginning of 21st century. This has resulted in a weakened baroclinic rim-current around the Gyre during this period. Here we show that the prominent increase in temperature and salinity in the Northeastern North Atlantic during the last decade is linked to the weakened Gyre conditions. In order to further explore the nature of the decadal scale variations of hydrography and volume transports of the pole-ward flowing Atlantic water crossing the Iceland-Scotland Ridge a numerical model is invoked. A 53 years hind-cast simulation with a regional version of the Nansen Center version of the Miami Isopycnic Coordinate Ocean Model (MICOM) indicates large variations in volume transport of Atlantic water through the Iceland-Faroe (IF) Gap. The mid 60s, the late 70s to the early 80s showed very weak transports while the periods 1971-1975 and 1989-1994 were characterized by abnormally strong transports. Strong/weak transport periods through IF Gap are concurrent with strong/weak deep convection periods in the Labrador Sea and an expanded/contracted Subpolar Gyre. The simulated transport through the IF Gap follows the temporal evolution of the sea level within the Subpolar Gyre.
OS34A-02 16:16h
Radiation Budget Over the Arctic Ocean: Influence of Aerosols and Clouds
The 1994 Arctic Ocean Section (AOS) and the 1997-98 Surface Heat Budget of the Arctic (SHEBA) experiments pioneered the deployment of advanced spectroradiometers for studies of clouds and climate, including Fourier Transform Infrared (FTIR) instruments. This type of instrumentation is now operating routinely at the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) North Slope of Alaska (NSA) site, and the multiyear time series of these measurements is now revealing key optical and microphysical properties of the pervasive cloud cover over the Arctic Ocean, as they influence the surface energy budget. The most recent analysis has confirmed an indirect effect of anthropogenic aerosol (Arctic "haze") on the radiation balance, through its influence on the size distribution of liquid and ice particles in clouds. The detection of this aerosol indirect effect in FTIR data, and its implications for the budget of shortwave and longwave radiation at the Arctic Ocean surface, are discussed.
OS34A-03 16:32h
Determination of Atmospheric Properties Over Arctic Polynyas
Due to the relatively sparse in situ and satellite-derived data collection at polar latitudes, model analyses rely primarily on coastal weather station time series for atmospheric initialization. Values from these sites are extrapolated over adjacent marine environments, including nearby polynyas - areas of open water within the ice pack. However, most weather stations are topographically-shadowed or under the influence of katabatic drainage, so that measurements are localized and unrepresentative of the area covered by large-scale analytical grids. Moreover, the surface fluxes over land are not comparable to those in a polynya environment, where there are increases heat and moisture exchange across the oceanic interface, often leading to the formation of convective cloud at the downwind polynya edge. To determine the best accuracy of analyzed moisture and temperature profiles, independent, ship-based radiosondes collected offshore of the flat, ponded, coastal plain at Barrow, Alaska are compared against upper air observations from a weather station, the ARM (Atmospheric Radiation Measurement) North Slope of Alaska-Adjacent Arctic Ocean site, MODIS retrievals, and the NCEP Final Analysis (FNL) 1-degree moisture and temperature profiles. While the atmospheric temperature structure agrees well amongst the five data sets, the analyzed and retrieved humidity profiles do not capture the boundary layer moisture, and are in fact drier than upper air observations despite a known "dry bias" in radiosonde data. Comparisons of surface fields are limited to ship-model match-ups and largely reflect limitations of coarse model resolution.
OS34A-04 16:48h
Observed 80-Years of Climate Change in Baffin Bay: Facts, Fiction, and the NAO
Climate change often appears dramatic when one averages the oscillatory system under investigation over short periods of its most extreme positive and negative stages and calls the difference change. While formally correct, the suggested change is often mistaken for a trend especially by the public and those not trained in the delicacies of statistics. Hence we reject this methodology and settle for a careful regression analysis of Baffin Bay and Davis Strait hydrographic data from 1916 through 2003. We estimate the degrees of freedom in the sampled temperature and salinity fields conservatively after discarding all data that do not fit within a few standard deviations from an iteratively calculated mean profile for suitably defined regions. Strong and significant subsurface warming emerges at locations where the water is deeper than 2000-m. We find weak and barely significant freshening of waters in the upper 300-m of the water column along the western, but not the eastern rim of Baffin Bay. Formal correlations of temperature and salinity anomalies with the often used North-Atlantic Oscillation (NAO) index are weak and generally not significant. Averaging hydrographic conditions along isobaths selectivly during positive and negative stages of the NAO, however, we find a fresher and cooler water column along the western rim of Baffin Bay during positive NAO as compared to the negative NAO phase. This contrasts with our finding along the eastern rim. Along Greenland during the positive NAO phase near bottom waters are both saltier and warmer than during the negative phase of the NAO. These finding are consistent with enhanced (positive NAO phase) and diminished (negative NAO) inflows of warm and salty waters from the North-Atlantic advecting heat and salt northward along the eastern rim of Baffin Bay and enhanced inflow of cold and fresh from the Arctic advecting ice and freshwater southward along the western rim of Baffin Bay. The effect on density results in lighter, more vertically and horizontally stratified waters during the positive NAO. Geostrophically, this implies a more intense circulation.
http://newark.cms.udel.edu/~muenchow
OS34A-05 17:04h
Fram Strait Ice Export: Variability and its Impact on Climate
The global coupled atmosphere-ocean-sea ice model ECHAM5/MPI-OM is used to investigate the interannual to decadal variability of the ice export through the Fram Strait. The interannual variability of the ice export in the model is mainly caused by variations in the pressure gradient across the Fram Strait. We show in this study that large scale atmospheric modes like the stratospheric polar vortex and the wave number one of the Arctic sea level pressure are responsible for a considerable part of the variability in the pressure gradient across Fram Strait. Decadal variability in the ice export is mainly caused by a coupled atmosphere-ocean-sea ice mode that is characterised by a clockwise propagation of ice thickness anomalies in the Arctic Basin and associated SLP anomalies. This is shown by using lag-EOFs of the SLP and the sea ice thickness of the 500-year control integration of the coupled model and by sensitivity experiments. Furthermore the impact of large anomalies in the ice export through the Fram Strait on the atmospheric and oceanic climate is analysed. Anomalous exported ice is propagating southward in the East Greenland Current, melting and flowing as freshwater signal into the Labrador Sea after one to two years. This freshwater anomaly leads to a negative salinity anomaly and to a decrease in the oceanic convection in the Labrador Sea and vice versa after low ice exports. As a consequence more ice is formed in the Labrador Sea and the heat flux to the atmosphere and thus the air temperature and the precipitation is changed in this region. Besides this local effect, the changes in the heat fluxes have an impact on the large scale atmospheric circulation. After large ice exports and corresponding low heat fluxes to the atmosphere in the Labrador Sea, a positive sea level pressure anomaly extends from the Labrador Sea over the North Atlantic to Europe while the pressure is reduced over the Nordic Seas. The related anomalous wind field is modifying the sea ice transport in the Arctic.
OS34A-06 INVITED 17:20h
Changing Arctic Ice Cover and Water Resources in the American West
Over the last century, Arctic sea ice cover has decreased dramatically and many researchers expect that future greenhouse warming will exacerbate this trend. The prospect of a warmer Arctic with less ice raises many environmental and economic questions, one of which is: How will reduced Arctic ice cover affect extrapolar climates? Previous research suggests that a reduction in Arctic sea ice corresponding to that projected for the year 2050 could drive a 50 - 100% increase in annual evaporation minus precipitation (E-P) in the American West. The projected 2050 ice cover that drives this response in E-P is characterized by a significant decrease in ice concentrations in the Greenland, Iceland, and Norwegian (GIN) seas and an increase in ice concentration in the Davis Strait. In addition, sea surface temperatures in the GIN seas increase and those in the Fram and Davis Straits decrease significantly. A recent series of sensitivity studies shows that changes in the precipitation regime over the American West, while sensitive to individual anomalies, are responding as much to the combined pattern of multiple anomalies as to the magnitude and sign of individual anomalies. This result highlights the complexity of predicting climate impacts of, or responses to, changes in Arctic surface conditions and suggests that continued research into the relationship between the Arctic and global climate is extremely important if we hope to quantify future climate change.
OS34A-07 INVITED 17:40h
Increased Heat Transport into the Arctic Ocean in a Climate Model of the 21st Century
Recently Holland and Bitz (2003) showed that most global climate models predict an increase in the northward oceanic heat transport north of about 60N in future climate scenarios compared to present day. This increase in heat transport is reminiscent of explanations posed for the recently observed warming in the Atlantic layer in the Eurasian Basin of the Arctic Ocean. An implication is that warmings like those that occurred in the recent past, which have been attributed to wind variations by some, could also occur in the future due to greenhouse gas forcing. This may seem surprising because the North Atlantic Meridional Overturning Circulation (MOC) decreases in most future climate scenarios (IPCC 2001). Our analysis of one model, the Community Climate System Model Version 3, shows that, although in future scenarios the MOC becomes shallower and weaker south of about 60N, it strengthens into the Nordic Seas and Arctic Ocean. This increased flow near the surface carries warmer water than present into the Arctic. We present evidence to argue that the increase in MOC and heat transport into the Arctic in future scenarios with the model is in response to the transition from perennial to firstyear ice in the Barents Sea and Arctic Ocean, and the attendant increase in heat loss and brine rejection in fall and winter.