ALBERT

Description: During the EU research project DAMOCLES 18 ice buoys were deployed in the region of the Arctic transpolar drift (TPD). Sixteen of them formed a square with 400 km side-length. The measurements lasted from 2007 to 2009. The properties of the TPD and the impact of synoptic weather systems on the ice drift are analysed. Compared to Nansen's drift with the vessel Fram the measured speed of the TPD is here almost twice as fast. Within the TPD, the speed increases by a factor of almost three from the North Pole to the Fram Strait region. The hourly buoy position fixes show that the speed is underestimated by 10–20% if positions were taken at only 1–3 days intervals as it is usually done for satellite drift estimates. The geostrophic wind factor Ui/Ug, i.e. the ratio of ice speed Ui and geostrophic wind speed Ug, in the TPD amounts to 0.012 on average, but with regional and seasonal differences. The constant Ui/Ug relation breaks down for Ug 〈 5 m s−1. The impact of synoptic weather systems is studied applying a composite method. Cyclones (anticyclones) cause cyclonic (anticyclonic) vorticity and divergence (convergence) of the ice drift. The amplitudes are twice as large for cyclones as for anticyclones. The divergence caused by cyclones corresponds to a 0.1–0.5%/6 h open water area increase based on the composite averages, but reached almost 4% within one day during a strong August 2007 storm. This storm also caused a~long-lasting (over several weeks) rise of Ui and Ui/Ug and changed the ice conditions in a way allowing ocean tidal motion to directly affect ice motion. The consequences of an increasing Arctic storm activity for the ice cover are discussed.

Description: During the EU research project Developing Arctic Modelling and Observing Capabilities for Long-term Environmental Studies (DAMOCLES), 18 ice buoys were deployed in the region of the Arctic transpolar drift (TPD). Sixteen of them formed a quadratic grid with 400 km side length. The measurements lasted from 2007 to 2009. The properties of the TPD and the impact of synoptic weather systems on the ice drift are analysed. Within the TPD, the speed increases by a factor of almost three from the North Pole to the Fram Strait region. The hourly buoy position fixes would show that the speed is underestimated by 10–20% if positions were taken at only 1–3 day intervals as it is usually done for satellite drift estimates. The geostrophic wind factor Ui / Ug (i.e. the ratio of ice speed Ui and geostrophic wind speed Ug), in the TPD amounts to 0.012 on average, but with regional and seasonal differences. The constant Ui / Ug relation breaks down for Ug 〈 5 m s−1. The impact of synoptic weather systems is studied applying a composite method. Cyclones (anticyclones) cause cyclonic (anticyclonic) vorticity and divergence (convergence) of the ice drift. The amplitudes are twice as large for cyclones as for anticyclones. The divergence caused by cyclones corresponds to a 0.1–0.5% per 6 h open water area increase based on the composite averages, but reached almost 4% within one day during a strong August 2007 storm. This storm also caused a long-lasting (over several weeks) rise of Ui and Ui / Ug and changed the ice conditions in a way which allowed large amplitudes of inertial ice motion. The consequences of an increasing Arctic storm activity for the ice cover are discussed.

Description: Auf Empfehlung der UNESCO sollen während der Internationalen Hydrologischen Dekade (IHD) sämtliche Süßwasservorkommen der Welt erfaßt werden. Als Beitrag zu diesen Bemühungen wird gegenwärtig am Geographischen Institut der ETHZ ein Schweizerisches Gletscherinventar aufgestellt. Die Arbeiten am Inventar werfen einige Probleme auf, die im folgenden dargestellt werden. Das gesammelte Datenmaterial bildet Grundlage für ein Forschungsprojekt, welches weitere Erkenntnisse über die Beziehung zwischen Gletscher und Klima liefern soll.

Description: This paper focuses on the dynamic mechanisms that create favorable conditions for the occurrence of frosts that affect large areas of Argentina and are denominated generalized frosts (GF). The hemispheric teleconnection patterns linked to extreme cold events affecting central and northeastern Argentina during winter are identified. The objective is to determine whether the conditions found in previous studies for the composite of winters with extreme (maximum and minimum) frequency of GF occurrence respond to typical characteristics of the austral winter or they are inherent to those particular winters. Taking the mean winter as basic state in the 1961–1990 period, a series of numerical experiments are run using a primitive equation model in which waves are excited with a thermal forcing. The positions of the thermal forcing are chosen according to observed convection anomalies in a basic state given by the austral winters with extreme frequency of GF occurrence. The wave trains excited by anomalous convection situated in specific regions may propagate across the Pacific Ocean and reach South America with the appropriate phase, creating the local favorable conditions for the occurrence of GF. However, the anomalous convection is, by itself, not sufficient since the response also depends on the basic state configuration. This is proved by placing the forcing over the region of significant anomalous convection for maximum and minimum frequency of GF occurrence and the response was very different in comparison to the mean winter. It is concluded that the conditions for a greater GF frequency of occurrence are inherent to these particular winters, so that such conditions are not present in the average winter.

Description: Based on previous results obtained from observations and linear wave theory analysis, the hypothesis that large-scale patterns can generate extreme cold events in southeast South America through the propagation of remotely excited Rossby waves was already suggested. This work will confirm these findings and extend their analysis through a series of numerical experiments using a primitive equation model where waves are excited by a thermal forcing situated in positions chosen according to observed convection anomalies over the equatorial region. The basic state used for these experiments is a composite of austral winters with maximum and minimum frequency of occurrence of generalized frosts that can affect a large area known as the Wet Pampas located in the central and eastern part of Argentina. The results suggest that stationary Rossby waves may be one important mechanism linking anomalous tropical convection with the extreme cold events in the Wet Pampas. The combination of tropical convection and a specific basic state can generate the right environment to guide the Rossby waves trigged by the tropical forcing towards South America. Depending on the phase of the waves entering the South American continent, they can favour the advection of anomalous wind at low levels from the south carrying cold and dry air over the whole southern extreme of the continent, producing a generalized frost in the Wet Pampa region. On the other hand, when a basic state based on the composites of minimum frosts is used, an anomalous anticyclone over the southern part of the continent generates a circulation with a south-southeast wind which brings maritime air and therefore humidity over the Wet Pampas region, creating negative temperature anomalies only over the northeastern part of the region. Under these conditions even if frosts occur they would not be generalized, as observed for the other basic state with maximum frequency of occurrence of generalized frosts.