Atmospheric Circulation in Antarctica: Analysis of Synoptic Structures via Measurement and Regional Climate Model

Concerning climate change Antarctica is a very sensitive region. Those changes are relevant for synoptic systems, the humidity coupled with them and precipitation. In this study measurements of relevant climatic variables like mean sea level pressure, temperature and humidity are analysed together with results of the regional climate model HIRHAM. This way the model results are validated and changes in the recent past can be monitored. HIRHAM is a regional climate model consisting of model dynamics from HIRLAM and model physics from ECHAM. It is run with boundary conditions from ERA40 (1994 to 1999) and ERAinterim (1996 to 2009). Measurements are provided by radiosondes and GPS-retrieval. HIRHAM data has been compared to reanalyses and measurements at station sites. It shows that the steep antarctic orography leads to di�erences between measurements and model. Nevertheless HIRHAM provides a good performance simulating the relevant variables on decadal time scales in general but also concerning the specialities of the continent like for example katabatic winds. The systemic di�erences between both measurement methods GPS-retireval and radiosonding have been analysed. While GPS measures humidity in a volume of air, the radiosonde integrates humidity on a path, which leads to di�erent results in total water vapour. After the validation of the model, its data has been analysed concerning synoptical pat- terns. It could be shown, that in the HIRHAM grid area the most active region is the Ross- and Amundsen-Sea region. The MSLP standard deviation shows that the main cylone tracks are north of the East Antarctic Coast stations. This is one reason for the humidity being lower in this region. In Antarctica the humidity is low and has a strong humidity gradient around the coast. This is because of the di�erent humidities over land and over and the steep orography in Antarctica. For a �ner analysis of pressure patterns, a �lter tool developed at the HZ Geesthacht has been used. Over the continent there is a high pressure zone, over the seas around it, especially in latitudes of ca. 60� mostly low pressure structures can be found. These patterns, reaching from Ross Sea to Amundsen Sea and the Peninsula, are synoptically active. To �nd AAO-positive and AAO-negative seasons for an analysis of the synoptical behaviour in di�erent AAO-phases, the �rst EOF of the Antarctic region has been calculated with ERA-interim data of the years 1996 to 2009. In AAO-positive phases the pressure is lower than in AAO-negative phases, especially in the region around West Antarctica. This e�ect is coupled to a higher humidity in this region on AAO-positive seasons. For a seasonal mean of daily P-E-budgets, the precipitation is stronger in AAO-positive phases. which is also consistent with the higher humidity and stronger lows in these seasons