The influence of synoptic scale fluctuations on the NAO-index
Abstract
The NAO is the dominante mode of variability of the atmospheric circulation over the North Atlantic on time scales of more than one month. The effects of the NAO are rather well investigated. But despite a growing amount of NAO-studies, its causes are not well understood. In the present studie we are investigating the role of synoptic scales. We aim to answer the question whether the shift of the cyclone tracks is responsible for changes of the NAO and whether variations in the intensity of the synoptic systems plays a minor role. The NCEP/NCAR reanalysis data are applied to determine the distribution of the synoptic systems over the North Atlantic. The synoptic scale fluctuations in the time series of pressure anomalies are determined by thresholds and their lifetime. The detected systems are described by fixed values to exclude their local intensity variations. Subsequently we compared the low frequency variability of the 'manipulated' pressure field in the questionable region with the observed NAO. The agreement of both is surprisingly high. This leads to the assumption, that the cyclone tracks resp. the position of the polar front is the main source for the development of the NAO-pattern. Investigations of the mean position of the front in the North Atlantic region support this point of view. Thus we conclude, that the well known NAO-pattern is generated through the varying position of the polar front and an additional low frequent background pattern is not involved.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA....10913L