On the relation between NAO and water vapour transport towards Europe

• Published in: Meteorologische Zeitschrift
• Main Authors: Ruprecht, Eberhard, Schröder, Susanne S., Ubl, Sandy
• Format: Article in Journal/Newspaper
• Language: English
• Published: Schweizerbart 2002
• Subjects: North Atlantic; North Atlantic oscillation
• Online Access: https://oceanrep.geomar.de/id/eprint/6144/; https://oceanrep.geomar.de/id/eprint/6144/1/Ruprecht.pdf; https://doi.org/10.1127/0941-2948/2002/0011-0395

In this study, the water vapour transport is investigated together with cyclone tracks across the North Atlantic Ocean and their relation to the North Atlantic Oscillation, NAO. During high NAO winters, the maximum of the water vapour transport across 10°W (defined as transport towards Europe) is observed between 50° and 55°N, during low NAO winters, the maximum reduces from 160 kg/m·s to 120 kg/m·s and shifts to about 44°N. In order to understand this change the NCEP/NCAR reanalysis data (1958-1998) are used and a composite is performed of the cyclone's water vapour transport relative to the cyclone centre. The transport is strongest in the south-east sector of a cyclone. Its maximum increases with decreasing central pressure of the cyclone. Thus, deep cyclones transport more water vapour than shallow ones. During high NAO winters, deep cyclones are more frequent than in low NAO winters. Additionally, cyclone tracks are concentrated north of 55°N during high NAO winters and are rather equally distributed in meridional direction over the North Atlantic in low NAO winters. Since the water vapour content of the atmosphere over the North Atlantic increases towards the tropics, we found that during low NAO winters, the cyclones south of 55°N transport 60% more water vapour than those north of 55°N. This fact and the relation between cyclone properties and NAO can explain the shift of the maximum water vapour transport towards Europe from 55°N during high NAO winters to 44°N during low NAO winters.