Improving the Gibraltar/Reykjavik NAO index

The longest instrumental index of the North Atlantic Oscillation (NAO) is based on pressure measurements from Gibraltar and Reykjavik. Recently two long pressure series from the town of Cádiz and the nearby San Fernando observatory in southern Spain have been digitized. As Gibraltar is situated within 100 kilometers from Cádiz and San Fernando, a long held suspicion that early pressure data from Gibraltar contains several problems can now be investigated. This leads to the creation of an improved version of the long NAO index in which the period from 1821 to 1856 has been revised.