These characteristics are 

 usually associated with the negative 

 phase of the North Atlantic Oscillation 

 (NAO), which describes the dominant 

 variability in the atmospheric circulation 

 in the North Atlantic. (See, for 

 example, Hurrell, 1995). A negative 

 NAO phase is associated with a 

 weakening of the Icelandic Low and the 

 southward displacement of its mean 

 position. In this state, the winter 

 storms track south of Newfoundland, 

 resulting in east and northeast winds 

 and relatively warm, moist maritime air 

 over Labrador and Newfoundland. 

 Seven of the eleven mildest years in 

 Ice Patrol's history had strongly 

 negative (< -1.0) NAO index values 

 (Table 2). Three years (1931, 1952, 

 and 1980) were neutral NAO years, 

 with the index between -1 .0 and +1 .0. 



Many of the common factors of 

 a mild season were present in 1999. 

 For example, in March, air 

 temperatures in Labrador and east 

 Newfoundland were 4°C to 6°C warmer 

 than normal, and the thickness and 

 area! extent of sea ice were much less 

 than normal. However, among the 11 

 mildest iceberg years in MP's history, 

 1999 stands alone in one regard. 

 Remarkably, the winter NAO index for 

 1999 was 1.70, indicating a positive 

 NAO phase. 



In the historical record, there 

 appear to be two kinds of mild years: 

 those in which very few icebergs are 

 found near Newfoundland (1951 and 

 1966) and those with significant 

 populations in the bight of 

 Newfoundland and the Strait of Belle 

 Isle (1924, 1931, 1952, and 1958). 

 This suggests two fundamentally 

 different processes, one related to the 

 supply of icebergs to the Newfoundland 

 area, and the other related to the 

 movement once they are in the vicinity 

 of the Strait of Belle Isle. 



One thing is certain: an iceberg 

 year like 1999 leads to a greater 

 respect for the variability of the iceberg 

 threat to transatlantic shipping and the 

 complexity of causal factors. 



Table 2. Years with the lowest number of icebergs estimated to 

 have drifted south of 48'N and winter (Dec-Mar) NAO index. 



Note: The NAO Index used here is based on the difference of 

 the normalized sea level pressures between Lisbon, Portugal 

 and Stykkisholmur/Reykjavik, Iceland^ (Source: University 

 Corporation for Atmospheric Research Climate and Global 

 Dynamics Division) 



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