Icelandic Low Cyclone Activity: Climatological Features, Linkages with the NAO, and Relationships with Recent Changes in the Northern Hemisphere Circulation
Output from a cyclone detection and tracking algorithm, applied to twice-daily sea level pressure (SLP) fields for the period 1966–93, is used to examine the characteristics of cyclone activity associated with the locus of the mean Icelandic low (IL), variability during extremes of the North Atlantic oscillation (NAO), and recent changes in relation to circulation over the Northern Hemisphere. Cyclone events within the climatological IL display a modest seasonal cycle with a winter maximum. However, winter systems are considerably deeper than their summer counterparts with much larger maximum deepening rates. During the cold season (October–March), IL cyclone intensities are typical of oceanic systems but exhibit lower maximum deepening rates. During the warm season (April–September), intensities are typical of Northern Hemisphere values with deepening characteristics similar to those for all extratropical oceans. Depending on the month, 10%–15% (13%–18%) of cyclone events in the IL region represent local cyclogenesis (cyclolysis). Roughly half of all IL cyclones correspond to systems showing their first appearance of a closed isobar north of 55°N, but some can be traced upstream as far as the southern and northern Rocky Mountains. There is a twofold decrease in cold season cyclone events within the climatological IL during negative extremes of the NAO, with accompanying reductions in intensity, but little change in maximum deepening rates or source regions. This is associated with modest increases in activity to the south over a large area from Labrador eastward to Portugal, reflected in the southward excursion and weakening of the subpolar low. Despite a change toward a more positive NAO index in recent years, no significant increases in cold season cyclone activity are observed in the IL region. However, there have been significant local increases within the region north of 60°N for both cold and warm seasons. These are most pronounced over the central Arctic Ocean, associated with decreases in high-latitude SLP of up to 4 mb. The regional patterns of altered cyclone activity and SLP are consistent with recent changes in high-latitude sea ice conditions and surface temperatures.