The strength of the Icelandic Low is the critical factor in determining path of the polar jet stream over the North Atlantic In the winter the IL is located at SW of Greenland (Subpolar Gyre) In summer the IL is to be found
much further north (most likely the North Icelandic Jet, formed by complex physical interactions between warm and cold
currents) These two run
under two different
regimes and two clocks (see the CET synthesis from 3 harmonics, one for each summer and winter, and one common — see the above link, bottom graph).