(a) EOF1 of storm track density interannual variability in JJA for period 1948–2011 and (b) the corresponding PC1
Figure 1. (a) EOF1 of storm track density interannual variability in JJA for period 1948–2011 and (b) the corresponding PC1. (c) EOF1 of Eady growth rate (d−1) and (d) the corresponding PC1. Black contours in (a) and (b) are the climatology and thick black lines in (c) and (d) are 11 year running mean. (e), (f) Composite track density for low and high PC1. Densities are in units of number density per month per unit area, where the unit area is equivalent to a 5° spherical cap (~106 km2). (g) Normalized time series of track density averaged in two regions shown in (e).
The summertime variability of the extratropical storm track over the Atlantic sector and its links to European climate have been analysed for the period 1948–2011 using observations and reanalyses. The main results are as follows. (1) The dominant mode of the summer storm track density variability is characterized by a meridional shift of the storm track between two distinct paths and is related to a bimodal distribution in the climatology for this region. It is also closely related to the Summer North Atlantic Oscillation (SNAO). (2) A southward shift is associated with a downstream extension of the storm track and a decrease in blocking frequency over the UK and northwestern Europe. (3) The southward shift is associated with enhanced precipitation over the UK and northwestern Europe and decreased precipitation over southern Europe (contrary to the behaviour in winter). (4) There are strong ocean–atmosphere interactions related to the dominant mode of storm track variability. The atmosphere forces the ocean through anomalous surface fluxes and Ekman currents, but there is also some evidence consistent with an ocean influence on the atmosphere, and that coupled ocean–atmosphere feedbacks might play a role. The ocean influence on the atmosphere may be particularly important on decadal timescales, related to the Atlantic Multidecadal Oscillation (AMO).