Winter-to-spring transition in Europe 48-54 degrees N: From temperature control by advection to control by insolation

As established in previous studies, and analyzed further herein for the yea rs 1988-1998, warm advection from the North Atlantic is the predominant con trol of the surface-air temperature in northern-latitude Europe in late win ter. This thesis is supported by the substantial correlation Cti between th e speed of the southwesterly surface winds over the eastern North Atlantic, as quantified by a specific Index Ina, and the 2-meter level temperature T s over central Europe (48-54 degrees N; 5-25 degrees E), for January, Febru ary and early March, In mid-March and subsequently,,the correlation Cti dro ps drastically (quite often it is negative). The change in the relationship between Ts and Ina marks a transition in the control of the surface-air te mperature. As (a) the sun rises higher in the sky, (b) the snows melt (the surface absorptivity can increase by a factor of 3.0), (c) the ocean-surfac e winds weaken, and (d) the temperature difference between land and ocean ( which we analyze) becomes small, absorption of insolation replaces the warm advection as the dominant control of the continental temperature. We defin e the onset of spring by this transition, which evaluated for the period of our study occurs at pentad 16 (Julian Dare 76, that is, March 16). The con trol by insolation means that the surface is cooler under cloudy conditions than under clear skies. This control produces a much smaller interannual v ariability of the surface temperature and of the lapse rate than prevailing in winter, when the control is by advection. Regional climatic data would be of greatest value for agriculture and fores try if compiled for well-defined seasons. For continental northern latitude s, analysis presented here of factors controlling the surface temperature a ppears an appropriate tool for this task.