The early major warming in December 2001 – exceptional?

Abstract

The early major warming in December 2001 is described and compared to the two other December major warmings in 1998 and 1987, showing a strong tropospheric‐stratospheric coupling in all three cases. We argue that the occurrence of free westward propagating Rossby waves interacting with a forced quasi‐stationary wave number 1 led to these three early events. The possible excitation of these waves is discussed with respect to the tropospheric circulation, which showed strong blockings over the northern Atlantic prior to the early major warmings.