Index Cycles in the Northern Hemisphere during the Global Weather Experiment

Abstract

An index cycle occurring in the Northern Hemisphere during the late winter and spring of 1979 has been examined in detail using the ECMWF Level IIIb data set. The cycle has an average period of around 25 days and is most prominent over the Atlantic sector at middle and high latitudes. It is seen as an alternation between zonal flow with weak poleward heat transport and a blocking phase where the increased meridional flow strengthens the poleward transport at midlatitudes. The negative correlation between the eddy heat flux and the meridional temperature gradient suggests that the cycle may play some part in the short term regulation of the temperature gradient between 52 and 70°N.