An interdecadal mode of Northern Hemisphere storm track variations

In this paper, interannual variations in the northern hemisphere winter storm tracks are examined based on 51 winters (December-January-February) of NCEP/NCAR reanalysis data. The leading EOF corresponds to the simultaneous strengthening/weakening of both the Pacific and Atlantic storm tracks. The principal component associated with this leading EOF exhibits pronounced inter-decadal variability. There appears to be a transition during the early 1970's -- from a weak storm track state prior to the early 1970's to a strong storm track state after that. Decadal mean storm track intensity during the 1990's is nearly 40% stronger than that during the late 1960's and early 1970's.

In order to show that the inter-decadal variations are not introduced into the reanalysis data by changes in the observation network, radiosonde observations along the storm track maxima are examined. The radiosonde observations are largely consistent with the reanalysis data, except for the suggestion that biases in the reanalysis data might have led to a slight overestimation of the magnitude of the variations. More will be said about storm track variations as seen directly from radiosonde data in a companion paper submitted to the poster session.

Relationship between storm track variations and low frequency (seasonal mean) flow anomalies are examined. Relations to several other modes of interdecadal variabilities are also explored. It is shown that parts of the storm track variations could be related to the Arctic Oscillation as well as the inter-decadal ENSO-like variabilities. However, even when storm track variations linearly dependent on these other modes are taken out, substantial inter-decadal variations still remains, suggesting that a large part of the storm track variability is unrelated to these other modes of variations.