H. Lin et J. Derome, The genesis and predictability of persistent Pacific-North American anomalies in a model atmosphere, TELLUS A, 51(5), 1999, pp. 686-697
Citations number
17
Categorie Soggetti
Earth Sciences
Journal title
TELLUS SERIES A-DYNAMIC METEOROLOGY AND OCEANOGRAPHY
The setup process of Pacific-North American (PNA) pattern anomalies that la
st more than 10 days and the role played therein by synoptic-scale transien
ts are investigated using a T21, 3-level quasi-geostrophic model. As there
is no time-dependent forcing in the model, the low-frequency PNA anomalies
are generated entirely by the internal dynamics. From a 300-winter integrat
ion, 100 cases of positive PNA anomalies and 118 cases of negative PNA anom
alies lasting at least 10 days are identified. The PNA composites reveal th
at 5 days before the setup of a positive (negative) PNA anomaly there is a
negative (positive) height anomaly over Canada. This anomaly moves westward
and stops over the North Pacific. It then intensifies and a wave train dev
elops downstream. The role played by the synoptic-scale transients is explo
red by prediction experiments starling from day - 5 (5 days before the appe
arance of a PNA anomaly). A 5-day average centered at day -5 is used to rem
ove the synoptic-scale transients from the initial state. The results indic
ate that even without these transients in the initial conditions, the setup
process of a positive PNA anomaly can be reproduced quite well. On the oth
er hand, the setup of a negative PNA pattern cannot be predicted if the syn
optic-scale transients are absent from the initial conditions. In this case
, the pattern evolves systematically, but differently from the way it evolv
es in the control run.