Hindcasting the NAO using diabatic forcing of a simple AGCM

Abstract

A primitive equation dry atmospheric model with a rudimentary representation of the model physics is used to hindcast the winter NAO using diabatic forcing diagnosed from NCAR/NCEP reanalysis data in the period 1949-1999. Using ensembles of experiments we are able to reproduce the observed NAO index in the ensemble mean with a correlation of 0.79. By prescribing time dependent forcing only in the tropics (30°S-30°N), or only in the extra-tropics, we show that the recent upward trend in the NAO is related to tropical forcing. The implication is that coupling with the mid-latitude or Arctic ocean is not important for the trend. The model also exhibits the recent eastward shift in the sea level pressure signature of the interannual NAO variability and shows that this is associated with non-linear dynamical processes.