Storm tracks and synoptic eddy dynamics in NASA model simulations

We present a statistical and dynamical assessment of the structure and subseasonal variability of storm tracks and in AMIP-type simulations of NASA/GSFC general circulation models (GCMs). These are complemented with parallel analyses of the life cycles of synoptic eddy events occurring within the storm tracks of interest. Focusing on the boreal cool season (November-April), simulation output from both NASA/NCAR and Aries (NSIPP) models are compared to parallel observational analyses derived from NCEP/NCAR and ERA-40 reanalyses. One interesting aspect of the storm track variability over the North Pacific is the so-called midwinter suppression. We find that both GCMs faithfully reproduce the observed Pacific midwinter suppression. Interestingly, however, the NSIPP model also produces a midwinter suppression of the Atlantic storm track in association with an anomalously strong upper level jet stream simulated in this region. In our analysis, the cool season is divided into three parts: the early, mid-, and late stages. For each stage we isolate the typical three-dimensional anomaly structures for growing cyclones and anticyclones within the Pacific and Atlantic storm tracks. Furthermore, we employ a suite of dynamical diagnoses that includes E-vectors, deformation analyses, potential vorticity inversions, and local energetics analyses to study large-scale dynamical characteristics of the simulated synoptic eddies. The results from the early and late stages of the cool season are contrasted with midwinter results. Our study is geared to simultaneously deduce (a) underlying physical mechanisms for observed and simulated midwinter suppression features and (b) variations in synoptic eddy dynamics linked to midwinter suppression.