Abstract
The model fidelity in simulating the Northern Hemisphere storm track interannual variability and the connections of this variability to the low frequency atmospheric variations and oceanic variations are examined based on the atmospheric European Centre for Medium-Range Weather Forecasts (ECMWF) model and coupled NCAR Community Climate System Model (CCSM) systems at different horizontal resolutions. The atmospheric general circulation model (AGCM) runs are forced by observed sea surface temperatures (SST) with varying atmospheric resolutions, while the coupled general circulation model (CGCM) runs have a fixed atmospheric resolution but varying oceanic resolutions. The phases, between the North Pacific (NP) and North Atlantic (NA) sectors, of the simulated hemisphere-scale Empirical Orthogonal Function (EOF) modes of the storm track fluctuations change with the model resolution, suggesting the storm track variability in NP and NA basins are largely independent. The models can qualitatively reproduce the basin-scale EOFs of both NP and NA storm track variability. These EOFs are not sensitive to either atmospheric or oceanic model horizontal resolutions, but their magnitudes from the CGCM runs are substantially underestimated. The storm track variations over NP basin are hybrid of internal atmospheric variations and external forcing from the underlying conditions, but the fluctuations over the NA basin are merely atmospheric internal variability. The NP storm track variability from SST forcing accounts for 4.4% of the total variance in observations, while it only has less than 2% of the total in all AGCM simulations. The external forcing to the storm track variations is more realistically reproduced in the higher atmospheric resolution runs. The air–sea coupling makes the SST feedbacks to the atmospheric internal variability, absent in the atmospheric ECMWF model hindcasts, emerge in the coupled CCSM simulations.
Highlights
The poleward transports of heat and momentum are enormous by the midlatitude atmospheric weather systems
By comparing the oceanic low frequency patterns linked to storm track variations between atmospheric and coupled model runs, we identify the nature of the storm track activities
One set is based on the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecast System (IFS) atmospheric general circulation model (AGCM) forced with observed sea surface temperatures (SST)
Summary
The poleward transports of heat and momentum are enormous by the midlatitude atmospheric weather systems. Wettstein and Wallace (2010) have performed the hemispheric EOF analyses on both hemispheric and sectorial domains using the ERA40 monthly variance fields of the high-pass filtered winds They reveal the mono-sign and dipole structures in their leading EOF modes of the hemispheric fields. Outputs from atmospheric European Center for MediumRange Weather Forcasts (ECMWF) Integrated Forecasting System (IFS) hindcasts at the resolutions of T159, T511, and T1279, as well as those from the Community Climate System Model (CCSM) HRC06 runs are used for the study of low-frequency variations of the Northern Hemisphere storm tracks in this paper. Following Wettstein and Wallace (2010), we use the high frequency band-pass filtered variance of the meridional wind component as a storm-track indicator For this purpose, the daily 250 hPa meridional winds from the model outputs, and from the ERA-40 atmospheric reanalysis are.
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