Dynamics of the interannual variability of the winter Pacific storm track

The leading mode in the interannual variability of the winter Pacific storm track is a strengthening (weakening) of the storm track, accompanied by weakening and broadening (strengthening and narrowing) of the Pacific jet stream. A previous study by Harnik and Chang has shown that a stronger but narrower jet inhibits eddy growth by meridional confinement of the eddies, suggesting that a weaker storm track is not inconsistent with a stronger but narrower jet. However, the question remains as to what leads to the changes in jet structure.

The maintenance of the anomalous jet structures is examined using stationary wave model experiments. Forcing due to anomalous tropical diabatic heating, as well as convergence of transient vorticity and heat fluxes, are diagnosed from reanalysis data and imposed to force a stationary wave response. When all the forcings are included, the model largely reproduces the observed changes in jet structure.

The stationary wave model results are further analyzed by diagnosing the response to each of the individual forcings, as well as partitioning the eddy forcings into local and remote forcings. Preliminary results suggest that different forcings are responsible for the maintenance of the narrow strong jet and broad weaker jet. For the strong jet case, much of the response comes from eddy vorticity forcing, with diabatic heating also contributing. For the weak jet case, eddy heat fluxes force the largest response, with very little response due to diabatic forcing. In addition, evidence that suggests that part of the Pacific jet response could be forced by eddy fluxes over the Atlantic is found -- in support of a recent suggestion by Nakamura, Izumi, and Sampe -- and perhaps offering an explanation to the recent finding that storm track variations in the Pacific and Atlantic are significantly correlated.