Storm Track Shifts Induced by Regional Stationary Wave Interference

In the Northern Hemisphere (NH) midlatitudes, there are two regions of major storm activity, one over the North Pacific Ocean and the other over the North Atlantic Ocean. Because of their impact on weather and climate, understanding their variability has been an active area of research. Observations and modeling studies show some evidence that the storm tracks and associated atmospheric jets may shift poleward in response to greenhouse gas warming. Using climate models, a previous study found that changes in the position of the jets, which is closely tied to the storm tracks, can be mostly realized through changes in the sea surface temperature (SST) rather than the direct response to the greenhouse gas forcing. In line with that result, another prior study found that planetary-scale waves excited by tropical convection can constructively interfere with climatological stationary waves and modulate storm track eddy activities.

In this study, we examine stationary wave interference over the North Pacific (NP) and North Atlantic (NA) sectors to investigate the relationship between stationary wave interference and storm track changes. Observational data analysis shows that the NP storm track tends to shift equatorward (poleward) upstream (downstream) of the climatological storm track during constructive interference events. The result is a storm track with an enhanced southwest-northeast tilt. The NP constructive interference is found to occur in association with enhanced heating over central North Pacific, which in turn is preceded by enhanced tropical Pacific warm-pool convection. The opposite features are seen during destructive interference. In the NA sector, constructive (destructive) stationary wave interference tends to be followed by a poleward (equatorward) shift over the entire Atlantic storm track with the southwest-northeast tilt remaining intact. Anomalous extratropical heating associated with the constructive interference also shows this tilted structure over the North Atlantic. This extratropical heating is preceded by enhanced tropical convection in both tropical Pacific warm pool and tropical Atlantic warm pool. Storm track shifts are sustained for about two weeks during the constructive interference events.

By performing initial value calculations with the dynamical core of a GCM, we examine the impacts of tropical and extratropical heating on stationary wave interferences and storm track shifts. Preliminary model results show that the model solution captures the observed transient wave field reasonably well. To a lesser extent, geopotential height variance (which is a measure of storm track activity) and sensible poleward heat flux also agree with the observation. Our results suggest a possible dynamical mechanism by which the observed storm track shifts can occur.