A New Look at the Pacific Storm Track Variability: Sensitivity to Tropical SSTs and to Upstream Seeding

Abstract

There is a fairly well defined stationary wave and storm track response to El Niño SSTs over the Pacific. In this paper, the case is made that this response is a direct result of increased baroclinicity in the central Pacific and that changes in the stationary wave pattern farther east are primarily forced by changes in these transient eddies. There is also a lot of natural variability that is not associated with El Niño. The paper also stresses the point that much of the variability can be understood as forced by variations in the upstream seeding of the storm track. The question of whether these seeding variations should be thought of as chaotic noise or forced by identifiable mechanisms is not addressed. Thus, the claim is that the storm track variability and its feedback to the quasi-stationary circulation depends on two key parameters: mid-Pacific baroclinicity, controlled by SSTs, and the strength of the upstream seeding. The approach is to first examine the effect of storm track seeding by waves entering from the Asian continent during normal years (non-ENSO years). The results show that two mechanisms operate to distribute eddy energy along the storm track: downstream development and baroclinic development. The large effect on baroclinic development at the storm track entrance results from a combination of factors: surface baroclinicity, land?sea contrast, and strong moist fluxes from the western subtropics. Experiments show that sensitivity to the seeding amplitude is large. The larger the seeding amplitude, the closer the more intense baroclinic waves flux energy downstream to upper-level waves. These barotropic waves tend to break anticyclonically and produce a ridge in the eastern Pacific. Sensitivity to SST anomalies shows qualitative and quantitative similarity with the observed anomalies. Simulations show increased mid-Pacific baroclinicity because stronger convection in the midtropical Pacific enhances a large pool of warm air over the entire mideastern subtropical ocean. Waves with sources at the storm track entrance break anticyclonically and produce the ridge in the eastern Pacific. On the other hand, baroclinic waves generated or regenerated in the mid-Pacific tend to break cyclonically, produce a trough tendency, and reduce the eastern ridge amplitude in the Pacific?North American (PNA) sector. These results strongly suggest that the variability of the quasi-permanent circulation indeed could be produced by the high-frequency eddy feedback, and two mechanisms are primarily responsible for the forcing of the quasi-permanent circulation: downstream development from the western Pacific and the anomalous baroclinicity in the mideastern Pacific. The intensity of these counteracting forcings gives the different flavors of the El Niño response over the PNA region. Regardless of the SST anomaly strength, the PNA patterns seem unique but obviously have different intensities.