Interaction Between Wintertime Storm Tracks And Extratropical Oceans In The Northern Hemisphere

Storm tracks are referred as the intense synoptic-scale eddies prevalent in themidlatitudes aloft, which is a crucial component of the midlatitude weather andclimate system. Moreover, from the perspective of the midlatitude air-sea interaction,the transient eddy feedback induced by the anomalous storm-track activitiesresponding to the extratropical sea surface temperature (SST) anomalies plays animportant role in regulating the large-scale air-sea coupling. Therefore, understandingof storm-track variations at different time scales and its potential association with SSTvariability will be helpful in understanding the extratropical climate variability and itspotential predictability. However, the direct relationship between the storm-trackanomalies and the underlying SST anomalies has not been clearly identified.Meanwhile, the long-term changes of storm tracks, which may indicate changes in themidlatitude air-sea coupling, have not been uncovered so far.In this study, significant seasonal and interannual couplings between wintertimestorm-track and SST anomalies in the North Pacific and North Atlantic are revealedbased on the oceanic assimilation datasets, the Twentieth-Century Reanalysis datasetversion2(20CRv2), and multiple statistical methods such as a lagged maximumcovariance analysis (MCA). As for the North Pacific:①At seasonal time scales, it isfound that the basin-scale cold SST anomalies in the subpolar region during thepreceding fall can significantly induce positive storm-track anomalies to the north of30°N, thus poleward intensify storm-track activities. Such cold SST anomalies in thepreceding fall indeed poleward strengthen the atmospheric baroclinicity andbaroclinic energy conversion in early winter, which is likely to intensify thecorresponding storm-track activities. The storm-track response pattern, however, is insharp contrast to the forcing pattern, with cold SST anomalies in the western-centralNorth Pacific and warm SST anomalies along the western coast of North America corresponding to an equatorward shift and intensification of storm tracks.②Atinterannual time scales, it is found that the wintertime (DJF) SST and storm-trackanomalies are mutually reinforced, manifesting as a horseshoe-like pattern in SSTanomalies similar to PDO (with warming in the western-central domain) coupled withbasin-scale positive storm-track anomalies to the north of40°N. This coupled modeexhibits significant decadal variability. Further inspection finds that such storm-trackresponse is closely related to the strengthening of the low-level troposphericbaroclinicity induced by the SST variations; on the other hand, anomalous net surfaceheat flux and Ekman advection induced by the storm-track variations mainlycontribute to the generation of SST anomalies.As for the North Atlantic:①At seasonal time scales, it is found that SSTanomalies in the preceding early winter can significantly influence storm tracks inearly spring; that is, an intensification of storm tracks in response to a midlatitudeSST dipole, with cold pole centered southeast of Newfoundland and warm pole in thewestern subtropical Atlantic. Such SST dipole in the preceding early winter indeedstrengthens the atmospheric baroclinicity and baroclinic energy conversion in earlyspring, which is likely to intensify the corresponding storm-track activities. In contrastto the North Pacific, however, this storm-track response pattern is similar to thestorm-track forcing pattern in early spring.②At interannual time scales, it is foundthat the wintertime (JFM) SST and storm-track anomalies are mutually reinforced,manifesting as a midlatitude SST monopole (with warm anomalies centered to thesouth and east of Newfoundland) coupled with a zonal-dipole-like pattern instorm-track anomalies (with dominant negative anomalies in the downstream). Thiscoupled mode exhibits significant decadal variability. Further inspection finds thatsuch SST monopole can significantly weaken the low-level tropospheric baroclinicityin the belt of40°N50°N, which is presumably responsible for the weakening ofstorm-track activities in the downstream; on the other hand, anomalous downward netsurface heat flux and Ekman warm advection induced by the storm-track variationsmainly contribute to the generation of SST monopole. Based on the individual56ensemble-members of the20CRv2dataset, centennialtrends in Northern Hemisphere winter storm tracks during the20th century are alsoinvestigated systematically. It is found that the20th century trends in the storm-trackactivities exhibit large discrepancies between the upper and lower troposphere. In theupper troposphere, a substantial intensification is identified at the poleward anddownstream regions of the North Pacific and North Atlantic storm tracks in terms ofthe synoptic eddy kinetic energy (EKE) and standard deviation of filtered geopotentialheight, which indicates a large northeastward expansion of storm tracks in the late20th century. In addition, meridional eddy flux of westerly momentum in the uppertroposphere tends to intensify over the North Pacific and United States. However, inthe lower troposphere the synoptic eddy activities, especially in terms of EKE andmeridional eddy heat flux, tend to be significantly weakened over the central-westernregion of the subpolar North Pacific and the upstream regions of the North Atlanticstorm tracks. Further inspections find that such strengthening (weakening) of stormtracks in the upper (lower) troposphere are strongly associated with the increase(decrease) of the baroclinic instability.In light of the above findings, we investigate modulation of the midlatitudeair-sea coupling over the North Pacific by global warming, with focus on theatmospheric response to SST anomalies in the Kuroshio-Oyashio Extension (KOE). Itis demonstrated that in early winter geopotential height over the Aleutian Low regiondisplays an equivalent barotropic ridge in response to warm SST anomalies in theKOE region. During the20th century, this warm SST-ridge response in early winter isfound to be enhanced significantly, which is probably attributed to global warming.Further analysis of IPCC-AR4multi-model projections indeed demonstrates that in awarmer climate such warm SST-ridge response is substantially enhanced. Thisindicates that global warming intensifies the midlatitude air-sea coupling over theNorth Pacific in early winter.