Study On Interaction Between North Pacific Storm Track And Midlatitude Oceanic Front

The midlatitude oceanic front is a zonally-elongated domain with a prominent sea surface temperature meridional gradient,which is a key region for the midlatitude air-sea interaction.The synoptic-scale transient eddy activity is prevalent over the midlatitude oceanic front,referred to as storm track,which plays an important role in the midlatitude weather and climate system.Therefore,it is of great significance to investigate the interaction between the storm track and the midlatitude oceanic front,which may help to understand and improve the theory of the midlatitude air-sea interaction.The high-resolution oceanic and atmospheric reanalysis datasets are utilized to investigate the relationship between the North Pacific storm track and the midlatitude oceanic front in each season.Then,based on the Twentieth-Century Reanalysis dataset version 2 and the oceanic assimilation datasets,a lagged Maximum Covariance Analysis(MCA)is used to examine the impact of North Pacific storm track on the midlatitude oceanic front.Furthermore,the sensitivity of the North Pacific storm track to midlatitude oceanic frontal intensity is illustrated by the Weather Research Forecasting(WRF)model version 3.4.Finally,this study evaluates the performances of Phase 5 of the Coupled Model Intercomparison Project(CMIP5)models in simulating the relationship between the North Pacific storm track and the midlatitude oceanic front and projects future changes under global warming.The main conclusions are as follows:(1)There exists a close association between the North Pacific storm track and the midlatitude oceanic front.The storm track strengthens(weakens)with the increased(decreased)oceanic front and shifts northward(southward)with the poleward(equatorward)movement of the oceanic front.The impact of the oceanic frontal intensity on the storm track is strongest in winter and spring,followed that by autumn,and it is weakest in summer;while the impact of the oceanic frontal meridional position on the storm track is strongest in winter and autumn,followed that by spring,and it is weakest in summer.The seasonal variations of the relationship between the two may be attributed to the near-surface baroclinicity and the baroclinic energy conversion anomalies.(2)The enhanced(reduced)North Pacific storm track tends to strengthen(weaken)the midlatitude oceanic front with a lag of 1–2 months.The forcing effect of storm track on the oceanic front is strongest in autumn,followed by that in summer and winter,and it is weakest in spring.Moreover,the mixed layer heat budget analysis suggests that sea surface temperature anomalies related to the oceanic front are primarily attributed to the storm-track-induced net surface heat flux and Ekman advection anomalies.The anomalies of both net surface heat flux and Ekman heat transport are presumed to be associated with storm-track-induced surface wind anomalies.(3)The North Pacific storm track has a high sensitivity to the midlatitude oceanic frontal intensity.When the oceanic front strengthens,the storm track intensifies significantly,with the largest enhancement located over and downstream of the oceanic front.Meanwhile,the largest storm-track response gradually shrinks in the meridional direction and gradually approaches the axis of the oceanic front.The enhanced oceanic front intensities the near-surface baroclinicity through increasing the near-surface air temperature meridional gradient,and more mean available potential energy is converted to the eddy kinetic energy,favoring the development of the storm track.The oceanic frontal intensity also has an impact on the maintenance of the near-surface baroclinicity.With the increased oceanic front,the near-surface baroclinicity experiences a slow but strong restoration.(4)All selected CMIP5 models can well reproduce the spacial distribution of the storm track,which performs best in autumn,followed by that in winter and spring,and poorly in summer.A model's ability in reproducing the storm track is closely related to its performance in the near-surface baroclinicity simulation.The meridional position of the storm track will remain unchanged under global warming while the storm track intensity is projected to decrease significantly in the whole troposphere except for a northward intensification of the upper-tropospheric storm track in winter and spring.(5)A majority of selected CMIP5 models can reproduce the northward intensification of the storm track accompanied by the strengthened oceanic front,which performs best in spring,followed by that in winter and winter,and poorly in autumn.A model's performance in capturing the relationship between the storm track and the midlatitude oceanic front partly depends on its ability in simulating the influence of the oceanic front on the near-surface baroclinicity.The model's performance in reproducing the wintertime storm-track response pattern is also related to its ability in simulating the upper-level jet intensity.The oceanic front will have a more significant influence on the wintertime storm track,especially in the upstream and downstream of the storm track;while the impact of the oceanic front on the storm track will weaken in other seasons,with the largest decrease in summer.