The Variabilities Of The Wintertime North Pacific Basin-scale Oceanic Fronts And Their Impact On The Overlying Atmosphere

It remains unclear how the midlatitude sea surface temperature(SST)anomaly affects the atmosphere.The oceanic fronts associated with SST anomalies are considered to be a key region for the midlatitude ocean affecting the overlying atmosphere.Investigating the variabilities of oceanic fronts and their impact on the overlying atmosphere is helpful not only in understanding the midlatitude ocean-atmosphere interaction,but also in predicting decadal climate change.In this dissertation,several sets of data are used including HadISST monthly mean sea surface temperature(SST)data during 1960-2011,OISST daily mean SST data during 1982-2011,and NCEP/NCAR daily mean reanalysis data during 1960-2011.This dissertation investigates the variabilities of wintertime North Pacific basin-scale oceanic fronts,the interaction between oceanic fronts and atmosphere,and the dynamic mechanism involved in,by diagnosing observational datasets and carrying out model experiments.The innovative results include:1.New indexes are defined to quantitatively characterize thevariabilities of wintertime North Pacific basin-scale oceanic frontsNew indexes are defined to characterize the intensity and meridional location of wintertime North Pacific basin-scale oceanic fronts:subtropic front(STF)and subarctic front(SAF).The frontal indexes include the intensity of STFZ(ITS_STFZ),the location of STFZ(LCT STFZ),the intensity of SAFZ(ITS_SAFZ),and the location of SAFZ(LCT_SAFZ),respectively.The variabilities of wintertime North Pacific basin-scale oceanic fronts are shown by the frontal indexes.There are significant interannual(2-3 years)and decadal(about 12 years)variabilities of oceanic fronts.2.Two modes are found in the impact of North Pacific basin-scale oceanic front variabilities on the overlying atmosphereOn seasonal mean time scale,two modes are found in the oceanic front variabilities as well as corresponding atmospheric anomalies,which are closely associated with Pacific Decadal Oscillation(PDO)and North Pacific Gyre Oscillation(NPGO),respectively.One is that a stronger STFZ occurs with a southward shifted SAFZ,corresponding to a positive PDO phase,which is accompanied with an intensification of the westerly jet.The other is that a stronger SAFZ occurs with a northward shifted STFZ,corresponding to a positive NPGO phase,which is accompanied a northward shift of westerly jet.Lead-lag correlation analyses imply that two-way interactions exist between the oceanic front variabilities and air-sea heat fluxes.The leading anomalies of surface heat fluxes act on the SST and result in the variabilities of oceanic fronts.This process is considered to be atsmpsheric forcing on the oceanic fronts via surface heat fluxes.On the other hand,when oceanic front varies ahead,it arouses the anomalies of surface heat fluxes in 20?40 days.Durding this process,the SST anomaly affecs the surface heat fluxes via ocanic fronts,and then forces on the overlying atmosphere.Two modes are found in the impact of North Pacific basin-scale oceanic front variabilities on the atmosphere by the lead-lag correlations between montly mean oceanic front indexes and the montly mean atmosphere.(1)The strengthened STFZ(or southward shift of SAFZ)impacts the surface heat fluxes in a month,and induces accelation of westerly jet around 40°N,with equivalent barotropic structure.(2)The strengthened SAFZ(or northward shifted STFZ)affects the sea surface flux in a month,and helps to maintain the northward shift of westerly jet.3.Dynamical mechanisms are proposed to be responsible for the impact of North Pacific basin-scale oceanic front variabilities on the atmosphereThe contributions of diabatic heating forcing,transient eddy heating and vorticity forcing are investigated to the impact of wintertime North Pacific basin-scale oceanic front variabilities on westerly jet,by dignosticing the quasigeostrophic potential vortivity(PV)equation.Climatogically,diabatic heating forcing and trasient eddy heating forcing affect the zonal wind with baroclinic sturcutre,and tent to subduce the vertical shear of westerly jet.On the contrary,the vorticity foring of transient eddy on the westerly jet is of equivalent barotropic structure,and drives a deep subarctic jet around 40°NThe anomalies of diabatic heating forcing and transient eddy heating forcing induced by oceanic front variabilities are with baroclinic structure.Therefore,these two factors are not the main contributors to the barotropic jet anomaly induced by oceanic front variabilities.By contrast,the transient eddy vorticity forcing anomaly induced by oceanic front variabilities is of barotropic structure,and leads the the anomaly of westerly jet throughout the troposphere.Specifically:(1)The strengthened STFZ(or southward shifted SAFZ)reinforces the low-troposphere atmospheric baroclinicity and the transient eddy activities.The strengthened transient eddy activities accelerates the westerly jet with equivalent barotropic structure,via their vortcitiy forcing.(2)The intensification of SAFZ(or northward shift of STFZ)maintains the northward shift of meridional gradient of air temperature and atmospheric baroclinicity in lower troposphere,and sustains the northward shift of westerly jet via transient eddy vorticity forcing.Therefore,the atmospheric baroclinicity in lower troposphere,transient eddy activities together with their vorticity forcing are considered to be the main contributor to the influence of oceanic front variabilities on westerly jet.The results of numerical simulations testify the mechanism proposed by observation analysis for the impact of subtropic front affecting the atmosphere.