The Impact of Super Typhoon Nuri (2014) on the Structure and Strength of the Stratospheric Polar Vortex

The impact of Super Typhoon Nuri (2014) on the structure and strength of the stratospheric polar vortex

ISS HD Earth Viewing Experiment Nov 2, 2014 Super Typhoon Nuri Hannah E. Aard and Andrea L. Lang University at Albany, SUNY Cyclone Workshop Paciﬁc Grove, CA 28 October 2015

R EPORTS

1998 - 1999 Northern Annular Mode A Weak Vortex Regimes

1 50

hPa 40 km 10 30

0 20 0 100

10 4

1000 0 2 Background Oct Nov Dec Jan Feb Mar Apr -10 Fig. 1. Time-height development of the northern annular mode during the winter of 1998–1999. The indices have daily resolution and are nondimensional. Blue corresponds to positive values (strong polar vortex), and red corresponds to negative values (weak polar vortex). The contour -1 interval is 0.5, with values between Ϫ0.5 and 0.5 unshaded. The thin horizontal line indicates the approximate boundary between the troposphere and the stratosphere. B Strong Vortex Regimes

A Composite of 18 Composite of 18weak Weak vortex events Vortex Events 10 30 30 km hPa 20 0

100 0 tropopause 300 10

1000 0 -90 -60 -30 0 30 60 90 -1 Lag (Days) −AO at 0 2 0 B Composite of 30 Composite of 30 Strongstrong Vortexvortex events Events the surface 10 30 30 km hPa 20

100 Fig. 3. Average sea-level pressure anomalies tropopause (hPa) for (A) the 1080 days during weak vortex 300 10 regimes and (B) the 1800 days during strong vortex regimes. 1000 0 -90 -60 -30 0 30 60 90 the PDFs, especially between the tails of the Lag (Days) curves.+AO at Values of AO or NAO index greater Fig. 2. Composites of time-height development of the northern annular mode for (A) 18 weak the surface than 1.0 are three to four times as likely during Baldwin and Dunkerton 2001 vortex events and (B) 30 strong vortex events. The events are determined by the dates on which strong vortex regimes than weak vortex re- the 10-hPa annular mode values cross –3.0 and ϩ1.5, respectively. The indices are nondimensional; gimes. Similarly, index values less than Ϫ1.0 the contour interval for the color shading is 0.25, and 0.5 for the white contours. Values between are three to four times as likely during weak Ϫ0.25 and 0.25 are unshaded. The thin horizontal lines indicate the approximate boundary vortex regimes than strong vortex regimes. Val- between the troposphere and the stratosphere. ues of the daily AO index greater than 1.0 and less than Ϫ1.0 are associated with statistically Stratospheric and tropospheric annular “strong vortex regimes,” as characterized by significant changes in the probabilities of mode variations are sometimes independent of the normalized AO index (22). The average weather extremes such as cold air outbreaks, each other, but (on average) strong anomalies value (1080 days) during weak vortex re- snow, and high winds across Europe, Asia, and just above the tropopause appear to favor tro- gimes is Ϫ0.44, and ϩ0.35 for strong vortex North America (25). The observed circulation pospheric anomalies of the same sign. Oppos- regimes (1800 days). The large sample sizes changes during weak and strong vortex regimes ing anomalies as in December 1998 (Fig. 1) are contribute to the high statistical significance are substantial from a meteorological viewpoint possible, but anomalies of the same sign dom- of these averages (23). During the weak and and can be anticipated by observing the strato- inate the average (Fig. 2). strong vortex regimes the average surface sphere. These results imply a measure of pre- To examine the tropospheric circulation pressure anomalies (Fig. 3) are markedly like dictability, up to 2 months in advance, for AO/ after these extreme events, we define weak opposite phases of the AO (11) or NAO (14), NAO variations in northern winter, particularly and strong vortex “regimes” as the 60-day with the largest effect on pressure gradients for extreme values that are associated with un- periods after the dates on which the Ϫ3.0 and in the North Atlantic and Northern Europe. usual weather events having the greatest impact ϩ1.5 thresholds were crossed. Our results are The probability density functions (PDFs) of on society. not sensitive to the exact range of days used the daily normalized AO and NAO indices (24) Since the NAO and AO are known to mod- and do not depend on the first few days after during weak and strong vortex regimes are ulate the position of surface cyclones across the the “events.” We focus on the average behav- compared in Fig. 4. More pronounced than the Atlantic and Europe, we examine the tracks of ior during these “weak vortex regimes” and shift in means are differences in the shapes of surface cyclones with central pressure less than

582 19 OCTOBER 2001 VOL 294 SCIENCE www.sciencemag.org The stratospheric polar vortex Zonal mean zonal wind (m s-1) at 10 hPa and 65°N

mean 80

20 Winter (Nov- Feb) 0 10 hPa height (m) −20 and wind (m s-1) −40 The winter of 2014/2015 zonal mean zonal wind

10 hPa 65°N The winter of 2014/2015 zonal mean zonal wind

10 hPa 65°N

Final warming

Zonal mean zonal wind rapid deceleraons The winter of 2014/2015 zonal mean zonal wind Karpetcko and Nikulin (2004)

10 hPa 65°N

Nov/Dec wave acvity is negavely correlated with Jan/Feb wave acvity 4446 JOURNAL OF CLIMATE VOLUME 17 4446 JOURNAL OF CLIMATE VOLUME 17 plane channel model. They found that, when wave forcing at the lower boundary isplane raised channel beyond model. a critical They found that, when wave forc- amplitude, the eddy componentsing at and the mean lower zonal boundary flow is raised beyond a critical in the stratosphere oscillateamplitude, quasiperiodically. the eddy In components their and mean zonal flow model vertically propagatingin planetary the stratosphere waves 1 oscillate and 2 quasiperiodically. In their decelerate westerly zonal flowmodel until vertically it turns into propagating east- planetary waves 1 and 2 erlies. Since planetary wavesdecelerate cannot propagate westerly in zonal east- flow until it turns into east- erlies (Charney and Drazin 1961),erlies. this Since results planetary in trapping waves cannot propagate in east- of the waves. In the absenceerlies of wave (Charney forcing and the Drazin strato- 1961), this results in trapping sphere relaxes to the radiativeof the equilibrium, waves. In thewesterlies absence of wave forcing the strato- are recovered, and waves cansphere againBackground relaxes propagate to the into radiative the equilibrium, westerlies stratosphere. This mechanismare suggests recovered, that and the waves period can again propagate into the 4446 of oscillationsJOURNAL is determined OFstratosphere. by CLIMATE the time that This the mechanism strato- suggests that theVOLUME period17 sphere takes to relax to theof radiative oscillationsJan/Feb zonal mean zonal wind equilibrium. is determined The by the time that the strato- plane channel model. They foundperiod that, of the when vacillations wave forc- is roughlysphereWeak takes 2–3 months, to Nov/Dec wave acvity relax which to the radiative equilibrium. The Strong Nov/Dec wave acvity ing at the lower boundary iscoincides raised beyond with the a period critical of theperiod heat of flux the anticorrelation vacillations is roughly 2–3 months, which amplitude, the eddy componentsfound and here. mean zonal flow coincides with the period of the heat flux anticorrelation in the stratosphere oscillate quasiperiodically.Aconnectionbetweeneddycomponentsandmean In their found here. model vertically propagatingzonal planetary flow oscillations waves 1 and was 2 afterwardAconnectionbetweeneddycomponentsandmean found in general decelerate westerly zonal flowcirculation until it turns models into and east- inzonal observations flow oscillations (Cristiansen was afterward found in general erlies. Since planetary waves1999, cannot 2001). propagate Cristiansen in east- (2001)circulation pointed models out that and these in observations (Cristiansen erlies (Charney and Drazin 1961),oscillations this results originate in trapping from competition1999, 2001). between Cristiansen zonal (2001) pointed out that these of the waves. In the absencewind of wave deceleration forcing the caused strato- by verticaloscillations convergence originate of from the competition between zonal sphere relaxes to the radiativevertical equilibrium, component westerlies of the EPwind flux deceleration and zonal wind caused ac- by vertical convergence of the are recovered, and waves canceleration again propagate caused into by relaxation the vertical toward component the radiative of the EP flux and zonal wind ac- stratosphere. This mechanismequilibrium. suggests that Figure the 3period showsceleration the JF wind caused anomalies by relaxation for toward the radiative of oscillations is determinedyears by the with time the that weak the strato- and strongequilibrium. ND heat flux Figure as well 3 shows as the JF wind anomalies for sphere takes to relax to thetheir radiative differences. equilibrium. Only Thewintersyears when with the the ND weak normalized and strong ND heat flux as well as period of the vacillations is roughlylatitudinal 2–3 averaged months, heatwhich fluxtheir anomalies differences. at 20 OnlyhPa were winters when the ND normalized coincides with the period of themore heat (1985, flux anticorrelation 1994, 1998, 2001,latitudinal 2002) averaged or less (1980, heat flux anomalies at 20 hPa were found here. 1981, 1989, 1990, 1996, 2000)more thanJanuary/ (1985, 1 s were 1994, included 1998, 2001, 2002) or less (1980, Aconnectionbetweeneddycomponentsandmeanin these statistics. It is seen1981, that, 1989, when 1990, wave 1996, flux is 2000) than 1 s were included zonal flow oscillations was afterwardweak in the found early in winter, general thein midwinter theseFebruary zonal statistics. polar night It is jet seen that, when wave flux is circulation models and inis observations well developed (Cristiansen between 60weak8 and in 70 the8N(Fig.3a).In early winter, the midwinter polar night jet contrast, when the early winteris well heatmean zonal developed flux is strong, between the 608 and 708N(Fig.3a).In 1999, 2001). Cristiansen (2001) pointed out that these 99% confidence oscillations originate from competitionmidwinter polar between night zonal jet is deceleratedcontrast,wind when between the 60 early8 and winter heat flux is strong, the wind deceleration caused by vertical708Nwithasecondarymaximumappearingatabout convergence of the midwinter polar night jet is decelerated between 608 and level vertical component of the EP30 flux8N(Fig.3b).ThepositiveJFwindspeeddifferences and zonal wind ac- 708Nwithasecondarymaximumappearingatabout celeration caused by relaxationbetween toward the winters the radiative with weak30 and8KarpetchkoN(Fig.3b).ThepositiveJFwindspeeddifferences strong ND and heat flux- Weak−Strong equilibrium. Figure 3 showses the are JF statistically wind anomalies significant for between throughoutNikulin the 2004 the winters majority with of weak and strong ND heat flux- years with the weak and strongthe ND extratropical heat flux as stratosphere well as es (Fig. are statistically 3c). There issignificant also a throughout the majority of their differences. Only wintersnegative when the statistically ND normalized significantthe wind extratropical speed anomaly stratosphere in (Fig. 3c). There is also a 21 latitudinal averaged heat fluxthe anomalies upper part at of20 the hPa plot were at aboutnegative 308N. statistically To see the significant wave F windIG.3.CompositemeansoftheJFzonal-meanzonalwind(ms speed anomaly in ) and EP flux vectors in the years with (a) weak and (b) strong wave 21 more (1985, 1994, 1998, 2001,propagation 2002) or behavior less (1980, for thesethe upper periods, part we of calculated the plot at about 308N. To see the wave FIG.3.CompositemeansoftheJFzonal-meanzonalwind(ms ) propagation behavior for theseactivity periods, during wethe ND calculated period and (c)and their EP difference. flux vectors Positive in the contour years with (a) weak and (b) strong wave 1981, 1989, 1990, 1996, 2000)the than EP fluxes 1 s were (see included Andrews et al. 1987) and superim- values are solid, negative values are dotted, and zero contours are the EP fluxes (see Andrews et al. 1987) and superim- activity during the ND period and (c) their difference. Positive contour in these statistics. It is seenposed that, them when on wave the fluxwind is fields. The arrows of the EP dashed. The arrow scale is four timesvalues larger are in solid, (c) than negative in (a) values and are dotted, and zero contours are weak in the early winter, theflux midwinter vector arepolar scaled night by jet densityposed tothem make on the wind strato- fields.(b). The 99% arrows confidence of the level EP for zonaldashed. wind The is arrow shaded. scale is four times larger in (c) than in (a) and is well developed between 60spheric8 and 70part8N(Fig.3a).In of the picture moreflux vector readable. are The scaled JF byEP density to make the strato- (b). The 99% confidence level for zonal wind is shaded. contrast, when the early winterflux heat differences flux is strong, between the thespheric winters part with of the weak picture and more readable. The JF EP midwinter polar night jet is deceleratedstrong ND between heat flux 60 are8 and shownflux in differencesFig. 3c. It is between seen that thepositive winters wind with difference. weak and The statistical significance of 708Nwithasecondarymaximumappearingataboutthe direction of the anomalousstrong EP flux ND is heat consistent flux are with shownthe in differences Fig. 3c. It is between seen that the EPpositive flux components wind difference. during The statistical significance of 308N(Fig.3b).ThepositiveJFwindspeeddifferencesthe correlation pattern shownthe in direction Fig. 1a. of It the is directed anomalousmidwinter EP flux is consistentin the years with withthe weak differences and strong between ND heat the EP flux components during between the winters with weakupward and strong throughout ND heat the flux- midlatitudethe correlation stratosphere pattern and tro- shownflux in Fig. can 1a.be seen It is in directed Fig. 4. Themidwinter picture in for the the years vertical with weak and strong ND heat es are statistically significantposphere throughout on the the majority southern of partupward of the throughout positive wind the dif- midlatitudecomponent stratosphere (Fig. 4a) and is tro- essentiallyflux can the be same seen as in that Fig. for 4. The picture for the vertical the extratropical stratosphereference, (Fig. 3c). where There significant is also a anticorrelationposphere on the between southern the partthe of theheat positive flux correlation wind dif- (Fig.component 1a). It shows (Fig. 4a) that is the essentially the same as that for negative statistically significantheat wind fluxes speed is observed anomaly in (Fig.ference, 1a). The where anomalous significant EP anticorrelationvertical component between of the the EPthe flux heat is flux stronger correlation in the (Fig. 1a). It shows that the heat fluxes is observed (Fig. 1a). The anomalous EP21 vertical component of the EP flux is stronger in the the upper part of the plot at aboutflux 30is8 directedN. To see downward the wave in theFIG.3.CompositemeansoftheJFzonal-meanzonalwind(ms polar stratosphere in midlatitude troposphere and) stratosphere and weaker in propagation behavior for thesethe periods, upper part we of calculated the plot onfluxand the EP is northern fluxdirected vectors side downward in the of years the inwiththe the (a) vicinity polar weak and stratosphere of (b) the strong subtropical wave in midlatitude jet stream troposphere in the years and stratosphere and weaker in theactivity upper during part the ofND the period plot and on(c) their the difference. northern Positive side of contour the the vicinity of the subtropical jet stream in the years the EP fluxes (see Andrews et al. 1987) and superim- values are solid, negative values are dotted, and zero contours are posed them on the wind fields. The arrows of the EP dashed. The arrow scale is four times larger in (c) than in (a) and flux vector are scaled by density to make the strato- (b). The 99% confidence level for zonal wind is shaded. spheric part of the picture more readable. The JF EP flux differences between the winters with weak and strong ND heat flux are shown in Fig. 3c. It is seen that positive wind difference. The statistical significance of the direction of the anomalous EP flux is consistent with the differences between the EP flux components during the correlation pattern shown in Fig. 1a. It is directed midwinter in the years with weak and strong ND heat upward throughout the midlatitude stratosphere and tro- flux can be seen in Fig. 4. The picture for the vertical posphere on the southern part of the positive wind dif- component (Fig. 4a) is essentially the same as that for ference, where significant anticorrelation between the the heat flux correlation (Fig. 1a). It shows that the heat fluxes is observed (Fig. 1a). The anomalous EP vertical component of the EP flux is stronger in the flux is directed downward in the polar stratosphere in midlatitude troposphere and stratosphere and weaker in the upper part of the plot on the northern side of the the vicinity of the subtropical jet stream in the years 4446 JOURNAL OF CLIMATE VOLUME 17 4446 JOURNAL OF CLIMATE VOLUME 17 plane channel model. They found that, when wave forcing at the lower boundary isplane raised channel beyond model. a critical They found that, when wave forc- amplitude, the eddy componentsing at and the mean lower zonal boundary flow is raised beyond a critical in the stratosphere oscillateamplitude, quasiperiodically. the eddy In components their and mean zonal flow model vertically propagatingin planetary the stratosphere waves 1 oscillate and 2 quasiperiodically. In their decelerate westerly zonal flowmodel until vertically it turns into propagating east- planetary waves 1 and 2 erlies. Since planetary wavesdecelerate cannot propagate westerly in zonal east- flow until it turns into east- erlies (Charney and Drazin 1961),erlies. this Since results planetary in trapping waves cannot propagate in east- of the waves. In the absenceerlies of wave (Charney forcing and the Drazin strato- 1961), this results in trapping sphere relaxes to the radiativeof the equilibrium, waves. In thewesterlies absence of wave forcing the strato- are recovered, and waves cansphere againBackground relaxes propagate to the into radiative the equilibrium, westerlies stratosphere. This mechanismare suggests recovered, that and the waves period can again propagate into the 4446 of oscillationsJOURNAL is determined OFstratosphere. by CLIMATE the time that This the mechanism strato- suggests that theVOLUME period17 sphere takes to relax to theof radiative oscillationsJan/Feb zonal mean zonal wind equilibrium. is determined The by the time that the strato- plane channel model. They foundperiod that, of the when vacillations wave forc- is roughlysphereWeak takes 2–3 months, to Nov/Dec wave acvity relax which to the radiative equilibrium. The Strong Nov/Dec wave acvity ing at the lower boundary iscoincides raised beyond with the a period critical of theperiod heat of flux the anticorrelation vacillations is roughly 2–3 months, which amplitude, the eddy componentsfound and here. mean zonal flow coincides with the period of the heat flux anticorrelation in the stratosphere oscillate quasiperiodically.Aconnectionbetweeneddycomponentsandmean In their found here. model vertically propagatingzonal planetary flow oscillations waves 1 and was 2 afterwardAconnectionbetweeneddycomponentsandmean found in general decelerate westerly zonal flowcirculation until it turns models into and east- inzonal observations flow oscillations (Cristiansen was afterward found in general erlies. Since planetary waves1999, cannot 2001). propagate Cristiansen in east- (2001)circulation pointed models out that and these in observations (Cristiansen erlies (Charney and Drazin 1961),oscillations this results originate in trapping from competition1999, 2001). between Cristiansen zonal (2001) pointed out that these of the waves. In the absencewind of wave deceleration forcing the caused strato- by verticaloscillations convergence originate of from the competition between zonal sphere relaxes to the radiativevertical equilibrium, component westerlies of the EPwind flux deceleration and zonal wind caused ac- by vertical convergence of the are recovered, and waves canceleration again propagate caused into by relaxation the vertical toward component the radiative of the EP flux and zonal wind ac- stratosphere. This mechanismequilibrium. suggests that Figure the 3period showsceleration the JF wind caused anomalies by relaxationHow do we determine if there is wave acvity? for toward the radiative of oscillations is determinedyears by the with time the that weak the strato- and strongequilibrium. ND heat flux Figure as well 3 shows as the JF wind anomalies for sphere takes to relax to thetheir radiative differences. equilibrium. Only Thewintersyears when with the the ND weak normalized and strong ND heat• flux Planetary scale EP flux as well as period of the vacillations is roughlylatitudinal 2–3 averaged months, heatwhich fluxtheir anomalies differences. at 20 OnlyhPa were winters when the ND normalized coincides with the period of themore heat (1985, flux anticorrelation 1994, 1998, 2001,latitudinal 2002) averaged or less (1980, heat flux anomalies at 20 hPa were found here. 1981, 1989, 1990, 1996, 2000)more thanJanuary/ (1985, 1 s were 1994, included 1998, 2001, 2002) or less (1980, Aconnectionbetweeneddycomponentsandmeanin these statistics. It is seen1981, that, 1989, when 1990, wave 1996, flux is 2000) than 1 s were included zonal flow oscillations was afterwardweak in the found early in winter, general thein midwinter theseFebruary zonal statistics. polar night It is jet seen that, when wave flux is circulation models and inis observations well developed (Cristiansen between 60weak8 and in 70 the8N(Fig.3a).In early winter, the midwinter polar night jet contrast, when the early winteris well heatmean zonal developed flux is strong, between the 608 and 708N(Fig.3a).In 1999, 2001). Cristiansen (2001) pointed out that these 99% confidence oscillations originate from competitionmidwinter polar between night zonal jet is deceleratedcontrast,wind when between the 60 early8 and winter heat flux is strong, the wind deceleration caused by vertical708Nwithasecondarymaximumappearingatabout convergence of the midwinter polar night jet is decelerated between 608 and level vertical component of the EP30 flux8N(Fig.3b).ThepositiveJFwindspeeddifferences and zonal wind ac- 708Nwithasecondarymaximumappearingatabout celeration caused by relaxationbetween toward the winters the radiative with weak30 and8KarpetchkoN(Fig.3b).ThepositiveJFwindspeeddifferences strong ND and heat flux- Weak−Strong equilibrium. Figure 3 showses the are JF statistically wind anomalies significant for between throughoutNikulin the 2004 the winters majority with of weak and strong ND heat flux- years with the weak and strongthe ND extratropical heat flux as stratosphere well as es (Fig. are statistically 3c). There issignificant also a throughout the majority of their differences. Only wintersnegative when the statistically ND normalized significantthe wind extratropical speed anomaly stratosphere in (Fig. 3c). There is also a 21 latitudinal averaged heat fluxthe anomalies upper part at of20 the hPa plot were at aboutnegative 308N. statistically To see the significant wave F windIG.3.CompositemeansoftheJFzonal-meanzonalwind(ms speed anomaly in ) and EP flux vectors in the years with (a) weak and (b) strong wave 21 more (1985, 1994, 1998, 2001,propagation 2002) or behavior less (1980, for thesethe upper periods, part we of calculated the plot at about 308N. To see the wave FIG.3.CompositemeansoftheJFzonal-meanzonalwind(ms ) propagation behavior for theseactivity periods, during wethe ND calculated period and (c)and their EP difference. flux vectors Positive in the contour years with (a) weak and (b) strong wave 1981, 1989, 1990, 1996, 2000)the than EP fluxes 1 s were (see included Andrews et al. 1987) and superim- values are solid, negative values are dotted, and zero contours are the EP fluxes (see Andrews et al. 1987) and superim- activity during the ND period and (c) their difference. Positive contour in these statistics. It is seenposed that, them when on wave the fluxwind is fields. The arrows of the EP dashed. The arrow scale is four timesvalues larger are in solid, (c) than negative in (a) values and are dotted, and zero contours are weak in the early winter, theflux midwinter vector arepolar scaled night by jet densityposed tothem make on the wind strato- fields.(b). The 99% arrows confidence of the level EP for zonaldashed. wind The is arrow shaded. scale is four times larger in (c) than in (a) and is well developed between 60spheric8 and 70part8N(Fig.3a).In of the picture moreflux vector readable. are The scaled JF byEP density to make the strato- (b). The 99% confidence level for zonal wind is shaded. contrast, when the early winterflux heat differences flux is strong, between the thespheric winters part with of the weak picture and more readable. The JF EP midwinter polar night jet is deceleratedstrong ND between heat flux 60 are8 and shownflux in differencesFig. 3c. It is between seen that thepositive winters wind with difference. weak and The statistical significance of 708Nwithasecondarymaximumappearingataboutthe direction of the anomalousstrong EP flux ND is heat consistent flux are with shownthe in differences Fig. 3c. It is between seen that the EPpositive flux components wind difference. during The statistical significance of 308N(Fig.3b).ThepositiveJFwindspeeddifferencesthe correlation pattern shownthe in direction Fig. 1a. of It the is directed anomalousmidwinter EP flux is consistentin the years with withthe weak differences and strong between ND heat the EP flux components during between the winters with weakupward and strong throughout ND heat the flux- midlatitudethe correlation stratosphere pattern and tro- shownflux in Fig. can 1a.be seen It is in directed Fig. 4. Themidwinter picture in for the the years vertical with weak and strong ND heat es are statistically significantposphere throughout on the the majority southern of partupward of the throughout positive wind the dif- midlatitudecomponent stratosphere (Fig. 4a) and is tro- essentiallyflux can the be same seen as in that Fig. for 4. The picture for the vertical the extratropical stratosphereference, (Fig. 3c). where There significant is also a anticorrelationposphere on the between southern the partthe of theheat positive flux correlation wind dif- (Fig.component 1a). It shows (Fig. 4a) that is the essentially the same as that for negative statistically significantheat wind fluxes speed is observed anomaly in (Fig.ference, 1a). The where anomalous significant EP anticorrelationvertical component between of the the EPthe flux heat is flux stronger correlation in the (Fig. 1a). It shows that the heat fluxes is observed (Fig. 1a). The anomalous EP21 vertical component of the EP flux is stronger in the the upper part of the plot at aboutflux 30is8 directedN. To see downward the wave in theFIG.3.CompositemeansoftheJFzonal-meanzonalwind(ms polar stratosphere in midlatitude troposphere and) stratosphere and weaker in propagation behavior for thesethe periods, upper part we of calculated the plot onfluxand the EP is northern fluxdirected vectors side downward in the of years the inwiththe the (a) vicinity polar weak and stratosphere of (b) the strong subtropical wave in midlatitude jet stream troposphere in the years and stratosphere and weaker in theactivity upper during part the ofND the period plot and on(c) their the difference. northern Positive side of contour the the vicinity of the subtropical jet stream in the years the EP fluxes (see Andrews et al. 1987) and superim- values are solid, negative values are dotted, and zero contours are posed them on the wind fields. The arrows of the EP dashed. The arrow scale is four times larger in (c) than in (a) and flux vector are scaled by density to make the strato- (b). The 99% confidence level for zonal wind is shaded. spheric part of the picture more readable. The JF EP flux differences between the winters with weak and strong ND heat flux are shown in Fig. 3c. It is seen that positive wind difference. The statistical significance of the direction of the anomalous EP flux is consistent with the differences between the EP flux components during the correlation pattern shown in Fig. 1a. It is directed midwinter in the years with weak and strong ND heat upward throughout the midlatitude stratosphere and tro- flux can be seen in Fig. 4. The picture for the vertical posphere on the southern part of the positive wind dif- component (Fig. 4a) is essentially the same as that for ference, where significant anticorrelation between the the heat flux correlation (Fig. 1a). It shows that the heat fluxes is observed (Fig. 1a). The anomalous EP vertical component of the EP flux is stronger in the flux is directed downward in the polar stratosphere in midlatitude troposphere and stratosphere and weaker in the upper part of the plot on the northern side of the the vicinity of the subtropical jet stream in the years Why EP flux?

Change in the mean zonal wind with me   du ∇•F ≈ dt (-) (-) convergence decrease Zonal-mean eddy heat ﬂux EP Flux Divergence v'T ' F = p afRcosφ z s HN 2

Vercal component of EP ﬂux Why EP ﬂux?

Change in the mean zonal wind with me   du ∇•F ≈ dt (-) (-) The 100 hPa convergencezonal-mean eddy heat flux decrease describes the vercal Zonal-mean eddy heat flux EP Flux Divergence component of EP flux and describes vercal wave acvity flux near the tropopause v'T ' F = p afRcosφ z s HN 2

Vercal component of EP ﬂux The winter of 2014/2015

Let's explore the 23 November deceleraon

-20

-40 The winter of 2014/2015

Let's explore Why the November deceleraon? the • Rapid zonal mean zonal wind deceleraons in November are 23 November uncommon deceleraon • Can have implicaons for the polar vortex later in the season

Recurvature of -20 Nuri -40 Data and Methods

• ERA-Interim data – ～80 km horizontal resoluon – 60 vercal levels from 1000 hPa to 0.1 hPa – 6-hourly data available from 1000 hPa to 1 hPa • 31-year climatology from 1979—2010 • Heat flux: – Zonal mean (v'T') • Anomalies with respect to climatology (e.g., Polvani & Waugh 2004) • Full heat flux (not anomaly) – Longitudinal distribuon (v'T') – Averaged from 45°-75°N, weighted by the cosine of latude 100 hPa heat flux anomaly & 40-day heat flux and zonal mean zonal wind These are all 10 hPa zonal mean zonal wind anomalies Zonal mean zonal wind 40-day averaged heat flux

100 hPa 45—75°N 10 hPa 65°N Heat flux anomaly Zonal mean averaged over the zonal wind previous 40 days 100 hPa heat flux anomaly & 40-day heat flux and zonal mean zonal wind These are all 10 hPa zonal mean zonal wind anomalies Zonal mean zonal wind

40-day averaged heat ﬂux 23 November 2014

-1 v'T'40-day = 7.4 K m s Zonal mean zonal wind

40-day averaged heat ﬂux 23 November 2014

100 hPa 45—75°N 10 hPa 65°N Heat flux anomaly 100 hPa 45—75°N Zonal mean averaged over the Daily heat flux zonal wind previous 40 days anomaly 100 hPa heat flux anomaly & 40-day heat flux and zonal mean zonal wind These are all 10 hPa zonal mean zonal wind anomalies

20 November Zonal mean zonal wind

10 November 40-day averaged heat ﬂux

100 hPa 45—75°N 10 hPa 65°N Heat flux anomaly 100 hPa 45—75°N Zonal mean averaged over the Daily heat flux zonal wind previous 40 days anomaly The tropospheric North Pacific synopc evoluon 0000 UTC 4 November 2014