Research Article Recent Intensified Winter Coldness in the ...

Research ArticleRecent Intensified Winter Coldness in the Mid-HighLatitudes of Eurasia and Its Relationship with Daily ExtremeLow Temperature Variability

Chuhan Lu Shaoqing Xie Yujing Qin and Jiewen Zhou

Key Laboratory of Meteorological Disaster Ministry of Education Joint International Research Laboratory ofClimate and Environment Change Collaborative Innovation Center on Forecast and Evaluation of Meteorological DisastersNanjing University of Information Science amp Technology Nanjing 210044 China

Correspondence should be addressed to Chuhan Lu luchuhannuisteducn

Received 14 December 2015 Revised 11 February 2016 Accepted 14 February 2016

Academic Editor Alex Cannon

Copyright copy 2016 Chuhan Lu et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Observational records in recent decades show a large-scale decrease in the cold-season temperature variance in the NorthernHemispheremidlatitudes under continuous global warming However severe low temperature events in winter frequently occurredin midlatitude Eurasia (MEA) in the last decade Here we define a new coldness intensity (CI) index for the near-surface basedon the amplitude of daily anomalously cold temperatures in winter to demonstrate the CI of the variability of low temperatureextremes The results show that a sign-consistent mode dominates the CI variation in MEA with a marked intensification duringthe last decade via empirical orthogonal function (EOF) analysisThis leadingmode is significantly related to the frequency ofwinterextreme events The associated circulations are characterized by a remarkable anomalous anticyclone in Northwest Eurasia whichinduced substantial cold advection inMEAThewidespread intensifiedCI inMEA is closely linkedwith strong surface anticyclonesand synoptic blocking in the mid-high latitudes (25∘Endash85∘E) Coincidently positive phase shifts of the first two leading modes ofthe extratropical circulation which feature similar blocking-like anomalies in the northwestern Eurasian subarctic jointly play animportant role in the recent frequency of severe winters

1 Introduction

During the past decade cold and anomalously heavy snowfallhas affected most midlatitudes of the Northern Hemisphereincluding major industrialized centers along with an overallgradual warming trend and an acceleration of Arctic sea-ice accumulation [1ndash3] Considerable interannual variabilityandmarkedly coldwinters have accompanied this unforeseencooling trend in midlatitudes of the Northern Hemisphere[4] One of the most impressive cold winters in recent yearsoccurred in themidlatitudes of Eurasia in January 2008whena freezing rain event in southern China disrupted traffic anddamaged infrastructure with an economic loss of more than100 billion Yuan [5 6]

A number of studies have focused on the recent increasein midlatitude cold extremes and their attributions Several

studies have demonstrated some possible connectionsbetween Arctic amplification midlatitude weather and cli-mate extremes [7ndash9] however the statistical robustness andphysical mechanisms of the connections are still unclear[10ndash12] The Arctic sea-ice losses and warm North Atlanticsea surface temperature (SST) may both play important rolesin the air temperature variability over the East Asia monsoonregion during cold seasons [3 13 14] by linking the changein the Arctic Oscillation (AO)North Atlantic Oscillation(NAO) Atlantic Multidecadal Oscillation (AMO) [15] andthe Siberian High [16] Persistent weather systems may alsocontribute to large-scale cold extremes [5 17ndash19] Zhanget al [20] reported that the intensified synoptic anticycloneactivity after the early 2000s had a prominent impact on theincreased frequency of cold weather events

Hindawi Publishing CorporationAdvances in MeteorologyVolume 2016 Article ID 3679291 11 pageshttpdxdoiorg10115520163679291

2 Advances in Meteorology

The subseasonal variance in the daily temperature canbe used to quantify the overall seasonal high-frequencytemperature fluctuation [21 22] The increased cold-seasontemperature variance suggests that cold outbreaks or warmevents occur more frequently However in contrast to therecent increased extreme cold weather in the midlatitudesof Eurasia decreases in the subseasonal all-longitude zonalmean temperature variability have been detected over thepast few decades [23] Schneider et al [24] argued thatsuch inconsistency resulted from a warmer mean climatewhich entails changes in the frequency with which fixedtemperature thresholds are exceeded The research questionis as follows how can the integrated intensity of the recentextreme cold events during the cold season be quantified

Here we defined a new objective index to quantify theintensity of winter coldness and focused on its contributionto the recent increased large-scale extreme cold events in themidlatitudes of Eurasia (MEA) The associated circulationpatterns and their relationship with synoptic weather systemswill be discussed to explain the change in the recent intensecold events in MEA

2 Data and Methodology

21 Data This study uses the daily data from the ECMWFERA-Interim reanalysis from January 1 1979 throughDecember 31 2013 [25] We use a full T255 (512 times 256)Gaussian grid which has an approximate global horizontalresolution of 07∘times 07∘ And the gridded daily minimum andmaximum surface air temperature (119879min and 119879max) are alsofrom the ERA-Interim associated forecast field Furthermorewe use119879min and119879max data at the 1331 Eurasianmeteorologicalstations from the daily global historical climatology networkfrom 1972 to 2012 (GHCN-DAILY httpsgisncdcnoaagovgeoportalcatalogsearchresourcedetailspageid=govnoaancdcC00861) Although there aremore temperature stationsin the Eurasianmid-high latitudes we only chose 1331 stationshaving sufficiently long record from 1973 to 2012 Consideringthe sparsity of stations over northernChina andMongolia weadded 215 stations over these regions from the daily Chinesesurface climate dataset (V3) and baseline meteorologicaldata in Siberia (V5)Themeanmonthly indices of the AO aretaken from httpwwwcpcncepnoaagovproductsprecipCWlinkdaily ao indexaoshtml In this study winter meanswere constructed by averaging the data for DecemberJanuary and February (DJF) which resulted in 35 winters(1979ndash2013)

22 Definition of Intensity of Winter Coldness Percentile-based indices that consider the occurrence of cold spell daysand cold days are widely applied to study changes in dailyextreme cold events However the frequency of extremesdepends on the choice of the percentile value (eg 5th and10th for cold events) the indices may not reflect the seasonalintegrated intensity of cold events well because they alsodepend on the amplitude and persistence of events On theother hand the traditional subseasonal temperature variancewould underestimate the seasonal intensity of coldness if apersistent cold condition dominates the cold season despite

an increase of extreme cold events Moreover the warmevents would also induce an amplification of the values ofvariance if we focus on the cold anomalies and associated coldextremes Therefore we define a new coldness intensity (CI)in winter as

CI = 190

90

sum

119894=1

(

1198791015840

119894

120590119894

)

2

1198791015840

119894

=

119879119894minus 119879119888119879119894minus 119879119888lt 0

0 119879119894minus 119879119888ge 0

(1)

Here 119879119894is the daily 925 hPa temperature from December 1st

in the preceding year to February 28th in the current year (90days for DJF) 1198791015840

119894

is a cold anomaly relative to the climatology(119879119888) of 90-day mean of 119879

119894from 1979 to 2013 when 119879

119894minus119879119888lt 0

When 119879119894is relative warm (119879

119894minus 119879119888ge 0) the value of 1198791015840

119894

becomes zero so that the anomalous warmth is excluded Tohomogenize the fluctuations among different locations thevalues were divided by the standard deviation (120590

119894) at each grid

point that is the multiyear standard deviation at a grid pointon the 119894th day Because the 1198791015840

119894

represents a cold deviationfrom climatology (1979ndash2013) the value of its intraseasonalvariability or CI measures a seasonal integrated magnitude ofcold anomalies in winter Accordingly the standardized valueof CI differs from its corresponding standardized subseasonalfluctuation especially in a persistent cold (warm) winterwhich may include more frequent cold (warm) extremes

3 Result

31 Recent Intensified Winter Coldness We used the 10thpercentile of the daily maximum (minimum) surface air tem-perature (SAT) over 1979ndash2013 based on ERA-Interim toobtain cold day (night) events The zonal mean (0∘ndash180∘E)occurrence of extreme low temperatures shows a clear inter-decadal fluctuation in the midlatitudes of Eurasia (MEA)since 1979 (Figures 1(a) and 1(b))The low temperature eventsat 30∘Nndash60∘N exhibited a notable decline in the 1990s andthe early 2000s which corresponded to the gradual warmingin the global surface temperature However a significantincrease in cold days and cold nights emerged after the mid-2000s in accordance with the recent severe winters overEurasia (0405 0506 0708 0910 1011 1112 and 1213)The mean frequencies of cold days (nights) in 1979ndash19891990ndash2004 and 2005ndash2012 are 134 (147) dy 117 (120) dy and157 (152) dy respectively Thus stronger cooling conditionsoccurred over Eurasia in recent years compared with the1980s

After the mid-2000s the subseasonal temperature vari-ance at 925 hPa displayed greatly contrasting anomaliesbetween 30∘Nndash45∘N and 45∘Nndash60∘N (Figure 1(c)) Note thatthe variance is also divided by 120590

119894to homogenize the fluctu-

ations among different locations The linear trend for 1996ndash2013 is 11 per decade at 30∘Nndash45∘N (exceeding the 01 sig-nificance) The intensified subseasonal fluctuation indicatesmore unstable weather changes during the cold seasons forexample more frequent cold air outbreaksmay occur In con-trast a decrease of minus10 per decade emerged at 45∘Nndash60∘N

Advances in Meteorology 3

325N

375N

425N

475N

525N

575N

625N

675N

20101990 1995 2000 200519851980

12 18 24 30 36

(a) Cold days

325N

375N

425N

475N

525N

575N

625N

675N

1985 1990 1995 2000 2005 20101980

12 18 24 30 36

(b) Cold nights

09 1 11 12 13

325N

375N

425N

475N

525N

575N

625N

675N

20101990 1995 2000 200519851980

(c) Standard variance05 07 09 11 13

325N

375N

425N

475N

525N

575N

625N

675N

1985 1990 1995 20101980 2000 2005

(d) CI

Figure 1 Changing extreme cold events 925 hPa temperature variability and CI Zonal mean (0∘ndash180∘E) winter cold days ((a) unit days)cold nights ((b) unit days) intraseasonal variance (c) and CI (d) during 1979ndash2013 The values are calculated at each grid point before areaaveraging for 5∘ latitude bands from 30∘N to 70∘NThe 1979ndash2013 mean is used as the climatological value in (d)The units for (a) and (b) aredays

which is inconsistent with the low temperature extremes inFigures 1(a) and 1(b) However the decrease trend is notsignificant Therefore the overall linear trend at 30∘Nndash60∘Ndisplays a weak increase (005 per decade) This indicatesa reduction of the synoptic temperature variance near thesurface during the cold season which is consistent with theall-longitude zonal mean results of Screen [23]

As shown in Figure 1(d) the variation in the CI greatlyresembles the change in the frequency of cold days and coldnights A remarkably strong CI is observed over 30∘Nndash45∘Nand 45∘Nndash60∘N after the mid-2000s which is coincidentwithmore frequent low temperature events Considering thatcolder conditions favor cool weather events the increasein the synoptic negative temperature variability physicallyindicates a better chance of low temperature extremes andvice versa The inconsistency between intraseasonal varianceand CI implies that more persistent cold events as well as

more frequent percentile-based low temperature extremesmay have occurred in recent years despite the possibly lessfrequent cold outbreaks in the context of global warming

To explicitly analyze the spatial-temporal characteristicsof the winter CI in MEA an EOF analysis is applied to thewinter CI over 0∘ndash180∘E and 30∘ndash55∘N during 1979ndash2013The variance contribution rates of the first two modes are32 and 23 which can be well discriminated accordingto the criterion proposed by North et al [26] The spatialdistribution of the leading mode (EOF1) of the CI and itscorresponding time series (PC1) are shown in Figure 2 Asdepicted in Figure 2(a) the EOF1 mode shows a patternof consistent positive CI departures in MEA indicatingconsistent intensifying or weakening coldness in winter Theareas with high values are mainly located west of 50∘Ewith the largest variability around northwestern China PC1exhibits clear interdecadal variation superimposed on large


002004

002 004

015

009

002

002

004

004

009 009

(a)2000 200519951985 1990 20101980

minus2

minus1

0

1

2

3

(b)

Figure 2 (a) EOF1 mode of the CI anomalies at 30ndash55∘N in Eurasia (b) Standardized time series of PC1 (dotted solid line) the regional (30ndash55∘N 0ndash180∘E) average CI (ECI bar) and its corresponding 9-yr runningmean (dashed line)The areas with positive values are shaded in (a)

interannual variability in recent decade (Figure 2(b)) Inparticular the positive phase occurs during the early andmid-1980s followed by dominantly negative values from thelate 1980s to the beginning of the 21st century then a positivephase with sharply higher values occurred Considering thespatial sign-consistency of EOF1 we calculated a regionalaverage CI over MEA and defined it as ECI for simplicity(gray bar in Figure 2(b)) Clearly the variations between PC1and ECI as a function of time are highly coincident whereofECI is in correlation with PC1 at a value of 094 Thusboth PC1 and ECI indicate the variability in the subseasonalcoldness fluctuation in MEA EOF2 is characterized by anorth-south opposing variation with positive values mainlylocated south of 40∘N (figure not shown) However the cor-relation coefficient between PC2 and ECI decreases to minus014Therefore the relationship between the EOF2 variability andwidespread cooling in MEA is not as close as that betweenEOF1 and ECI Thereafter we focus on EOF1 and ECI

To show the representativeness of ECI to CI in MEA wefurther correlated the time series of ECI with the time seriesof CI at each grid point A significant correlation betweenECI and coldness intensity is observed in MEA and in largeareas of China except the Tibetan Plateau (Figure 3(a)) Inparticular the high positive correlation with values above06 (exceeding the 001 119905-test significance) are mainly locatedin Central Europe Central Siberia and Northwest ChinaThis is consistent with the large areas of EOF1 variabilityFurthermore significant positive correlations also emergeover East China the Korean Peninsula and North JapanThe overwhelming significant positive correlations suggestgenerally homogeneous large-scale winter CI variability overMEA during the cold season as indicated by ECI

As shown in Figures 3(b) and 3(c) the correlationbetween the ECI and the frequency of cold days and coldnights based on gridded 119879min and 119879max from ERA-Interimis generally identical to that between the ECI and CI(Figure 3(a)) which exhibits a dominantly large positivecorrelation at 30∘Nndash60∘N with a westward shift centeredat Kazakhstan Significant areas also spread to the westernBering Sea and west of the Ural Mountains Note thatwe used 1979ndash2013 as a reference period in defining coldevents the results may be influenced by the warmer mean

temperature in the context of the warming climate [24]To examine the robustness of the relationship between theECI and temperature extremes we redefined the cold daysand cold nights as days for which the stations 119879min and119879max were less than the corresponding 10th percentile value(1973ndash1992) following Zhang et al [27] As documentedin Figures 4(a) and 4(b) a widespread significant positivecorrelation occurs despite the data gap inMongolia Althoughthe significance level between the ECI and the frequency ofcold nights is relatively lower in East China the correlationsobtained by the reanalysis data and station data display ahighly consistent geographical distribution Therefore thecoincident strong correlations of both the coldness intensityand low temperature extremes with the ECI indicate thatthe recent frequent low temperature extremes can be wellmanifested by the intense coldness in the past few years

32 Associated Atmospheric Circulation Anomalies Thechange in the local SAT is expected to be predominantlygenerated by the horizontal advection of air masses fromwarmer or colder regions The components of zonal(minus119906(120597119879120597119909)) and meridional (minusV(120597119879120597119910)) temperatureadvection and their sum (minus119906(120597119879120597119909) minus V(120597119879120597119910)) arecalculated by the daily 925 hPa temperature and horizontalwinds which include both the stationary and nonlineartransient wave contributions Both the zonal advection andmeridional advection present a physical consistency withthe change in the CI and the frequency of low temperatureextremes at MEA (Figures 5(a) and 5(b)) In general theamplitudes of anomalous zonal and meridional componentsare comparable The intense coldness in the past few yearsis physically linked with a remarkably cold advection due toboth minus119906(120597119879120597119909) and minusV(120597119879120597119910) components A significantweakening of westerlies in MEA during the recent decadeweakenswarmzonal advection from thewest part of Eurasiancontinent Also the decreased westerlies favor persistenceand further intensification of surface anticyclone systems[20] Therefore the weak westerlies may play a physicallyimportant role in the recent decade of minus119906(120597119879120597119909) ForminusV(120597119879120597119910) component significant intensification northerliesare shown in mid and northern Siberia (Figure 6(a)) andthe cold advection because northerly becomes more efficient


06

06

06

04

04

0402

02

005 001

(a) CI

04

0202

02

02

0406

0606

04

04

04

04

minus02

005 001

(b) Cold days

06

04

0404

04

04

02

02

02

02

02

06

0406

minus02

005 001

(c) Cold nights

Figure 3 Correlation coefficients between ECI and the coldness intensity (CI) (a) and frequency of station-based cold days (b) and coldnights (c) The shading indicates areas with significant two-tailed Studentrsquos 119905-test exceeding 005 and 001

minus01 0 01 005 001minus005minus001

(a) Cold daysminus01 0 01 005 001minus005minus001

(b) Cold nights

Figure 4 Statistical significance of correlation coefficients between the ECI and the occurrence of station cold days (a) and cold nights (b)Here we also used the 10th percentile threshold but substituted a stationary climatology for 1973ndash1992 following Zhang et al [27]The valuesof the colored bars represent the significance of the two-tailed Studentrsquos 119905-test at each station (dot) the positive (negative) sign denotes apositive (negative) correlation

despite a possible decrease of meridional temperaturegradient from the recent arctic amplification Therefore therecent cold advection by both minus119906(120597119879120597119909) and minusV(120597119879120597119910)components plays an important role in the change in thehorizontal temperature advection (Figure 5(c)) and wintercoldness variation

To investigate the atmospheric circulation anomaliesassociated with the change in the temperature advectionand CI a composite difference in the sea-level pressure

(SLP) and horizontal wind at 925 hPa between the high andlow ECI years is shown in Figure 6(a) Note that all of theselected highlow ECI years are greater (less) than + (minus) the05 standard deviation (SD) from the time series mean Inparticular we identified 13 low years and 10 high yearsincluding 5 high years that occurred after the 0506 winterThis highlights the notable intensification of coldness inMEA in recent years Significant positive SLP differences arevisible over the northwestern Eurasian subarctic (NWEAS)


325N

375N

425N

475N

525N

575N

625N

675N

1985 1990 1995 2000 2005 20101980

08

minus08 0

02

05

minus05

minus065

065

minus02

035

005

minus035

minus005

(a) Zonal Tadv

325N

375N

425N

475N

525N

575N

625N

675N

1985 1990 1995 2000 2005 20101980

08

minus08 0

02

05

minus05

minus065

065

minus02

035

005

minus035

minus005

(b) Meridional Tadv

1985 1990 1995 2000 20051980 2010

325N

375N

425N

475N

525N

575N

625N

675N

08

minus08 0

02

05

minus05

minus065

065

minus02

035

005

minus035

minus005

(c) Total Tadv

Figure 5 Changing zonal mean zonal ((a) unit Klowastdayminus1) meridional ((b) unit Klowastdayminus1) and total ((c) unit Klowastdayminus1) temperatureadvection anomalies at the 925 hPa level (1979ndash2013) Values are calculated at each grid point before area averaging and are calculated for5∘ latitude bands

with a maximum centered around Scandinavia Anticyclonicnear-surface wind differences are consistently apparent overthe anticyclonic SLP anomalies As expected cold zonaladvection differences are observed over most of Europeindicating weakening zonal warm and moisture flow fromthe North Atlantic (Figure 6(b)) Significant cold meridionaladvection is shown over Siberia (Figure 6(c)) This reflectsthe anomalous northerlies west of the Ural Mountainswhich bring cold air masses from the high latitudes Coldmeridional advection is also apparent along the coastal areaof East Asia suggesting an associated intensified East Asianwinter monsoon Accordingly the total horizontal tempera-ture advection shows a dominant widespread cooling effect inMEA during the high ECI years (Figure 6(d)) Consequentlythe anticyclonic SLP anomalies over NWEAS associated withmidlatitudinal cold advection drove the recent intensifiedwinter coldness and increased the low temperature extremes

33 Active Eurasian Anticyclone and Blocking Activity Dra-matically intensified or unusually persistent weather systemsare usually linked with regional extreme cold events [18 19]For example the passing or abrupt intensification of a surfacecold-core high could directly cause cold surges in the winter[28] To assess the link between the activity of synopticweather systems and the change in the ECI we introducedan anticyclone intensity index (ACI) over Eurasia (0∘ndash110∘E40∘ndash75∘N) from Zhang et al [20] which is defined by theintegration of the central 6-hourly SLP anomalies of everysynoptic anticyclone during winter (DJF mean)

The association of surface anticyclonic activity with theoccurrence of severe winters over Eurasia was examined bya scatter plot of the ECI and ACI scores (Figure 7) Thereis a significant relationship between these two indices (07with significance exceeding 001) More interestingly 88of the low temperature winters (ECI gt 05 SD) including


8minus2 6 10minus4 2 4

(a) SLP and UV925

2

minus2

075

025

125

275

minus075

minus125

minus025

minus275

(b) Zonal Tadv

2

minus2

075

025

125

275

minus075

minus125

minus025

minus275

(c) Meridional Tadv

2

minus2

075

025

125

275

minus075

minus125

minus025

minus275

(d) Total Tadv

Figure 6 Composite difference between high and low ECI years (plusmn05 SD) from 1979 to 2013 for the SLP (shaded) and 925 hPa wind field(vector) in ((a) the units are hPa for SLP msdotsminus1 for 925 hPa wind) the zonal temperature advection ((b) unit Klowastdayminus1) the meridionaltemperature advection ((c) unit Klowastdayminus1) and the total horizontal temperature advection ((d) unit Klowastdayminus1) The dotted areas and windvectors indicate a significant difference that exceeds 005 for the two-tailed Studentrsquos 119905-test

0506Cor = 07 1112

83840708

7879

8485

10117980

ACI

minus3

minus2

minus1

0

1

2

3

minus3

minus2

minus1

0

1

2

3

0 1 3minus3 2minus2 minus1

ECI

minus3 minus2 minus1 0 1 2 3

Figure 7 Scatter plot of the ECI and ACI for reanalysis The blueand red marks indicate high and low ECI years (plusmn05 SD) respec-tively the grey marks indicate the ECI that were neither high norlow years

4 severe winters after 2005 coincided with positive ACIswhich denote an intensified surface anticyclone over EurasiaThis indicates that the synoptic anticyclonic activity and itspossible cold air outbreaks may be physically linked with therecent Eurasian cold winters

To compare broad atmospheric circulation patterns weexamined the difference map of the SLP and 500 hPa geopo-tential height (Z500) by subtracting the composites of highACIs from those of low ACIs (Figure 8(a)) An anticyclonicSLP anomaly together with a similar blocking-like Z500center is clearly seen in NWEAS This is identical to theabove composite SLP pattern by the high-low ECI with aslightly southward shifted maximum center The significantSLP differences associated with ACIs suggest intensified sur-face anticyclonic activity closely connected with the sta-tionary large-scale atmospheric circulation pattern In con-junction with the intensified anticyclones cold conditionswere widespread across much of MEA negative temper-ature differences extending eastwardsoutheastward fromnorthwestern Europe to eastern China range from minus1∘C tominus5∘C (Figure 8(b)) which favors the increase in extremecold events We also defined the frequency of blocking


minus20

0

0

minus20

minus10

minus30

40

10203040

minus2 2minus4 6 84 10

(a) SLP and Z5002minus3 minus2 minus1 1minus4 3 4 5minus5

(b) SAT

Figure 8 Same as Figure 6 but for SLP and Z500 in ((a) the units are hPa for SLP gpm for Z500) and SAT in ((b) unit K) between high andlow ACI years (plusmn05 SD) The dotted areas indicate significant difference exceeding 005 for the two-tailed Studentrsquos 119905-test

40E0 20E 120E80E 100E60E 140E 160E 180012345678

(a) ECI40E0 20E 120E80E 100E60E 140E 160E 180

012345678

(b) ACI

Figure 9 Composites of winter blocking frequency associated with high-low ECI years ((a) unit days) and high-low ACI years ((b) unitdays)The solid curve (long dashed curve) denotes composites for the high (low) index years and the dotted curve denotes the climatologicalvalues during 1979ndash2013 The shaded area indicates longitudes where the difference between composites exceeds the 01 significance

events over Eurasia along 30∘Endash85∘E based on an objectiveblocking-detection method by Charney et al [29] Both theamplified ECI and ACI are accompanied by more frequentblocking events (Figures 9(a) and 9(b)) suggesting barotropicinteractions between the surface anticyclone and upper-levelblocking high connected with the development of a longplanetary wave This is consistent with previous studiesindicating the important role of synoptic weather systems inthe mid-lower troposphere in explaining the extreme coldevents over MEA [9 20]

34 Changes in the Extratropical Leading Mode Previousstudies reported that the AONAO significantly affectsextreme weather events such as cold air outbreaks frozenprecipitation and strong wind events over large areas of theNorthern Hemisphere [16 21 30] To examine the extent ofextreme cold events associated with the AO we conductedan EOF analysis on the DJF-mean area-weighted 1000 hPageopotential height (Z1000) anomalies (1979ndash2013) north of20∘N The variance contribution rates of the first two modesare 298 and 209 which can also be well discriminatedaccording to the criterion proposed by North et al [26]

The spatial pattern of the leading mode (EOF1) of Z1000resembles the negative-phased monthly AO (Figure 10(a))The correlation of its corresponding time series (ZPC1) withECI is significantly different from zero (Table 1)These resultssuggest that the positive phase of the EOF1 is associated withintensified cold conditions in MEA arising from the positive

Table 1 Correlation coefficients between all the indices studiedNote that the bold (asterisks) numbers and asterisks indicate sig-nificant difference exceeding 005 (001) for the two-tailed Studentrsquos119905-test

AO ZPC1 ZPC2 ZPC1 + ZPC2ECI minus045 041 038 055lowast

Z1000 anomalies around the Arctic Ocean and its coastalareas in the Eurasia sector Interestingly compared with thetraditional monthly data constructed AO (httpwwwcpcncepnoaagovproductsprecipCWlinkdaily ao indexaoshtml) the Arctic center in EOF1 (individual seasonal AO)shows a clear northeastward shift from north of Iceland tothe Barents Sea together with a positive shift of the ZPC1(Figure 10(d)) This indicates a spatially deformed extra-tropical atmospheric circulation pattern in the NorthernHemisphere during the past decade which is consistent withthe rapid changes in the Arctic climate system [1]

As shown in Figure 10(b) the second mode (EOF2) alsoexhibits positive Z1000 anomalies around the Barents Seawith a weaker Aleutian low and a weak negative Atlanticcenter The corresponding time series (ZPC2) shows highvalues since the mid-2000s (Figure 10(e)) The correlationcoefficient between ZPC2 and ECI is 038 (passing the005 significance 119905-test) indicating a close linkage betweenEOF2 and the intensity of midlatitudinal Eurasia cold events


minus3 4 62 3minus4 5minus2 69minus5

(a)minus3 4 62 3minus4 5minus2 69minus5

(b)205 1 25minus2 minus1

minus1

minus3

minus3

minus005

minus2

minus1

2

0

04

1

3

0

1

(c)

minus3

minus2

minus1

0

1

2

3

4

20101990 1995 2000 200519851980

ZPC1

(d)

minus3

minus2

minus1

0

1

2

3

4

1985 1990 1995 2000 2005 20101980

ZPC2

(e)

minus3

minus2

minus1

0

1

2

3

4

1985 1990 1995 2000 2005 20101980

ZPC1 + ZPC2

(f)

Figure 10The spatial distribution of the first two leading modes of the extratropical Z1000 in the Northern Hemisphere and their associatedcirculation anomalies (a d) are for EOF1 and its corresponding ZPC1 and (b e) are EOF2 and ZPC2 respectively The units are gpm forZ1000 in (a d) (c) The regression coefficients of SAT and Z1000 in relation to ZPC1 + ZPC2 (f) The units are gpm for Z1000 and K for SATDashed lines in (d e f) denote the corresponding 9-yr running mean for the original time series (solid line) The dotted areas in (c) indicatesignificantly regressed SAT exceeding 005 for the 119865-test

We further constructed a multivariate linear regression timeseries (ZPC1 + ZPC2) by jointly regressing ZPC1 and ZPC2onto ECI The correlation between ZPC1 + ZPC2 with ECIis 055 which is clearly higher than the individual PCs withECI The regression of Z1000 onto ZPC1 + ZPC2 displaysstriking anticyclonic anomalies in NWEAS leading to pro-nounced negative SAT anomalies in the mid-high latitudesof Eurasia This pattern is identical to the ECI-associatedsurface circulation anomalies in Figure 8 suggesting that thesevere winters in midlatitudinal Eurasia can be explainedby a combination of these two extratropical leading modesparticularly in recent decades

4 Conclusion and Discussion

In this study we defined a new objective index to quantify theintensity of winter coldness (CI) based on the amplitude ofdaily cold temperature anomalies The relationship betweenthe change in the CI with the recent frequent low temperatureextremes in themidlatitudes of Eurasia (MEA) the associatedcirculation patterns and the possible contributing factors wasanalyzed Our major results are summarized as follows

(1)The change in the CI in MEA is highly consistent withthe recent intensified low temperature extremes Howeverthe subseasonal variance of temperature at 45∘Nndash60∘N ofMEA shows a decreased trend with minus10 per decade that isinconsistent with increase of cold extremes in the recentdecades The dominant mode of the CI in MEA features asign-consistent variation with a marked intensification since2005 according to empirical orthogonal function (EOF)analysis This leading mode is significantly related to the fre-quency of extreme cold days (nights) in winter over largeareas of the mid-high latitudes of Eurasia Therefore aregional integrated index ECI is defined to quantify the over-all intensity of winter coldness in MEA which is highly cor-related with PC1

(2) The differences in the SLP between high and lowECI values are characterized by a remarkable anomalousanticyclone in northwestern Eurasia This blocking-like cir-culation dynamically weakens the warm-wet air from theNorth Atlantic to west of Europe and transports cold denseair from the high latitudes to Siberia and East Asia thussubstantial cold advection is induced across MEA


(3) We found that the large-scale strengthening andincreasing of synoptic weather systems including surfaceEurasian anticyclones and synoptic blocking over 25∘Endash85∘Esubstantially corresponds to the widespread intensificationof the CI in MEA Additionally the frequent severe wintersare also likely coupled with a positive phase shift of the firsttwo extratropical circulation leading modes which feature asimilar blocking-like anomaly in the northwestern Eurasiansubarctic

There are several other issues related to winter Eurasiancoldness that need to be studied in the future The detaileddynamical coupled processes influencing synoptic weathersystems and the stationary extratropical leading mode ofwinter temperature extremes across MEA are unknownFurthermore external forcing deserves further investiga-tion based on observations and simulations In Section 34we show a higher correlation with ECI using multivariateregressed ZPC1 and ZPC2 Since the ZPC1 is closely linked tothe AO this suggests there are other independent potentialdriven factors linked with the recent Eurasian cold wintersMori et al [14] argued that the atmospheric response tosea-ice decline is approximately independent of the ArcticOscillation The atmospheric response to both the ArcticOscillation and sea-ice decline favors cold-air advection toEurasia and hence severe winters Li et al [31] also arguedthat Arctic sea-ice loss does not drive systematic changesin the Northern Hemisphere large-scale circulation in thepast decades but enhances the variability of Eurasian winterclimate and thus increases the probability of an extremeEurasian winter cooling trend Therefore the recent severercold winter in Eurasia may be due to the jointly influence ofatmospheric internal variability (eg AONAO) and externalforcing (eg Arctic sea-ice decline [32] and snow cover [33])More data analysis and high-resolution model simulationsand projections will be pursued in the future

Competing Interests

The authors declare that there is no competing interestsregarding the publication of this paper

Acknowledgments

This work is supported jointly by the National Basic ResearchProgram of China (2015CB953904) the Natural ScienceFoundation of Jiangsu Province grant (BK2012465) theNational Natural Science Foundation of China (4157508141575057) and a project funded by the Priority AcademicProgram Development of Jiangsu Higher Education Institu-tions (PAPD)

References

[1] X Zhang A Sorteberg J Zhang R Gerdes and J C ComisoldquoRecent radical shifts of atmospheric circulations and rapidchanges in Arctic climate systemrdquo Geophysical Research Lettersvol 35 no 22 Article ID L22701 2008

[2] J E Overland ldquoPotential Arctic change through climate ampli-fication processesrdquo Oceanography vol 24 no 3 pp 176ndash1852011

[3] B Y Wu J Z Su and R H Zhang ldquoEffects of autumn-winterArctic sea ice onwinter SiberianHighrdquoChinese Science Bulletinvol 56 no 30 pp 3220ndash3228 2011

[4] L Wang and W Chen ldquoDownward Arctic Oscillation signalassociated with moderate weak stratospheric polar vortex andthe cold December 2009rdquo Geophysical Research Letters vol 37no 9 Article ID L09707 2010

[5] M Wen S Yang A Kumar and P Zhang ldquoAn analysis of thelarge-scale climate anomalies associated with the snowstormsaffecting China in January 2008rdquoMonthly Weather Review vol137 no 3 pp 1111ndash1131 2009

[6] C Bueh N Shi and Z Xie ldquoLarge-scale circulation anomaliesassociatedwith persistent low temperature over SouthernChinain January 2008rdquo Atmospheric Science Letters vol 12 no 3 pp273ndash280 2011

[7] V Petoukhov and V A Semenov ldquoA link between reducedBarents-Kara sea ice and cold winter extremes over northerncontinentsrdquo Journal of Geophysical Research vol 115 no 21Article ID D21111 2010

[8] J A Francis and S J Vavrus ldquoEvidence linkingArctic amplifica-tion to extreme weather in mid-latitudesrdquoGeophysical ResearchLetters vol 39 no 6 Article ID L06801 2012

[9] J Liu J A CurryHWangM Song andRMHorton ldquoImpactof decliningArctic sea ice onwinter snowfallrdquo Proceedings of theNational Academy of Sciences of theUnited States of America vol109 no 11 pp 4074ndash4079 2012

[10] E A Barnes ldquoRevisiting the evidence linking Arctic amplifica-tion to extreme weather in midlatitudesrdquo Geophysical ResearchLetters vol 40 no 17 pp 4734ndash4739 2013

[11] J A Screen and I Simmonds ldquoCaution needed when linkingweather extremes to amplified planetary wavesrdquo Proceedings ofthe National Academy of Sciences of the United States of Americavol 110 no 26 p E2327 2013

[12] P Hassanzadeh Z Kuang and B F Farrell ldquoResponses of mid-latitude blocks andwave amplitude to changes in themeridionaltemperature gradient in an idealized dry GCMrdquo GeophysicalResearch Letters vol 41 no 14 pp 5223ndash5232 2014

[13] Z Han S Li and M Mu ldquoThe role of warm North AtlanticSST in the formation of positive height anomalies over theUral Mountains during January 2008rdquoAdvances in AtmosphericSciences vol 28 no 2 pp 246ndash256 2011

[14] M Mori M Watanabe H Shiogama J Inoue and M KimotoldquoRobust Arctic sea-ice influence on the frequent Eurasian coldwinters in past decadesrdquo Nature Geoscience vol 7 pp 869ndash8732014

[15] Y Peings and G Magnusdottir ldquoForcing of the wintertimeatmospheric circulation by the multidecadal fluctuations of theNorth Atlantic oceanrdquo Environmental Research Letters vol 9no 3 Article ID 034018 2014

[16] S Chen W Chen and K Wei ldquoRecent trends in wintertemperature extremes in eastern China and their relationshipwith theArcticOscillation andENSOrdquoAdvances inAtmosphericSciences vol 30 no 6 pp 1712ndash1724 2013

[17] W Zhou J C L Chan W Chen J Ling J G Pinto and YShao ldquoSynoptic-scale controls of persistent low temperatureand icy weather over Southern China in January 2008rdquoMonthlyWeather Review vol 137 no 11 pp 3978ndash3991 2009

[18] J Inoue M E Hori and K Takaya ldquoThe role of barents seaice in the wintertime cyclone track and emergence of a warm-Arctic cold-Siberian anomalyrdquo Journal of Climate vol 25 no 7pp 2561ndash2569 2012


[19] Y J Orsolini R Senan R E Benestad and A MelsomldquoAutumn atmospheric response to the 2007 low Arctic seaice extent in coupled ocean-atmosphere hindcastsrdquo ClimateDynamics vol 38 no 11-12 pp 2437ndash2448 2012

[20] X Zhang C Lu and Z Guan ldquoWeakened cyclones intensifiedanticyclones and recent extreme cold winter weather events inEurasiardquo Environmental Research Letters vol 7 no 4 Article ID044044 2012

[21] D Gong S Wang and J Zhu ldquoArctic Oscillation influenceon daily temperature variance in winter over Chinardquo ChineseScience Bulletin vol 49 no 6 pp 637ndash642 2004

[22] Y Xie Y Liu and J Huang ldquoThe influence of the autumnArcticsea ice on winter air temperature in Chinardquo Acta MeteorologicaSinica vol 72 no 4 pp 703ndash710 2014 (Chinese)

[23] J A Screen ldquoArctic amplification decreases temperature vari-ance in northern mid- to high-latitudesrdquo Nature ClimateChange vol 4 no 7 pp 577ndash582 2014

[24] T Schneider T Bischoff and H Płotka ldquoPhysics of changes insynoptic midlatitude temperature variabilityrdquo Journal of Cli-mate vol 28 no 6 pp 2312ndash2331 2015

[25] D P Dee S M Uppala A J Simmons et al ldquoThe ERA-Interimreanalysis configuration and performance of the data assim-ilation systemrdquo Quarterly Journal of the Royal MeteorologicalSociety vol 137 no 656 pp 553ndash597 2011

[26] G R North T L Bell R F Cahalan and F J Moeng ldquoSamplingerrors in the estimation of empirical orthogonal functionsrdquoMonthly Weather Review vol 110 no 7 pp 699ndash706 1982

[27] X Zhang E Aguilar S Sensoy et al ldquoTrends in Middle Eastclimate extreme indices from 1950 to 2003rdquo Journal of Geophys-ical Research D Atmospheres vol 110 no 22 Article IDD221042005

[28] Y Ding and T N Krishnamurti ldquoHeat budget of the Siberianhigh and the winter monsoonrdquo Monthly Weather Review vol115 no 10 pp 2428ndash2449 1987

[29] J G Charney J Shukla and K C Mo ldquoComparison of a baro-tropic blocking theory with observationrdquo Journal of the Atmo-spheric Sciences vol 38 no 4 pp 762ndash779 1981

[30] KGuirguis A Gershunov R Schwartz and S Bennett ldquoRecentwarm and cold daily winter temperature extremes in theNorthernHemisphererdquoGeophysical Research Letters vol 38 no17 Article ID L1770 2011

[31] C Li B Stevens and J Marotzke ldquoEurasian winter cooling inthe warming hiatus of 1998ndash2012rdquo Geophysical Research Lettersvol 42 no 19 pp 8131ndash8139 2015

[32] Z Guan Q Zhang andM Li ldquoInterannual variations in atmo-spheric mass over liquid water oceans continents and sea-ice-covered arctic regions and their possible impacts on the borealwinter climaterdquo Journal of Geophysical Research Atmospheresvol 120 no 23 pp 11846ndash11861 2015

[33] J L Cohen J C Furtado M A Barlow V A Alexeev and JE Cherry ldquoArctic warming increasing snow cover and wide-spread boreal winter coolingrdquo Environmental Research Lettersvol 7 no 1 Article ID 014007 2012

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ClimatologyJournal of

EcologyInternational Journal of


EarthquakesJournal of


Hindawi Publishing Corporationhttpwwwhindawicom

Applied ampEnvironmentalSoil Science

Volume 2014

Mining


Journal of

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal of

Geophysics

OceanographyInternational Journal of


Journal of Computational Environmental SciencesHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering


GeochemistryHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Atmospheric SciencesInternational Journal of


OceanographyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in


MineralogyInternational Journal of


MeteorologyAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Paleontology JournalHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014


Geological ResearchJournal of


Geology Advances in


The subseasonal variance in the daily temperature canbe used to quantify the overall seasonal high-frequencytemperature fluctuation [21 22] The increased cold-seasontemperature variance suggests that cold outbreaks or warmevents occur more frequently However in contrast to therecent increased extreme cold weather in the midlatitudesof Eurasia decreases in the subseasonal all-longitude zonalmean temperature variability have been detected over thepast few decades [23] Schneider et al [24] argued thatsuch inconsistency resulted from a warmer mean climatewhich entails changes in the frequency with which fixedtemperature thresholds are exceeded The research questionis as follows how can the integrated intensity of the recentextreme cold events during the cold season be quantified

Here we defined a new objective index to quantify theintensity of winter coldness and focused on its contributionto the recent increased large-scale extreme cold events in themidlatitudes of Eurasia (MEA) The associated circulationpatterns and their relationship with synoptic weather systemswill be discussed to explain the change in the recent intensecold events in MEA

2 Data and Methodology

21 Data This study uses the daily data from the ECMWFERA-Interim reanalysis from January 1 1979 throughDecember 31 2013 [25] We use a full T255 (512 times 256)Gaussian grid which has an approximate global horizontalresolution of 07∘times 07∘ And the gridded daily minimum andmaximum surface air temperature (119879min and 119879max) are alsofrom the ERA-Interim associated forecast field Furthermorewe use119879min and119879max data at the 1331 Eurasianmeteorologicalstations from the daily global historical climatology networkfrom 1972 to 2012 (GHCN-DAILY httpsgisncdcnoaagovgeoportalcatalogsearchresourcedetailspageid=govnoaancdcC00861) Although there aremore temperature stationsin the Eurasianmid-high latitudes we only chose 1331 stationshaving sufficiently long record from 1973 to 2012 Consideringthe sparsity of stations over northernChina andMongolia weadded 215 stations over these regions from the daily Chinesesurface climate dataset (V3) and baseline meteorologicaldata in Siberia (V5)Themeanmonthly indices of the AO aretaken from httpwwwcpcncepnoaagovproductsprecipCWlinkdaily ao indexaoshtml In this study winter meanswere constructed by averaging the data for DecemberJanuary and February (DJF) which resulted in 35 winters(1979ndash2013)

22 Definition of Intensity of Winter Coldness Percentile-based indices that consider the occurrence of cold spell daysand cold days are widely applied to study changes in dailyextreme cold events However the frequency of extremesdepends on the choice of the percentile value (eg 5th and10th for cold events) the indices may not reflect the seasonalintegrated intensity of cold events well because they alsodepend on the amplitude and persistence of events On theother hand the traditional subseasonal temperature variancewould underestimate the seasonal intensity of coldness if apersistent cold condition dominates the cold season despite

an increase of extreme cold events Moreover the warmevents would also induce an amplification of the values ofvariance if we focus on the cold anomalies and associated coldextremes Therefore we define a new coldness intensity (CI)in winter as

CI = 190

90

sum

119894=1

(

1198791015840

119894

120590119894

)

2

1198791015840

119894

=

119879119894minus 119879119888119879119894minus 119879119888lt 0

0 119879119894minus 119879119888ge 0

(1)

Here 119879119894is the daily 925 hPa temperature from December 1st

in the preceding year to February 28th in the current year (90days for DJF) 1198791015840

119894

is a cold anomaly relative to the climatology(119879119888) of 90-day mean of 119879

119894from 1979 to 2013 when 119879

119894minus119879119888lt 0

When 119879119894is relative warm (119879

119894minus 119879119888ge 0) the value of 1198791015840

119894

becomes zero so that the anomalous warmth is excluded Tohomogenize the fluctuations among different locations thevalues were divided by the standard deviation (120590

119894) at each grid

point that is the multiyear standard deviation at a grid pointon the 119894th day Because the 1198791015840

119894

represents a cold deviationfrom climatology (1979ndash2013) the value of its intraseasonalvariability or CI measures a seasonal integrated magnitude ofcold anomalies in winter Accordingly the standardized valueof CI differs from its corresponding standardized subseasonalfluctuation especially in a persistent cold (warm) winterwhich may include more frequent cold (warm) extremes

3 Result

31 Recent Intensified Winter Coldness We used the 10thpercentile of the daily maximum (minimum) surface air tem-perature (SAT) over 1979ndash2013 based on ERA-Interim toobtain cold day (night) events The zonal mean (0∘ndash180∘E)occurrence of extreme low temperatures shows a clear inter-decadal fluctuation in the midlatitudes of Eurasia (MEA)since 1979 (Figures 1(a) and 1(b))The low temperature eventsat 30∘Nndash60∘N exhibited a notable decline in the 1990s andthe early 2000s which corresponded to the gradual warmingin the global surface temperature However a significantincrease in cold days and cold nights emerged after the mid-2000s in accordance with the recent severe winters overEurasia (0405 0506 0708 0910 1011 1112 and 1213)The mean frequencies of cold days (nights) in 1979ndash19891990ndash2004 and 2005ndash2012 are 134 (147) dy 117 (120) dy and157 (152) dy respectively Thus stronger cooling conditionsoccurred over Eurasia in recent years compared with the1980s

After the mid-2000s the subseasonal temperature vari-ance at 925 hPa displayed greatly contrasting anomaliesbetween 30∘Nndash45∘N and 45∘Nndash60∘N (Figure 1(c)) Note thatthe variance is also divided by 120590

119894to homogenize the fluctu-

ations among different locations The linear trend for 1996ndash2013 is 11 per decade at 30∘Nndash45∘N (exceeding the 01 sig-nificance) The intensified subseasonal fluctuation indicatesmore unstable weather changes during the cold seasons forexample more frequent cold air outbreaksmay occur In con-trast a decrease of minus10 per decade emerged at 45∘Nndash60∘N


325N

375N

425N

475N

525N

575N

625N

675N

20101990 1995 2000 200519851980

12 18 24 30 36

(a) Cold days

325N

375N

425N

475N

525N

575N

625N

675N

1985 1990 1995 2000 2005 20101980

12 18 24 30 36

(b) Cold nights

09 1 11 12 13

325N

375N

425N

475N

525N

575N

625N

675N

20101990 1995 2000 200519851980


325N

375N

425N

475N

525N

575N

625N

675N

1985 1990 1995 20101980 2000 2005

(d) CI







002004

002 004

015

009

002

002

004

004

009 009

(a)2000 200519951985 1990 20101980

minus2

minus1

0

1

2

3

(b)








06

06

06

04

04

0402

02

005 001

(a) CI

04

0202

02

02

0406

0606

04

04

04

04

minus02

005 001

(b) Cold days

06

04

0404

04

04

02

02

02

02

02

06

0406

minus02

005 001

(c) Cold nights




(b) Cold nights






325N

375N

425N

475N

525N

575N

625N

675N

1985 1990 1995 2000 2005 20101980

08

minus08 0

02

05

minus05

minus065

065

minus02

035

005

minus035

minus005

(a) Zonal Tadv

325N

375N

425N

475N

525N

575N

625N

675N

1985 1990 1995 2000 2005 20101980

08

minus08 0

02

05

minus05

minus065

065

minus02

035

005

minus035

minus005

(b) Meridional Tadv

1985 1990 1995 2000 20051980 2010

325N

375N

425N

475N

525N

575N

625N

675N

08

minus08 0

02

05

minus05

minus065

065

minus02

035

005

minus035

minus005

(c) Total Tadv







(a) SLP and UV925

2

minus2

075

025

125

275

minus075

minus125

minus025

minus275

(b) Zonal Tadv

2

minus2

075

025

125

275

minus075

minus125

minus025

minus275

(c) Meridional Tadv

2

minus2

075

025

125

275

minus075

minus125

minus025

minus275

(d) Total Tadv


0506Cor = 07 1112

83840708

7879

8485

10117980

ACI

minus3

minus2

minus1

0

1

2

3

minus3

minus2

minus1

0

1

2

3


ECI






minus20

0

0

minus20

minus10

minus30

40

10203040



(b) SAT


40E0 20E 120E80E 100E60E 140E 160E 180012345678

(a) ECI40E0 20E 120E80E 100E60E 140E 160E 180

012345678

(b) ACI













minus1

minus3

minus3

minus005

minus2

minus1

2

0

04

1

3

0

1

(c)

minus3

minus2

minus1

0

1

2

3

4

20101990 1995 2000 200519851980

ZPC1

(d)

minus3

minus2

minus1

0

1

2

3

4

1985 1990 1995 2000 2005 20101980

ZPC2

(e)

minus3

minus2

minus1

0

1

2

3

4

1985 1990 1995 2000 2005 20101980

ZPC1 + ZPC2

(f)










Competing Interests


Acknowledgments


References












































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in


325N

375N

425N

475N

525N

575N

625N

675N

20101990 1995 2000 200519851980

12 18 24 30 36

(a) Cold days

325N

375N

425N

475N

525N

575N

625N

675N

1985 1990 1995 2000 2005 20101980

12 18 24 30 36

(b) Cold nights

09 1 11 12 13

325N

375N

425N

475N

525N

575N

625N

675N

20101990 1995 2000 200519851980


325N

375N

425N

475N

525N

575N

625N

675N

1985 1990 1995 20101980 2000 2005

(d) CI







002004

002 004

015

009

002

002

004

004

009 009

(a)2000 200519951985 1990 20101980

minus2

minus1

0

1

2

3

(b)








06

06

06

04

04

0402

02

005 001

(a) CI

04

0202

02

02

0406

0606

04

04

04

04

minus02

005 001

(b) Cold days

06

04

0404

04

04

02

02

02

02

02

06

0406

minus02

005 001

(c) Cold nights




(b) Cold nights






325N

375N

425N

475N

525N

575N

625N

675N

1985 1990 1995 2000 2005 20101980

08

minus08 0

02

05

minus05

minus065

065

minus02

035

005

minus035

minus005

(a) Zonal Tadv

325N

375N

425N

475N

525N

575N

625N

675N

1985 1990 1995 2000 2005 20101980

08

minus08 0

02

05

minus05

minus065

065

minus02

035

005

minus035

minus005

(b) Meridional Tadv

1985 1990 1995 2000 20051980 2010

325N

375N

425N

475N

525N

575N

625N

675N

08

minus08 0

02

05

minus05

minus065

065

minus02

035

005

minus035

minus005

(c) Total Tadv







(a) SLP and UV925

2

minus2

075

025

125

275

minus075

minus125

minus025

minus275

(b) Zonal Tadv

2

minus2

075

025

125

275

minus075

minus125

minus025

minus275

(c) Meridional Tadv

2

minus2

075

025

125

275

minus075

minus125

minus025

minus275

(d) Total Tadv


0506Cor = 07 1112

83840708

7879

8485

10117980

ACI

minus3

minus2

minus1

0

1

2

3

minus3

minus2

minus1

0

1

2

3


ECI






minus20

0

0

minus20

minus10

minus30

40

10203040



(b) SAT


40E0 20E 120E80E 100E60E 140E 160E 180012345678

(a) ECI40E0 20E 120E80E 100E60E 140E 160E 180

012345678

(b) ACI













minus1

minus3

minus3

minus005

minus2

minus1

2

0

04

1

3

0

1

(c)

minus3

minus2

minus1

0

1

2

3

4

20101990 1995 2000 200519851980

ZPC1

(d)

minus3

minus2

minus1

0

1

2

3

4

1985 1990 1995 2000 2005 20101980

ZPC2

(e)

minus3

minus2

minus1

0

1

2

3

4

1985 1990 1995 2000 2005 20101980

ZPC1 + ZPC2

(f)










Competing Interests


Acknowledgments


References












































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in


002004

002 004

015

009

002

002

004

004

009 009

(a)2000 200519951985 1990 20101980

minus2

minus1

0

1

2

3

(b)








06

06

06

04

04

0402

02

005 001

(a) CI

04

0202

02

02

0406

0606

04

04

04

04

minus02

005 001

(b) Cold days

06

04

0404

04

04

02

02

02

02

02

06

0406

minus02

005 001

(c) Cold nights




(b) Cold nights






325N

375N

425N

475N

525N

575N

625N

675N

1985 1990 1995 2000 2005 20101980

08

minus08 0

02

05

minus05

minus065

065

minus02

035

005

minus035

minus005

(a) Zonal Tadv

325N

375N

425N

475N

525N

575N

625N

675N

1985 1990 1995 2000 2005 20101980

08

minus08 0

02

05

minus05

minus065

065

minus02

035

005

minus035

minus005

(b) Meridional Tadv

1985 1990 1995 2000 20051980 2010

325N

375N

425N

475N

525N

575N

625N

675N

08

minus08 0

02

05

minus05

minus065

065

minus02

035

005

minus035

minus005

(c) Total Tadv







(a) SLP and UV925

2

minus2

075

025

125

275

minus075

minus125

minus025

minus275

(b) Zonal Tadv

2

minus2

075

025

125

275

minus075

minus125

minus025

minus275

(c) Meridional Tadv

2

minus2

075

025

125

275

minus075

minus125

minus025

minus275

(d) Total Tadv


0506Cor = 07 1112

83840708

7879

8485

10117980

ACI

minus3

minus2

minus1

0

1

2

3

minus3

minus2

minus1

0

1

2

3


ECI






minus20

0

0

minus20

minus10

minus30

40

10203040



(b) SAT


40E0 20E 120E80E 100E60E 140E 160E 180012345678

(a) ECI40E0 20E 120E80E 100E60E 140E 160E 180

012345678

(b) ACI













minus1

minus3

minus3

minus005

minus2

minus1

2

0

04

1

3

0

1

(c)

minus3

minus2

minus1

0

1

2

3

4

20101990 1995 2000 200519851980

ZPC1

(d)

minus3

minus2

minus1

0

1

2

3

4

1985 1990 1995 2000 2005 20101980

ZPC2

(e)

minus3

minus2

minus1

0

1

2

3

4

1985 1990 1995 2000 2005 20101980

ZPC1 + ZPC2

(f)










Competing Interests


Acknowledgments


References












































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in


06

06

06

04

04

0402

02

005 001

(a) CI

04

0202

02

02

0406

0606

04

04

04

04

minus02

005 001

(b) Cold days

06

04

0404

04

04

02

02

02

02

02

06

0406

minus02

005 001

(c) Cold nights




(b) Cold nights






325N

375N

425N

475N

525N

575N

625N

675N

1985 1990 1995 2000 2005 20101980

08

minus08 0

02

05

minus05

minus065

065

minus02

035

005

minus035

minus005

(a) Zonal Tadv

325N

375N

425N

475N

525N

575N

625N

675N

1985 1990 1995 2000 2005 20101980

08

minus08 0

02

05

minus05

minus065

065

minus02

035

005

minus035

minus005

(b) Meridional Tadv

1985 1990 1995 2000 20051980 2010

325N

375N

425N

475N

525N

575N

625N

675N

08

minus08 0

02

05

minus05

minus065

065

minus02

035

005

minus035

minus005

(c) Total Tadv







(a) SLP and UV925

2

minus2

075

025

125

275

minus075

minus125

minus025

minus275

(b) Zonal Tadv

2

minus2

075

025

125

275

minus075

minus125

minus025

minus275

(c) Meridional Tadv

2

minus2

075

025

125

275

minus075

minus125

minus025

minus275

(d) Total Tadv


0506Cor = 07 1112

83840708

7879

8485

10117980

ACI

minus3

minus2

minus1

0

1

2

3

minus3

minus2

minus1

0

1

2

3


ECI






minus20

0

0

minus20

minus10

minus30

40

10203040



(b) SAT


40E0 20E 120E80E 100E60E 140E 160E 180012345678

(a) ECI40E0 20E 120E80E 100E60E 140E 160E 180

012345678

(b) ACI













minus1

minus3

minus3

minus005

minus2

minus1

2

0

04

1

3

0

1

(c)

minus3

minus2

minus1

0

1

2

3

4

20101990 1995 2000 200519851980

ZPC1

(d)

minus3

minus2

minus1

0

1

2

3

4

1985 1990 1995 2000 2005 20101980

ZPC2

(e)

minus3

minus2

minus1

0

1

2

3

4

1985 1990 1995 2000 2005 20101980

ZPC1 + ZPC2

(f)










Competing Interests


Acknowledgments


References












































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in


325N

375N

425N

475N

525N

575N

625N

675N

1985 1990 1995 2000 2005 20101980

08

minus08 0

02

05

minus05

minus065

065

minus02

035

005

minus035

minus005

(a) Zonal Tadv

325N

375N

425N

475N

525N

575N

625N

675N

1985 1990 1995 2000 2005 20101980

08

minus08 0

02

05

minus05

minus065

065

minus02

035

005

minus035

minus005

(b) Meridional Tadv

1985 1990 1995 2000 20051980 2010

325N

375N

425N

475N

525N

575N

625N

675N

08

minus08 0

02

05

minus05

minus065

065

minus02

035

005

minus035

minus005

(c) Total Tadv







(a) SLP and UV925

2

minus2

075

025

125

275

minus075

minus125

minus025

minus275

(b) Zonal Tadv

2

minus2

075

025

125

275

minus075

minus125

minus025

minus275

(c) Meridional Tadv

2

minus2

075

025

125

275

minus075

minus125

minus025

minus275

(d) Total Tadv


0506Cor = 07 1112

83840708

7879

8485

10117980

ACI

minus3

minus2

minus1

0

1

2

3

minus3

minus2

minus1

0

1

2

3


ECI






minus20

0

0

minus20

minus10

minus30

40

10203040



(b) SAT


40E0 20E 120E80E 100E60E 140E 160E 180012345678

(a) ECI40E0 20E 120E80E 100E60E 140E 160E 180

012345678

(b) ACI













minus1

minus3

minus3

minus005

minus2

minus1

2

0

04

1

3

0

1

(c)

minus3

minus2

minus1

0

1

2

3

4

20101990 1995 2000 200519851980

ZPC1

(d)

minus3

minus2

minus1

0

1

2

3

4

1985 1990 1995 2000 2005 20101980

ZPC2

(e)

minus3

minus2

minus1

0

1

2

3

4

1985 1990 1995 2000 2005 20101980

ZPC1 + ZPC2

(f)










Competing Interests


Acknowledgments


References












































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in



(a) SLP and UV925

2

minus2

075

025

125

275

minus075

minus125

minus025

minus275

(b) Zonal Tadv

2

minus2

075

025

125

275

minus075

minus125

minus025

minus275

(c) Meridional Tadv

2

minus2

075

025

125

275

minus075

minus125

minus025

minus275

(d) Total Tadv


0506Cor = 07 1112

83840708

7879

8485

10117980

ACI

minus3

minus2

minus1

0

1

2

3

minus3

minus2

minus1

0

1

2

3


ECI






minus20

0

0

minus20

minus10

minus30

40

10203040



(b) SAT


40E0 20E 120E80E 100E60E 140E 160E 180012345678

(a) ECI40E0 20E 120E80E 100E60E 140E 160E 180

012345678

(b) ACI













minus1

minus3

minus3

minus005

minus2

minus1

2

0

04

1

3

0

1

(c)

minus3

minus2

minus1

0

1

2

3

4

20101990 1995 2000 200519851980

ZPC1

(d)

minus3

minus2

minus1

0

1

2

3

4

1985 1990 1995 2000 2005 20101980

ZPC2

(e)

minus3

minus2

minus1

0

1

2

3

4

1985 1990 1995 2000 2005 20101980

ZPC1 + ZPC2

(f)










Competing Interests


Acknowledgments


References












































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in


minus20

0

0

minus20

minus10

minus30

40

10203040



(b) SAT


40E0 20E 120E80E 100E60E 140E 160E 180012345678

(a) ECI40E0 20E 120E80E 100E60E 140E 160E 180

012345678

(b) ACI













minus1

minus3

minus3

minus005

minus2

minus1

2

0

04

1

3

0

1

(c)

minus3

minus2

minus1

0

1

2

3

4

20101990 1995 2000 200519851980

ZPC1

(d)

minus3

minus2

minus1

0

1

2

3

4

1985 1990 1995 2000 2005 20101980

ZPC2

(e)

minus3

minus2

minus1

0

1

2

3

4

1985 1990 1995 2000 2005 20101980

ZPC1 + ZPC2

(f)










Competing Interests


Acknowledgments


References












































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in





minus1

minus3

minus3

minus005

minus2

minus1

2

0

04

1

3

0

1

(c)

minus3

minus2

minus1

0

1

2

3

4

20101990 1995 2000 200519851980

ZPC1

(d)

minus3

minus2

minus1

0

1

2

3

4

1985 1990 1995 2000 2005 20101980

ZPC2

(e)

minus3

minus2

minus1

0

1

2

3

4

1985 1990 1995 2000 2005 20101980

ZPC1 + ZPC2

(f)










Competing Interests


Acknowledgments


References












































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in




Competing Interests


Acknowledgments


References












































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in


























Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in

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