decadal variability of droughts and ﬂoods in the yellow...

INTERNATIONAL JOURNAL OF CLIMATOLOGYInt. J. Climatol. 33: 3217–3228 (2013)Published online 14 February 2013 in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/joc.3662

Decadal variability of droughts and floods in the YellowRiver basin during the last five centuries and relations

with the North Atlantic SST

Jie Zhang,a,* Dongliang Li,a Laurent Lib and Weitao Denga

a Key Laboratory of Meteorological Disaster of Ministry of Education (KLME), Nanjing University of Information Science & Technology,Nanjing, China

b Laboratoire de Meteorologie Dynamique, IPSL/CNRS, Universite Pierre et Marie Curie, Paris, France

ABSTRACT: Droughts and floods are frequent disasters in the Yellow River basin in northern China. They have a strongimpact on agriculture and water resource management. To explore the physical mechanisms of these droughts and floods,influences exerted by the sea surface temperature (SST) anomalies in the mid-latitude North Atlantic on the wetness anddryness in the Yellow River basin have been investigated. The drought/flood index (DFI), derived from a reconstructeddataset covering the last five centuries, is used. Numerical simulations are conducted with the Community AtmosphericModel version 3.0 (CAM3) to study the influence of various mechanisms. It is found that DFI in the entire Yellow Riverbasin experienced oscillations at about 50–60 years during the past five centuries. Droughts and floods have inconsistentpatterns in different areas of the Yellow River. The periodic variation of DFI in Xi’an, a station in a semi-humid subarea,is in contrast to the North Atlantic SST (10◦N–55◦N and 70◦W–10◦W) oscillation in spring and summer; the periodicvariation of the Yinchuan DFI in a semi-arid subarea (the upper reaches of Yellow River) correlates with the North AtlanticSST oscillation variation in spring and summer. The North Atlantic SST is probably one of the key sources of internalvariability of the climate system, which results in Rossby wave adjusting. The warm phase of the North Atlantic SST isrelated to the operation of a Northern Atlantic Oscillation (NAO) like pattern conducive to easterly wind anomalies innorthern China and enhances anti-cyclones around Lake Baikal that leads to less precipitation or more frequent droughtsin the semi-arid subarea in the upper reaches of the Yellow River, but wet conditions in the semi-humid subarea in themiddle reaches, monsoon subarea in the lower reaches and plateau subarea in the upper reaches of the river.

KEY WORDS drought/flood; the North Atlantic SST; Yellow River; decadal variability

Received 5 April 2012; Revised 22 October 2012; Accepted 11 January 2013

1. Introduction

Droughts and floods are two of the most devastatingnatural disasters, affecting economics, society and theenvironment (Woodhouse and Overpeck, 1998). In theYellow River basin, the second largest river in China,droughts and floods are probably the most importantnatural phenomena for water resources and for humanbeings, both historically and in the present. The middlereaches of the Yellow River burst almost every year in thelast Qing dynasty (Lu, 2003). In the past 25 years, severedroughts occurred frequently, causing discontinuous flowaround 226 days in total, and the non-flowing portionof the river reached about 700 km in 2007 (Ma, 1998).It is important to understand the physical mechanismof droughts and floods, in order to improve the watermanagement in those highly populated regions.

* Correspondence to: J. Zhang, Key Laboratory of MeteorologicalDisaster of Ministry of Education(KLME), Nanjing University ofInformation Science & Technology, Nanjing, China.E-mail: [email protected]

Dry and wet conditions in the Yellow River vary atmultiple scales with different mechanisms. The seasonalcycle of precipitation is significant in the Yellow Riverbasin with dryness in winter and spring and wetness insummer and fall. This is mainly due to its geographicallocation at the edge of the Asian summer monsoon. Morephysical processes that may contribute to relatively shortduration droughts and floods during the instrumentalperiod in the Yellow River basin have been extensivelystudied (Chen et al., 1996; Wang et al., 2006), includingremote effects such as forcing by sea surface temperatures(SSTs) and local effects such as land surface feedbacksin relation to soil moisture or snow cover. SST anomaliesin the tropical Pacific and North Atlantic (NA) areoften related to droughts in the Yellow River basin(Dong and Ren, 1996). Many major droughts are foundto be positively/negatively correlated with warm SSTanomalies in the east tropical Pacific (i.e. El Nino) (Wanget al., 2006; Rong et al., 2010). Studies show that SSTanomalies in the North Pacific Ocean and Indian Oceanexert influences on precipitation in East Asia, and onwet/dry conditions in the Yellow River basin (Moy et al.,

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3218 J. ZHANG et al.

2002). Precipitation in the upper reaches of the YellowRiver is associated with water vapour transportation bythe southwestly winds from the tropical ocean (Dong andRen, 1996). Precipitation in the lower reaches of theriver is associated with water vapour transportation bysoutheasterly winds from the mid-latitude Pacific Ocean(Ropelewsi and Halpert, 1987; Shi and Chen, 2003).

Besides the strong seasonal variability, droughts andfloods have interannual and decadal behaviours in theriver basin. Recent studies (Li et al., 2011) showed obvi-ous oscillations at 26 and 73 years, and the extendeddrought period in the early 1930s is believed to be a man-ifestation of these oscillations. Zhu (2003) showed thatthere are periodicities of 50 years in the middle reachesof the river, and of 70–80 years in the lower reaches.Over the last 50 years, the wet period appeared in the1950s and 1960s, and the dry period in the 1990s. It isclear that multiple scales and complex physical mecha-nisms exist in controlling the wet/dry conditions of theYellow River basin. Low-frequency variability of precip-itation is sometimes associated with the internal dynam-ics of the ocean’s circulation, and/or by greenhousewarming or other radiative forcing (Delworth and Mann,2000; Hu et al., 2011). Therefore, droughts and floodsmay have different origins: not only from anthropogenicglobal warming (Woodhouse and Overpeck, 1998), butalso from the natural variability in the climate system.Research showed that droughts and floods in the middlereaches of the Yellow River may have some correla-tion with the variation of sun spots (Li, 2005), but nophysical mechanisms is provided. Observations show thatdifferential and persistent large-scale circulation regimesexist at decadal timescales in association with decadalvariations of SST in both the North Pacific and theNA (Sutton and Hodson, 2007; Hu and Feng, 2008; Huet al., 2011). Most major historical droughts in the mid-reaches of the river in the 1320s, 1480s, 1520s, 1580s,1640s,1690s,1720s,1780s and 1870s are also associatedwith warm SSTs in the NA (Jiang et al., 1997).

Decadal droughts and floods in the river basin alsohappened on a paleo time scale. Paleoclimatic recordsbased on tree rings, archeological remains, lake sedimentand geomorphic data indicate that the droughts in the past2000 years have been exceeded several times recently(Zhang, 1991; Jiang et al., 1997). For the last 500 years,droughts and floods in the Yellow River basin showedremarkably strong signals in the Little Ice Age (LIA) andthe Medieval Warm Period (MWP, 950 until 1100 A.D.)(Zhang, 1991). The cold temperatures of the LIA agreewell with negatively abnormal precipitation fluctuatingin central China (Paulsen et al., 2003) and the TibetanPlateau (Yang et al., 2003) while MWP agrees withpositively abnormal precipitation in central China. Themain mechanism of how the LIA affected droughtsand floods in the Yellow River basin is explained asfollows: when it is cooled in the northern high latitudes,a negative state of the NAO or Arctic Oscillation (AO)leads to weakened, southward displaced westerlies withreduced water vapour delivered to the Asian hinterland;

when combined with an intensified, southward-expandedpolar vortex, and Asian summer monsoon circulation, theresults are atmospheric stability and weak water vapourtransport. In contrast, MWP leads to a positive state of theNAO/AO, which increases atmospheric instability andmore water vapour is delivered to the Asian hinterland(Menocal et al., 2000; Shindell et al., 2001; Hendy et al.,2002; Yu et al., 2006). Most of the studies have lookedfor correlations on seasonal and inter-annual scales (Chenet al., 1996; Yu et al., 2006). No reliable mechanism hasbeen reported. The impact of decadal North Atlantic SSTvariability on the Yellow River has not been well studied.Some case studies reported that the North Atlantic SSTlikely impacts Hetao drought in the upper reaches (Chenet al., 1996).

To further explore periodical changes of droughts andfloods and associated mechanisms, the Chinese Academyof Meteorological Science (CAMS, 1981) initiated animportant project to investigate water resources andwater-related natural hazards in the Yellow River basin.Instrument records are the primary dataset to study thedynamics of droughts and floods and associated mech-anisms of variation at multi-decadal timescales. Butinstrument records are often not long enough to studylong-term variability. Precisely dated tree-ring chronolo-gies, ice cores and other proxies, as well as documen-tary records, help to augment instrument informationon droughts and floods back to a few centuries (Yanget al., 2003). In the database of the CAMS (1981), thedrought/flood index (DFI) is compiled for 120 stationsbased on local documentary records and other historicaldocuments. The data cover 531 years (1470–2000), pro-viding an unprecedented opportunity for understandinghistorical drought/flood periods and their driving factors.In this study, special attention is paid to the role ofthe North Atlantic SST for droughts and floods in theYellow River basin. The instrument data from the last60 years are used for identifying the relationship betweenthe North Atlantic SST and dryness/wetness, DFI, in theYellow River basin.

This paper is organized as follows. The model descrip-tion and experimental setup are described in Section 2.The temporal variability of DFI and the relation with theNorth Atlantic SST are shown in Section 3. Finally, adiscussion is provided in Section 4.

2. Study areas and methods

2.1. Study area

The Yellow River is the second largest river in China.Its drainage basin (752 443 km2) covers about 12.5% ofthe Chinese territory. The Yellow River valley has hostedmajor Chinese civilizations since ancient times, providingrich water resources necessary for the development ofhuman societies. The Yellow River originates from theTibetan Plateau and flows through the semi-arid area ofthe Loess Plateau before reaching more humid areas ineastern China. Figure 1 shows the schematic coverage for

2013 Royal Meteorological Society Int. J. Climatol. 33: 3217–3228 (2013)

DROUGHT/FLOOD IN THE YELLOW RIVER IS CORRELATED TO NA SST 3219

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Figure 1. Location of the Yellow River basin and the China droughts and floods stations. DRS is the station (from right to left, it is the stationof Doudaoguai and Huayuankou) dividing the river into upper reaches, middle reaches and lower reaches, RS is the representative station(fromright to left, Jinan, Xi’an, Yinchuan and Xining, respectively), CS is the Chinese station about drought and flood index, PB is the provinceboundary, YRV is the Yellow River valley, YR is the Yellow River, BYRV is the boundary of the Yellow River valley, SCZ is the sub climatezone, which shows four climate zones around the Yellow River, from left to right, the climate zone are the plateau zone, transition zone, arid

zone and monsoon zone, respectively.

the drainage basin of the Yellow River. The whole basinexperiences a typical continental climate. The YellowRiver is divided into three parts according to valleycharacteristics: the upper reach, middle reach and lowerreach. The hinge stations are Toudaoguai (111◦04′E,40◦16′N) and Huayuankou (113◦39′E, 34◦55′N), whichare labelled in Figure 1(b). The upper reaches havea plateau climate and semi-arid, the middle reacheshave semi-humid climate, the lower reaches of the riverhave a monsoon climate. The annual mean surface airtemperature varies from about 2.6–13.9 ◦C, from theupper reaches to the lower reaches. The annual meanprecipitation varies from about 410–650 mm. The YellowRiver basin is located in a transition zone between theEast Asia Monsoon (EAM) and the northern hemispherewesterly winds. It is a region with climate variability andfrequent droughts and floods.

2.2. Materials and methods

The main data used in this study are from a compila-tion by the Chinese Academy of Meteorological Sciences(1981) covering more than five centuries. This datasetuses both historical documents and instrument measure-ments for most of the Chinese territory. It is more relevantfor the summer season from May to September. Thereare 120 locations with a complete time series of 531 years(1470–2000). Thirty-three of them are located in the Yel-low River basin. The DFI is defined as five grades:

Grade 1 severe flood: Ri > (R + 1.17σ );Grade 2 flood: (R + 0.33σ ) < Ri ≤ (R + 1.17σ );Grade 3 normal: (R – 0.33σ ) < Ri ≤ (R + 0.33σ );Grade 4 drought: (R – 1.17σ ) < Ri ≤ (R – 0.33σ );Grade 5 severe drought: Ri ≤ (R – 1.17σ )

where R is the climatological average of precipita-tion from May to September for all years, Ri is the

corresponding precipitation for each individual year andσ is the standard deviation. The DFI data are down-loaded from http://new-cdc.cma.gov.cn:8081/home. TheDFI show a large timescale background, which affectsseasonal and interannual dryness and wetness. Observa-tional data from the last 60 years are used for testing therelationship between the dryness and wetness in the Yel-low River basin and the North Atlantic SST oscillation.The variation of each circulation factor is investigated byusing the anomalies of wind and geopotential height at500 hPa from the NCEP/NCAR reanalysis (http://www.esrl.noaa.gov/psd/data/reanalysis/reanalysis.shtml).

The Met Office Hadley Centre’s sea ice and SSTdataset, HadISST1 is a unique combination of monthlyglobal fields of SST and sea ice concentration on a1◦ latitude–longitude grid from 1870 to date. Theprimary purpose of HadISST1 is to drive atmosphericmodels (AGCMs) in the simulation of the recent climateand to evaluate coupled atmosphere–ocean models,thereby improving our understanding of natural andhuman-induced climatic variations and allowing evalua-tion of model performance. HadISST1 has also been usedto supply information for the ocean surface for the period1958 through 1981 in the 40 years ECMWF Reanalysis(ERA40). HadISST1 compares well with other publishedanalyses, capturing trends in global, hemispheric andregional SST as well, containing SST fields with moreuniform variance through time and better month-to-month persistence than those in GISST (global sea-iceand SST) (Rayner et al., 2003). The SST data are fromhttp://www.metoffice.gov.uk/hadobs/hadisst/.

2.3. Model description and experimental setup

The atmospheric general circulation model used in thisstudy is the Community Atmospheric Model (CAM) ver-sion 3.0 (Collins et al., 2006), developed at the NationalCenter for Atmospheric Research. It is a global spectral



model with triangular truncation. This paper applies itsEuler dynamic framework T85 at a resolution of about1.4◦, which corresponds to 256 grid points in zonal direc-tion and 128 points in meridional direction with a total of26 layers in the η vertical coordinate. The model includesradiation processes, cloud effects, convection, land sur-face processes, boundary layer effects and other physicalprocesses (these processes are the default options), and anoptional slab ocean/thermodynamic sea ice model com-bination (not used for our study); a detailed description isavailable at http://www.ccsm.ucar.edu/models/atm-cam/.The dynamics and physics in CAM3 have been changedsubstantially compared to previous versions (Collinset al., 2006; Hack et al., 2006). We chose this optionbecause our goal was to investigate the impact of spe-cific, prescribed SST anomalies on drought/flood. It ispossible that there could be feedbacks between changesdue to the drought and the SST anomalies.

3. Results and discussions

3.1. Drought/flood variability in the Yellow Riverbasin

Climate in the Yellow River basin is complicateddue to its large basin size and varying surface alti-tudes. To better study its climate, the Yellow Riverbasin is divided into four sub-areas: the plateausubarea, semi-arid subarea, semi-humid subarea andmonsoon-dominant subarea (shown in Figure 1(b), dashline). Four stations are used as representative for thesub-areas: Xining (101.787◦E, 36.609◦N), Yinchuan(38.468◦E,106.272◦N), Xi’an (108.949◦E, 34.262◦N)and Jinan (117.006◦E, 36.667◦N). A temporal series ofDFI for the four stations are shown in Figure 2. Theresults from a wavelet analysis are shown in Figure 3.Due to missing data, the temporal series for Yinchuanand Xining stations are shorter. The temporal variabilityof DFI over the four stations is different (Figure 2).Whereas similarity exists between the monsoon subarea(Figure 2(a)) and the semi-humid subarea (Figure 2(b)),the semi-humid and semi-arid subareas have patternsthat are the opposite (Figure 2(c)). Most of the wet (dry)periods in the semi-humid subarea correspond to periodsof dryness (wetness) in the semi-arid subarea. The DFIin the plateau subarea is different from the other threeareas; the wet/dry phases reflect inconsistent behaviourover the entire valley with distinct spatial patterns. Forthe whole Yellow River basin, oscillations at roughly18–30 years and 60–80 years are observed during thepast 531 years (Figure 3). For Jinan, oscillations of DFIat roughly 26–30, 60, 120 and 210 years are observedduring the past 531 years (Figure 3(a)); Mann–Kendalltesting showed a 26–30-year dominant period (figureomitted). For Xi’an, oscillations of DFI at roughly 26, 60and 170 years are observed (Figure 3(a)); Mann–Kendalltesting showed a 60-year dominant period (figureomitted). For Yinchuan, oscillations at roughly 18–20and 50–60 years are observed during the past 300 years

(Figure 3(a)); Mann–Kendall testing showed an 18-yeardominant period (figure omitted). For Xining, oscillationsat roughly 18 and 80 years are observed during the past150 years (Figure 3(a)); Mann–Kendall testing showedan 18-year dominant period (figure omitted). It is clearthat multiple dynamics of water vapour transport operatefor a complex climate to occur in this basin. Xi’an inthe semi-humid subarea was in wet phases in the 1530s,1600–1620s, 1640–1660s, 1750s, 1840s, 1900s and1960s and in dry phases in the 1520s, 1580s, 1630s,1730s, 1800s, 1850s, 1910s and 1990s. Comparing withXi’an, dry/wet phases in Jinan are different. The dryphase occurred in the 1670s, 1780s and 1910s, and thewet phase occurred in the1540s, 1840s and 1990s. Thedry phase in Yinchuang occurred in the 1740s, 1810s,1860–1880s and 1950s, and the wet phase occurredin the 1740s, 1800s, 1840s, 1900s and 1960s. Thesepatterns are also observed in the past reconstructionsfrom the tree ring results (Wang et al., 2002).

3.2. Atmospheric circulation characteristics duringdrought/flood periods

In order to investigate the causes of wetness and dryness,atmospheric circulation characteristics under wet and dryconditions are studied with data from the NCEP/NCARreanalysis. We performed a composite analysis of theanomalous atmospheric general circulation at 500 hPafollowing the DFI and summer precipitation in theYellow River basin. We selected ten drought casesand ten flood cases at the station corresponding tothe upper and middle reaches of the Yellow River(water resources are crucial in this part of the riverbasin including semi-arid and semi-humid areas). Thecases are listed in Table 1. Droughts and floods areclosely associated with distinctive regional atmosphericcirculation regimes over the Eurasian continent. Duringdrought periods (Figure 4(a)), more frequent northeastwind anomalies occur over northern China, eastern Chinaand over Lake Baikal. A continental anti-cyclone existsnear Lake Baikal. The periphery of the anti-cycloneextends south to the Yellow River. Positive geopotentialheight anomalies cover Lake Baikal and most of China.This circulation anomaly is not favourable for northwardsummer monsoon water vapour transport, and reducesmonsoonal rainfall in northern China. Moreover, thewesterly flow is also weakened that limits the westerlywater vapour transport and the rainfall. Under theseconditions, atmospheric stability is enhanced that furtherdecreases the rainfall in the semi-arid of the upper reachesof the river.

For wet conditions in the semi-arid of the Yellow River(Figure 4(b)), more frequent westerly wind anomaliesare found in northern and northwestern China and overLake Baikal, and more frequent southerly wind anomaliesin East Asia. A continental cyclone exists near LakeBaikal. The periphery of the cyclone extends southwardto the semi-arid of the Yellow River, which enhancesatmospheric instability and convection, and enhances



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Table 1. Summer precipitation anomaly in the middle and upper reaches of the Yellow River basin.

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Figure 4. Composite analysis of the atmospheric circulation at 500 hPa (geopotential height and wind) for dry (a) and wet (b) conditions in themiddle reaches of the Yellow River. Data are from the NCAR/NCEP reanalysis.

rainfall. Negative anomalies of geopotential height coverLake Baikal and most of China. This circulation enhancesthe monsoon water vapour transport toward the north, andbrings more rainfall to northern China. Furthermore, theatmospheric circulation anomaly enhances westerly flowand westerly water vapour transport and rainfall. Underthese circulation conditions, the anti-cyclonic circulationover the mid-latitude Pacific Ocean is enhanced, whichcontributes to water vapour transport from ocean toland. The subtropical anti-cyclone over the western North

Pacific is the major system providing water vapour forfloods in the middle reaches of the Yellow River (Dongand Ren, 1996; Wang et al., 2013).

Over Asia and Europe, there are two centres ofabnormal geopotential height and wind. The shape ofthe anomalous circulation is similar to that of mid-latitude Rossby waves, which probably indicates thatdroughts and floods in the Yellow River basin arestrongly related to anomalies of large-scale atmosphericgeneral circulation, manifested as Rossby wave trains.



Circulation changes over the continent of North Americaalso indicate geopotential height anomalies, as in Suttonand Hodson (2005). As mentioned above, the NorthAtlantic SST anomalies may play an important role inmodulating rainfall in the Yellow River basin by affectingmid-latitude atmospheric circulation.

3.3. The relation of drought/flood variability in theYellow River to SST

In order to investigate the role of the North AtlanticSST in creating the wetness and dryness in the YellowRiver basin, Figure 5 shows a composite map of theNorth Atlantic SST differences between wet and dryconditions in the semi-arid of the Yellow River from Juneto September. Flood in the semi-arid of the Yellow Rivercorresponds to negative anomalies of SST in most ofthe NA, with maximum in mid-latitudes. For the wholesummer, the North Atlantic SST (10–55◦N) decreases,and the maximum decreasing centre is near 55◦N. TheSST in high latitudes of the NA (more than 55◦N)increases, and the maximum increasing centre is near62◦N. These basin-wide SST variations are also recordedin tree rings around the Atlantic (Gray et al., 2004), and insediments on the continental shelf of Venezuela (Grosfeldet al., 2007). The North Atlantic SST variability is anatural mode of variability. Some studies argued thatthe North Atlantic SST likely impacts North Americandrought during the medieval times (MT) (Conroy et al.,2009). In this study, the North Atlantic SST (10◦N–55◦N,70◦W–10◦W) is selected due to obvious SST variationin correlation to drought/flood in the Yellow River innorthern China.

The correlation coefficient between the North AtlanticSST and precipitation in the Yellow River basin fromJune to August for the last 60 years is shown in Figure 6.SST is negatively correlated to the summer precipitationin the western part of the upper reaches (semi-aridsubarea) of the Yellow River. The correlation becomespositive in the lower reaches of the Yellow River and inthe eastern part of the middle reaches (monsoon subareaand semi-humid). Warm SSTs in the NA result in wetconditions in the lower reaches of the Yellow River anddry conditions in the western part of the river’s middlereaches. In contrast, cold SSTs result in the exact oppositeconditions. The drought/flood phases reflect behaviourover the entire valley, but distinct spatial patterns do exist.The fact that the pattern is not only shown in the recent60 years but also in historical series suggests that it mightrepresent a natural mode of variability.

In order to further explore the effect of the NorthAtlantic SST on the wet/dry conditions in the Yel-low River basin, the DFI in the semi-humid subarea(Xi’an, Figure 7(a)) and semi-arid subarea (Yinchuan,Figure 7(b)) are plotted in Figure 7, together with theaverage of the North Atlantic SST from June to August(Figure 7(c)) and from March to May (Figure 7(d)). Theresults for the last 140 years are the same as in Figure 3.From large timescales, it is found that the DFI in the

semi-humid area has roughly a 50–60 year cycle, whichis different from the North Atlantic SST oscillation inspring and summer. The DFI in the semi-arid area hasa roughly 50–60 year cycle, which is consistent with theNorth Atlantic SST oscillation in spring and summer.These results are consistent with Figures 5 and 6. Instru-ment records show that the North Atlantic SSTs haverisen and fallen in a roughly 60–80-year cycle duringthe past 150 years (Schlesinger and Ramankutty, 1994)(Figure 2(c)), which is known as the Atlantic multi-decadal oscillation (AMO) (Kerr, 2000). These resultssuggest that the AMO plays a major role on decadal tocentennial time scales.

3.4. Role of the North Atlantic SST anomalies

To study the effect of anomalous North Atlantic SST(10◦N–55◦N and 70◦W–10◦W) on the drought/floodconditions in the Yellow River basin, two numericalexperiments were conducted. One was the control runfrom May of the first model year to September of thetenth model year (hereafter referred to as CTL). Theother was a sensitivity experiment with modified NorthAtlantic SSTs (hereafter referred to as NAP). It wasdesigned by adding 0.1 × SST (in its unit of degreesCelsius) to the observed North Atlantic SST for allseasons. Because SST variation ranged 2.4 ◦C, whichis closed to the average value ±1.2 ◦C, it also seemedabnormal 10% SST. Therefore, in order to simulateabnormal SST, an additional 10% SST is added to theoriginal SST, meaning 0.1 × SST is defined as abnormalSST which can reflect real fluctuations in SST. Otherforcing such as the greenhouse gases and the total solarirradiance remain the same as in the (present day) controlrun. CTL and NAP were run for 10 years (1995–2005).To remove the possible impact of initialization andequilibration on the model results, only the output forthe last 5 years of each run was analyzed in this study.

The mechanism of the North Atlantic SST on atmo-spheric circulation in Asia and the Yellow River isexplored. The analysis by Wang et al. (2013) showsthat more than 70% of the precipitation in the mid-dle reaches and upper reaches occurred from May toSeptember, more than 80% of the anomalous precipita-tion in the middle reaches and upper reaches happenedfrom July to August 2, and anomalous precipitation inthe lower reaches happened during the first 20 days inAugust. Therefore, July is selected to study the influenceof the North Atlantic SST on the atmospheric circulationin the upper and middle reaches of the Yellow River.We propose an explanation of the dynamical responseof the atmosphere to the thermodynamic SST forcing.Simulated results of increasing the North Atlantic SSTshow that the warm phase of the North Atlantic SST isassociated with a different regional circulation regime inthe northern hemisphere and over the Yellow River. Theatmospheric circulation responses to the North AtlanticSST warming are obvious. The averaged differences ofgeopotential height and wind at 500 hPa in July between



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Figure 6. Correlation coefficient between the North Atlantic SST andsummer precipitation in the most recent 60 years.

the sensitivity experiment and the control run are shownin Figure 8(a). With a warming of the North AtlanticSST, increased geopotential height occurs in the middleof the NA and over the western coast of Europe, north-ern Asia and North America. In particular, the bound-ary of increased geopotential height from Lake Baikalextends southward to the upper reaches of the YellowRiver, and to the semi-arid subarea of the river basin. Adecrease of geopotential height occurs over Siberia, NAand North Pacific. Along with geopotential height anoma-lies, wind vector differences also show large-scale circu-lation changes, with cyclones in the middle of the NA,Siberia, as well as the western coast of North America.Anti-cyclones occur over the western coast of Europe,northern Asia and North America. Differences in verticalvelocity at 500 hPa between the sensitivity experimentand the control run are shown in Figure 8(b). Negative

anomalies occur in the upper and lower reaches of theYellow River, but weak positive anomalies occur in themiddle reaches.

This anomalous circulation reduces the monsoon watervapour transport toward the north and reduces the rainfallover the semi-arid subarea of the upper reaches of theYellow River basin. At the same time, it also reduces thewesterly flow, the westerly water vapour transport and theassociated rainfall. In addition, under these conditions,atmospheric stability is enhanced over the semi-aridsubarea, which reduces the possibility of rainfall inthe semi-arid subarea of the Yellow River. However,precipitation increases in the semi-humid subarea andthe lower reaches of the Yellow River due to ascendingmovement and abundant water vapour from the east,transported by the subtropical anti-cyclone over the JapanSea.

July precipitation differences between the sensitivityexperiment and the control run are shown in Figure 9.The large-scale precipitation (Figure 9(a)) response tothe North Atlantic SST warming shows three atmospherecirculation patterns in China, with smaller precipitationvariability in northwest and southeast China, decreasedprecipitation in the upper reaches of the Yellow Riverand the Yangze River. Compared with the large-scale pre-cipitation, convective precipitation shows strong regionalfeatures. A significant reduction in precipitation is foundin the semi-arid subarea of the upper reaches of theYellow River basin and in southeast China. Increasedprecipitation occurs in northeast China, the semi-humidsubarea and the monsoon subarea, and the source regionof the Yellow River and north of the Yangtze River.Increased precipitation reaches more than 8 mm day–1,and the reduction is larger than 6 mm day–1. It is evident



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Figure 7. Temporal variations of DFI in semi-humid (a) and semi-arid (b) areas. Panel (c) is the averaged SST in the NA in summer (from Juneto August). Panel (d) is the same as in (c) but for Spring (from March to May).

that a warming in the North Atlantic SST can changethe atmospheric circulation and therefore large-scale pre-cipitation, especially in the upper reaches of the YellowRiver. At the same time, it can also modify the regional-scale atmosphere stability and the convection precipita-tion. From these experiments, the warming in the NorthAtlantic SST may lead to the drought in the upper reachesof the Yellow River or semi-arid subarea, and flood in thelower reaches of the Yellow River or semi-humid subarea.However, the drought of the semi-arid subarea and floodin monsoon subarea of the lower reaches or semi-humidsubarea is rather weak because anomalous precipitationis <10 mm day–1. However, it qualitatively explains theseverity and longevity of the drought in the semi-aridarea of the upper reaches of the river and flood in thesemi-humid area in the middle reaches and the monsoonarea in the lower reaches of the river. These results areconsistent with Xu et al. (2001).

This anomaly pattern changes during the warm phaseof the North Atlantic SST, result in below-averageprecipitation for northwest and southeast China, butabove-average precipitation for northeast China and northof the Yangze River. These changes effectively describean alternating monsoon regime that follows the phasesof the North Atlantic SST. The North Atlantic SST isnot only linked to drought/flood in the Yellow Riverbasin, it has also been linked to recent decadal climatechanges over North America (Robert et al., 2012). Allthese findings indicate that the North Atlantic SST

anomaly may modify atmospheric circulation, and in turn,it changes location and intensity of the precipitation.Some observational and modelling studies showed thata warm SST in the NA may induce a sea level pressure(SLP) pattern that resembles the negative phases of theNAO, suggesting that the North Atlantic SST and NAOmay be manifestations of the same basic climate systemphenomenon, just manifested somewhat differently forthe atmosphere and the ocean (Feng et al., 2009).

4. Conclusions and discussions

This study focuses on exploring decadal timescale pre-cipitation and its possible mechanism. The timescale ofabnormal precipitation is analyzed by using a 531-yearDFI. Aiming at abnormal precipitation, two key factorsof atmospheric stability and water vapour transport areconsidered to explore the possible mechanism.

Dry or wet conditions in the Yellow River basinexperienced oscillations roughly every 50–60 years and18–26 years during the past five centuries. Droughts andfloods appear to have a mode of natural variability, whichhappened frequently during the most recent 60 years.Drought and flood have inconsistent trends between themonsoon subarea and semi-arid subarea, semi-humidsubarea and plateau subarea, the four parts of the YellowRiver.

In the last 140 years, the semi-humid and monsoonsubarea DFI have been inconsistent with the North



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Figure 8. Simulated July averaged geopotential height (above sea level, shadow) and wind speed (m s–1, vectors) at 500 hPa level differencebetween experiments and control run (a), and vertical velocity (pressure, shadow) and wind speed (m s–1, vectors) at 500 hPa level difference

between experiments and control run (b).

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Figure 9. Simulated July averaged daily precipitation (mm day–1) difference between experiments and control run. (a) Large-scale (stable)precipitation and (b) convective precipitation.

Atlantic SST oscillation in the spring and summer. Incontrast, the semi-arid subarea DFI have been consistentwith the North Atlantic SST oscillation in spring andsummer.

During flood periods in the semi-arid subarea in theupper reaches of the river, the SST of NA is low. In addi-tion, more frequent westerly wind anomalies appeared inNorthwest China and over Lake Baikal, and the edge ofthe cyclone extends south to the middle reaches of the

Yellow River, which enhances atmospheric instability andconvection, and enhances rainfall. The subtropical anti-cyclone is also the major system providing water vapourto the middle reaches of the Yellow River. During droughtperiods, the atmospheric circulation patterns reverse.

The warm (cold) phase of the North Atlantic SST alter-nates in the monsoon regime, resulting in water vapourtransport toward the north and atmospheric stability,which will result in abundant precipitation distribution in



China. The warm phase of the North Atlantic SST resultsin more frequent easterly wind anomalies in northernChina and over Lake Baikal, which indicate poor pre-cipitation or frequent drought in the semi-arid subarea inthe upper reaches of the Yellow River. However, highfrequency precipitation or flood in the sub-humid in themiddle reaches and the monsoon subarea in the lowerreaches of the Yellow River are due to more frequentsubtropical anti-cyclone anomalies in the middle PacificOcean.

Simulation results show that the North Atlantic SSTcan induce geopotential height and SLP anomalies in themid-latitudes and high-latitudes; this pattern indicates thenegative (positive) phases of NAO. Many researches alsoshow that the North Atlantic SSTs operate in conjunctionwith other key SST phenomena such as ENSO, as wellas Pacific decadal oscillation (PDO). Whether the NorthAtlantic SST combined with Pacific SST informationinfluences drought/flood in Yellow River remains to beexplored.

Acknowledgements

This research is financially supported by the Meteorolog-ical Foundation of China (Grant No. GYHY201006038)and National 973 Project (2013CB956004), a projectfunded by the Priority Academic Program Developmentof Jiangsu Higher Education Institutions (PAPD). We sin-cerely thank an anonymous reviewer for valuable sugges-tions and comments that greatly improved the manuscript.

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