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ReviewofGlobalPrecipitationfor2017

RobertAdler,GuojunGuandJian-JianWangUniversityofMarylandJanuary30,2018

Headlines:1)LaNinaeffectscontinueinto2017forglobaltotalsandpattern 2)PositiveglobalmeanprecipitationtrendbecomingevidentwithGPCP 3)Globallandprecipitationatornearall-timehighin2017 4)2017contributestoongoingtrendsinprecipitationintensityinthe tropicsTheGlobalPrecipitationClimatologyProject(GPCP)monthlydatasetisalong-term(1979-present)analysis(Adleretal.,2003;Huffmanetal.,2009:Adleretal.,2017)usinga combinationof satelliteandgauge information. An interimGPCPanalysiscompleted within 10 days of the end of the month allows for use in climatemonitoringand, inthiscase,aquicklookatthejustcompletedyear(2017). ThecurrentVersion2.3 of theGPCPMonthly analysis is aNOAAClimateDataRecord(CDR) and is part of the Global Energy and Water EXchanges (GEWEX) projectundertheWorldClimateResearchProgram(WCRP).Fig. 1 shows the GPCP climatology map (1979-2016), the annual mean map for2017,andanomalymapsfor2017,forbothrainratemagnitudesandpercentages.Theclimatologymapshowstheusualmaximaofthetropicsandmid-latitudes,withsimilar features, of course, for2017, butwith fairly obvious greatermagnitudeofmean rainfall over the Maritime Continent and surrounding areas. The annualanomalymaps(Fig.1cand1d)emphasizethosefeatures,showingadefiniteLaNinapattern with the strong positive anomaly over the western Pacific and a rainfalldeficitover thecentralandeasternPacificnear theEquator. Indeed, theseasonalanomalypatternsduring2017(notshown)indicatethattheLaNinafeaturesinthePacific Ocean/Maritime Continent area existed in varying strengths during theentiretyof2017. ThisfollowedatransitionfromElNinotoLaNinaduring2016.TypicalLaNinaanomalyfeaturescanalsobeseeninsomeotherlocationssuchasthe tropical Atlantic, the negative anomaly off of Baja Mexico extending into thesouthwest cornerof theU.S., anda typicalLaNinapatternalong theeast coastofAfrica. A negative anomaly feature across the very southern tip of the Africancontinent canbe seen,whereCapeTown,SouthAfricaexperienced itsdriestyearsince1933,probablydue,inpart,toLaNina.InFig.1dtheCapeTownareaisshowntohaveaspatiallysmall,butverystrongnegativepercentagefeature,emphasizingtheconnectionbetweenlarge-scalefeaturesandlocalimpacts.NorthernCalifornia,OregonandWashingtonshowthestrongpositiveanomalyrelatedtoheavyrainintheearlypartoftheyear.Athigherlatitudes,forexampleacrossnorthernEurasiaandAlaska,positiveanomaliesdominatetheinter-annualchanges,butalsomayberelatedtoanestimatedpositivetrendovertheGPCPrecordintheseareas.

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Table1givestheestimatedglobalmeanrainfallratefor2017(2.72mm/d)andthesub-totals for landandocean. Theglobalnumber isslightly(1%)higher thanthe1979-2016climatology,withlandareascontributingavalueabout4%higherthanthelandclimatology.Forthisexercisethelandprecipitationvaluesforyears2014-2017fromVersion2.3havebeenadjustedupward2%.Thisadjustmentisbasedona difference in the version of gauge analysis that is used for the recent period ascompared to the pre-2014 era. This is a small, but significant adjustment, and isbased on an inter-comparison of the gauge analyses for an overlapping period.However,theresultscouldchangeslightlywhenupdated,soshouldbeconsideredcarefully.To put the global numbers for 2017 in context, Fig. 2a shows plots of the annualanomalyoftheglobaltotal(andoceanandlandtotals)from1979-2017,withFig.2cshowingannualmeanvaluesforNino3.4asameasureofENSOforcomparisonwiththeannualanomalyvalues.TheoceanandlandvaluesinFig.2a“flip-flop”betweenElNinoandLaNinayears,with theglobal totalvaluehavingsmalleryear-to-yearvariations, although larger during El Nino years (e.g., 1998, 2010, 2015-2016)(Adleretal.2017).2017,aweakLaNinayear,hasanestimatedrecord-settinghighGPCPlandvalue,compensatedbyarelativelylowoceanvalue.Theglobalvaluefor2017isnotarecordhighvalue,althoughthecombinationof2016and2017hasanestimated mean that is higher than any other sequential two-year mean. Theestimated trend is calculated for the three curves in Fig. 2a and is a very slightpositiveforallthree,withavalueof0.009mmday-1decade-1(0.33%perdecade)fortheglobaltrend(significantatthe5%level).Thetrendvaluesforbothlandandocean are very similar, but are not significant due to the larger inter-annualvariations. The second panel (Fig. 2c) removes the ENSO effect on the annualanomalies,andresultsinreducedvariations,butthetrendvaluesstaythesameandthesignificanceresultsarealsounchangedrelativetothe5%threshold.Thesmallcalculated global precipitation trend compared to the global surface temperaturetrend(0.16Kdecade-1)forthesameperiodgivesa1.3%K-1fortherateofincreaseinglobalprecipitationinrelationtoglobalwarming.Thisvalueisclosetothevalueoften quoted coming from climate models, but the GPCP-based value is verysensitivetothelength(andhomogeneity)oftherecord.AsthelengthofrecordforanalyseslikeGPCPincreases,itisobviousthattheirvaluewillincreasedramatically.AlthoughtheglobaltrendsinFig.2areverysmall,thetrendsarelargerandvariableinthespatialdomain(seeFig.3),withthepatternshowinganareaofpositivetrendalongmost of the ITCZ, especially across the Pacific and Indian Oceans. OceanicdecreasesnorthandsouthofthePacificITCZareadjacentandweaklyconnectedtodecreases over land, as in the southwesternU.S. A general scenario ofwet areasgetting wetter, dry areas getting drier is evident. At high northern latitudes thepositive trendsnotedacrossEurasiaand theArcticOcean toAlaskaaresimilar tothepositiveanomalyfeaturesseeninFig.1for2017.Addingoneyear(2017)tothedata set does not significantly change the pattern in Fig. 3, but also serves as astarting point for examining regional trends in relation to ENSO, inter-decadalvariationsandglobalwarming(seeGuetal.,2016;Adleretal.,2017).

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Data from 2017 have also been incorporated into the investigation of trends inrainfallintensityusingthemonthlyGPCPanalyses(GuandAdler,2018). Focusingon the tropics and the post-1987period (the satellitemicrowave era) percentiles(andotherparametersrelatedtointensity)ateachgridwerederivedandcomparedto thepreviousyears (seeFig.4). Themean tropical rainfallhasapositive trendovertheperiod(Fig.4d),butitisalsoevidentthatthereareevenstrongerpositivetrends in the higher percentiles (more intense rainfall),with significant trends atpercentilesgreaterthan70%(seeFig.4a).Atintermediatepercentiles(30-40%)adownward trend is noted (Fig. 4c). At low percentiles (not shown) trends wereindeterminate, but defined “dry areas” showed increases during the period. Theyear2017resultshelpedsolidifythetrendresults,butanENSOcomponentiseasilyrecognizableinthe95thpercentileresultswiththeElNinosof1998,2010and2016.Thereisalsoadecadalshiftevidentaround1998relatedtotheshift inthePacificDecadal Oscillation (PDO). Monitoring rainfall intensity changes, even on themonthly time-scale, will henceforth be a focus of our routine monitoring andresearchwiththeGPCPdata.References:Adler, R. F., G. J. Huffman, A. Chang, R. Ferraro, P. Xie, J. Janowiak, B. Rudolf, U.Schneider,S.Curtis,D.Bolvin,A.Gruber,J.Susskind,andP.Arkin,2003:Theversion2 Global Precipitation Climatology Project (GPCP) monthly precipitation analysis(1979-present).J.Hydrometeor,4,1147-1167.Adler, R., G. Gu, M. Sapiano, J. Wang, G. Huffman 2017. Global Precipitation: Means, Variations and Trends During the Satellite Era (1979-2014). Surveys in Geophysics 38: 679-699, doi:10.1007/s10712-017-9416-4Adler et al., 2018. An Update (Version 2.3) of the GPCP Monthly Analysis. (inPreparation).Gu, G., R. Adler, and G. Huffman, 2016: Long-Term Changes/Trends in Surface Temperature and Precipitation during the Satellite Era (1979-2012). Climate Dynamics 1-15. DOI 10.1007/s00382-015-2634-x Gu, G., and R. Adler, 2018. Precipitation Intensity Changes in the Tropics from Observations and Models. J. Climate. In review.Huffman,G. J., R. F. Adler, D. T. Bolvin, and G. Gu, 2009:Improvements in the GPCP global precipitation record: GPCP Version 2.1. Geophys. Res. Lett., 36, L17808, doi:10.1029/2009GL040000.

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Table1.GlobalmeanGPCPprecipitation(mmday-1)during1979-2016andin 2017. Also estimated are the annual anomalies in 2017 and correspondingpercentagechanges.

Meanrain-rateduring1979-

2016(mmday-1)

Meanrain-ratein

2017(mmday-1)

Annualanomalyin2017(mm

day-1)

Percentagechangein2017(%)

Land+Ocean 2.69 2.72 0.03 1.12Land 2.24 2.34 0.10 4.46Ocean 2.90 2.89 -0.01 -0.34

Figure1. (a)GPCPclimatologicalmeanprecipitation (mmday-1), (b)annualmeanprecipitationin2017, (c)annualprecipitationanomalies(mmday-1) in2017,and(d)annualprecipitationanomaliesinpercentagesfor2017(withareashavinglessthan0.5mmday-1ofmeanprecipitationalsoshowninwhite).

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Figure2.Timeseriesof(a)global,annualmeanprecipitationanomalies,(b)noENSOeffect,and(c)annualmeanNino3.4.Alsoshownin(a)and(b)arecorrespondinglineartrends(mmday-1perdecade),andthosefollowedby“*”arestatisticallysignificant.

Figure3.LineartrendofGPCPprecipitation(mmday-1perdecade)during1979-2017.

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Figure4.Annualanomaliesof(a,b,c)precipitationpercentilesand(d)meanprecipitationdeterminedbyGPCPmonthlyrain-ratesbetween30oN-30oS(land+ocean).Alsoshownaretheircorrespondinglineartrends(mmday-1perdecade),andthosefollowedby“*”arestatisticallysignificant.