Impact of common SST anomalies on global drought and pluvial frequencyKirsten Findell and Tom DelworthGeophysical Fluid Dynamics LaboratoryPrinceton, NJ

33rd Climate Diagnostics and Prediction Workshop/CLIVAR Drought Workshop (CDPW)October 2008

SST forcing patterns used for Clivar Drought Working Group StudyClimatology derived from Hadley Centre Monthly SST data, 1901 to 2004 (HADISST1, Rayner et al 2003)Rotated EOF analysis: first three modes:Linear trend pattern (27.2%), scaled by 1sPacific pattern (20.5%), scaled by 2sNorth Atlantic pattern (5.8%), scaled by 2s

SST forcing patterns used for Clivar Drought Working Group StudyREOF 1REOF 3REOF 2All runs: fixed SSTs, 50 years longControl run: Climatological SSTs

OutlineSST forcing patternsDrought index used: SDDIExtensive look at Cold Pacific experimentGFDL model to demonstrate the methodologyMulti-model analysis (GFDL, NSIPP, CAM)The three models tend to yield similar results in terms of drought/pluvial frequency and intensityPac/Atl combination experiments, Multi-model meansPositive/negative forcings tend to yield opposite resultsTrend and Trend/Pac/Atl combinationsWhat do they tell us about the relative strengths of these different forcings in the present day?Trend impacts are generally overwhelmed by Pac/Atl impactsAdditional experiments to look at future scenariosTrend impacts should not be overwhelmed by Pac/Atl impacts in the futureConclusions

Supply-Demand Drought Index (SDDI)From Rind et al. (JGR, 1990)Difference between moisture supply (precip) and demand (potential evap)Similar in construct to PDSI:dSDDI = P Ep (P Ep)climZSDDI = d/sY(i) = 0.897 Y(i-1) + Z(i)(P Ep)clim, s: seasonal cycle (monthly values) determined from Control runBenefits:Tied to soil moisture (through evaporative demand), but since soil moisture is very model dependent (e.g., depth saved) this index is good to use when looking at multiple modelsNo grid-specific empirical coefficients to estimate (might these change if climate changes?)

Cold Pacific: Changes in Mean SDDIGFDL model: Annual mean SDDI differences,where significantly different from control (95% level, modified t-test)

Cold Pacific: Drought FrequencyGFDL model: differences in meanAverage number of months/year in drought(SDDI < -2.0)

Cold Pacific: Changes in Drought FrequencyGFDL model: differences in meanDifference from Ctl, months/year in drought(SDDI < -2.0)Locations with reductions in mean SDDI also show increased drought frequency.There are some additional locations with minor increases in drought frequency,despite a lack of significant change in the mean SDDI (Central Asia, Central Africa).

Cold Pacific: Pluvial FrequencyGFDL model: differences in meanAverage number of months/year withSDDI > 2.0Locations with increased mean SDDI also show increased pluvial frequency.There are some additional locations with minor increases in pluvial frequency,despite a lack of significant change in the mean SDDI (high latitudes).

Cold Pacific: Drought FrequencyMulti-modelmean

Cold Pacific: Drought IntensityMulti-modelmean

Cold Pacific: Pluvial FrequencyMulti-modelmean

Cold Pacific: Pluvial IntensityMulti-modelmean

Model agreement in Cold Pacific runDrought is: Much more frequent in Continental US and southern South AmericaSlightly more frequent in eastern Canada, from 35-50N in Europe and Asia, and along the east coast of Asia Pluvials are:Much more frequent in Central America, northern South America, and OceaniaSomewhat more frequent in Arabia and Australia

Model differences in Cold Pacific runCAM is much more sensitive than GFDL and NSIPP throughout Africa (greater tendency towards pluvials in this run) African pluvial pattern in CAM run extends into Mediterranean region, pushes European drought region further northGFDL model more sensitive in high latitudes (more pluvials in this run)The three models differ in the central location and intensity of the drought response in the US

Drought Frequency, MMM, Pac/Atl runsCold PacificCold AtlanticWarm AtlanticWarm Pacific

Pluvial Frequency, MMM, Pac/Atl runsCold PacificCold AtlanticWarm AtlanticWarm Pacific

SDDI time series in Central US Pac/Atl combinations; GFDL model

SDDI time series in Central US Do we see linearity in the responses?GFDL model

SDDI time series in Central US Do we see linearity in the responses?GFDL model

Warm Linear Trend: Drought FrequencyMulti-modelmean

Warm Linear Trend: Pluvial FrequencyMulti-modelmean

Model agreement in Warm Trend runModest increases in drought frequency:~30-45N in North America (Central US) and ~35-50N in Europe and Asia (Mediterranean region and extending eastward) Central and Southern AfricaModest increases in pluvial frequency:North of ~55N, particularly in AsiaCentral America and NE coast of South AmericaEastern Australia

Drought in Trend, Pac, Atl combinationsCold Trend, Cold Pac, Warm AtlCold Trend, Warm Pac, Cold AtlCold TrendGFDL modelWarm Trend, Warm Pac, Cold AtlWarm TrendWarm Trend, Cold Pac, Warm Atl

Pluvials in Trend, Pac, Atl combinationsCold Trend, Cold Pac, Warm AtlCold Trend, Warm Pac, Cold AtlCold TrendGFDL modelWarm Trend, Warm Pac, Cold AtlWarm TrendWarm Trend, Cold Pac, Warm Atl

Trend/Pac/Atl combinationsImpacts of Trend are overwhelmed by Pacific/Atlantic impactsSince these standard runs had the Trend EOF multiplied by 1s, and Pac/Atl multiplied by 2s, this result is in part due to the experimental designPresent-day runs: composites of the three EOFs all multiplied by 1sShould indicate if Trend impacts are overwhelmed by ENSO/NAO in current worldFuture-scenario runs: composites of the Pac/Atl EOFs multiplied by 1s, Trend by 2sShould indicate if Trend impacts will be overwhelmed by ENSO/NAO in the future, assuming that ENSO/NAO dont change much in the future

Droughts and pluvials in the presentWarm 1s Trend, Cold 1s Pac, Warm 1s AtlWarm 1s TrendGFDL modelWarm 1s Trend, Warm 1s Pac, Cold 1s AtlPluvial increases bottom rowDrought increases top row

Present-day scenario resultsDrought/pluvial frequency is a strong function of Pacific and Atlantic conditionExceptions:Central Africa has frequent droughtsSouthern India and SE Australia have frequent pluvialsCalls into question the validity of fixed SST experiments for the Indian Ocean

Droughts and pluvials in the futureWarm 2s Trend, Cold 1s Pac, Warm 1s AtlWarm 2s TrendGFDL modelWarm 2s Trend, Warm 1s Pac, Cold 1s AtlPluvial increases bottom rowDrought increases top row

Future scenario resultsDroughts become more frequent in the future, independent of ENSO/NAO:Mediterranean Sea region and east past Caspian Sea (~30-50N in Africa, Europe, and Asia)Central AfricaPluvials become more frequent in the future:North of 50N (esp. in Europe and Asia)Coastal AsiaIndia and Australia (esp. SE)Caveat: experimental validity in Indian Ocean region?Drought/pluvial frequency remains a strong function of Pacific and Atlantic condition:Continental USSouthern and Northern South America

ConclusionsThe three models (GFDL, NSIPP, CAM) generally yield similar drought/pluvial results:Cold Pacific: Increased drought in US, southern SAm., (S Europe, SE coast Asia)Increased pluvials in Cent. Am., northern SAm., Oceania, (Arabia, Australia)Warm Pacific: In general, opposite of Cold PacificCold Atlantic:Increased droughts in Cent. Am., northern SAm., Central Africa, (Oceania)Increased pluvials in cent. SAm., (cent. US)Warm Atlantic:In general, opposite of Cold AtlanticWarm Trend:Modest changes that are overwhelmed by Pac/Atl impacts in combination runs

Conclusions (cont.)Areas with an increase (decrease) in the mean SDDI tend to also show an increase (decrease) in the frequency of extreme SDDI valuesAreas with more frequent extremes also tend to show higher intensity extremesPresent day experiments suggest:The impact of the background warming trend is generally overwhelmed by the ENSO and NAO signalsQuestionable use of fixed SST models in Indian Ocean BasinFuture experiments suggest:The impact of the trend will be more dominant than the ENSO and NAO signals in many areas of the globe when the magnitude of the trend has doubled, assuming ENSO and NAO characteristics remain relatively stable.