ENSO Forcing of ENSO Forcing of Streamflow Conditions in Streamflow Conditions in

the Pearl River Basinthe Pearl River Basin

R. Jason Caldwell, LMRFCR. Jason Caldwell, LMRFCandand

Robert J. Ricks, WFO LIXRobert J. Ricks, WFO LIX

Regional HydrologyRegional Hydrology Pearl River BasinPearl River Basin

• Yockanookany RiverYockanookany River• Pearl RiverPearl River• Bogue Chitto RiverBogue Chitto River• Among others…Among others…

Yockanookany

Upper Pearl

Lower Pearl

Bogue Chitto

Streamflow EventsStreamflow Events Daily mean streamflow in cfsDaily mean streamflow in cfs 7575thth/25/25thth Percentile values from USGS Percentile values from USGS

used to denote above/below normal used to denote above/below normal streamflow daysstreamflow days

Monthly counts for high/low flow Monthly counts for high/low flow events calculatedevents calculated

Missing data were considered to be Missing data were considered to be non-events for conservative non-events for conservative estimates of actual event daysestimates of actual event days

ENSO Impacts on PrecipitationENSO Impacts on Precipitation

Dry winter across the north, wetter south

Transitions to wetter spring season

Dry winter with increasing moisture across north in

spring season

Near normal winter condition trends to dry

conditions in spring season

Composite Analysis MethodologyComposite Analysis Methodology ONI values and ENSO episode assigned to ONI values and ENSO episode assigned to

monthly streamflow event countsmonthly streamflow event counts Terciles computed to determine Terciles computed to determine

above/near/below normal conditionsabove/near/below normal conditions Counts for each ENSO episode-category Counts for each ENSO episode-category

performed (e.g. ENa, LNb, NUn)performed (e.g. ENa, LNb, NUn) Probabilities for each event define the Probabilities for each event define the

historical compositeshistorical composites Only statistically significant (90% Only statistically significant (90%

confidence) and at least half of sites confidence) and at least half of sites withing the Mainstem, Yockanookany, or withing the Mainstem, Yockanookany, or Bogue Chitto must indicate similar Bogue Chitto must indicate similar relationshipsrelationships

Cool Season Wet Tendency

Cool Season Wet Tendency

DRY AT COAST MIGRATES NORTH IN SPRING

DRY CONDITIONS NEARLY PREDOMINATE COOL SEASON

DRY CONDITIONS NEARLY PREDOMINATE COOL SEASON

MIXED SIGNALS IN THE SUMMERTIME

Cool Season Wet Tendency at Coast Migrates Northward in Spring

Cool Season Wet

Tendency at Coast

DRY NORTH EARLY IN COOL

SEASON

2525thth Percentile Composites Percentile Composites No signal in seasons: MAM, AMJ, JJA, No signal in seasons: MAM, AMJ, JJA,

JAS, SON, OND, and NDJJAS, SON, OND, and NDJ Increased/decreased probability of Increased/decreased probability of

below/above normal low flow days in below/above normal low flow days in Bogue Chitto during El NinoBogue Chitto during El Nino

Forecast VerificationForecast Verification 1982-2005 CPC Nino 3.4 SST Forecast 1982-2005 CPC Nino 3.4 SST Forecast

probabilities combined with historical probabilities combined with historical composites to produce hindcasts for each composites to produce hindcasts for each monthmonth

Only 0.5 month leads utilizedOnly 0.5 month leads utilized• Decreasing forecast accuracy with increasing Decreasing forecast accuracy with increasing

lead timelead time• 90-day outlooks for SFPO90-day outlooks for SFPO

Contingency tables used to compute Contingency tables used to compute statistical scoresstatistical scores

Heidke Skill ScoresHeidke Skill Scores

Least skill in Least skill in southernmost southernmost basinsbasins• BXAL1, TYTM6, BXAL1, TYTM6,

and BSHL1and BSHL1

Mainstem and Mainstem and Yockanookany Yockanookany ranged from ranged from 0.13 to 0.250.13 to 0.25

Heidke Skill Score 75th Percentile Flow Day Forecasts

Based on ENSO Composite Analysis(Perfect = 1, No Skill = 0)

0.13318

0.170668

0.216392

0.2475550.231548

0.094829

0.048804 0.053382

0

0.05

0.1

0.15

0.2

0.25

0.3

KSCM6 OFAM6 ENBM6 JACM6 MTCM6 BXAL1 TYTM6 BSHL1

Site ID

Heid

ke S

kill

Scor

e

Heidke

Skill relative to random chance

Ranked Probability ScoresRanked Probability Scores

Marginal Marginal skill skill indicated indicated with RPS with RPS of 0.40 to of 0.40 to 0.460.46

ENSO Composite Performance Using Ranked Probability Score(Perfect = 0, No Skill = 1)

0.417874 0.420369 0.407035 0.412881 0.436875 0.455066 0.436195 0.432091

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

KSCM6 OFAM6 ENBM6 JACM6 MTCM6 BXAL1 TYTM6 BSHL1

Site ID

RPS RPS

Skill in predicting the proper category

Ranked Probability Skill ScoreRanked Probability Skill Score

Limited Limited improvement improvement over climo in over climo in upper Pearl; upper Pearl; little or no little or no skill across skill across lower Pearllower Pearl

ENSO Composite Performance Using Ranked Probability Skill Score

(Perfect = 1, No Skill <=0)

0.046397

0.0766430.063382

0.005853 0.010495 0.019804

0.052056

-0.03231

-0.2

-0.1

0

0.1

0.2

KS

CM

6

OFA

M6

EN

BM

6

JAC

M6

MTC

M6

BX

AL1

TYTM

6

BS

HL1

Site ID

RPSS RPSS

Skill relative to climatology

ConclusionsConclusions Precipitation departures attributed to Precipitation departures attributed to

ENSO episode correlate well with ENSO episode correlate well with streamflow events streamflow events

Locations near the Gulf of Mexico exhibit Locations near the Gulf of Mexico exhibit least improvement over climatology least improvement over climatology (localized forcing)(localized forcing)

ENSO-based composites yield greatest ENSO-based composites yield greatest forecast improvement in upper Pearl River forecast improvement in upper Pearl River BasinBasin

Smaller basins can produce contradictory Smaller basins can produce contradictory signals compared to larger mainstem signals compared to larger mainstem basinsbasins

Future ResearchFuture Research Include lag-time response in Include lag-time response in

computationscomputations 2525thth percentile hindcast verification percentile hindcast verification Ensemble streamflow prediction and Ensemble streamflow prediction and

Spring Flood Potential Outlook Spring Flood Potential Outlook applicationsapplications

Web access to composites and Web access to composites and forecastsforecasts