ENSO Forcing of

Streamflow Conditions in

the Pearl River Basin

R. Jason Caldwell, LMRFC

and

Robert J. Ricks, WFO LIX

Regional Hydrology Pearl River Basin

• Yockanookany River

• Pearl River

• Bogue Chitto River

• Among others…

Yockanookany

Upper Pearl

Lower Pearl

Bogue Chitto

Streamflow Events

Daily mean streamflow in cfs

75th/25th Percentile values from USGS used to denote above/below normal streamflow days

Monthly counts for high/low flow events calculated

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

ENSO 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 Methodology

ONI values and ENSO episode assigned to monthly streamflow event counts

Terciles computed to determine above/near/below normal conditions

Counts for each ENSO episode-category performed (e.g. ENa, LNb, NUn)

Probabilities for each event define the historical composites

Only statistically significant (90% confidence) and at least half of sites withing the Mainstem, Yockanookany, or Bogue Chitto must indicate similar relationships

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

25th Percentile Composites

No signal in seasons: MAM, AMJ, JJA, JAS, SON, OND, and NDJ

Increased/decreased probability of below/above normal low flow days in Bogue Chitto during El Nino

Forecast Verification

1982-2005 CPC Nino 3.4 SST Forecast probabilities combined with historical composites to produce hindcasts for each month

Only 0.5 month leads utilized • Decreasing forecast accuracy with increasing

lead time

• 90-day outlooks for SFPO

Contingency tables used to compute statistical scores

Heidke Skill Scores

Least skill in southernmost basins • BXAL1, TYTM6,

and BSHL1

Mainstem and Yockanookany ranged from 0.13 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

kil

l S

co

re

Heidke

Skill relative to random chance

Ranked Probability Scores

Marginal skill indicated with RPS of 0.40 to 0.46

ENSO Composite Performance Using Ranked Probability Score

(Perfect = 0, No Skill = 1)

0.417874 0.420369 0.407035 0.4128810.436875 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

RP

S

RPS

Skill in predicting the proper category

Ranked Probability Skill Score

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

ENSO Composite Performance

Using Ranked Probability Skill Score

(Perfect = 1, No Skill <=0)

0.046397

0.0766430.063382

0.005853 0.0104950.019804

0.052056

-0.03231

-0.2

-0.1

0

0.1

0.2

KS

CM

6

OF

AM

6

EN

BM

6

JA

CM

6

MT

CM

6

BX

AL1

TY

TM

6

BS

HL1

Site ID

RP

SS

RPSS

Skill relative to climatology

Conclusions

Precipitation departures attributed to ENSO episode correlate well with streamflow events

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

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

Smaller basins can produce contradictory signals compared to larger mainstem basins

Future Research

Include lag-time response in computations

25th percentile hindcast verification

Ensemble streamflow prediction and Spring Flood Potential Outlook applications

Web access to composites and forecasts