enso forcing of streamflow conditions in the pearl river basin · composite analysis methodology...
TRANSCRIPT
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