homework
lym503829712
GEO344Lecture23-ENSO.pdf
GEO 344 Weather and Climate Prof. Stuart Evans
Lecture 23 El Niño and the Southern Oscillation
El Niño and the Southern Oscillation (ENSO) Atmosphere-ocean variability on interannual timescale
Large observable changes in temperature, precipitation, etc.
Lasts a few years at a time The atmosphere and ocean interact to make it happen
The warmest part of the Pacific is the western part ocean ocean ocean
Because ocean currents push warm water there trade winds
The water is pushed by easterly trade winds
Deep cold water rises up in the east
Wa rm
Wa ter
Slo wly
Lea ks t
o H igh
er
Lat itud
es
Every few years, the trade winds weaken…
HL
El Niño—Warm phase of ENSO La Niña—Cold phase of ENSO
El Niño and the Southern Oscillation (ENSO)
Warm water in central/eastern Pacific Convection follows warm water
Warm water in western Pacific Convection follows warm water
ENSO Sea Surface Temperature (SST) Anomalies
El Niño La Niña
Why we care about ENSO in a weather class
The first thing they mention when making a seasonal prediction
Seasonal prediction
NOAA CPC 3 month outlooks
• No longer a specific forecast of temperature or rainfall amount.
• Shows whether above normal, below normal, or near-normal is most likely
• Buffalo this summer: • Above average temperatures
occur in more than 50% of estimates
Seasonal prediction
NOAA CPC 3 month outlooks
• No longer a specific forecast of temperature or rainfall amount.
• Shows whether above normal, below normal, or near-normal is most likely
• Buffalo this summer: • Above average rainfall occurs
in 33-40% of estimates
Seasonal prediction
NOAA CPC summer drought outlook
• Uses the seasonal temperature and precipitation forecasts
• Makes prediction relative to present conditions
Measuring ENSO – Southern Oscillation High and low pressure shifts back and forth across the Pacific with the shifting ocean temperatures.
Noticed by Gilbert Walker in early 1900s. Wasn’t able to
completely explain it at the time.
Surface Air Pressure Anomaly
Gilbert Walker 1868-1958
“Walker’s hope was….to discover a starting point for a theory of world weather. It hardly appears to be working out like that.” Obituary in Quarterly Journal of the Royal Meteorological Society
Measuring ENSO – Southern Oscillation
Method 1: Use atmospheric pressure • Compare the difference in pressure
between two sites on opposite sides of the Pacific (Darwin, Australia and Tahiti)
Tahiti minus Darwin surface pressure: A measure of trade
wind strength
Darwin
Tahiti
Weak Trade Winds
Strong Trade Winds
Measuring ENSO – El Niño
Method 2: Use sea surface temperatures • Pick a region in the Pacific – calculate the
temperature anomaly. • Various regions exist (some for historical
reasons), the most popular is the 3.4 region
• The temperature anomaly is the Niño3.4 index
El Niño=Niño3.4 ≥ 0.5°C for 5 months
La Niña=Niño3.4 ≤ -0.5°C for 5 months
Measuring ENSO – El Niño
El Niño=Niño3.4 ≥ 0.5°C for 5 months
La Niña=Niño3.4 ≤ -0.5°C for 5 months
Method 1: Use sea surface temperatures • Pick a region in the Pacific – calculate the
temperature anomaly. • Various regions exist (some for historical
reasons), the most popular is the 3.4 region
• The temperature anomaly is the Niño3.4 index
Nino3.4 SST (5N-5S, 120W-170W)
major El Niño events
ENSO
Weak Trade Winds
Strong Trade Winds
Warm eastern waters
Cold eastern waters
Why do we call it both El Niño and the Southern Oscillation?
Because one is the ocean and the other is the atmosphere and they happen at the same time
What’s ENSO like today?
A weak/near neutral El Niño
We can forecast the evolution of ENSO
Predictions for rest of year
We can forecast the evolution of ENSO
The last time there was a big El Niño: 2015-16
ENSO QUICK LOOK August 20, 2015 A monthly summary of the status of El Niño, La Niña and the Southern Oscillation, or “ENSO”, based on NINO3.4 index (120-170W, 5S-5N) During late July through early-August 2015 the SST was at a strong El Niño level. All atmospheric variables support the El Niño pattern, including weakened trade winds and excess rainfall in the east- central tropical Pacific. The consensus of ENSO prediction models indicate continuation of strong El Niño conditions during the August-October 2015 season in progress. Some further strengthening into fall is likely, with the event lasting into spring 2016.
Early-Aug CPC/IRI Consensus Forecast1
JAS ASO SON OND NDJ DJF JFM FMA MAM 0
10
20
30
40
50
60
70
80
90
100
Time Period
Pr ob
ab ili
ty (%
)
ENSO state based on NINO3.4 SST Anomaly Neutral ENSO: −0.5oC to 0.5oC
2015 2016
Mid-Aug IRI/CPC Plume-Based Forecast2
ASO SON OND NDJ DJF JFM FMA MAM AMJ 0
10
20
30
40
50
60
70
80
90
100
Time Period
ENSO state based on NINO3.4 SST Anomaly Neutral ENSO: −0.5oC to 0.5oC
2015 2016
El Nino Neutral La Nina
Climatological Probability:
El Nino Neutral La Nina
Historical NINO3.4 SST Anomaly
1982 1986 1990 1994 1998 2002 2006 2010 2014 −2.5
−2 −1.5
−1 −0.5
0 0.5
1 1.5
2 2.5
3
←
NI NO
3. 4
SS T
An om
al y
(o C)
Time Period
MJJ Jul JAS ASO SON OND NDJ DJF JFM FMA MAM AMJ -2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
NI NO
3. 4
SS T
An om
al y
(O C )
DYN AVG STAT AVG CPC CON
Dynamical Model:
Statistical Model:
NCEP CFSv2 NASA GMAO JMA SCRIPPS LDEO AUS/POAMA ECMWF UKMO KMA SNU IOCAS ICM COLA CCSM3 MetFRANCE SINTEX-F CS-IRI-MM GFDL CM2.1 CMC CANSIP GFDL FLOR
CPC MRKOV CDC LIM CPC CA CPC CCA CSU CLIPR UBC NNET FSU REGR UCLA-TCD UNB/CWC
2015 2016
Mid-Aug 2015 Plume of Model ENSO Predictions
OBS FORECAST
IRI/CPC
Historically Speaking
El Niño and La Niña events tend to develop during the period Apr-Jun and they: - Tend to reach their maximum strength during Dec-Feb - Typically persist for 9-12 months, though occasionally persisting for up to 2 years - Typically recur every 2 to 7 years
1Based on a consensus of CPC and IRI forecasters, in association with the official CPC/IRI ENSO Diagnostic Discussion. 2Purely objective, based on regression, using equally weighted model predictions from the plume.
What actually happened
“There is a greater than 90% chance that El Niño will continue through Northern Hemisphere winter 2015-16, and around an 85% chance it will last into early spring 2016.”
NOAA/NCEP 13 August 2015
Forecast Verification
þ
ý
þ
þ
ý
þ
þ
Rainfall forecasts for the 2015-16 El Niño
þ
The forecast did pretty good!
A few other impacts of ENSO
Record Coral Bleaching Event 2014-16
• Longest, most widespread bleaching on record
• > 40% of global reefs affected • Previous global bleaching events
in 1997-98 & 2009-10
93% bleached 22% dead
2015: A Record Year for Tropical Storms in the Pacific
Chan-Hom
RaquelThree Category 4s
§ Record ACE § 20 storms ≥ Cat4
30 Aug 2015
22 Oct 2015
“…caused by El Niño...but more extreme because of anthropogenic forcing...” --Zhang et al, 2016, BAMS
A nc
ho vy
C at
ch (m
ill io
ns o
f to
ns )
1972-73
Anchovy fishmeal used as a feed supplement for cattle and poultryà worldwide impacts on food supply and prices
upwelling
1972-73 El Niño Leads to Collapse of Peruvian Anchovy Fishery
